Author: PETER BRIERE
Company: US GEOLOGICAL SURVEY

EARTH-SURFACE PROCESSES, MATERIALS USE, AND URBAN DEVELOPMENT -- A CASE STUDY OF THE SAN JUAN METROPOLITAN AREA, NORTHEASTERN PUERTO RICO

W. R. Osterkamp

ABSTRACT
	The San Juan Metropolitan Area (SJMA) was selected for case study as part of an investigation of "Earth-surface processes, materials use, and urban development:  understanding the human contributions to global geomorphological change” (ESPROMUD).  The ESPROMUD investigation, commissioned by the Scientific Committee on Problems of the Environment (SCOPE), has the goal of evaluating and understanding the effects of urbanization and resource extraction on earth-surface processes.
	Puerto Rico, a partially closed system (oceanic island) relative to water and mineral resources, and its capital city, San Juan, are struggling to meet the demands for water, building materials, and infrastructure development by an expanding population and unchecked urbanization.  The SJMA case study suggests a need for long-term, inventory-based policies and practices regarding the use and conservation of natural resources to avoid shortages of water and building supplies for housing and infrastructure expansion.  Resource inventories, both in proximity to the SJMA and island-wide, must be evaluated relative to projected demand in order to achieve realistic constraints for a stable urban economy and healthy ecosystems.  

INTRODUCTION
	In recent decades, stresses imposed by human activities on global systems have emerged as concerns to planners, earth scientists, environmental managers, and citizens.  Since glacial times, land-surface changes often have been dominated by human activities, particularly agriculture.  In addition to deforestation, land degradation, and soil erosion associated with farming, major transformations of landscapes and of fluvial and coastal systems continue to occur in areas of human settlements and mineral extraction.  The environmental effects of urbanization and related activities of mining, quarrying, and infrastructure development are principal factors promoting anthropogenic change in the nature and processes of the earth’s surface.
	The ESPROMUD (earth-surface processes, materials use, and urban development: understanding the human contribution to global geomorphological change) project is part of the Global Changes cluster within the scientific program of SCOPE -- the Scientific Committee on Problems of the Environment -- and is designed to evaluate the effects of urbanization and extractive industries on earth-surface processes.  An objective of the ESPROMUD activity is to compile and evaluate available information regarding these effects and thus to identify gaps in the understanding of the impacts.  The Río Grande de Loíza Basin/San Juan, Puerto Rico, exemplifies problems related to urban expansion and was selected for this pilot study.  
	Puerto Rico is an island far enough removed from other land areas that importation of water and rock resources is impractical (fig. 1A).  The island (fig. 1B) can be viewed as a partially closed system relative to availability of resources for urban development, and therefore the difficulties of accommodating the needs of an expanding population may be more easily recognized than they are elsewhere.  Moreover, the San Juan metropolitan area (SJMA) is typical of large cities worldwide by (1) having a coastal setting of fragile, near-shore ecosystems vulnerable to the stresses of human development, (2) being dependent on inland water and rock resources of nearby drainage basins, and (3) experiencing an urban growth rate greater than the regional rate.
	The 802-km2 (square kilometer) Loíza drainage basin, the largest of Puerto Rico, and the adjacent 67-km2 Río Piedras Basin were selected to analyze the effects that growth of the capital city, San Juan, has had on these rivers and the coastal zone into which they discharge (fig. 1C).  The growth was intensified after 1953 by the construction of Carraizo Dam on the Río Grande de Loíza, thereby increasing the potential supply of municipal water by impounding streamflow in Loíza Reservoir (or Lago Loíza).  The dam and reservoir originally were intended for the generation of hydroelectricity, but storage of water and extractions to satisfy as much as 58 percent of the metropolitan needs of the San Juan area (Fields, 1972) quickly stopped the generation of electricity.  The use of Loíza Reservoir, which collects runoff and sediment from the upper 540 km2 of the basin, as a storage facility for a municipal water supply was also complicated by sedimentation resulting in a 13-percent reduction in storage during the first 10 years and a 47-percent reduction in 1994 (Webb and Soler-López, 1997).  Urbanization was stimulated also, mostly in the 1960s, by ground-water development in support of industrial expansion.  The ground-water resources were recovered from karstic limestone that occurs in near-coast areas of Puerto Rico.  
	Increased land degradation and alteration of the drainage network prompted accelerated erosion and rapid sedimentation in the reservoir and the release of solid wastes and chemical contaminants to the surface- and ground-water systems.  Thus, the effects of urbanization and resource development now are impeding further development of land and water resources.   The Loíza Basin, a principal source of rock and water for many Puerto Ricans, has had (1) a high rate of urban expansion in recent decades, (2) active quarrying of building stone and excavation of sand and gravel to support population increase, and (3) abundant documentation of the impacts that urbanization has exerted on the fluvial and coastal systems.  These features make the Loíza Basin/San Juan area an instructive case-study example.

CHARACTERISTICS OF PUERTO RICO AND THE RÍO GRANDE DE LOIZA AND RIO PIEDRAS WATERSHEDS  
	Puerto Rico, an oval-shaped island, lies in the West Indies chain between Hispaniola and the Virgin Islands.  Centered at latitude 18 15’ N, longitude 66 30’ W, Puerto Rico has an east-west length of 176 km (kilometers) and a north-south width of 56 km;  the area is about 8780 km2.  Population centers are mostly within 7 km of the coast, a zone in which about 85 percent of the 3.8-million inhabitants live (Rodriguez, 1994).  A central mountain chain, the Cordillera Central, extends eastward through much of Puerto Rico’s length and typically exceeds 600 m (meters) above sea level (fig. 1B);  peaks are as high as 1338 m.  An eastern segment, the Sierra de Cayey, forms part of the southern drainage divide of the Loíza Basin, and western portions of the Sierra de Luquillo provide runoff to the eastern part of the basin (fig. 1C). 

Physiography, Geology, and Soils 
 	Major physiographic units of Puerto Rico, largely based on geology, are Upland, Northern Karst, and Coastal Plain (Monroe, 1980).  The upper four-fifths of the Loíza Basin are in the mountainous Upland province, an erosional landscape of granodiorite and volcaniclastic rocks of late Cretaceous and early Tertiary age (fig. 2, B and C).  In the Piedras and lower Loíza Basins, thick limestones with basal clastic beds of Tertiary age (fig. 2, D) unconformably overlie the fractured igneous rocks, and in some near-coast areas are covered by a veneer of silt and sand (fig. 2, E).  The clastic and carbonate beds occur locally along the southern coast of Puerto Rico, but the limestones that define the Northern Karst province are mostly exposed as a band, up to 23 km wide, along the northern coast west of the Piedras and Loíza Basins (Glover, 1971; Monroe, 1980).
	The Coastal Plain province consists predominantly of surficial material resting unconformably on the older rocks.  The sediment, mostly poorly consolidated Quaternary-aged silt, sand, and gravel, is a variety of flood-plain, coastal-plain, and beach deposits (fig. 2, E).  In the lower Loíza Basin, Coastal Plain deposits generally blanket karstic limestone, thereby minimizing surficial expression of the Northern Karst province.	
	Most soils of the Upland province are acidic, clay-rich, and highly erodible, reflecting deep weathering of the volcanic rocks from which they are derived.  The southeastern Loíza Basin has loamy soils derived from weathered granitic rocks.  Where flood-plain, terrace, and coastal-plain sediment overlies volcanic or carbonate rocks of the Loíza and Piedras Basins, soils are well drained and sandy.  Detailed descriptions of soil characteristics of the upper Loíza Basin were given by Hunt (1976), and Boccheciamp (1978) mapped the soils of the San Juan area.

Climate
	 Puerto Rico has a tropical marine climate of average temperatures ranging from 20 C in the mountains to 26 C in coastal areas (Picó, 1969).  The central mountain range is a barrier to northeast trade winds and casts a rain shadow over most of southern Puerto Rico.  Rainfall in the northern island averages about 2000 mm (millimeters) annually, whereas southern areas average about 1100 mm (Calvesbert, 1970).  Annual precipitation in the Loíza Basin generally averages between 1570 and 1700 mm;  the basin-wide mean is about 1600 mm but rainfall locally reaches about 2500 mm due to an orographic effect of the mountains.  San Juan, representative of the Río Piedras drainage basin, has a mean annual precipitation of about 1530 mm. 
	Rain in Puerto Rico typically falls as short-duration showers of 5 to 10 mm throughout the year, the 4-month period of August through November generally accounting for nearly half of the annual total.  Hurricanes and other tropical storms, such as Hurricane Hortense, September, 1996, may yield as much as 70 percent of total rainfall for a year during a one- or two-day period (Palm and Hodgson, 1993; Larsen, 1997).  

Population, Land Use, and Economy 
	The population of Puerto Rico, presently about 3.8 million, rose 84 percent from about 1.9 million in 1940 to 3.5 million in 1990, and in the Loíza Basin population rose from 93,200 in 1971 (Hunt, 1976) to 350,000 in 1990.  From 1950 to 1990 the population in the Caguas area increased 460 percent, from about 34,000 to 191,000.  About 1.22 million people, mostly in the Río Piedras Basin, lived in the San Juan, Bayamón, Guaynabo, and Carolina districts (fig. 1C) in 1990 (U. S. Department of Commerce, 1992).  By 1984, 75 percent of the Piedras Basin was urban, and projections suggest that the entire basin will be developed by the year 2000 (Gellis, 1991). 
	The rapid rise in Puerto Rico’s population, and that of the SJMA in particular, is related to economic changes of recent decades.  Through nearly 70 years following American occupation in 1898, about three-fourths of Puerto Rico, including the Loíza Basin, was cropland and pasture converted from forest.  Sugar cane, pineapple, coffee, and tobacco accounted for 31 percent of net income and 80 percent of exports in 1940 (Brockmann, 1952).  In the 1950s, tax-exemption programs aided industry and manufacturing, such as pharmaceutical companies, and by the mid-1960s northeastern Puerto Rico especially had changed from an agrarian to an industrial economy.  The shift caused abandonment of many farms and thus re-conversion to a 35-percent forest cover by 1985 (Gellis and others, 1998).  In the Upland and Northern Karst provinces, urbanization into adjacent woodlands or renewed agriculture will be hampered by topography.  Half of Puerto Rico has slopes greater than 45 percent, which, if developed, would be unstable and highly susceptible to failure and erosion (Cerame Vivas, 1989; Larsen and Torres-Sánchez, 1996; Larsen, 1997).      
	The changes in the economic base prompted migration to urban areas, especially San Juan and Caguas (fig. 1C).  By 1991, the Loíza Basin was 8 percent urban (Larsen and Torres-Sánchez, 1992), resulting in congestion in San Juan, spillover into nearby towns, and decentralization of industry.  In 1940, 70 percent of the island’s population was rural as opposed to about 21 percent now.  Furthermore, many rural residents commute to city jobs so that few in northeastern Puerto Rico escape the urban influence (Hunter and Arbona, 1995).   

FLUVIAL SYSTEM, RIO GRANDE DE LOIZA AND RIO PIEDRAS 
	Puerto Rico is drained by streamflow except in the karst area, where much of the runoff infiltrates and recharges limestone aquifers.  The Río Grande de Loíza, which enters the Atlantic Ocean about 10 km east of San Juan (fig. 3), supplies much of the water for the SJMA.  The less extensive and largely urbanized Río Piedras Basin (figs. 1C; 3), immediately west of the Loíza Basin, contributes a small proportion to the water needs of the SJMA.  Data summarizing mean discharge, Q, in cubic meters per second (m3/s), unit discharge, Qu, in cubic meters per second per square kilometer (m3/s/km2), and sediment yield, in (metric) tons per square kilometer per year (Qsy), are listed by U. S. Geological Survey (USGS) station number (Sta. No.) for gage sites of the two basins in Table 1.  Also given are station names and the drainage area (DA) of each, in square kilometers.  Instantaneous peak discharges (Qp) are provided for selected gage sites. 

Streamflow
	Unit runoff from the Loíza and Piedras Basins, respectively, averages about 0.025 and 0.035 m3/s/km2 (table 1).  Compared to average precipitation for the two basins, these rates suggest that about 50 and 72 percent of rainfall, respectively, results in streamflow in the two basins.  The remainder of rainfall is mostly lost to the atmosphere as evapotranspiration.  The relatively high unit discharges of the Piedras Basin are assumed to be caused in part by high runoff rates typical of urban settings with extensive paved areas.  Mean discharge into Loíza Reservoir is about 13 m3/s and recent releases of water from Loíza reservoir have averaged about 6.4 m3/s (table 1, number 15);  the difference is mostly extractions averaging about 3.5 m3/s for municipal water use (Gellis and others, 1998), infiltration, and evapotranspiration.  Based on runoff relations for non-urbanized areas of the Loíza Basin, mean discharge from the Piedras Basin prior to urbanization was about 1.7 m3/s.  Extrapolating recent discharge records for the gage site Río Piedras at Hato Rey, PR (table 1, number 19), runoff to the ocean from the Río Piedras Basin now averages about 2.3 m3/s.  Thus, urban growth of the SJMA has, through extractions for public water supplies, reduced streamflow in the Río Grande de Loíza, but has increased flow in the Río Piedras through increased waste water originating in the Loíza Basin, reduction of infiltration, and increased runoff from paved areas.      

Floods
	Flooding in the Loíza and Piedras Basins can occur any time but is most likely during the hurricane season.  An exception occurred in January, 1992, when flooding was due to a cold front combined with an upper-level trough (Torres-Sierra, 1996).  Instantaneous peak discharges of the last 36 years in the Loíza Basin at the Caguas, Juncos, Gurabo, and Carolina gage sites (fig. 3; table 1, numbers 6, 12, 13, and 16) occurred during Hurricane Donna on September 6, 1960.  Peak discharges during a hurricane of August 4, 1945, probably exceeded those of 1960 at many sites in the Loíza Basin, but accurate records are not available (Fields, 1972).  Other peak discharges listed in Table 1, for numbers 1, 4, 5, 6, 7, 9, and 10, occurred during the frontal storm of January 6, 1992 (Torres-Sierra, 1996).  Estimates of peak discharges of September, 1996, during Hurricane Hortense suggest that most flows did not exceed those of a return period of 50 years (Torres-Sierra, 1997).
  	Streamflow records at the Río Piedras at Hato Rey gaging station (table 1, number 19) began in 1970 and indicate a peak discharge on June 17 of that year.  For the period 1960 through 1993, 49 and 57 percent of the floods, respectively, at the Caguas and Gurabo gage sites occurred during the August through November period (Gellis and others, 1998).
	Mean discharges for 18 gaging stations and instantaneous peak discharges for ten gaging stations are given in Table 1;  at several sites, numbers 4, 5, 7, 9, 10, and 16, the period of record is short and poorly representative (at number 16, mean discharge is unavailable owing to tidal conditions of measurement).  Nevertheless, at eight sites of the Loíza Basin, the peak discharge ranged from about 160 to 1050 times mean discharge;  peak discharge for a 26-year period at the Río Piedras at the Hato Rey site (number 19) was about 170 times mean discharge.  This measure of discharge variability signifies the flashy streamflows that are typical of areas in the tropics that are subject to intense storms and hurricanes.    
Sediment Yield

	Sampling for suspended sediment at two gage sites in the Loíza Basin began in 1984, and therefore undisturbed conditions are not represented.  Sediment yields in the Loíza and Piedras Basins are listed in Table 1 for 18 sampling sites.  Average annual yields for the Caguas and Gurabo sites, numbers 6 and 13 (table 1) are given for the 1984 through 1994 water years.  Most other sediment yields are for the 1991, 1992, and 1993 water years only, a period of slightly deficient runoff and sediment discharge in Puerto Rico (Gellis, 1991;  Gellis and others, 1998).  Sediment yields for Río Cayaguas (no. 3) and Río Canóvanas (no. 17) are for the 1991 through 1995 water years (Larsen, 1997).       
	Sediment-transport curves developed by Gellis and others (1998) and Larsen (1997) show that 80 percent or more of sediment moved by the Río Grande de Loíza typically occurs naturally during the several days each year of highest streamflow.  The anomalously high sediment yield measured in Río Piedras Basin at El Señorial (table 1, number 18) is for 1988 and 1989, when active housing construction accounted for about 6 percent of the basin area (Gellis and others, 1998).  Number 19 (table 1) refers to a very small watershed of a construction area and also illustrates the erosion and sediment movement that are possible in northeastern Puerto Rico under disturbed conditions (Gellis, 1991).  Thus, the greatest potential for destructive erosion and large amounts of sediment delivered to and deposited in Loíza Reservoir is when a hurricane or other intense-rainfall event occurs in disturbed areas.
	Sediment-yield data of Table 1 are too variable and not numerous enough to show strong relations to high-intensity storms or land disturbance, particularly because comparative data from relatively pristine drainage basins are sparse.  In the now undisturbed Luquillo Experimental Forest (fig. 2), which borders part of the Loíza Basin to the east, the sediment yield, 1991 through 1995 water years, from a 3.3-km2 watershed of the upper Icacos River was 525 tons/km2/yr.  In a 17.8-km2 watershed of the Mameyes River, which flows northward from the experimental forest, the sediment yield was 117 tons/km2/yr (Larsen, 1997).  Compared to sediment yields of the Río Grande de Loíza, from watersheds of similar size, these yields may not be high.  Relative to yields from basins in other tropical areas, the sediment yields of the Icacos and Mameyes Rivers are high and possibly reflect the effects of landslide disturbance along road corridors and of recent hurricanes (Larsen, 1997; Larsen and Parks, 1997). 
	Using erosion-prediction technology (the Universal Soil Loss Equation), Molinelli (1982) estimated soil loss in the Loíza Basin for forested, pasture, cropland, and construction areas, respectively, to be 1950, 2490, 7750, and 7870 t/km2/yr.  Gellis and others (1996) measured sediment discharges from small plots at construction sites that yielded sediment averaging about 30 to 40 times that of forest and pasture sites and 10 times that of croplands.  These estimates, which are partly supported by data of Table 1, are useful for distinguishing variation in sediment yield with land use, but do not account for sediment entrained by overland flow and later stored at the bases of hillslopes or on bottomland surfaces.  Hence, measured sediment yields are generally lower than estimates of erosion rates.  The estimates suggest that sediment yields of the last century in the Loíza Basin may have been greatest prior to 1950, when farming dominated land use.  Sediment yields during the last 30 years possibly decreased as farms were abandoned and the lands reverted to forest.  As development related to the urban areas of San Juan and Caguas continues in the Loíza Basin, accelerated sediment discharge from construction sites may in turn accelerate the sedimentation rate in Loíza Reservoir and thereby further threaten the water supply that has facilitated urbanization.

Water Quality
	Due to a generally granitic/volcanic geology and moist climate, dissolved-solids concentrations in runoff of northeastern Puerto Rico are typically less than 300 mg/l (milligrams per liter);  pH values of streamflow are consistently in the range of 7.0 to 7.9 (Díaz and others, 1995).  Concentrations of specific dissolved constituents generally do not exceed health-standard limits, but locally and for short durations the concentrations of constituents such as manganese, iron, and various organic compounds may exceed recommended limits (Díaz and others, 1995).  Where ground-water inputs to streams have been contaminated by municipal landfills, hazardous-waste dump sites, and various illegal disposal sites, streamflow may be highly contaminated with mercury and other heavy metals, solvents, and pesticides (Hunter and Arbona, 1995).    
	High concentrations of fecal-coliform (FC) and fecal-streptococci (FS) bacteria are a major surface-water quality concern throughout Puerto Rico, but the problem is most acute in streams draining urban areas, industrial parks, and suburban communities lacking sewage systems.  As noted by Hunter and Arbona (1995, p. 1336), the combined processes of urbanization and industrial growth have "been so rapid as to create a serious lag in the provision of infrastructure services;  unauthorized domestic hookups always exceed already-saturated plant capacities.  Thus at the majority of sewage facilities, the supply of human waste outstrips treatment capacity and raw sewage is consequently discharged into streams.  The risk to human health is obvious.”   
	Díaz and others (1995) found that the highest concentrations of FC and FS in surface waters of Puerto Rico generally occur in streams draining densely populated and industrialized areas of the island.  In 1994 the highest levels of surface-water contamination by FC and FS occurred at sites in the SJMA.  At the Quebrada Blasina near Carolina, PR, water-quality sampling site (fig. 3, number 0603), in the eastern San Juan area, for example, concentrations of FC and FS in water samples collected during the 1994 water year were as great as 530,000 and 120,000 colonies per 100 milliliters, respectively.  Similarly, at the Río Piedras at Hato Rey (fig. 3, number 0491; table 1, number 19), water-sample concentrations of FC in 1994 varied from 37,000 to 600,000, and FS ranged from 7,200 to 730,000.  These concentrations compare to a standard maximum microbiological-contaminant level of 1000 colonies FC per 100 milliliters for raw recreational water in the United States (Hunter and Arbona, 1995).  
	In addition to receiving wastes in runoff from the metropolitan area, streams of San Juan also receive waste-laden effluent from sewage-treatment plants and untreated sources in the upper parts of the Loíza and Piedras drainage basins.  Elsewhere in Puerto Rico, the main sources of contamination in surface-water systems are liquid wastes from industrial and municipal sources (Díaz and others, 1995).  

ENVIRONMENTAL ISSUES OF THE LOIZA BASIN 
	Puerto Rico is susceptible to earthquakes, tsunamis, hurricanes and coastal floods, riverine flooding, landslides, and subsidence (Palm and Hodgson, 1993);  natural hazards are exacerbated by urbanization.  Water pollution, accumulation of solid wastes, and erosion accelerated by development, all direct consequences of urban growth, however, may be of greater economic and social significance than are natural hazards.  Cerame Vivas (1989, p. 109) commented that the "main ecological problem of Puerto Rico by the year 2000 will be population.  All of the environmental problems that we confront emanate from our overpopulation.” 

Natural Hazards 
	Puerto Rico is near the subduction zone and boundary of the North American and Caribbean tectonic plates and thus is susceptible to large earthquakes and tsunamis (Hays and Gori, 1984).  In 1918 a magnitude-7.5  earthquake destroyed many buildings on stream-valley alluvium and coastal-plain sediment and was accompanied by a tsunami on the west coast (Reid and Taber, 1919).  Parts of Puerto Rico most subject to damage by earthquakes and tsunamis are coastal and flood-plain areas of Río de Bayamón (in the Toa Baja, Cataño, and Bayamón districts, fig. 1C), Río Piedras, and Río Grande de Loíza, where urban expansion has been most intense (Molinelli, 1987).   
	Hurricanes are common in Puerto Rico and other coastal areas of the Caribbean region where they cause storm surge, high wind and rainfall,  tornadoes, and flooding.  Storm surge (a rapid rise in sea level) is the main danger to coastal population centers of Puerto Rico and accounts for a large majority of hurricane-related deaths (Pielke, 1990, p. 59).  Storm surge of hurricanes and other tropical storms also causes coastal and bottomland flooding.  Effects of high wind and heavy rain, as much as 400 mm in a day, include extensive erosion, flooding, and plant-biomass reduction (Scatena and others, 1993).
	The most recent major storm to affect Puerto Rico was Hurricane Hortense, September 9 and 10, 1996, which yielded up to 600 mm of rain in the Sierra de Luquillo (fig. 1C) and generally in excess of 400 mm throughout the rest of eastern Puerto Rico (Torres-Sierra, 1997).  Flooding was extensive along the Río Grande de Loíza and other streams draining northeastern Puerto Rico, causing major damage to urban areas and coastal-plain infrastructure.  In some urban areas the flooding and resulting property damage were compounded by inadequate or clogged drainage systems.  Channel alterations in the Loíza Basin included scour, aggradation, and widening.  Estimates suggested that storm-related sediment loads in the Loíza Basin, partly due to numerous debris flows, soil slips, and landslides, equaled the mean annual sediment yield.  Much of the sediment was deposited in Loíza Reservoir, further reducing its storage capacity.  Damage to forests was minor, but crop damage was extensive.  Power and water were disrupted to nearly all of Puerto Rico, and damage to streets, highways, and distribution systems for food and water was also substantial.  Coastal-plain flooding caused only limited damage to the San Juan municipal landfill, but movement of solid and toxic wastes to the fluvial network from other landfill sites may have been  extensive (M. C. Larsen, USGS, written commun., 1996; F. N. Scatena, USDA Forest Service, oral commun., 1996).     

Hazards Aggravated by Urban Development
	Deep weathering and soil development in eastern Puerto Rico causes steep, unstable slopes that are vulnerable to mass movement, especially when induced by heavy rainfall and treefall.  Hurricane Hugo, with rainfalls exceeding 200 mm in September, 1989, triggered numerous landslides;  71 percent of the landslides that occurred during and shortly following the hurricane in the Sierra de Luquillo were also related to road construction (Larsen and Torres-Sánchez, 1992).  Landslide activity was absent in the Loíza Basin, immediately to the west, where rainfall did not exceed 200 mm (Larsen and Torres-Sánchez, 1992).  Hundreds of landslides, however, were triggered by Hurricane Hortense in the upper Loíza Basin (M. C. Larsen, USGS, written commun., 1996).  Road construction and related urban disturbances strongly increase sediment yield through mass-movement processes as well as by normal hillslope and fluvial erosion (Molinelli, 1984; Larsen and Parks, 1997).
	Dissolution of limestone by ground-water circulation occurs  naturally in the Northern Karst province, resulting in caverns and sinkholes.  In urban areas of Puerto Rico these processes are enhanced by fracturing induced during earthquake activity, and collapse may be induced where ground-water withdrawals or drainage of wetlands reduce pore pressures in the aquifer.  The imposed weight of  large buildings, highway structures, and artificial fill can be sufficient to cause bedrock failure in karstic areas (Griggs and Gilchrist, 1983).     
	Sediment is volumetrically the biggest contaminant of water in Puerto Rico.  Sediment increases water turbidity of reservoirs and lakes, which degrades water for consumptive use, fouls water-distribution networks, and destroys aquatic habitat.  Riverine sedimentation decreases the runoff capacity and affects flood frequency, fills navigable channels, and reduces soil fertility on agricultural land.  Sedimentation in reservoirs throughout Puerto Rico is reducing storage capacity and water availability (Molinelli, 1982; Webb and Soler-López, 1997). 

Environmental Effects of Urbanization
	Among the inevitable by-products of urban expansion are loss of wildlife habitat, solute loading of water supplies, air pollution, soil loss, and the generation of solid and toxic wastes.  As previously noted, pollution of surface and ground waters in the Loíza and Piedras Basins is in some cases extreme, and accelerated erosion due to soil disturbance adversely affects agricultural production, generation of hydropower, and public water supplies.  In the San Juan area, environmental degradation due to air pollution is minimized by strong trade winds and consequently is probably less intense than for most urban areas of the world (Calvesbert, 1970).
	Besides the impact on water resources, the most problematic environmental effect of urbanization in Puerto Rico may be the handling of solid wastes, including toxic substances.  The population of 3.8 million, about 430 persons/km2, generates a daily average of about 5000 tons of solid waste, one of the highest amounts, per capita, in the United States (Hunter and Arbona, 1995).  Almost all of these wastes are derived from imported foods, raw materials, and manufactured goods, and about a third, by weight, mostly metals, glass, plastics, and some wood products, is not readily subject to biodegradation (Cerame Vivas, 1989).
	Thirty of 61 landfills in Puerto Rico, including the San Juan landfill, will be full within 4 years and nearly two-thirds may be unusable within 10 years.  The San Juan landfill, in an urban marsh, receives about 1000 tons of solid waste daily and has reached its legal height limit of 22 m;  a section of the city’s main water-supply pipe underlies the landfill, causing concerns of contamination by toxic leachates (Hunter and Arbona, 1995).  At least 14 dumps, two of which are Superfund sites (Cerame Vivas, 1989), containing hazardous waste are in the Loíza Basin;  materials in the dumps are not well documented but probably include pharmaceutical chemicals, petrochemicals, and wastes from paint production and the apparel industry (Hunter and Arbona, 1995).
	The area of Puerto Rico and its distance from other potential disposal or recycling sites, the population density, and an undeveloped recycling capability combine to cause a steady increase of solid wastes.  Assuming a 1989 cost of $60 per person/year to handle and dispose of wastes, the annual cost, adjusted for inflation, for refuse management by the year 2000 may approach $1 million per day (Cerame Vivas, 1989).  Much of this expense will be borne by the SJMA, and does not include the disposal of presently stored wastes in landfills of the Loíza Basin.

IMPACTS OF QUARRYING AND MINING, LOIZA AND PIEDRAS BASINS
	Historic mining activity in Puerto Rico has included the extraction of lime for cement but started with the mining of gold placers by Spanish settlers and the development of iron and copper deposits (Cardona, 1984).  In the Loíza Basin, known metalliferous deposits are limited to lenses of magnetite in beach sands (Monroe, 1977) and small amounts of copper, molybdenum, gold, and silver associated with sulfide minerals disseminated in quartz diorite and volcanic rocks or concentrated along shear zones (Pease, 1968).   Potential metallic ores in the Loíza Basin may include iron and copper skarns and copper or copper-gold bearing porphyries (Orris and others, 1992).   The only significant recent extractions of metal ores in the Río Grande de Loíza Basin have been at the Keystone iron-skarn and Island Queen copper-skarn mines near Juncos, in the southeastern part of the basin (fig. 3).  About 8 km apart, the two mines yielded a total of about 200,000 tons of ore in 1951 through 1953.  Assuming a weight of 2 tons/m3, the extracted ore amounted to about 100,000 m3, and a similar volume of deposited wastes (Hooke, 1994).    
		Most quarrying in Puerto Rico presently is related to the production of cement and concrete, and is nearly restricted to industrial (non-metallic) minerals, limestone, volcanic rocks, and sand from weathered igneous-intrusive rocks.  Continuing urbanization and conversion to an economy based on tourism and industry depend on the availability of construction materials.  Especially during a construction boom of the 1950s and early 1960s, alluvial deposits along the Río Grande de Loíza and elsewhere (including beaches) were extensively mined (Rodriguez, 1994).  Downstream from Loíza Reservoir and along the lower reaches of the Río Guaynabo (in the Río Bayamón Basin) and the Río Piedras, large volumes of sand and gravel were taken from steadily expanding pits dug into alluvium of the bottomlands;  an analysis of 1990 aerial photography showed 64 sand and gravel pits in the basin (Gellis and others, 1998).  
	By 1969, when the estimated rate of extraction of sand and gravel from the channels and flood plains of the Río Grande de Loíza stream network was 51,500 m3/month, channel deposits were nearly depleted and operations were shifting to alluvial-terrace sources (Hunt, 1976).  Despite claims only 2 decades ago of "virtually unlimited supplies of sand and gravel” from alluvium of the lower Loíza Basin (Monroe, 1977), the onshore resource of Puerto Rico has been essentially depleted, and increasingly the coastal and offshore supplies are being developed;  projections suggest that terrestrial sources of sand and gravel will be exhausted by the year 2000.  Results have been inadequate supplies of coarse fluvial sediment to replace the bottomland and coastal-zone losses, beach erosion, and environmental degradation that is proving harmful to the tourist-based economy (Rodriguez, 1994). 
	The depletion of sand and gravel resources has led to the development of alternative sources of building materials.  Crushed stone is mined from large open-pit quarries in unweathered volcaniclastic rocks near Loíza Reservoir, and sand is extracted from highly weathered granodiorite in the vicinity of San Lorenzo (fig. 3).  An inevitable consequence of mining sand from saprolitic horizons of the granodiorite is release of fine sediment to the upper Río Grande de Loíza and thus to Loíza Reservoir.  In lower parts of the Loíza Basin, limestone of the Northern Karst province yields construction stone.  The limestone, however, is also a principal source of ground water in near-coast areas, and quarrying of the limestone has reduced ground-water availability and the potential for urban and industrial development.
		Availability of industrial minerals has been essential to the growth and development of Puerto Rico.  Data from the Puerto Rican Department of Natural Resources indicate that the material moved from sand and gravel pits and rock quarries active in 1996 ranged between 100 and 3000 m3/day/pit and averaged a total of about 17,000 m3/day.  Assuming a mean density of 2 tons/m3 and 250 working days per year, about 34,000 tons of material were moved daily and about 8 x 106 tons, or 4 x 106 m3, are extracted annually.  Dividing these estimates by the population of the SJMA, approaching 1.3 x 106, suggests an annual per-capita consumption of industrial minerals of 6 tons, or 3 m3, about a sixth that of the per-capita consumption in the United States (17 m3).
	Most of the population growth of Puerto Rico presently occurs in the SJMA, and therefore the major use of building and construction materials also occurs in the San Juan area.  If a mean thickness of 0.2 m of concrete, building stone, and other rock construction materials is assumed for the San Juan area (K. G. Renard, Consulting Engineer, oral commun., 1997), a total volume for the metropolitan area is roughly 1 x 108 m3, or nearly 80 m3/person.  Based on recent estimates of population increases (U. S. Department of Commerce, 1992) and a population density approaching 3300 persons/km2, the annual increase in size of the SJMA is about 15 km2, and the annual use of industrial minerals is about 4 x 106 m3, the same volume, estimated from records of the Department of Natural Resources, of sand, gravel, and rock extracted annually from the Río Grande de Loíza Basin.	
	A 1992 mineral assessment (Orris and others, 1992) indicated that there were 19 active quarrying sites and about 40 sites of previous activity in the Loíza Basin.  Recent data, acquired from the Puerto Rico Department of Natural Resources, list 29 active quarries in the basin.  In addition to sand (from weathered granodiorite) and limestone, current extractions are crushed and broken rock of several kinds, volcanic aggregate, and surficial "fill”.  The 1992 assessment indicated that other sources of construction materials are calcareous clastic rocks, beach and swamp deposits, and landslide and breccia areas.  Marine volcaniclastic rocks are quarried at various sites in the Loíza Basin.
	Importation of industrial minerals to support construction has proven costly to neighboring islands in the Caribbean region (Rodriguez, 1994).  In contrast to most metals, industrial minerals are rarely traded on the world market.  Transportation costs of such bulk materials require that they be obtained within reasonable proximity to the location of their intended use.  This economic aspect poses a particular burden for small island nations with rapidly growing economies and populations.  Governmental officials of Puerto Rico are beginning to realize prospective limits to growth imposed by depletion of its mineral-resource base.

URBAN GROWTH IN THE LOIZA AND PIEDRAS BASINS 
	The Río Piedras Basin is almost totally urban, and coastal-plain portions of the Río Grande de Loíza Basin are rapidly approaching total urbanization.  Presently, however, about 8 percent of the Loíza Basin, mostly near the Atlantic coast, is urban, the remainder, in the Upland and Northern Karst provinces having forest (about 21 percent), agriculture and pasture (about 58 percent), and other rural land uses (about 11 percent) (Gellis and others, 1998).  Further urbanization of the Piedras Basin is not feasible, and urban expansion in the Loíza Basin during the next few decades is likely to be concentrated in presently suburban communities of the Coastal Plain province east of San Juan and Carolina.
	Table 2 lists projected water-use needs, relative to those of 1993, for the years 2000, 2020, and 2050;  the projections were compiled by Ocasio (1996).  Entries for the San Juan metropolitan area (SJMA, table 2) are sums of anticipated needs for the Bayamón, Canóvanas, Carolina, Cataño, Guaynabo, San Juan, Toa Baja, and Trujillo Alto districts of the combined urban area.  Residential water use, reflecting expected population increase, is likely to increase the most from 1993 to 2050, an average of 46.8 percent for all of Puerto Rico.  Although some of the increase, 28.3 percent to 302,000 m3/day, will be in the SJMA, much of the remaining expected increase will be in areas that become urbanized during the next half century.  Like that of the Río Piedras Basin, water use in the present SJMA cannot increase greatly because the area is already almost totally developed.  There is, however, significant potential for increased urban growth and water use in parts of the Loíza Basin to the east of San Juan.
	Presently available water resources are inadequate to accommodate significant increased usage in the next 50 years by industry, whereas commercial use necessarily must increase to support an expanding population.  Projected total water needs will not increase as strongly as will residential needs, in part because progressively smaller percentages of a finite water resource will be used for other purposes (table 2).  If new technologies for water development, such as desalinization of sea water (Zack and Larsen, 1993), are implemented (which, owing to cost, is unlikely in the near-term), water use by the various components of the population and economy could increase significantly.
	Although Puerto Rico is a water-rich island, present water demands during periods of normal streamflow are approaching the capacity of those streams to supply water of adequate quantity and quality.  Recent drought conditions have demonstrated the precarious nature of water availability and its distribution in Puerto Rico and to the SJMA.  The New York Times (June 22, 1997) reported that ".....Puerto Rico ordered water rationing in San Juan and nearby areas last week because of low rainfall, heavy use of water, and limited reservoir space.  It is the second time that rationing has been ordered in three years.”  The report continued that officials of the Puerto Rico Aqueduct and Sewer Authority began ".....to shut off water service every other day in sections of San Juan and nearby Carolina, Trujillo Alto, and Canóvanas.  Government officials estimated that about 500,000 people would be subject to the rationing.”  The water crisis prompted statements by the governor, Pedro Rossello, that he would press for long-term projects to remedy the problem, but the newspaper did not record how the projects might alleviate water shortages or increase the available supply of water.  Recently, a $1.8 billion program of the Puerto Rico Commonwealth was initiated to alleviate water shortages through surface-water and ground-water extractions (M. C. Larsen, USGS, written commun., 1998).

IMPACT OF URBANIZATION ON EARTH-SURFACE PROCESSES 
	The main changes to earth-surface processes by urbanization in the Río Grande de Loíza and Río Piedras drainage basins have been land disturbance, erosion, sediment transport (including the movement of contaminants adsorbed on sediment), and elimination of slope-stabilizing vegetation.  Landfills and dump sites contain materials susceptible to fluvial transport during high-intensity storms, and quarries become sites of stored sediment and generally have areas of bare surface vulnerable to erosion.  Urbanization creates impervious surfaces that yield high rates of runoff during rainfall, and thus tends to increase flood peaks.
	The impacts to surface processes by changes in sediment discharge and storage are self evident and have been described in previous sections of this discussion.  The consequences of flashy streamflow include increased flooding, possible channel erosion, and flood-plain deposition.  The effects of these changes to biota are unclear, but the entrainment and bottomland deposition of contaminated sediment certainly may be injurious to human health, the food network, and agricultural productivity.     

QUANTITATIVE CHANGE TO RIVER SYSTEMS
	The main modification made to the channel network of the Río Grande de Loíza, was the construction of Carraizo Dam in 1953 about 22 km upstream from the river mouth.  Results have been storage of water and sediment in Loíza Reservoir, channel scour downstream from the dam, possible sedimentation in the lower reaches of the Río Grande de Loíza, and interbasin diversion of streamflow to the San Juan area.  Regulation of streamflow has minimized flood peaks except at times of extreme events (such as Hurricane Hortense), assured downstream water users of low-flow discharges during periods of drought, and increased the potential for flooding through induced sedimentation in the lowest reaches of the Loíza channel.  Reduced flows below Loíza Reservoir, by decreasing channel conveyance, have increased flood potential in densely-populated bottomlands along the downstream reaches.   
	The original storage capacity of Loíza Reservoir was about 26.9 million m3, and was increased to nearly 30 million m3 by flashboard additions in 1977, but by 1995, due to sedimentation, had been reduced to about 14.2 million m3 (Webb and Soler-López, 1997).  In recent decades the reservoir has provided nearly 60 percent of the water needs for the SJMA, but that percentage is likely to decrease as new engineering approaches to import water to San Juan are implemented.  Among these plans are the development of a large aqueduct to San Juan from the western end of the island and construction of water-supply dams and reservoirs upstream from Loíza Reservoir on the Río Grande de Loíza at Quebrada Arenas, Río Cayaguas at Cerro Gordo, and Río Valenciano upstream from Juncos (fig. 3). 
	Lago Las Curias, in a headwater part of the Piedras Basin, provides minor regulation of Río Piedras streamflow, and all urban reaches of the lower Río Piedras have been channelized or otherwise altered.  A water-intake and water-treatment plant on the Río Piedras provided the original source of municipal water for San Juan.  Except during periods of water shortage, these facilities are no longer used owing to pollution of the Río Piedras (F. N. Scatena, USDA Forest Service, oral commun., 1996).    

COASTAL PROCESSES AND TRANSFORMATIONS 
	Puerto Rico’s north coast at San Juan is a microtidal, storm-dominated area of Quaternary alluvial and nearshore deposits overlying the Tertiary limestones of the Northern Karst province.  Where they remain, sandy pocket beaches, lagoons, mangrove forests, and grass marshes protect inland coastal-plain areas from wave erosion.  Geomorphic features include coral reefs and eolianite and cuspate-beach ridges near the mouths of Río Grande de Loíza and other major streams entering the Atlantic Ocean (Webb and Morton, 1996).
	Mangrove species concentrate fine sediment and organic matter in tidal-flat and shallow-water lagoonal environments.  The mangroves provide high species diversity of marine fauna and flora, form habitat for native and migratory birds, help to buffer inland areas from storm-surge and related wave-action processes, and provide recreational and tourist income to the Puerto Rican economy.  
	Principal lagoons and estuaries of the San Juan area are Bahia de San Juan, Caño de San Antonio, Laguna del Condado, Laguna San José, Laguna La Torrecilla, and Laguna de Piñones.  These shallow-marine brackish-water bodies, prior to development, were generally adjacent to and interactive with mangrove swamps.  In large part their positions are determined by coastal-zone outcrops of eolianite, ancient beach sand that is mildly cemented by calcium carbonate.  Fine sediment of the lagoons, and to a lesser degree of the deeper Bahia de San Juan estuary, is subject to continuous reworking and resuspension by tidal processes;  thus, the lagoons appear to be partially self-cleaning.  Sand spits and other barrier islands do not occur along the north coast of Puerto Rico, probably because shelf and coastal-plain areas are too narrow and the gradient from the island to deep water of the Puerto Rico Trench is too great.         
	Under natural conditions, coastal sand dunes of northern Puerto Rico were at least as high as 10 m above mean sea level and formed a band as wide as 160 m or more (Nichols and others, 1987).  The dunes provided a barrier between the Atlantic Ocean and the coastal plain, offering protection from waves, flooding, and in particular storm surge related to hurricanes and other tropical cyclones.  In recent decades much of the protection provided by the dunes has been eliminated by mining of the dune sand for a variety of purposes.
	Near-shore coral reefs are extensive off the north coast of Puerto Rico, many being within a few tens of meters of San Juan.  Marine waters in proximity to the reefs support abundant fish and crustaceans and are also important for recreation and tourism. Adding to the protection afforded by mangrove swamps and sand dunes, the reefs form a breakwater against wave erosion of beach deposits and provide a setting in which deposition augments beach sand lost during hurricanes and by high-intensity storms.  Where reefs adjacent to coastal-plain areas of Puerto Rico have been destroyed by streams discharging large loads of fine sediment and other contaminants, as much as 1 km of inland shoreline retreat has occurred (M. C. Larsen, USGS, written commun., 1998).  

QUANTIFICATION OF COASTAL CHANGE 
	Urbanization in the San Juan area of Puerto Rico has had measurable effects on coastal dunes, tidal-zone stability, and lagoons.  The effects of urban growth on marshes, mangroves, and coral reefs are apparent but are difficult to quantify.

Mining of Coastal Sand 
	Following decades of sand mining along beaches of the north coast of Puerto Rico, few natural dunes persist.  Especially from the beach of Carolina, in the eastern part of the SJMA, enormous quantities of sand were removed in the 1950s to construct the San Juan airport.  Within a few years accelerated beach erosion necessitated the construction of artificial breakwaters to protect the tidal zone.  Estimates of dune volumes for the north coast of Puerto Rico decreased from 27 million m3 in 1950 to 16 million m3 in 1980 (Castillo and Cruz, 1980).
	Where beach and coastal-plain sand of leeward areas separating shoreline dunes from lagoonal mangrove forests has been removed, dune heights have been reduced as much as 12 m, and widths of foredune ridges have been narrowed to 8 m or less.  Storm waves now easily overtop the beach sand and breach the destabilized ridges that remain.  Washover by storm-wave swash has altered the ecological conditions of the mangroves by redepositing sand in the mangroves, in some cases suffocating them.  Repair to roads and other public facilities in the SJMA is needed frequently after storm erosion and deposition of sand and debris in low-lying areas.  Annual erosion rates at a line of dunes along the shore at Carolina, for example, over periods of 27 to 40 years, averaged 2.0 m (Nichols and others, 1987).  When overtopping and dune-face erosion occurs, mixing of saltwater ponded in the excavated lowland areas contaminates freshwater of the coastal-plain aquifer, continuing shoreline retreat is likely, and buildings, marshes, mangrove forests, and other areas of the tidal zone become vulnerable to damage or destruction (Zack, 1986; Nichols and others, 1987).   
	Various measures of protection, such as seawalls and revetments, have been employed where erosion damage has been pronounced.  A result has been a narrowing of the dry-beach width for stabilized shorelines versus greater widths along destabilized shorelines (Wright and Pilkey, 1991).  Many sites of San Juan that are protected by seawalls, for example, have no beach, and unprotected shorelines of Carolina, where sand has been extracted, have narrow beaches that have been migrating inland and displacing mangrove forests. 

Dredging and Filling of Lagoons
	All of the lagoons of the San Juan area except for Laguna de Piñones,                                                                                                                                                                     which is protected by mangrove forests, have been extensively modified by shoreline structures, dredging of sand from the interior of the lagoon or channels entering it, filling in shoreline areas, and construction of railroads, bridges, and buildings for housing and commercial purposes.  Laguna La Torrecilla, at the eastern end of the San Juan airport, increased  in volume by an estimated 110 percent due to dredging of about 3.1 million m3 of sand and silt from only 25 percent of the lagoon area.  Later filling along the shorelines reduced the area of the lagoon by about 10 percent.  In places Laguna La Torrecilla was deepened to as much as 18 m, whereas Laguna San José and Laguna del Condada, both surrounded by urban area of San Juan, were deepened to about 11 m (Ellis, 1976).  The dredging was accompanied by destruction of adjacent mangrove swamps to permit the construction of hotels and marine clubs in the vicinities of the airport and the old section of San Juan.  

DISCUSSION 
	The social, economic, and environmental problems that have accompanied urban growth in Puerto Rico are obvious.  Solutions, too, are easily identified if only specific objectives are considered, but remediation of an environmental problem may exacerbate a social or economic problem.

Urbanization and Resource Availability 
	Water development, especially the construction and filling of Loíza Reservoir, encouraged expanding urbanization, and incentives for drug industries provided jobs that resulted both in massive contamination of surface-water and ground-water supplies and population increases in the Río Grande de Loíza drainage basin.   Construction of new homes and businesses increased erosion and sedimentation in the reservoir.  Erosion and pollution that accompany urbanization, moreover, are seriously impairing the ability of the reservoir to provide water of adequate quantity and quality.  
	Despite hazards such as landslides, hurricanes, and earthquakes, sedimentation is the greatest threat to continuing water storage in Loíza Reservoir.  As an expanding urban population in Puerto Rico increases its dependence on stored water for domestic supplies and power generation, the rate of reservoir sedimentation becomes pivotal to the social and economic well being of the urban community (L’vovich and others, 1990).  For any reservoir, this rate is indicative of general basin conditions -- climate, geology and soils, topography, vegetation, and especially land use.  Reservoir sedimentation, however, is also influenced by specifics such as rainfall intensity and distribution, tectonics, fire frequency, and the extent to which soil erosion due to human disturbance is minimized.  The longevity of present and planned reservoirs in Puerto Rico, those in support of urban growth, can be increased if presently available erosion technology and land-use planning are applied in a thoughtful and conscientious manner. 
	Future discoveries of metallic-mineral resources in Puerto Rico, based on identification of permissive geologic environments, could include various types of deposits containing copper, gold, iron, zinc, lead, manganese, and molybdenum (Orris and others, 1992).  In the Loíza Basin in particular, porphyry copper-gold and copper-iron skarn deposits were deemed permissive by the geologic setting and characteristics.  Were such deposits to be discovered and developed, additional demands and impacts on the fluvial and ground-water systems would be anticipated.
	Environmental impacts of mining sand, gravel, and other building materials extend beyond the effects of development and denudation to modifications in stream regimen.  Loss of alluvial-terrace and flood-plain deposits has an adverse effect on agriculture and other riparian-zone land uses.  Alluvial barrow pits adjacent to river courses are subject to flooding and piracy of the stream channel with obvious impact on river-bank property.  Perhaps most serious is the loss of sand and gravel for beach nourishment and replenishment.
	Assessment of the environmental costs of exploiting industrial minerals and sand and gravel has been slow in Puerto Rico.  The principal concern at present appears to be the identification of additional resources for infrastructure development, with little effort directed toward remedial work at active or abandoned quarry sites.  Attention is being directed toward the exploitation of lagoons and off-shore deposits of sand and gravel (Rodriguez, 1994), which could create new environmental problems for coastal and shallow-marine areas.  Mitigation of the environmental impacts by regrading sites and revegetation, as well as by restrictions in site development, would help to alleviate continued erosion and reservoir siltation.  Dredging of sediment from Loíza Reservoir is planned to extend the useful life of the reservoir (Webb and Morton, 1996), an activity that also may provide a small supply of sand and gravel relative to urban demand.     	  
	Problems related to mining and quarrying experienced in the Río Grande de Loíza drainage basin are not unique.  In many urban areas of the world, a supply of industrial minerals at reasonable cost for continued urban growth is of increasing concern.  In the San Juan area of Puerto Rico, as elsewhere, the available resources that were unencumbered by site problems or land-use conflicts have been depleted.  Scars remaining are in need of remediation, and limestone that comprised a major fresh-water aquifer has been extensively quarried and in some areas the aquifer severely degraded.  The rapid expansion of urban development further restricts the resource base while increasing demand for the resources.  Runoff increases and ground-water recharge decreases.  The amount of construction material moved and used, on the order of 8 million tons annually, has significantly affected the fluvial and coastal environments in the Loíza Basin, and these changes have serious potential consequences for the tourist industry.
	Constraints imposed by the coastal position of San Juan are representative of those that affect mainland coastal urban centers as well.  The World Resources Institute (1996) estimates that by the year 2000, 3.3 billion people -- more than half of the global population -- will live in urban areas.  About 60 percent of the global population presently lives within 100 km of a coastline, and fully half of the world’s coastal ecosystems are at significant risk of degradation from development-related activity.  By 2025, when the globe will support a projected 8 billion people, the urban population is expected to be more than 5 billion and 90 percent of that increase will occur in developing countries.  Coastal urban centers are experiencing unprecedented growth, the sort of expansion that requires vast quantities of resources from outlying areas.  As demonstrated by growth of the San Juan metropolitan area, the ecological footprints of cities extend far beyond their geographic boundaries.  As world population heads toward 10 billion in the next century, we are obliged to consider the enormity of the demand that burgeoning urbanization and coastal infrastructures will impose on such basic and commonly used resources as water, rock, and sand and gravel. 

Controls on Urban Expansion
	The example of the San Juan area, Puerto Rico, seems to suggest that urbanization tends to exploit available resources in such a manner as to be self-limiting.  Puerto Rico, a water-rich island, continues to be burdened with a deficiency of municipal water supplies;  importation of water to the SJMA may insure controls on growth of other urban areas in Puerto Rico.  A purposeful change from an agrarian economy to that having an industrial base has promoted rapid urban-population increases, and infrastructure in support of the population growth has been inadequate to keep pace.  Thus, deficiencies in the municipal water-delivery and sewage systems may be discouraging further urban growth.
	Global population, however, continues to increase.  Where water and mineral resources of urban areas such as San Juan are limited, the increasing demand for available resources by an expanding population inevitably will impose disproportionate impacts on urban areas by normal climate variability and possibly by induced climate change.  Agriculture and other land-use activities of rural areas normally adapt to temporary excesses or deficiencies in water supply, but a dependence on infrastructure limits the ability of urban areas to maintain similar flexibility with changes in water availability.  Recent periods of water rationing due to drought in Puerto Rico provide stark evidence of this unfortunate situation.
	Growth is also being slowed by a dwindling resource of construction materials.  Limestone, for cement and concrete, is abundant in northeastern Puerto Rico, but quarrying this resource restricts further urban expansion.  Fluvial sand and gravel supplies, once thought to be nearly infinite, are essentially exhausted (Rodriguez, 1994), and the mining of limited off-shore deposits will be expensive and may result in unacceptable environmental degradation.  In Puerto Rico, frame housing is subject to damage by termites and hurricanes, and therefore cement construction is common;  the lack of sand and gravel may have a serious retarding effect on future housing construction on the island.  	
	Importation of industrial minerals to support construction has proven costly to neighboring Caribbean islands such as Saint Thomas (Rodriguez, 1994).  In contrast to most mineral resources, industrial minerals are not, with few exceptions, traded on the world market.  Transportation costs of these bulk materials require that they be obtained within reasonable proximity of their intended use.  This economic aspect poses a particular burden for small islands with rapidly growing economies and expanding populations.  Puerto Rico is beginning to experience the limits to growth that are imposed by the prospective depletion of an industrial mineral-resource base.  
	In recent decades, an important part of the economy of Puerto Rico has become dependent on tourism.  The adverse environmental effects of poorly planned urban growth -- water pollution, deforestation, inadequate infrastructure, accumulation of solid wastes, beach erosion, reef damage -- will impact the tourist industry, and further constrain urban growth.  	

COMMENTS AND RECOMMENDATIONS     
 	As noted by Larsen (1997, p. 45), "Puerto Rico is a microcosm of the developing world.”, and as such, stresses on resources due to development are similar to those being experienced in other parts of the world.  In Puerto Rico, and elsewhere, it seems likely that many problems attendant upon urban growth, such as inadequate supplies of construction materials and depletion of sand and gravel resources, could have been minimized by zoning policies and long-range planning designed to permit growth in a manner consistent with available resources.  Anticipation of resource depletion could have led to more enlightened land-use planning.
	Policy and planning conforming with an anticipated expansion of any urban area should be consistent with the available resources.  Through the 1940s, Puerto Rico largely had an agrarian economy.  Policy changes in the 1950s encouraged industrialization, which resulted in mass population migration from rural to urban settings.  Economic policies that stimulated industrial growth were not accompanied, however,  by social policies that regulated the growth and protected environmental attributes.
	Regardless of the accuracy of this historical interpretation, the San Juan/Loíza Basin example demonstrates a need for urban planners to embrace a long-term, cause-and-effect perspective if resource shortages and urban infrastructure deficiencies are to be avoided.  Projections of future growth of the SJMA, leading to urban policy and planning, should be developed within a context of social and economic goals for the entire island of Puerto Rico.  To be effective, long-term planning by managers of San Juan must be done in concert with neighboring communities, such as Bayamón, Toa Baja, Guaynabo, and Carolina, and with upstream communities of Caguas and San Lorenzo.  Inventories of renewable and non-renewable resources, both in the Loíza Basin and island-wide, must be weighed against projected needs for those resources if urban stability is to be achieved.
	Various actions can reduce present shortages of rock and water resources, but these actions should be applied cautiously so as not to re-stimulate an increase in urban population and thereby perpetuate a resource shortfall.  For example, where sand and gravel can be extracted from off-shore sites without causing adverse environmental damage, pressure to deplete remaining terrestrial and coastal supplies can be reduced.  Dredging of impounded sediment from reservoirs and lagoons, as is presently occurring at Loíza Reservoir, can (1) supplement dwindling supplies of sand and gravel needed for cement, (2) increase the fresh-water storage potential of the reservoir or the utility of the lagoon, and (3) provide replacement fill at riverine and marshland sites where sediment was extracted previously.  
	Short-term supplies of water in the SJMA can be enhanced through at least three strategies:  reduction of line loss, conservation incentives, and recycling (F. N. Scatena, USDA Forest Service, oral commun., 1997).  
1.  An estimated 40 to 59 percent of water treated for delivery to the SJMA from 1988 through 1993 did not reach intended users, presumably owing to the combined effects of seepage loss from pipes, loss from damaged pipes, inadequate monitoring of water-delivery, and illegal tapping or diversion of water from supply lines (Morris, 1994; Dopazo and Molina-Rivera, 1995).  Measures to reduce these losses have been limited, but it is estimated that water loss can be reduced to about 15 percent by increased surveillance and infrastructure repair (Morris, 1994).
2.  The use of tax advantages to encourage water conservation in the SJMA could lead to savings by both homeowners and businesses.  Included for water conservation might be incentives for the collection of water from rooftops and other high-runoff surfaces for use in gardening, evaporative cooling, and waste disposal.  Streamflow diversion for storage and water treatment should be scheduled to maximize ecological processes, such as spawning patterns of aquatic wildlife.
3.  Treated sewage effluent can be used advantageously for watering at sites such as parks, golf courses, and preserved wetlands.  Most treated waste water released by the SJMA presently is discharged to the Atlantic Ocean.  This water typically has concentrations of nutrients that could be beneficial to green areas but might be injurious to corals and other marine biota. 	
	A comprehensive, long-term policy for urban change in the SJMA may require definition and perhaps adjustment of an environmental ethic.  If a resource evaluation suggests that urban expansion cannot be sustained, policies of resource use must be adopted that recognize this constraint.  For example, as recently as about a decade ago, years after general acknowledgement that fluvial sand and gravel in northeastern Puerto Rico were nearly depleted and that removal of coastal dunes was resulting in disastrous environmental damage, Nichols and others (1987) wrote that a prohibition of sand-dune mining along coastal areas of Puerto Rico was.....”not acceptable because sand is needed to support the local construction industry and alternative sources as river sand and crushed rock aggregate, are scarce or costly.”  Obviously, if limited supplies dictate an unsustainable use of resources, policies must change accordingly.
	An environmental ethic for Puerto Rico and the SJMA cannot be imposed on the population by government, but government can, through policy, promote a prudent use of resources.  One possible approach is to modify present inducements that encourage industrialization and an economy dependent on global markets.  National and multi-national corporations, such as pharmaceutical companies, that were attracted to Puerto Rico after World War II have had little incentive to be protective of the island’s natural-resource base.  Island-based industries and agricultural cooperatives, especially those that produce foodstuffs intended primarily for local distribution and consumption, are much more likely than global conglomerates to be attentive to long-term care and management of the land and water upon which their continued production depends.  Although neither San Juan nor any other large city reasonably can be or should be detached from world trade, the relatively isolated position of Puerto Rico suggests that the more that foods, goods, and services are provided locally, the more efficient will be the economy and the more protective of resources will be the population.  Thus, a renewed emphasis on production for Puerto Rico as opposed to export may help the economic self-sufficiency of the island, and slow the trend of recent decades toward urban growth at the expense of the rural population. 
	A resource inventory, or an accounting of the likely availability of water and rock, must ultimately serve as a guide for the limits of urban growth and population in the SJMA.  An inventory of available resources must be compared to resources needed to support projected increases in population and urbanization with the realization that before the depletion or exhaustion of any required resource occurs, the effects of over-consumption will impose constraints on continued growth.  Just as extraction of coastal dunes has resulted in environmental and economic degradation, further appropriations of surface-water and ground-water resources, beyond those needed to maintain biophysical processes of fluvial and coastal systems, will destroy the very features upon which an urban economy depends.  Whether accomplished through policy, which seems preferable, or through exploitation of resources, which seems irresponsible, the limits on growth in cities such as San Juan will be determined by the population that can be supported.  Regardless of how efficiently resources are managed and conserved or how well urban officials anticipate growth, no city can accommodate expansion indefinitely.

ACKNOWLEDGEMENTS 
	This case-study analysis of resources and urbanization in Puerto Rico has been a team effort, with various but substantial contributions to the fieldwork and manuscript by Carroll Ann Hodges, Matt Larsen, Rick Webb, and Allen Gellis (USGS);  Fred Scatena (USDA Forest Service);  Robert Morton (Texas Bureau of Economic Geology);  and Maria Angeles Alonso (Madrid, Spain).  Significant field assistance, publications, and data were provided through the USGS Water, Energy, and Biochemical Budgets (WEBB) project.  The USDA Forest Service/Institute of tropical Forestry, Rio Piedras, PR, provided housing and transportation during field activities.  

REFERENCES
Blanco de Galinanes, M. T., 1977, Geovision de Puerto Rico. Editorial 	Universitaria, Universidad de Puerto Rico.    
Boccheciamp, R. A., 1978, Soil survey of the San Juan area of Puerto Rico:  	U. S. Department of Agriculture, Soil Conservation Service, 141 p.
Brockmann, V. W., 1952, The rural land classification program of Puerto 	Rico:  Northwestern University Studies in Geography Number 1, p. 	115-161.  
Calvesbert, R. J., 1970, Climate of Puerto Rico and U. S. Virgin Islands. In  	Climates of the States:  U. S. Department of Commerce, 	Environmental Science Services Administration, Climatography of 	the United States No. 60-52, 28 p.   
Cardona, W. A., 1984, El Yunque mineral prospects, eastern Puerto Rico:  	Caribbean Journal of Science, v. 20, no. 1-2, p. 79-87.
Castillo, J. B., and Cruz, H. M., 1980, Sand study, subtask 4.3, Coastal 	Management Program:  Puerto Rico Department of Natural Resources, 	144 p.
Cerame Vivas, M. J., 1989, Puerto Rico 2000:  Acta Científica, v. 3, no. 2-3, 	p. 109-112.
Commonwealth of Puerto Rico, 1984, Water quality standard development 	division water quality area. Evaluation of the water quality 	classification of Río Grande de Loíza Basin with respect to dissolved 	oxygen, 39 p.
Census of population and housing, 1980, Puerto Rico nos. 45-53, U. S. Dept. 	of Commerce, Bureau of the Census.
Díaz, P.L., Aquino, Z., Figueroa-Alamo, C., Vachier, R. J., and Sánchez, A. V., 	1995, Water resources data, Puerto Rico and the U. S. Virgin Islands, 	water year 1994:  U. S. Geological Survey Water-data Report PR-94-	1, 516 p.
Dopazo, Teresa, and Molina-Rivera, W. L., 1995, Estimated water use in 	Puerto Rico, 1988-89:  U. S. Geological Survey Open-File Report 95-	380, 30 p.
Ellis, S. R., 1976, History of dredging and filling of lagoons in the San Juan 	area, Puerto Rico:  U. S. Geological Survey Water-Resources 	Investigations 38-76, 25 p.
Fields, F. K., 1972, Floods at Caguas, Gurabo, Juncos, and San Lorenzo, 	Puerto Rico:  U. S. Geological Survey Hydrologic Investigations Atlas 	HA-438.
Gellis, A. C., 1991, Construction effects on sediment for two basins in 	Puerto Rico:  Proceedings, Fifth Federal Interagency Sedimentation 	Conference, Federal Energy Regulatory Commission, p. 4-72  to 4-78.
Gellis, A. C., Webb, R. M., Wolfe, W. J., and McIntyre, S. C. I., 1998, Effects 	of land use on upland erosion, sediment transport, and reservoir 	sedimentation, Lago Loíza Watershed, Puerto Rico:  U. S. Geological 	Survey Water Resources Investigations 	(in press). 
Glover, Lynn, III, 1971, Geology of the Coamo area, Puerto Rico, and its 	relation to the volcanic arc-trench association:  U. S. Geological 	Survey Professional Paper 636, 102 p.
Gomez-Gomez, Fernando, 1984,  Sinkhole development in limestone areas 	as related to rainfall and ground-water development in Puerto Rico. 	In:  Walter Hays and Paula Gori, eds, A workshop on Geologic Hazards 	in Puerto Rico, Reston, Virginia U.S. Geological Survey, Open-File 	Report no. 84-761, pp. 101-103. 
Griggs, G. B., and Gilchrist, J. A., 1983, Geologic hazards, resources, and 	environmental planning, 2nd edition:  Hedsworth, Belmont, CA, 502 p.
Guzmán-Ríos, Senén, 1989, Suspended-sediment data in the upper Río de 	Loíza Basin, Puerto Rico:  U. S. Geological Survey Open-File Data 	Report 88-324, 42 p.
Hays, W. W., and Gori, Paula, 1984, Background and summary of the 	workshop on geologic hazards in Puerto Rico, In:  A workshop on 	geologic hazards in Puerto Rico:  U. S. Geological Survey Open-File 	Report 84-761, p. 12-24.
Hays, W. W.,  and Gori, P., 1985,  A workshop  on "Reducing  Potential 	Losses from Earthquake Hazards in Puerto Rico”. Reston, Virginia: U. 	S. Geological Survey Open-File Report 84-761. 
Hooke, R. LeB., 1994, On the efficiency of humans as geomorphic agents:  	GSA Today, v. 4, no. 9, p. 217.
Hunt, J. L., 1976, Sedimentation of Loíza Reservoir, Puerto Rico:  U. S. 	Department of Agriculture, Soil Conservation Service, SCS-TP-153,  	19 p.
Hunter, J. M., and Arbona, S. I., 1995,  Paradise lost: an introduction to the 	geography of water pollution in Puerto Rico: Soc. Sci. Med.  v. 40, no. 	10, p. 1331-1355.
Larsen, M. C., 1997, Tropical geomorphology and geomorphic work:  a study 	of geomorphic processes and sediment and water budgets in montane 	humid-tropical forested and developed watersheds, Puerto Rico:  	Unpublished PhD dissertation, University of Colorado, Boulder, CO, 	341 p.  
Larsen, M. C., and Parks, J. E., 1997, How wide is a road?  The association 	of roads and mass-wasting disturbance in a forested montane 	environment:  Earth Surface Processes and Landforms, v. 22, p. 835-	848.
Larsen, M. C., and Torres-Sánchez, A. J., 1992, Landslides triggered by 	Hurricane Hugo in eastern Puerto Rico, September, 1989:  Caribbean 	Journal of Science, v. 28, no. 3-4, p. 113-125.
_____, 1996, Geographic relations of landslide distribution and 	assessment of landslide hazards in the Blanco, Cibuco, and Coamo 	basins, Puerto Rico:  U. S. Geological Survey Water-Resources 	Investigations Report 95-4029, 56 p.
L’vovich, M. T., and others, 1990, Use and transformation of terrestrial 	water systems, In: Turner, B. L., II, and others, (eds.), The earth as 	transformed by human action -- global and regional changes in the 	biosphere over the past 300 years:  Cambridge University Press, 	Cambridge.
McIntyre, S. and Gellis, A., 1993, Effects of land-use changes on reservoir 	sedimentation, Lake Loíza, Puerto Rico. 13th International 	Symposium of the North American Management Society, Seattle, 	Washington.
Molinelli, José, 1982, Soil erosion, sediment sources, and sediment 	loadings in the Loíza and La Plata lake watersheds Puerto Rico.  A 	Study Prepared For the Environmental Quality Board, 34 p.
_____, 1984, Rapid mass movement as a geologic hazard in Puerto Rico, 	In:  Proceedings, Workshop on geologic hazards in Puerto Rico:  	U.S. 	Geological Survey Open-File Report 84-761, p. 80-85.
_____, 1987, Earthquake vulnerability study for the metropolitan area of 	San Juan, Puerto Rico:  Proceedings, Hazard-mapping meeting, 	Kingston, Jamaica, p. 71-86.
Monroe, W. H., 1977, Geologic map of the Carolina Quadrangle, Puerto Rico:  	U. S. Geological Survey Miscellaneous Geologic Investigations Map I-	1054, scale 1:20,000.
Monroe, W. H., 1980, Some tropical landforms of Puerto Rico:  U. S. 	Geological Survey Professional Paper 1159, 39 p.   
Morris, G. L., 1994, Ten concepts on water supply and drought in Puerto 	Rico:  Dimension, Segundo Trimestre, p. 7-14.
Nichols, M., Cerco, C., Clinton, G., and Martinez, R., 1987, Coastal dunes for 	protection and sand resources:  Proceedings, Fifth Symposium on 	Coastal and Ocean Management, v. 2, Washington, D. C., p. 1302-1311.
Ocasio, F. M., 1996, Estudio de necesidad de producción de agua para Puerto 	Rico:  Autoridad de Acueductos y Alcantarillados, Departamento de 	Recursos de Agua, Puerto Rico, 64 p.
Orris, G. J., Carbonaro, Marguerite, Allen, M. S., Bliss, J. D., Ruiz, A. H., 	Kibbe, Richard, Paidakovich, M. E., Page, N. J., Pierce, H. A., and 	Staude, J. M. G., 1992, Puerto Rico: industrial mineral mines, 	prospects, and occurrences in the Mineral Resources Data System 	(MRDS):  U. S. Geological Survey Open-File Report 92-0244, 300 p.
Palm, R. I., and Hodgson, M. E., 1993, Natural hazards in Puerto Rico--  	attitudes, experience, and behavior of homeowners:  Monograph 55,  	Institute of Behavioral Science, University of Colorado, Boulder, CO, 	151 p. 
Pease, M. H., Jr., 1968, Geologic map of the Aguas Buenas Quadrangle, 	Puerto Rico:  U. S. Geological Survey Miscellaneous Geologic 	Investigations Map I-479, scale 1:20,000. 
Picó, Rafael, 1969, Nueva geografia de Puerto Rico--fisica, economica, y 	social:  Editorial Universitaria, Universidad de Puerto Rico, San Juan, 	460 p.  
Pielke, R. A., 1990, The hurricane:  Routledge, London, 220 p.  
Reid, H. F., and Taber, Stephen, 1919, The Porto Rico earthquakes of 	October-November, 1918:  Bulletin of the Seismological Society of 	America, v. IX, No. 4, p. 95-127.
Rodriguez, Rafael, 1994, Sand and gravel resources of Puerto Rico, In:  	Selected Issues in the USGS Marine and Coastal Geology Program:  U. 	S. Geological Survey, San Juan, Puerto Rico, 2 p. 
Scatena, F. N., 1989, An introduction to the physiography and history of the 	Bisley Experimental Watersheds in the Luquillo Mountains of Puerto 	Rico. Gen. Tech. Rep. SO-72. US Department of Agriculture, Forest 	Service, Southern Forest Experiment Station, 22 p.
Scatena, F. N., Silver, W., Siccama, T., Johnson, A., and Sánchez, M. J. 	1993,  	Biomass and nutrient content of the Bisley Experimental 	Watersheds, Luquillo Experimental Forest, Puerto Rico, before and 	after Hurricane Hugo, 1989:  Biotropica, v. 25, no. 1, p. 15-27. 
Smith, R. M., and Ambruna, F., 1955, Soil and water conservation research 	in Puerto Rico, 1938 to 1947: University of Puerto Rico Agricultural 	experiment station Bulletin 124, 51 p. 
Torres-Sierra, Heriberto, 1996, Flood of January 5-6, 1992, in Puerto 	Rico:  U. S. Geological Survey Open-file Report 95-374, 13 p.
_____, 1997, Hurricane Hortense -- Impact on surface water in Puerto 	Rico:  U. S. Geological Survey Fact Sheet FS-014-97, 4 p.
U. S. Department of Agriculture, 1990, Loíza 	Lake Water Quality Project 	Puerto Rico:  Soil Conservation Service  34 p.
U. S. Department of Commerce, 1992, 1990 census of population, general 	population characteristics, Puerto Rico:  Bureau of the Census CP-1-	53, p. 1-30.
Webb, R. M. T., and Morton, R. A., 1996, Impacts of mining and urbanization 	along the north coast of Puerto Rico:  a case study of the San Juan 	metropolitan area:  Abstracts with Programs 1996, 109th Annual 	Meeting, Geological Society of America.
Webb, R. M. T., and Soler-López, L. R., 1997, Sedimentation history of Lago 	Loíza, Puerto Rico, 1953 to 1994:  U. S. Geological Survey Water 	Resources Investigation Report 97-4108, 18 p.
World Resources Institute, 1996, World resources -- a guide to the global 	environment, 1996-97:  World Resources Institute, Washington, D. C., 	365 p.  
Wright, E. E., and Pilkey, O. H., 1991, The effect of shore-parallel hard 	stabilization on the shoreline of Puerto Rico:  Abstracts with 	Programs, 1991, 26th Annual Meeting, Northeastern Section, 	Geological Society of America, Baltimore, Maryland, p. 152.
Zack, Allen, 1986, Effects of sand removal on the shallow aquifer in the 	vicinity of the Camuy mangrove forest, Puerto Rico:  Proceedings, 	3rd Caribbean Islands Water Resources Congress, Puerto Rico Water 	Resources Association, p. 41-48.
Zack, Allen, and Larsen, M. C., 1993, Puerto Rico nad the U. S. Virgin 	Islands:  Research and Exploration, National Geographic Society, 	Water Issue, v. 9, p. 126-134.

Table 1.--Water and sediment discharges at gage sites, northeastern 		Puerto Rico.  The periods of record are variable but mostly 		range from 5 to 20 years.  Except as noted, data for stations 1 		through 17 are from Gellis and others (1998);  data for 		stations 18 and 19 are from Gellis (1991).  (DA = drainage 		area;  Q = mean discharge;  Qu = unit discharge;  Qsy = sediment 		yield;  Qp = peak discharge for period of record.)     

          Sta. No.		 DA		  Q		 Qu		  Qsy                 Qp
			          (km2)    (m3/s)    (m3/s/km2)    (t/km2/yr)   (m3/s)                         
 1	50051150 		 8.4		 .20		.024		    350	210
 2	50051180  	 9.7		 .29		.030		    670
 3	50051310		26.4		1.23		.047		  1540a	
 4	50051800		64.8		3.80		.059		    670	 1150
 5	50053025 		18.5		 .53		.029		    340
 6	50055000	       233		6.09		.026	           1080         2030
 7	50055100		13.7		  .31		.023		     430            50
 8	50055170		21.4		 .46		.022	           1200
 9	50055225		30.3		 .77		.026		     360	   380	 
10	50055390		13.2		 .21		.016		     140            45
11	50055750		57.8		1.12		.019		     380
12	50056400		42.5		1.39		.033		     690        1050
13	50057000	       156		3.71		.024		   1050	 1700	
14	50058350		 19.5		 .35		.018		     840
15     50059050            541            6.43           .012                440 
16     50061000            629                                                                  5580
17     50061800             25.5           .76            .030                142a                   
18	50048770 		 19.4	          .68		.035		11,600
19	50049100		 39.9		1.42		.036	          (7600)         245
Río Grande de Loíza drainage basin 
 1:  Quebrada Blanca at El Jagual
 2:  Quebrada Salvatierra near San Lorenzo
 3:  Río Cayaguas at Cerro Gordo
 4:  Río Grande de Loíza at Highway 183 near San Lorenzo
 5:  Río Turabo above Borinquen
 6:  Río Grande de Loíza at Caguas
 7:  Río Cagüitas near Aguas Buenas
 8:  Río Cagüitas near Caguas
 9:  Río Cagüitas at Villa Blanca at Caguas
10:  Río Bairoa at Bairoa     
11:  Río Gurabo below El Mango
12:  Río Valenciano near Juncos
13:  Río Gurabo at Gurabo
14:  Río Cañas at Cañas
15:  Río Grande de Loíza below damsite
16:  Río Grande de Loíza at Carolina
17:  Río Canóvanas near Campo Rico
Río Piedras drainage basin 
18:  Río Piedras at El Señorial
19:  Río Piedras at Hato Rey
	a Larsen (1997)

Table 2.-- Recent and projected water use, San Juan Metropolitan Area (SJMA) and Puerto Rico (PR), in cubic meters per day

		1993			2000			2020			2050

Industrial
  SJMA	19,400	 	19,500		19,400	      19,800
  PR		26,200		26,600		28,200	      28,700

Commercial
  SJMA	64,500		56,300		63,600	       76,700
   PR		85,000   		74,700		86,000	     108,300

Residential
  SJMA   235,400	     	       252,500              280,800	      302,000
   PR	     383,500                418,000              491,400             563,000

Total needed (does not include government, water loss, etc.)
  SJMA   620,600                 650,200              685,400             719,100
  PR       950,200              1,003,000           1,089,600          1,183,300

(Data from Ocasio, 1996)

________________________________________________________

Figure 1. -- Maps showing:  Puerto Rico relative to the Atlantic Ocean and Caribbean Sea, and to parts of North, Central, and South America (A);  Puerto Rico, with elevation contours generalized at 200-m intervals (B); and portions of east-central Puerto Rico with drainage divides (indicated by dashed lines) of the Río Piedras and Río Grande de Loíza Basins, and urbanized districts (bounded by solid lines) of San Juan, Toa Baja, Cateño, Bayamón, Guaynado, Carolina, and Caguas (C)

Figure 2. -- Generalized geologic map of Puerto Rico.  Letters refer to a serpentine-amphibolite-chert complex (A), volcaniclastic rocks of late-Cretaceous and early-Tertiary age (B), granodiorite and other igneous intrusive rocks (C), carbonate and clastic beds of middle-Tertiary age (D), and surficial silt, sand, and gravel of Quaternary age (E);  LEF identifies the Luquillo Experimental Forest (modified from Glover, 1971, and Monroe, 1980). 

Figure 3. -- Map of Río Piedras and Río Grande de Loíza drainage basins, with locations of stream-gage and sampling sites by the U. S. Geological Survey (modified from Díaz and others, 1995).