Home Biscayne aquifer Hydrologic system Water quality Ground-water withdrawal Vulnerability of Biscayne aquifer Future problems of potable supplies Other drinking water sources Summary & References PDF Version

The Biscayne aquifer is vulnerable to contamination from several sources other than by saltwater intrusion: (1) Direct infiltration of runoff from buildings, yards, paved areas and agricultural areas; (2) infiltration from septic-tank drainfields, soakage pits or galleries dug into the upper part of the aquifer; (3) canals and tack pits cut into the upper part of the aquifer; (4) solid-waste disposal areas (landfills or dumps); (5) sewage treatment plant disposal ponds; (6) wells used for drainage of storm water; and (7) wells used for disposal of industrial waste. Disposal wells had been permitted by the local Health Department where ground water contains chloride concentrations more than 1,500 mg/L.

Contaminants that have entered the aquifer flow system travel toward the ocean, unless they are diverted by pumping wells, utilized by vegetation, are adsorbed by the limestone sand or marl that compose the aquifer, or chemically precipitate to form insoluble compounds. Travel time to the ocean may range from days to years, depending upon the location of the source and the nature of the contaminant, ground-water gradients, the hydraulic conductivity of the aquifer, and rainfall. Travel to the ocean is more rapid during the rainy season than during the dry season, and more rapid in areas near canals than in areas distant from canals. Movement of contaminants in the aquifer will have lateral and vertical components, influenced by the ground-water gradient, the density of the contaminant-containing liquid if it is different from that of the ambient ground water, and the relative horizontal and vertical hydraulic conductivity of the aquifer.

In July 1976, 97 treatment plants having a total design capacity of 140 Mgal/d were treating 90 Mgal/d of waste water in Broward County (fig. 22). The majority of these were discharging secondary treated effluent at the coast or into major canals. Three of the largest plants discharge directly to ocean outfalls.

In 1977 only 47 percent of the developed area of Dade County was served with public sewers. Thirty-eight major waste-water treatment plants have a total capacity of 226 Mgal/d; the 3 largest of these comprise nearly 58 percent of the total treatment capacity of the county. In addition, 100 small plants have a total capacity of nearly 6.5 Mgal/d. Of the total, effluent, 233 Mgal/d, about 115 Mgal/d is discharged directly to the ocean (of which 30 Mgal/d is untreated) and 7 Mgal/d is injected into saline aquifers through wells about 3,000 ft deep. The remaining 111 Mgal/d of secondary and tertiary treated effluent, is discharged to controlled and uncontrolled reaches of canals, soakage pits, or drainfields dug into the Biscayne aquifer.

The major waste-water treatment franchise areas in Broward County are shown in figure 23. Figure 23 also shows areas in Dade County served by sewers.

Figure 22. -- Location of waste-water discharge facilities (Broward County Land Use Plan, 1977, p. 115; Metropolitan Dade County Department of Environmental Resources Management facilities map, 1977). [larger image]	Figure 23. -- Areas of public sewage systems under franchise in Broward County and in service in Dade County (Broward County Land Use Plan, 1977, p. 111; Metropolitan Dade County Master Plan, 1977, p. 54). [larger image]

A significant part of the total sewage effluent generated in southeast Florida from residences and some industries is discharged into septic tanks. Pitt and others, (1975, p. 9), estimated that in 1970 nearly 175,000 septic tanks were discharging about 40 Mgal./d of domestic waste water to the aquifer in Dade County. In Broward County the Department of Pollution Control and the Area Planning Board estimated that about 80,000 septic tanks were discharging more than 28 Mgal/d into the aquifer in May 1972. Some of these septic tanks were eliminated as sewer systems were expanded while others were being installed in new residential areas.

One of the problems that continues to confront health and water-supply officials is that thousands of residences in southeast Florida still depend on individual shallow wells for water supplies in areas served by septic tanks. These individual supply wells usually range in depth from 20 to 50 ft in Dade County and more than 50 ft in Broward County. Pitt and others (1975) found measurable amounts of septic-tank effluent in the upper 20-30 ft of aquifer at five study sites in Dade County. Dispersion, dilution, and chemical processes presumably obscure direct evidence of septic-tank effluent at greater depths.

The conclusions by Pitt and others (1975) may not apply to the overall ground-water quality of densely populated urban parts of Dade and Broward Counties where the upper sections of the aquifer are primarily sand. Many of these older residential sections are still served by septic tanks. Because of lack of regular maintenance, septic tanks can become overloaded and the drainfields clog causing raw effluent and solids to be temporarily backed up. Pitt and others (1975, p, 20-21) inferred that along much of eastern Dade County where the water table is 8 ft or more below the drainfields, recognizable effluent from single family residential septic tanks may reach the water table only during times of heavy rainfall. Part of the effluent would be evaporated or utilized by lawn grass, shrubs and tress, and the remainder would filter into unsaturated sediments which may remove some of the constituents in the liquid, In low lying urban areas, effluent from residential septic tanks may reach the water table during dry seasons as well as wet seasons.

Wastewater is also disposed of through soakage pits or drainfields, by direct discharge to canals and injection into deep wells. Soakage pits and drainfields are excavations in the upper part of the aquifer which are backfilled with gravel and sand to expedite vertical infiltration of effluents. These receive effluents from small treatment plants in interior areas where regional sanitary sewer systems are not available. The design capacity for these waste plants varies from 0.05 to 0 .2 Mgal/d. Level of treatment is usually tertiary. The plants serve large condominiums and apartment complexes, shopping centers, small housing developments, hospitals, and large office buildings.

Some of the larger and older sewage treatment plants discharge effluent into canals. Treatment is usually secondary. Points of discharge for most of the plants are upstream of control structures. Because little or no flow occurs in those canals during the dry season, the canal water tends to become enriched by nutrients and at times algal blooms form. No studies have been made and no data have been obtained concerning extent of the lateral and vertical movement of nutrient-enriched water from the canals to the aquifer during dry periods when water levels in canals are higher than adjacent ground-water levels.

Injection of secondary treated sewage effluent into deep wells began in 1972 in a rapidly growing housing area southwest of Miami. The discharge is into cavernous dolomite and limestone at a depth of about 3,000 ft where the ground water has the salinity of sea water. The zone receiving the effluent is separated from the Biscayne aquifer by an 800-ft thick section of relatively impermeable fine sand and clay, and about 2,000 ft of permeable and poorly permeable limestone. Two deep well disposal systems are operating in Dade County and one in Broward County. A large capacity system is under construction (1978) in the near-coastal part of south Dade County for the Miami-Dade Water and Sewer Authority.

Figure 24. -- Locations of solid-waste facilities, 1977, (Broward County Land Use Plan, 1977, p. 128; Metropolitan Dade County Environmental Resources Management facilities map, 1977). [larger image]

The disposal of solid waste represents a potential source of contamination to the Biscayne aquifer because of the increasing volume of waste and trash generated and the need for disposal areas, the rapidity of aquifer recharge, and the shallow depth of the water table. The distribution of solid-waste disposal sites in the two-county area is shown in Figure 24. The distribution includes closed, transfer, and active disposal sites. Even though a site is closed, it may contribute leachates because of continued decomposition of waste materials.

Investigations of ground-water quality have been made in the vicinity of solid-waste disposal facilities 5 mi northwest of Pompano Beach, and near the western city limits of Hialeah. The facilities have been in operation more than 15 years. The site near Pompano Beach is underlain by unconsolidated medium to fine sand which permits vertical movement of leachate. Water from 20-ft deep wells less than 100 ft downgradient of that facility contained average dissolved solids concentrations ranging from 1,587 mg/L to 1,677 mg/L. Water from wells 90-ft deep at the same site contained 475 mg/L, about the same concentration as water from shallow and deep wells upgradient from the disposal area. Shallow ground water in the upgradient, uncontaminated area contained 413 mg/L dissolved solids (Pitt, 1975, Table 2).

The solid-waste disposal facility near Hialeah is 3 mi upgradient (west) from the largest municipal well field in southeast Florida (fig. 20). The direction of ground-water flow in the vicinity of the well field suggests that leachate from the disposal area is migrating toward the pumping wells. The calculated average rate of ground-water movement was 2.9 ft/d (feet per day) (Mattraw, and others, 1978). The distribution of specific conductance of ground water along a west-east profile beneath the disposal area in October 1974 is shown in fig. 25 (Mattraw and others, 1978, fig. 20). Conductance values are greatest immediately beneath the disposal area, and decrease with depth. The pattern of the distribution shows the downgradient migration of the leachate, influenced partly by the heavy well-field pumpage to the east. Mattraw and others (1978) indicate that the occurrence of leachate at distances greater than 0.5 mi from the source is difficult to determine because dispersion and recharge dilute contaminant concentrations in the plume to concentrations that are virtually equivalent to ambient concentrations.

In general, the extent, shape, and concentration of the leachate plumes that form depend upon the following: (1) The vertical and horizontal permeability of the aquifer; (2) the types and volumes of waste; (3) the duration of the waste-disposal operation; (4) amount of infiltrated rainfall; (5) the adsorbant properties of the geologic materials that compose the aquifer, and (6) the hydraulic gradient.

Figure 25. -- Lines of equal specific conductance in vicinity of the Northwest 58th Street solid-waste disposal area, October 1974 (after Mattraw and others, 1978). [larger image]

Runoff from urban and agricultural areas has caused deterioration of the quality of canal water and to a lesser extent, ground water. Runoff enters the aquifer by surface infiltration, infiltration from canals, soakage pits, infiltration galleries or drainfields, french drains, and by drainage wells. Runoff enters canals and lakes by overland flow, through outfalls of storm-sewer systems, and by ground-water discharge.

Urban runoff contains a large variety of substances and chemicals including residues from automobile traffic on hundreds of miles of heavily traveled roads, household refuse, fertilizers and insecticides, and animal waste. A summary of water-quality data from sites on selected canals in urban parts of Dade County for 1976 is given in Table 3. These data were collected monthly by the Dade County Department of Environmental Resources Management. The sites on the Coral Gables waterway and Wagner Creek (near downtown Miami) are in uncontrolled (tidal) reaches of canals; the remaining sites are along controlled reaches of canals.

Table 3. -- Water quality in selected canals, 1976 [Data from files of Department of Environmental Resources Management, Dade County]
Sample Station Location	Dissolved Oxygen (DO) Mg/L			Biochemical Oxygen Demand (BOD) Mg/L			Phos Phosphate (P04) Mg/L			Coliform Bacteria per 100 mL (MPN)			pH
	1976 DATA
	MIN	MAX	AVG	MIN	MAX	AVG	MIN	MAX	AVG	MIN	MAX	AVG	MIN	MAX
Miami Canal	0.9	6.6	3.0	0.4	2.8	1.6	0.09	0.40	0.28	45	17,000	3,210	7.1	7.6
										Median 1,300
Red Road Canal	1.5	5.4	3.3	.4	2.8	1.8	.09	.45	.27	18	240,000	37,266	7.2	7.8
										Median 1,245
Red Road Canal	1.5	6.3	3.3	.8	3.0	1.8	.09	.24	.18	490	35,000	7,582	7.3	7.7
										Median 3,300
Miami Canal	1.4	6.8	3.3	.6	3.3	2.8	.07	.11	.09	130	11,000	2,872	7.1	7.6
										Median 1,045
Coral Cables Waterway	2.0	6.6	4.7	.8	3.4	2.1	.02	.18	.11	45	7,900	1,539	7.2	8.2
										Median 640
C-100 A Canal	2.5	15.8	8.8	1.0	3.6	2.2	.09	.54	.31	78	92,000	10,992	7.1	7.6
										Median 2,750
Wagner Creek (tributary of Miami River)	0.4	2.6	1.6	0.2	9.2	2.7	.09	.34	.22	2,300	240,000	111,618	7.1	7.6
										Median 92,000
Snapper Creek	1.5	7.0	3.6	1.8	15.2	3.9	.25	.42	.39	330	24,000	3,900	7.0	8.0
										Median 2,300
C-100 Canal	3.2	8.6	6.3	0.4	2.8	1.8	.09	.61	.25	230	3,300	1,578	7.1	7.9
										Median 1,300
Coral Cables Waterway	4.5	9.3	7.0	1.2	17.2	3.9	.02	.50	.23	330	24,000	7,919	7.1	8.7
										Median 3,300

The quality of the storm runoff from a low density (4 single-family residences per acre) residential area in Broward County is compared with a comparable acreage high density (multi-family apartments) residential area in Dade County in Table 4. The nutrient content in the runoff from the Broward County area was generally higher than in Dade County probably because of the large percentage of lawn area contrasted with the large paved vehicle-parking area in the Dade County site. Much of the nutrient content comes from fertilized lawns and shrubs in low density residential areas.

Table 4. -- Quality of runoff in high and low density residential areas.
	Broward County Low Density Residential				Dade County High Density Residential
	No.*	Mean	Minimum	Maximum	No.*	Mean	Minimum	Maximum
Organic Nitrate-N (mg/L)	344	1.23	0.18	9.4	27	0.913	0.37	2.6
Ammonia-N (mg/L)	342	0.343	.01	2.6	27	.031	.01	0.07
Nitrate-N (mg/L)	344	.484	.0	2.1	27	.067	.0	.15
Total Phosphorus as P (mg/L)	344	.317	.06	2.4	27	.204	.12	.50
Total Orthophosphate as P (mg/L)	345	.218	.03	1.8	27	.094	.03	.23
Copper (ug/L)	-	8.3	0	41	51	9.34	0	45
Iron (ug/L)	275	317	0	5,300	51	476	100	2,600
Lead (ug/L)	-	167	30	1,100	51	213	20	1,100
Zinc (ug/L)	-	86.7	10	560	51	168	20	790
*Number of samples

Pollution from urban areas is greatest near the coast where housing density is greatest and paved areas are maximum. Storm runoff in the coastal urban areas is generally to storm sewer systems connected to treatment plants or to tidal reaches of the nearest canals and then to tidal water. In near coastal areas not served by sewers, storm water infiltrates the aquifer directly and carries contaminants downward. In these areas, the residence time of contaminants that reach the saltwater zone of the aquifer is short because of the proximity to points of discharge to tidal waters.

In less dense populated areas farther inland, storm-water runoff from residential areas, small shopping centers, and roadways infiltrates directly into the aquifer. The thin soil, yard grasses and the shallow sections of limestone and sand above the water table tend to take up nutrients or filter or adsorb part of the pollutants before they reach the water table. The pollution loads are greatest at the early part of the rainy season when the "first flush" of the adsorbed or filtered constituents occurs. Subsequent rainfalls produce decreasing loads. Runoff from large shopping centers with multi-acre parking facilities is usually piped to a large sump to remove solids and other debris and then to a soakage pit or drainfield where it infiltrates the aquifer. If the area is near a controlled canal, the runoff may be discharged there.

Pollutant concentrations increase only temporarily in the canal water during the rainy season because the control structures in canals are opened repeatedly to discharge surplus water to the coast and pollutants are flushed to the ocean. A large part of that surplus comes from shallow ground water in areas adjacent to the canals. Flushing of shallow ground water occurs annually during the rainy season; the amount of flushing is dependent on the frequency and amount of rainfall and the locations of pollutants with respect to drainage canals. Flushing in shallow parts of the aquifer is less effective in areas distant from canals than in areas adjacent to canals.

Contribution of pollutants to the aquifer and to canals in the urban areas is minimal during the dry seasons. However, moderate-to-heavy, infrequent storms and attendant runoff can result in build-up of nutrients and other pollutants in shallow parts of the aquifer and in canals. These infrequent contributions tend to build up because control structures are closed. The only other contributions to canals are from inflows of ground water in the upgradient areas which contain nutrients from the use of fertilizers in residential areas and from septic tank effluent. In the downgradient reaches of canals where the canal water is higher than the ground water level, some filtration and adsorption of the nutrient-rich canal water takes place as the canal water seeps into the aquifer.

A part of the nutrient enrichment of ground water and canal water is from agricultural areas of interior Broward County and south Dade County. Analyses of ground-water showed no pesticide content but an occasional low concentration of the herbicide, silvex. The canal water in Broward and Dade Counties frequently contains detectable but low concentrations of silvex and 2-4D, and occasionally dieldrin. diazinon, lindane, and polychlorinated biphenyls. In contrast, canal-bottom sediments, contain a greater variety of herbicides and insecticides at much greater concentrations (Mattraw, 1975).

Pitt and others, (1975, p. 51) suggested that uniformly high nitrate concentrations in water from certain observation wells of different depths in south Dade County are probably related to the extensive and long-term agricultural activity in south Dade County. Citrus, avocados, and mangos have been grown for many years in south Dade County. Fertilizer and pesticides are used regularly and irrigation is from thousands of shallow wells throughout the dry seasons. Also, in south and inland Dade County and part of north Broward County, truck crops grown during October - February require frequent irrigation and regular fertilization. Irrigation in both counties is primarily by overhead sprinkler. The root systems are moistened, but little of the applied water leaks to the water table because of high evapotranspiration. Salts from fertilizers tend to accumulate in the soil zone and are washed downward into the aquifer during the first heavy rains of the rainy season.

Infiltration of water is rapid in south Dade County because solution-riddled limestone is at the surface. Infiltration is less rapid in north Dade County and most of Broward County where fine sand and other unconsolidated sediments underlie the surface to a depth of several feet. These sediments tend to filter out and concentrate pollutants at shallow depths within the saturated zone of the aquifer.

A method of storm-water disposal, used more in Dade County than in Broward County, is through wells cased to saltwater zones in coastal parts of the Biscayne aquifer. An undetermined number of these wells along the coastal zone are receiving runoff from parking lots, shopping centers, and other paved areas. These wells are usually cased to deep parts of the aquifer which contain water whose chloride concentration is greater than 1,500 mg/L. The storm water is untreated. No information is available concerning the specific paths of storm-water in the immediate area in the saltwater zone of the Biscayne aquifer.

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