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publications > wri > 02-4050 > use of geochem tracers > dist. sources of recharge
Interactions between Surface Water and Ground Water and Effects on Mercury Transport in the North-central EvergladesBy Judson W. Harvey, Steven L. Krupa, Cynthia Gefvert, Robert M. Mooney, Jungyill Choi, Susan A. King, and Jefferson B. Giddings
Distinguishing Sources of Recharge Using Major-ion CompositionMajor-ion chemistry was useful to identify some of the more important sources of water to Everglades surface and ground water. Analyses from a sampling event in December 1997 are shown as points on a Piper diagram in figure 23. Piper diagrams show the effects of various factors, including major-ion composition of possible source waters, as well as the proportions of mixing between those source waters in samples. The effects of geochemical interactions between water and soil or aquifer material also may be indicated on a Piper diagram. Possible source waters, or endmembers, are identified on Piper diagrams by water samples that plot at the extreme edges of the sample distribution. Waters that are mixtures between two endmembers are identified as samples that plot on lines connecting the endmembers. Often, more than two endmembers are present, complicating what would otherwise be a simple mixing analysis. Even if only two endmembers are likely to be present, accurately classifying those endmembers still may be difficult because of spatial and temporal variability of chemistry at a limited number of sampling sites.
There is no single widely accepted method of classifying water types by their major ion composition. For the present study, available classifications of water types in the Everglades were considered first. Howie (1987) used Stiff diagrams to classify ground waters in the southern part of the study area in Broward County. Upchurch (1992) suggested a descriptive classification of natural waters in Florida based on Piper diagrams, using the separate triangular graphs for cations and anions. Frazee (1982) developed a more specialized interpretive classification of south Florida waters using the combined cation and anion graph on Piper diagrams. In this study, the keys for both the Upchurch (1992) and Frazee (1982) classifications were used to prepare the Piper diagrams presented here (fig. 23). Calcium and/or sodium generally dominate the cation composition of south Florida waters. The anion composition is dominated by bicarbonate and/or chloride ions. If a third cation (for example, magnesium) or anion (for example, sulfate) is important, the sample is referred to as "mixed" in its cation or anion composition. Using the descriptive classification scheme of Upchurch (1992), along with water-stable isotopic data, nine major water types relevant to the study area were delineated (fig. 23):
Background - Source and Transitional WatersInterpretations of water sources to ENR and WCA-2A are summarized briefly here. Interpretations follow those of Frazee (1982) or are slight modifications thereof. Terms used to classify the waters for this study that were defined by Frazee (1982) are italicized.Water types A1, F1, E5, and B2 are the relevant source waters in the north-central Everglades. Type A1 waters are Fresh Recharge Waters derived from rainfall and interaction with aquifer sediments, principally limestone. Type F1 waters are similar to A1 waters but are higher in ionic strength and have a greater importance of sodium either because of different aquifer sediments, greater reaction times, or mixing with different water. Type E5 waters are Relict Seawater derived from seawater intrusion into the fine sands of the bottom half of the Surficial aquifer during an earlier geologic time. The term Relict Seawater, therefore, has the same meaning as Frazee’s term Lateral Inflow. Type B2 waters are Fresh Formation Waters derived from Fresh Recharge Waters that have much longer contact times with soils and aquifer sediments. Consequently, B2 waters have a greater effect of magnesium and sulfate in their ion makeup. Transitional Water is a general term used by Frazee (1982) to refer either to waters that are evolving by geochemical reactions with aquifer sediments, or waters that changed their geochemical character by mixing with other geochemically distinct waters. The most frequent classification of water in ENR and WCA-2A were F6 waters, followed by G6, F1, E5, E6, and G1 waters. Many of those waters are the product of mixing among two or more endmembers. For example, the most common water type was F6, which is a mixture between Fresh Recharge Waters (F1) and older Relict Seawater from the base of the aquifer (E5). The progressively saltier mixtures along the mixing continuum are F1, F6, E6, and, finally, E5 waters. Another important mixed water type was G6, which shares a fresh-salt mixture in common with F6 and E6 waters, but also has inputs of magnesium and sulfate from mixing with B2 waters. A good example of Transitional Waters that evolved over centuries or millennia in contact with aquifer sediments, with little mixing with other waters, are B2 waters. B2 waters are otherwise known as Fresh Formation Waters. Fresh Recharge WaterWater type A1 is freshwater that originated as rainfall-derived recharge to the aquifer. Type A1 water interacted with peat sediments and sand and carbonate layers of the Surficial aquifer over a relatively short period of geologic time (decades to centuries). Dissolution of limestone was the most important water-rock interaction affecting the chemistry of A1 waters. Type F1 waters are similar to A1 waters except that F1 waters have a higher ionic strength and more importance of sodium, which reflects either exposure to different aquifer sediments, more prolonged contact time, or a minor amount of mixing with Relict Seawater. Frazee (1982) refers to both A1 and F1 water types as Fresh Recharge Water.A1 waters are represented at the study sites by shallow ground waters sampled from wells along the Atlantic Coastal Ridge. Surface water and shallow ground water from WCA-2A and ENR are F1 or F6 waters, because of a greater contribution from sodium and/or chloride ions. Whereas A1 waters probably represent waters recharged through unsaturated sediments of sandy coastal ridges, F1 waters are more likely derived from recharge of rainfall that fell directly onto the wetlands themselves. Because water-stable isotopes indicated that Everglades ground water principally was derived from recharge of wetland surface water, F1 waters appear to be a much more important source of Fresh Recharge Water to the Everglades than A1 waters. A good model for the source of F1 waters to the study sites is water recharged within the rainfall-dominated wetlands of WCA-1. Wetlands outside of WCA-1 have other sources of water, such as deep ground-water discharge from the Surficial aquifer, drainage from Lake Okeechobee and from the Everglades Agricultural Area (EAA), with distinctly different geochemical signatures. The first of those other source waters discussed is discharge of Relict Seawater from the lower half of the Surficial aquifer. Relict SeawaterType E5 water is high ionic strength water that is dominated by the ion signature of seawater; for example, sodium and chloride. In surface water and ground water of the north-central Everglades, this signature is acquired by mixing with a large component of Relict Seawater that originally intruded the fine sands in the lower half of the Surficial aquifer during a geologic time of a relatively higher sea-level stand. E5 waters mostly are represented by the deep ground waters of the Surficial aquifer (60-200 ft deep), but E5 waters also are found at shallow depths in the aquifer (less than 60 ft deep), and on the wetland surface, because of mixing caused by upward discharge of Relict Seawater from the aquifer to the wetland surface. The presence of E5 waters in surface water and shallow ground water attests to the importance of deep vertical mixing between surface water and ground water in some locations. Near levees, vertical mixing is especially deep, extending through at least the top half of the Surficial aquifer.Fresh Formation WaterWater type B2 has greater effects of magnesium and sulfate in its ion composition, often because of longer periods of weathering in contact with soils and aquifer sediments (compared with other water types). Frazee (1982) refers to the water type as Fresh Formation Water, with long contact times being the main difference separating this water type from the Fresh Recharge Water dominated by calcium bicarbonate. Water type B2 is believed to be present in ground water beneath Lake Okeechobee and in areas surrounding Lake Okeechobee, including the Everglades Agricultural Area (EAA) (Frazee, 1982). Even longer contact times of B2 waters in the aquifer eventually would produce Connate Water (for example, classification C or D for cations and 3 or 4 for anions), a saltwater distinctly different from marine water in its ion balance. Connate Water also is believed to be present in ground water beneath Lake Okeechobee (Frazee, 1982).Mixed Waters and Other Transitional WatersMany of the ground waters from ENR and WCA-2A plotted on the Piper diagram on a mixing line between F1 and E5 waters. This mixing line represents the continuum between rainfall-driven recharge of wetlands in the Everglades (F1), and Relict Seawater (E5). Another group of water samples, mostly surface waters and a few ground waters, had a mixed cation signature (for example, a G classification for cations), because of either mixing between water types or longer weathering times in the aquifer. For example, mixing of F6 type water with Fresh Formation Water or Connate Water probably accounts for classification of Lake Okeechobee water as G6 type water. The presence of Fresh Formation Water in ground water of the Lake Okeechobee and EAA is evident from water analyses reported in Parker and others (1955). Those water samples were collected in the 1940s from ditches, canals, and ground waters in the EAA. The range of those samples on the combined cation and anion portion of a Piper diagram is shown as a trapezoidal shaded area in figure 23. A number of surface waters and shallow ground waters had the mixed cation signature of G6 or G1 waters. Because G6 waters only were found in surface water or shallow ground water, it was concluded that these waters are mixtures of various waters, such as A1, F1, E5, B2, and possibly Connate Water. Lake Okeechobee contains G6 type water, making it possible that some of the G6 waters from ENR and WCA-2A, most likely surface water, could be predominantly drainage water from Lake Okeechobee. In contrast, G1 waters were found both very shallow (15 ft) and relatively deep (60 ft or greater). The deeper G1 waters evolved by geochemical interactions with aquifer sediments. That geochemical change involved a transition from calcium-sodium bicarbonate water (F1) to calcium-sodium bicarbonate-chloride water (F6) and, finally, to G1 water with proportionately greater importance of magnesium (and sometimes sulfate) compared with G6 waters. The shift in anion composition usually is not as great as the cation shift, possibly because of bacterial sulfate reduction that removes sulfate from solution. Most of the shallow G-1 ground waters have the unique "light" stable isotopic composition that was interpreted earlier in this section as a signature of rainfall-driven recharge through agricultural soil. The mixed ion classification of that water may, in part, be the result of chemical inputs associated with agricultural fertilizer (Bates and others, 2002). For that reason, a specialized classification was created for shallow G1 waters called G1a, which uniquely associates the water with rapid recharge of precipitation that fell on wetlands converted for agricultural use. |
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 13 January, 2005 @ 12:27 PM (KP)
