Geochemical Tracing of Surface-Water and Ground-Water Interactions

It was believed that geochemical data would help delineate ground-water discharge and recharge in the north-central Everglades. Geochemical tracers did support previous hydraulic analysis by identifying recharge in WCA-1 and flow beneath the levee as an important source of ground water to the aquifer beneath ENR and WCA-2A. Support for the above interpretation comes from cross-sectional plots of specific conductivity and water type (figs. 24 and 25). Those plots show that Fresh Recharge Water (Type-F1) has been recharged to substantial depths (60 ft or more) beneath the levees separating the study sites from WCA-1. This observation is consistent with hydraulic interpretations that water ponded at higher elevations on the WCA-1 side of the levee drives recharge and flow beneath the levee.

Geochemical data provide a long-term integrated view of levee underflow. Of particular value is the information about the depth of underflow that would be difficult to discern from hydraulic data alone. At greater depths beneath the levee (100 ft) is Fresh Formation Water (type G1) that also is derived from freshwater recharge, but which has had considerably longer contact time with aquifer sediments.

Figure 24. Ground-water flow paths delineated by specific conductivity (contours) and geochemical water type (color codes) at Everglades Nutrient Removal Area (ENR), north-central Everglades, south Florida. [larger image]

Ground-water geochemistry changes rapidly with distance away from the levee, with Relict Seawater present in the aquifer at elevations well above the fine sands in the bottom half of the Surficial aquifer (below 100 ft) where it was once thought to be restricted (fig. 25). Upward movement of Relict Seawater from the bottom half of the Surficial aquifer is caused by ground-water underflow beneath the levees. The potential energy that drives recharge also drives a corresponding discharge flux on the down gradient side of the levee. Those flow paths are referred to as "underflow" paths. The deepest of those underflow paths are deep enough to have contacted the fine sands that contain the relict saltwater, mixing with the saltwater and advecting some of the salts upward toward the wetland surface. As a result, the presence of Relict Seawater (type E5) or Transitional Relict Seawater (type E6) delineates zones of discharge. In WCA-2A, the zone of discharge extends for 2 mi on the down gradient side of the levee. In ENR, the zone of discharge that is delineated by relict seawater does not occur directly down gradient of the levee, but instead is more centralized beneath the 2.5-mi-wide ENR wetland.

Are sea salts from the deeper aquifer actually discharged to surface water? Two surface-water samples from ENR are classified as Transitional Relict Seawater (type E6), indicating that relict sea salts discharge to surface water. The best example may be discharge to the seepage canal on the western side of ENR. The purpose of the seepage canal is to collect ground water that recharges within the ENR treatment wetland and return it to the wetland. The seepage canal not only captures ground water recharged in ENR but also ground-water discharge from the deeper parts of the aquifer. In contrast to ENR, discharge of relict sea salts to the wetland surface is not indicated by analysis of surface-water samples in WCA-2A. However, the relatively high salinity and geochemical makeup (type E5 and E6) of the shallowest ground waters beneath site F1 in WCA-2A, as well as similar data from peat porewater (unpublished data), suggest that relict sea salts are discharging to surface water at low rates.

Shallow ground water indicated additional variability in geochemical type, in addition to Fresh Recharge Water, Relict Seawater, and Transitional Relict Seawater. A fourth common type in shallow ground water beneath ENR was the variation of Fresh Formation Water classified as type G1a. Although possessing the same overall geochemical signature as the water found at 100 ft beneath the levees at both ENR and WCA-2A, it is clear that the shallow ground water at approximately 15 ft beneath central ENR fundamentally is different in origin. This shallow ground water is the water with the very light water-stable isotopic signature that was discussed earlier in this section. This interpretation, also discussed earlier, is that this water is derived from precipitation that infiltrated rapidly on farm fields prior to construction of ENR.

Figure 25. Ground-water flow paths delineated by specific conductivity (contours) and geochemical water type (color codes) at Water Conservation Area 2-A (WCA-2A), north-central Everglades, south Florida. [larger image]

Surface waters at ENR and WCA-2A always represented water mixtures rather than pure endmembers. The most common types were Transitional, Relict Seawater, and Transitional Fresh Formation Water. Surface waters always showed some effect of mixing with relict sea salts determined by the relative importance of chloride (type F6). The relict sea salts were derived from discharge of ground water from a deep source in the Surficial aquifer. A major component of drainage or runoff from Lake Okeechobee or the EAA was traceable by the increased relative importance of magnesium in water samples (type G6). Taken alone, however, major ions generally will not be sufficient to separate water predominantly derived from Fresh Formation Water from Agricultural Recharge Water. For example, using major ions alone, a ground water with long residence time in the aquifer cannot necessarily be distinguished from precipitation that recharged quickly in an agricultural field of the EAA (possibly because of the confusing signature introduced by chemical additives in fertilizer). Water-stable isotopes had considerable value in making that distinction, however, with lighter isotopes identifying waters recharged on agricultural fields without substantial evaporation.

Geochemical evidence provides a long-term integrated picture of hydraulic processes in the aquifer and exchange with surface water. Water-stable isotopes indicated that the major source of water in Everglades ground water is recharge of evaporated surface water from the wetlands rather than infiltration of rainfall through unsaturated sediments. This observation indicates that recharge always has occurred in the north-central Everglades. Ground-water recharge is balanced by discharge, which, although small in magnitude, is a persistent source of water to flow in the Everglades over the long term.

Major ion chemistry and water-stable isotopes identified eight major water types. Four of those were considered to be source waters (types F1, G1, G1a, and E5). The other four are mixtures of the source waters. The distribution of those water types identified long-term average flow pathways of recharge and discharge.

Particularly evident as a major factor driving recharge and discharge in the present-day Everglades were the effects of wetland compartmentalization by levees. Geochemical water typing provided information that was consistent with hydraulic information. The geochemical tracers, however, particularly were useful in showing the extent to which water management has caused vertical mixing throughout essentially the entire depth of the Surficial aquifer.

The effect of levees is indicated by the presence of Fresh Recharge Water recharged from the up gradient side of the wetland at a depth of 90 ft beneath the levee, whereas down gradient of the levee, relict sea salts have been entrained by the deepest underflow paths and transported upward to be discharged to surface water. The effect of levees on interactions between surface and ground water, therefore, extends vertically throughout at least the top three-fifths of the Surficial aquifer, and horizontally for at least 1 mi away from the levees. These observations support the hydraulic interpretation earlier in this report that water management has increased interactions between surface water and ground water in the north-central Everglades.