The origin and fate of nutrient-rich ground water is being addressed in three ways: 1) by direct subsurface measurement of direction and rate of flow using dye tracers (Shinn et al.), 2) by detection and measurement of methane, radon, and an artificial tracer, sulfur hexafluoride (Chanton et al.), and 3) by measurement of environmental isotopes (H, He, O, C, N, S) and tracers (chlorofluorocarbons, SF6, coprostanol) (Bohlke et al.). Combined results indicate a potential for rapid local ground-water movement of anthropogenic contaminants along with a tendency for export of Florida Bay ground water toward the Atlantic.

1) Two circular well clusters were core drilled in 1 to 2 m of water on either side of an undeveloped portion of Key Largo. Each cluster consists of 8 equally spaced wells arranged in a 200-ft-diameter circle. The wells contain two piezometers; one screened between -6 and -7.6 m and one between -12 and -13.7 m. The zones are separated by a Portland cement plug and a subaerial unconformity. An identical well is located in the center of each cluster. A fluorescein dye solution was pumped into the shallow zone of the central well and rhodamine dye was injected into the deep zone. These zones were periodically sampled in the 8 monitoring wells and dye concentrations were determined with a fluorometer. Movement of ground water was found to be most rapid in the shallow zone. Both rhodamine and fluorescein were detected in the shallow zone of two bay-side monitoring wells 18 days after injection. Rhodamine in the shallow zones of monitoring wells indicates upward movement of ground water and is consistant with seepage discussed later. Dyes were detected only in the seawardmost monitoring wells in each well cluster, confirming that net flow is toward the Atlantic on both sides of Key Largo. Rates of flow range from 0.5 to 2.0 m/day. Tidal pumping combined with elevated water level in Florida Bay apparently drive net flow toward the Atlantic.

Pressure measurements taken from piezometers at 15 minute intervals show that bay ground-water pressure, where tides are minimal, is precisely tuned to Atlantic tides and ground-water pressures in the Atlantic. Ground-water pressure is negative under the bay when the tide is low in the Atlantic and reverses during high tide. The difference in elevation, 1 m during low tide in the Atlantic, causes seaward flow of bay-side ground water through the permeable limestone underlying the Keys. The gradient is slightly less during high tide, resulting in reduced flow in the opposite direction. Tidal pumping also causes seepage of ground water into the overlying water column. Other chemical data, summarized below, were obtained from water collected from seepage meters, well clusters, and other monitoring wells.

2) Direct measurements of seepage confirmed that areas with high concentrations of natural tracers also exhibit high seepage rates. The natural tracers, 222Rn and CH4, are elevated in ground water relative to surface water and serve as indicators of the release of ground water into Florida Bay. Two independent surveys confirmed that concentrations of both tracers are significantly enriched in areas of Florida Bay near the Keys (especially near Key Largo), relative to the northern, middle, and northeastern portions of the bay. Direct measurements of seepage were consistent with this assessment. At a site near Key Largo, seepage was observed to change in harmony with Atlantic tidal stage. Water seeped into Florida Bay when ground-water pressure was positive and reversed when negative. Rates of seepage varied from 15 to 40 ml/m-2/min-1. Measurements over a tidal cycle indicate that variations of tracer concentrations within Florida Bay waters are controlled by tidal stage on the Atlantic side of Key Largo.

Sulfur hexafluoride, an inert, artificial, non-reactive, non-toxic tracer, was injected into a well at the Key Largo Ranger Station during a rising Atlantic tide. Located about 1 m above bay level, the well is screened from -1 to -12.2 m. The tracer moved ~50 m and was detected in Florida Bay surface waters 6 hours later. The experiment was replicated with similar results. During low Atlantic tide, the plume moved toward the Atlantic and was detected in a monitoring well 3 m away within 3 hours. After several more hours, the plume passed through the well a second time. The maximum extent of plume movement toward the Atlantic could not be measured due to lack of additional monitoring wells. These results, however, confirm oscillation and lateral movement of ground water driven by Atlantic tides. The well, which is open to the limestone 1 m below the surface, is considered more representative of a septic tank or cesspool system than a modern disposal well.

Sulfur hexafluoride was injected into a modern disposal well between -18 and -27.4 m at the Keys Marine Laboratory on Long Key. Within 1 hour, the tracer was detected at -18 m, 5 m away on the Atlantic side of the injection well. The tracer was observed 4 hours later at -4.5 m, 10 m away from the injection well. At other surrounding wells, the tracer was observed at a variety of depths. These results indicate mobility of the tracer associated with channels or conduits within the karst aquifer, rising of the freshwater plume in a saline aquifer, diffusion, and a hydrologic gradient dipping toward the Atlantic. These results are consistent with dye, microbial, and phosphate studies being conducted jointly at this site by the USGS, University of South Florida, and Pennsylvania State researchers.

3) Although small-scale tracer studies indicate rapid, local, lateral movement of water in the subsurface, and water level monitoring studies indicate hydraulic potential for ground-water flow from the bay side to the ocean side, those results do not address directly the large-scale extent of N-S ground-water transport and origin of nutrients observed in ground water far offshore. We are testing the use of a variety of environmental isotopes (He, H, C, O, S, and N) and tracers (chlorofluorocarbons, SF6, coprostanol) to see what can be learned from them about ground water sources, the scale of the ground-water flow systems, and the fate of injected contaminants from the Keys. In February 1996, 33 sites were sampled to provide a preliminary comprehensive survey of surface and shallow ground water representing the potential recharge sources and the major regional types of ground water in the vicinity of the Keys and offshore areas to the north and south. Analyses of those samples for isotopes and tracers, plus major ions, nutrients, and dissolved gases, are in various stages of completion in various laboratories.

Measurements of H and O isotope ratios and salinities indicate at least 4 mixing components: 1) seawater, 2) meteoric water, 3) evaporated seawater, and 4) evaporated meteoric water. Bay-side ground water generally was enriched in 2H, 18O, and salinity compared to offshore marine surface water. These data are consistent with recharge by evaporated marine bay water during times of relatively high bay salinity. Ocean-side ground water had 2H, 18O, and salinity values generally equal to those of offshore marine surface water and consistent with ground water recharge by normal seawater. Several ground water samples from under the Keys and from short distances (less than a few hundred meters) offshore had isotope compositions consistent with transport of bay water to the ocean side, and one nearshore sample indicated transport of seawater to the bay side. Isotope data for tap water, waste water, and injected waste water all are consistent with a common freshwater source on the Florida mainland, and with mixing of waste water and marine ground water in the subsurface near a waste-water injection site.

Concentrations of CFC-12 indicate that the residence times of marine ground water on both sides of the Keys range from years to decades or more, and that the apparent ages generally are stratified. Degradation apparently has altered the concentrations of CFC-11 and CFC-113 in some samples. Minor CFC contamination was detected in water from the Port Largo canal and in waste water at the Keys Marine Laboratory, but it does not appear to be widespread (though degradation may have altered some occurrences). Additional samples are being analyzed for tritium and He isotopes that, when combined with CFC results, should provide more insight into the ground-water age distributions in the region. Ambient concentrations of SF6 are also being investigated for comparison with CFC data.

Nutrient analyses confirm that reduced ground water throughout the area contains significant amounts of ammonium. Concentrations of sulfide, methane, and bicarbonate were also elevated. The concentrations and isotopic compositions of sulfate and sulfide in the reduced ground waters are consistent with minor sulfate reduction. The concentrations and isotopic compositions of dissolved inorganic carbon (DIC) indicate varying contributions from both organic carbon oxidation and carbonate mineral recrystallization. Additional samples are being analyzed for 14C, which may provide evidence about the ages of carbon sources for the DIC and methane. The concentrations and isotopic compositions of dissolved N2 were nearly consistent with atmospheric equilibration over a small range in temperature, but there was evidence for small amounts of excess N2 in many samples that may have been derived from denitrification (reduction of nitrate to N2). Relatively large amounts of excess N2 in waste water and in mixed ground water near an injection site apparently were the result of denitrification of nitrate in the wastewater. Isotope analyses of nitrate and ammonium are underway and should provide important additional constraints on the fate of anthropogenic nitrate and the origin of ground-water ammonium.

(This abstract was taken from the Florida Bay Science Conference Proceedings, 1996)

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