Ground water is a potential source, sink, and carrier of nutrients and other contaminants beneath the Florida Keys and nearby offshore regions. Ground-water flow patterns and nutrient sources need to be understood to assess and predict the dispersal of anthropogenic contaminants such as injected wastewater. Small-scale tracer studies indicate rapid local lateral movement of ground water near the Keys, and water-level monitoring studies indicate hydraulic potentials for ground-water flow. Those results, however, do not address directly the large-scale extent of ground-water transport and the origin of nutrients observed in offshore ground water. A variety of environmental isotopes and geochemical tracers are being used to provide information on the sources, flow directions, exchange rates, and chemical characteristics of ground water underlying the region at depths of about 3 to 20 m below the sediment surface. In this study a multi-tracer approach was used to address a variety of scientific problems: (1) salinity data combined with stable H and O isotopes of water provide information about ground-water sources; (2) CFC's, ³H, and He isotopes provide information about the subsurface residence times of ground water; and (3) ¹⁴C and stable isotopes of C-, N-, and S-bearing constituents provide information about sources of solutes, and chemical reactions among ground water, nutrients, and aquifer materials. Two major offshore sampling surveys were completed in 1996 and 1997, and additional samples were obtained from wells on land in 1998. In total, these constitute a set of representative samples of surface water and ground water from a large part of Florida Bay, the Keys, and offshore areas extending to the reef tract in the south.

Stable H- and O-isotope ratios and salinities indicate at least four mixing components: seawater, meteoric water, evaporated seawater, and evaporated meteoric water. Isotopic data for tap water, wastewater, and injected wastewater were consistent with a common freshwater source on the Florida mainland, and with mixing of wastewater and bay-type marine ground water in the subsurface near wastewater injection sites. Atlantic Ocean side ground water had values of ²H (+10 + 2 ‰), ¹⁸O (+1.15 + 0.15 ‰), and salinity (35 + 1 ‰) that generally were equal to those of offshore seawater. Those results are consistent with Atlantic Ocean side ground water having originated from recharge of normal seawater. Bay-side ground water generally had higher values of ²H (+13 to +20 ‰), ¹⁸O (+1.5 to +2.7 ‰), and salinity (35-39 ‰) compared to offshore seawater, with the highest values occurring in the eastern part of the bay and more marine values toward the west. The regional patterns of H and O isotope variations in bay-side ground water are similar to the patterns in bay-side surface water. Those results are consistent with the bay-side ground water having originated from recharge of evaporated bay water during times of high bay salinity, and they indicate little regional-scale lateral ground-water transport beneath the bay. Ground-water samples from beneath the Keys and from short distances (less than a few hundred meters) offshore to the south had isotopic compositions consistent with transport of bay water toward the ocean side in the subsurface. However, the salinity and isotope results so far have not supported long-distance transport (more than a few hundred meters) of bay-derived ground water to sites far offshore on the Atlantic Ocean side (for example, to the reef) within the depth range investigated.

Concentrations of CFC-12 in marine ground water on both sides of the Keys were consistent with atmospheric equilibration and subsequent isolation (recharge) at times ranging from 0 to more than 50 years ago. Degradation may have altered significantly the concentrations of CFC-11 and CFC-113 in most samples. Minor CFC contamination was detected in water from a canal on Key Largo and in wastewater at the Keys Marine Laboratory, but it did not appear to be widespread. Apparent recharge ages derived from analyses of ³H and He isotopes ranged from 0 to more than 30 years, and reconstructed values of initial ³H were in the range of 0-10 TU. ³H-³He ages and CFC-12 ages were generally correlated, but the CFC-12 ages commonly were on the order of 10-50 percent older than those ages based on ³H-³He. Apparent ages derived from both CFC and ³H-³He methods were stratified. Wherever more than one depth could be sampled at the same site, the deeper water appeared to be older. Estimated concentrations of non-atmospheric radiogenic ⁴He were on the order of 0-1 x 10^-8 ccSTP/g (cc at Standard Temperature and Pressure/g). The ¹³C values of dissolved inorganic carbon (DIC) indicate varying contributions from both organic carbon oxidation and carbonate mineral recrystallization. Un-normalized ¹⁴C abundances in DIC (mainly bicarbonate) range from less than 10 percent to about 115 percent "modern," consistent with a large range of apparent radiocarbon ages. Much of the variation in apparent ages, however, can be accounted for by chemical reactions between seawater with high ¹⁴C and carbonate sediments with low ¹⁴C. Combined results of chronologic tracers indicate that the time scales of ground-water/surface-water exchange are on the order of years to decades at depths up to about 20 m below the sediment-water interface. Within those time scales, ground-water compositions are altered significantly by ground-water-aquifer interactions that may include release or consumption of nutrients.

Nutrient analyses confirmed that anoxic marine ground water throughout the area contained significant amounts of ammonium (10-80 µM). Concentrations of sulfide, methane, and bicarbonate also were elevated in the reduced water samples. The ¹⁵N values of ammonium in ground water near a site where treated sewage effluent is injected to the ground-water system were high (+9 to +12 ‰) and could be consistent with a wastewater source. The ¹⁵N values of ammonium in all other ground water were low, but variable (+3 to +8 ‰). There was not a strong correlation between ammonium concentrations and ¹⁵N values. It is possible that much of the ground-water ammonium was produced naturally by anaerobic degradation of N-bearing organic matter in sediments. Nitrate concentrations in most samples were low (< 1 µM). Nitrate concentrations greater than 5 µM were detected in wastewater and in ground-water samples that contained low-salinity components either from wastewater or from precipitation recharge. The highest nitrate concentrations were measured in treated wastewater and in shallow mixtures of wastewater and saline ground water. A minor source of N also was identified in the Keys water supply. Concentrations and isotopic compositions of dissolved nitrogen gas in most ground water were generally consistent with atmospheric equilibration betweeN₂0 and 28 °C before recharge. There was evidence in some samples for small quantities of excess N₂ that may have been derived from denitrification (reduction of nitrate to N₂ ). Excess N₂ in wastewater and in mixtures of ground water and wastewater near an injection site was apparently the result of denitrification of nitrate in the wastewater.

This project is providing quantitative information about the origins, residence times, and nutrient characteristics of marine ground water beneath the Florida Keys and nearby offshore areas. Chronologic and isotopic data for ground water are useful in assessing the extent and rate of movement of potential contamination in the subsurface. Nutrient chemistry and isotopic data are useful in distinguishing natural and anthropogenic sources of chemical constituents in ground water. These data and interpretations aid decision-making about wastewater treatment and disposal in the region. In addition, the study includes testing some new combinations of methods and sample types that could expand the list of approaches available to the scientific community for future investigations of similar problems.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)