Ground waters (GW) are potential sources, sinks, and carriers of nutrients and other contaminants beneath the Florida Keys and nearby offshore regions to the north and south. Although small-scale tracer studies indicate rapid local lateral movement of water in the subsurface, and water level monitoring studies indicate that some hydraulic potential may exist for GW flow from the bay side to the ocean side, those results do not address directly the large-scale extent of GW transport and the origin of nutrients observed in ground waters far offshore. We are testing the use of environmental isotopes and tracers, combined with geochemical modeling, to provide new data on the sources, flow directions, exchange rates, and chemical characteristics of ground waters underlying the region at depths of about 3 to 20 m below the sediment surface. The empirical approaches being tested include: (1) analyses of chlorofluorocarbons (CFCs), SF6, 14C, 3H, and He isotopes for information about the residence times of water and dissolved species in the subsurface; (2) analyses of stable isotopes of H, O, C, N, and S for information about sources of, and chemical reactions among, waters, nutrients, and other species; and (3) modeling of chemical reactions and residence time distributions.

A set of representative samples of surface waters and ground waters was collected in 1996 in the Keys and offshore areas to the north and south. Measurements of H- and O-isotope ratios and salinities of those samples indicate at least four mixing components: seawater, meteoric water, evaporated seawater, and evaporated meteoric water. Oceanside GW had values of d2H (+10 2 ø), d18O (+1.15 0.15 ø), and salinity (36 1 ø) values generally equal to those of offshore marine surface waters, consistent with recharge of normal seawater. Bayside GW generally had higher values of d2H (+13 to +20 ø), d18O (+1.5 to +2.7 ø), and salinity (36-41 ø) compared to offshore marine surface waters, consistent with recharge of evaporated bay water during times of relatively high bay salinity. Several GW samples from the Keys and from short distances (less than a few hundred meters) offshore had isotopic compositions consistent with transport of bay water to the ocean side, and one nearshore sample indicated transport of seawater to the bay side. However, the salinity and isotopic results so far do not support long-distance transport (more than a few hundred meters) of evaporated bay water to sites far offshore on the ocean side (for example, to the reef), within the depth range investigated. Isotopic data for tap water, wastewater, and injected wastewater all were consistent with a common freshwater source on the Florida mainland, and with mixing of wastewater and bay-type marine GW in the subsurface near the waste-water injection site at the Keys Marine Lab (KML).

Concentrations of the chlorofluorocarbon CFC-12 in marine GW on both sides of the Keys were consistent with atmospheric equilibration and subsequent isolation (recharge) at times ranging from the present to more than 50 years ago. Degradation apparently had 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 KML site, but it did not appear to be widespread (though degradation may have altered some occurrences). 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 to 10 TU. 3H-3He ages and CFC-12 ages generally correlated, but the CFC-12 ages commonly were in the order of 10 to 50 percent larger. Apparent ages derived from both CFC and 3H-3He methods were consistently stratified; wherever more than one depth could be sampled at the same site, the deeper waters appeared to be older. Total concentrations of 4He were between about 3.7 and 4.9 x 10-8 ccSTP/g; whereas the estimated concentrations of non-atmospheric radiogenic 4He were in the order of 0-1 x 10-8 ccSTP/g. The concentrations and d13C values of dissolved inorganic carbon (DIC) indicate varying contributions from both organic carbon oxidation and carbonate mineral recrystallization. Unnormalized 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. However, much of the variation in apparent ages can be accounted for by chemical reactions between seawater with relatively high 14C and carbonate sediments with relatively low 14C.

Nutrient analyses confirmed that reduced marine GW throughout the area contained significant amounts of ammonium (10-80 mM). Concentrations of sulfide, methane, and bicarbonate also were elevated in the reduced waters. The d15N values of ammonium in ground waters near the KML injection site were relatively high (+9 to +12 ø) and could be consistent with a wastewater source. The d15N values of ammonium in all other ground waters were relatively low, but variable (+3 to +8 ø). There was not a strong correlation between ammonium concentrations and d15N values. It is possible that much of the GW ammonium is from anaerobic degradation of N-bearing organic matter in sediments. Nitrate concentrations in most samples were low (< 1 mM). Concentrations between 5 and 120 mM were detected only in a few samples, all of which contained low-salinity water components. The highest nitrate concentrations were in treated waste water and in a shallow mixture of waste water and saline ground water at KML. Unused tap waters collected from faucets at several different times and places on Key Largo also contained significant amounts of both ammonium (40 10 mM) and nitrate (50 10 mM) and may represent an unappreciated source of N in the Keys. Concentrations and isotopic compositions of dissolved nitrogen gas in most samples were approximately consistent with atmospheric equilibration between 20 and 28 oC. There was evidence in some samples for small amounts of excess N2 that may have been derived from denitrification (reduction of nitrate to N2). Relatively large amounts of excess N2 in wastewater and in mixed ground waters near the KML injection site apparently were the result of denitrification of nitrate in the wastewater.

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