BEHAVIOR OF SELECT TRACE ELEMENTS AND NUTRIENTS ACROSS A LARGE CARBONATE DISPERSIVE MIXING ZONE

Dispersive mixing of fresh and saline groundwater (i.e. subterranean estuary) can influence the cycling of dissolved metals and nutrients, ultimately affecting the role that coastal groundwater discharge can have on nearshore material budgets. To date, subterranean estuary research has focused mostly on the cycling of dissolved constituents in siliciclastic aquifers where fresh and saline groundwater mix laterally over a few meters. We examined the behavior of select dissolved metals, major cations and anions, and nutrients within a kilometer-scale, carbonate subterranean estuary in south Florida: the Shark River/Gulf of Mexico ecotone in Everglades National Park (ENP) during dry-season conditions. Ground- and surface-water samples were collected from a network of U.S. Geological Survey and Long-Term Ecological Research sites. Estuarine mixing models were used to examine elemental behavior within this large-scale mixing zone. Calcium enrichment is observed in all brackish groundwaters relative to mixing between fresh groundwater and Gulf of Mexico seawater, indicating that dissolution of the carbonate aquifer is an important geochemical process within this subterranean estuary. Dissolution may account for slight enrichment of strontium and phosphate in some of the groundwater samples. Barium demonstrates non-conservative release within the subterranean estuary; however, barium enrichment occurs at a lower salinity (< 10) than does maximum calcium enrichment (15 – 20). Uranium exhibits a non-conservative removal across the large-scale mixing zone. Dissolved iron (Fe) and manganese (Mn) concentrations are fairly constant in all groundwater samples (1.8 and 1.2 μM, respectively) and do not show a systematic source/sink behavior across the salinity gradient. Thus, Fe and Mn redox cycling may be less influential in this carbonate subterranean estuary than is typically observed in siliciclastic, redox-controlled systems. Similar magnitudes of calcium enrichment and uranium depletion in the surface waters reflect the strong coupling between the surface- and groundwater system in the ENP. Additional sampling will be conducted during the wet season to determine whether precipitation/recharge patterns influence the geochemical framework of this large-scale, carbonate subterranean estuary.