IMPACTS OF GROUNDWATER EXTRACTION AND SEAWATER INFILTRATION ON A SUBMERGED CAVE ECOSYSTEM

Karst springs support important surface ecosystems and provide valuable services to humans. Some spring conduits are large enough to support rich biological communities adapted to subsurface conditions. Coastal spring systems can carry high-salinity groundwater up from deep aquifer layers, potentially threatening spring and cave communities as well as drinking water quality. High hydraulic head pressure of shallow aquifer layers resists vertical infiltration of deep-sourced water, but head pressure can be reduced by anthropogenic groundwater extraction. Groundwater is being extracted from Sulphur Springs, a phreatic cave underneath Tampa, Florida. The primary conduit in Sulphur Springs Cave carries fresh groundwater, but a secondary conduit which is recharged by deeper, high-salinity groundwater also contributes to cave discharge, making the spring slightly brackish. In our current study, we assess the impacts of groundwater extraction and vertical infiltration on the Sulphur Springs ecosystem. Acoustic Doppler velocimeters and water quality multiprobes were deployed by divers in the cave conduits to record water discharge and physical parameter profiles. Nutrient profiles were also generated through spectrophotometric analysis of cave water samples. Cave sediment, water column, and biofilm microbial communities are being characterized through DNA analysis. We have found that hydraulic gradients in the Sulphur Springs system increase as extraction lowers spring pool level, and as precipitation recharges aquifer levels. Increased hydraulic gradients increased the salinity, decreased the pH, and increased the temperature of Sulphur Springs Cave water, indicating greater contribution of deep, saline aquifer layers. Sulfide concentrations also increased with hydraulic gradient. Groundwater extraction from Sulphur Springs during Florida’s dry season reduces the head pressure of the spring pool, allowing for vertically-infiltrating, nonpotable groundwater to rapidly enter the cave system. During the wet season, deep aquifer layers are recharged more quickly than shallow layers, also facilitating vertical infiltration of saline water. This research will identify ecological factors that can be managed for the conservation of coastal cave ecosystems.