Cave Diving Documents Spatial and Temporal Water Quality Variability in a Phreatic, Karst Cave System

Abstract

Karst aquifers have evolved secondary porosity features that facilitate heterogeneous recharge and groundwater flow dynamics. These dynamics affect the natural spatial and temporal variability of water quality in the aquifer. However, when recharge occurs near urban and agricultural land use that can introduce contamination, the contamination can conflate natural water quality variability, generating convoluted signals in time and space. Most water quality investigations in karst aquifers rely on groundwater sampling at discrete locations such as wells or springs, which do not always capture the magnitude of water quality heterogeneity. Cave diving in phreatic caves can be used to explore this variability by using water quality sensors and discrete water chemistry samples to explore spatial and temporal water quality changes for improved and targeted water resource management. Our study uses cave diving to document the spatial and temporal variation in water quality within a phreatic cave system in the Floridan Aquifer System (FAS), a karst aquifer in northern Florida. We collect continuous 15-s measurements of dissolved oxygen (DO), temperature, pH, and specific conductance along a 1.1 km transect, which intersects multiple cave passages that drain into the primary cave passage. We also collect discrete water chemistry samples in three separate cave passages within the phreatic cave, as well as at the spring vent, to document spatial and seasonal variability in nutrients, organic matter, and major groundwater ions. Our results show that specific conductance, DO, temperature, and pH vary together spatially in consistent ways, which we use to identify cave passages that receive more direct recharge. Spatial and temporal variability across the cave system was most pronounced for NO_x-N (nitrate + nitrite), DO, and dissolved organic carbon, while major ions showed minimal spatial variability but greater temporal variability. Relationships derived between specific conductance and NO_x-N show a positive correlation, while relationships between ions associated with carbonate mineralogy and specific conductance are negatively correlated, which likely reflects the impact of recharge from agricultural land use surrounding the cave system. Our results highlight water quality complexity in phreatic caves and have implications for local water quality restoration efforts, interpreting water quality data collected at a discrete location, and provide guidance for future water quality studies in phreatic cave systems.