The fate of urban springs: Pumping-induced seawater intrusion in a phreatic cave

Sulphur Springs Cave is an extensive phreatic cavity that produces a large, historic spring in the middle of metropolitan Tampa, Florida, USA. The city of Tampa extracts groundwater from the spring to supplement municipal water supply and to support low-salinity habitat in the estuarine Hillsborough River. Extraction at this site has occurred for many decades, but has intensified since the early 2000s, rapidly increasing the salinity of the spring and cave water. The purpose of this study was to address the potential sources and mechanisms of saltwater intrusion at this site using historical and current hydrochemical data published in the literature and online by government agencies. We also explored the cave to identify point-sources of intrusion, and collected water and biological samples from inside the cave to identify potential ecosystem impacts of increasing cave salinity. From 1946 to present, Sulphur Springs water shifted from being fresh (specific conductance <500 µS cm−1) and of calcium-sulfate type to being brackish (specific conductance ∼5000 µS cm−1 and higher) and of sodium-chloride type. We found numerous vents in the cave that issue saline, thermal, sulfidic water and host distinct microbial mat communities. These vents are likely connected to bedrock fractures that provide preferential flow-paths along which confined, deep-sourced saline water enters the freshwater portion of the aquifer, probably originating from the coastal mixing zone. Salinity increased at the spring during dry-season pumping activity and after wet-season recharge events, which likely increased artesian pressure in confined saline aquifer units. Salinization of Sulphur Springs may disrupt the cave microbe and stygobite communities and eventually make the spring unsuitable to maintain low-salinity habitat in the Hillsborough River.