Cross-Correlation of Hydrologic Data to Assess Water Exchange between Conduits and the Surrounding Rock Matrix in Karst Aquifers

The Santa Fe River in north-central Florida sinks underground at River Sink, where it then flows through conduits that are ~20 m in diameter and ~30 m below the surface in the Upper Eocene Ocala Limestone with relatively high porosity and permeability characteristic of eogenetic karst. The water periodically reemerges at the surface at karst windows and finally reemerges at River Rise, a spring with a discharge that ranged from 3.9 to 97 m3/s during our study and that is a straight-line distance of just under 5 km south-southwest of River Sink. We collected water level, electrical conductivity, and temperature data at River Sink, River Rise, and five intermediate karst windows over a three-year period with a data output interval of two to five minutes. Water levels change up to three meters during recharge events, which also produce electrical conductivity and temperature event-scale variations. There are also diurnal variations in electrical conductivity and temperature as well as seasonal temperature variations. We compute cross-correlation on the electrical conductivity and temperature time series from multiple stations along the flow path to determine travel time, which may vary from true flow-through time because of transport processes beyond flow velocity. We use the difference in calculated travel times from electrical conductivity and temperature to determine thermal retardation for all possible station pair combinations. We compare this to discharge records at River Sink and River Rise provided by the USGS. When discharge is greater at River Rise, water flows from the rock matrix into the conduits. However, discharge may be greater at River Sink following large recharge events, causing water to flow from the conduits into the surrounding rock. The variable flow conditions around the conduits uniquely modify the input signals of water flowing into River Sink. We compare our thermal retardation analysis to the difference in the two discharge records to identify relationships and assess the degree to which the thermal retardation serves as a proxy relevant to the exchange of water between conduits and the surrounding rock matrix, which has important implications for water quality. The long record allows us to evaluate how relationships change through time as well as during individual recharge events.