COMPARISON OF SPRING AND WELL MONITORING DATA IN A CARBONATE AQUIFER

Water in karst aquifers flows through both a fractured matrix and conduits (or enlarged fractures). The contribution of these components to water along the flow path and at discharge points is difficult to determine, but has implications for travel times and contaminant concentrations. We monitored a spring discharging from an enlarged fracture into Bushkill Creek in SE PA and a well 350 m upgradient that intersected a 1 m wide conduit. Samples were collected monthly for 17 months, and loggers monitored temperature, stage, and conductivity at both locations for 9 months. In addition, a non-quantitative dye trace was performed by injecting dye into the well and monitoring 9 springs in the discharge area using charcoal packs.

The dye trace showed a connection between the well and springs. However, the dye was visible in the well for about 3 months and only appeared in dilute concentrations at the springs after a large precipitation event also at 3 months. The dye was distributed along a 50 m stretch, indicating the connection between the well and the creek is a series of networked fractures and conduits rather than a single large conduit. This suggests a diffuse flow path from the well to the springs.

Water level in the well typically peaked about half a day after the spring and recovered more slowly from precipitation events. Temperature was constant in the well, but varied in the spring with storm events. Constant temperature is evidence for longer flow paths, while variable temperature suggests shorter flow paths. Conductivity, Ca, and HCO3 were higher in the well, but also showed more variability. For example, HCO3 averaged 400 mg/L in the well, with a standard deviation of 85 mg/L, while the spring averaged 170 mg/L with a standard deviation of 40 mg/L. The PCO2 and the SI of calcite in the well were generally higher, although both the spring and the well were undersaturated except for several well samples in June and July of the second year.

There is an apparent paradox in that the well has higher conductivity and slower water level recovery but more ion variability. The paradox is explained by a longer residence time for the water in the well, but with periodic flushing by more dilute water. The spring has a more constant mix of matrix and conduit water, suggesting diffuse recharge but shorter flow paths occur in this spring.