OUTGASSING AS A GEOCHEMICAL SIGNATURE OF RECHARGE WATER IN A KARST AQUIFER

Recharge distribution in a karst aquifer can be difficult to determine where it is diffuse rather than a point sink. Understanding recharge areas is important in planning aquifer protection. We used groundwater geochemistry to infer changes in recharge in a karst aquifer in southeastern PA that discharges at Lititz Spring. We sampled the spring and a nearby domestic well for a year. Although both locations are connected to conduits (as evidenced by tracer test), most of the year they were saturated with respect to calcite, which is more typical of matrix flow. We used geochemical modeling (PHREEQC) to explain this apparent paradox. The saturation index varied from 0.5 to 0 most of the year, with a few samples in springtime dropping below saturation. The log PCO2 varied from –2.5 to -1.7. The lower log PCO2 (closer to the atmospheric value of –3.5) occurred when the solutions were at or above saturation with respect to calcite. In contrast, samples collected in the springtime had high PCO2, low saturation indices, and high water levels. Geochemical modeling showed that water at high PCO2 will become supersaturated with respect to calcite when outgassing occurs. We suggest the recharge water travels through the soil zone, where it picks up CO2 from soil gas. Excess CO2 is outgassed when this recharge water reaches the conduit. At times of high water level (pipe full), the ougassing did not occur, but instead dilution occurs, reducing the saturation index.