THE INFLUENCE OF LOCAL RECHARGE ON RIVER INTRUSION INTO AN EOGENETIC KARST AQUIFER

Surface runoff from storms may allow river water to intrude into aquifers through spring vents if river stage exceeds the hydraulic head of the groundwater sourcing the spring. The transient storage of river water in the aquifer during these spring reversals results in chemical reactions that can cause calcite dissolution and affect groundwater quality. These reactions are especially prominent in eogenetic karst aquifers, i.e., those with high primary and secondary porosity, because of the large surface area provided by their matrix porosity. The intrusion of river water into eogenetic aquifers may be moderated or eliminated based on the magnitude of diffuse recharge. To assess the influence of diffuse recharge on spring reversals, we examined changes in head and chemical composition of water during and after two storms at Madison Blue Spring in north Florida, using grab samples of river and spring water along with continuously logged depths from sensors deployed in the river, inside phreatic conduits, and in adjacent groundwater wells. Aquifer water table elevations were comparable prior to both storms; however, recharge patterns differed. In March 2012 rain fell mostly in the upper confined portion of the river drainage basin, triggering a 2.5 m rise in river stage, with only about 0.02 m of recharge over the springshed. The spring began reversing after the river stage rose 0.7 m, exceeding the hydraulic heads in the spring, the conduits, and the monitoring wells in the aquifer matrix. The river water that entered the aquifer was enriched in organic carbon (OC) and undersaturated with respect to calcite. Calcite dissolution, enhanced by the breakdown of OC, occurred in the matrix adjacent to the conduits. In contrast, rain in June 2012 fell predominantly on the unconfined springshed. The June 2012 rainfall triggered a rise in river stage of 0.9 m along with an estimated 0.5 m of diffuse recharge over the springshed. The hydraulic head of the groundwater remained above river stage because of the diffuse recharge, and the spring did not reverse. No river water entered the aquifer to cause calcite dissolution adjacent to the conduits. These events illustrate that distribution of precipitation across a watershed impacts the exchange of surface water and groundwater and associated chemical reactions caused by the exchange.