EPIKARST ROLE IN CONTROLLING THE QUALITY OF KARST AQUIFER RECHARGE

Epikarst, or the region of vegetation, soil, and weathered bedrock lying between the land surface and soluble bedrock, offers water retention capacity that does not exist in deeper, more mature sections of karst aquifers. Thus, the epikarst can act as a temporary reservoir for surface-applied contaminants and naturally-occurring chemical species. Long-term multi- parameter records of precipitation, soil water, epikarst drip water, and in-cave stream-water at James Cave in Dublin, VA, allow us to examine the role of epikarst in controlling the quality and geochemical evolution of recharge water as it passes through the epikarst.

Data collected since September 2007 are being used to identify trends in the temporal and spatial distribution of recharge to the karst aquifer. Drip rates indicate that recharge occurs during late winter/spring, but is minimized by evapotranspiration in summer. Precipitation over James Cave passes through the epikarst where its composition is modified by water-rock interactions. Chemical species such as Na, K, Cl, NO3, SO4, and DOC can serve as tracers to assess the timing and mechanisms of recharge. Geochemical differences between sites indicate that hydrologic and geochemical processes in the epikarst are spatially heterogeneous. However, temporal variations in major ion concentrations can be correlated between sites as a function of recharge.

Specific conductance, pH, alkalinity, and major ion concentrations in epikarst drip water increase during low flow due to increased water-rock interaction. During high flow, however, younger recharge pushes older geochemically saturated water through the epikarst. High flow also inhibits the potential for natural attenuation of contaminants. The structural orientation of the epikarst, the presence of microbial activity, climate, and amount and timing of recharge are all factors in determining the extent to which epikarst controls the quality of recharge to the karst aquifer.