CHARACTERIZING THE GEOCHEMISTRY OF GROUNDWATER FLOW THROUGH THE EPIKARST OF NORTHWESTERN ARKANSAS

Blowing Spring discharges from a cave in Mississippian limestone of northwestern Arkansas. Land use above the cave is mixed suburban and forest, and the spring has experienced degraded water quality since the 1990’s, including increased nitrate and chloride concentrations. Karst recharge above the cave was monitored as the water traveled from the atmosphere, through the soil and epikarst zones, into cave conduits, and discharged at the spring. Water samples from the soil and cave were collected from March 2012 to May 2013 and included background sampling during base-flow conditions and storm event sampling following rain events. Groundwater geochemical evolution was characterized by comparing soil water, cave drip water, and cave stream water (flowing from a sump at the back of the cave) as groundwater moved from the soil into the cave. Soil water chemistry was heterogeneous between two dry valleys above the cave for most major cations and anions (calcium, magnesium, sodium, potassium, nitrate, chloride, and sulfate concentrations all varied between the valleys). The concentration and carbon isotopic composition of dissolved inorganic carbon (DIC) was similar between the two soil sites. Soil water had median DIC concentrations of 2.5 and 1.7 mg/L at the two soil sites, compared to a median value of 23.8 mg/L for cave drip water. The isotopic composition of DIC (δ¹³C-DIC) had a median value of ‑19.5‰ at both soil sites and -11.7‰ for cave drip water. The cave stream had slightly higher DIC concentration (25.5 mg/L) and intermediate δ¹³C-DIC values (‑14.5‰). Major cation and anion geochemistry varied among the soil and cave water; for example, nitrate was not significantly different between soil water (median value of 0.25 mg/L) and cave drip water (median value of 0.91 mg/L). But, cave stream water closest to the sump had concentrations of nitrate (median value of 3.01 mg/L) that were higher than the soil or cave drip water. The recharge area for the cave stream has not been identified, but drip water from immediately above the cave may not be the source of nitrate to the spring as the cave stream values were greater than the soil or cave drip water. Nitrate is likely sourced from the aquifer feeding the cave stream, with regional groundwater flow from northeast to southwest, coexistent with mixed forest and suburban land use.