WATER QUALITY VARIABILITY THROUGHOUT STORM EVENTS AT A KARST SPRING IN NORTHWESTERN ARKANSAS

Blowing Spring is the focal point of a park located in northwestern Arkansas and the spring has experienced degraded water quality since the 1990’s, including increased nitrate and chloride concentrations. The spring discharges from a cave in a Mississippian limestone that hosts abundant karst features, and the land use above the cave is mixed suburban and forest. The research assessed contaminant effects (bacteria and anions) on cave- and spring-water quality through a range of hydrologic conditions (base flow and storm events) from March 2012 to May 2013. During storm events, discharge at the spring increased from an average base flow of 0.01 m³/s to 0.28 to 0.57 m³/s, depending on rainfall intensity and antecedent conditions, with the highest recorded discharge of 2.0 m³/s. During base flow, E. coli concentration at the spring had a median value of 15 MPN/100 mL. Throughout three storm events (March 2012, January 2013, and April 2013), as discharge increased, E. coli concentration also increased, with a maximum value of 2,420 MPN/100 mL at a discharge of 1.9 m³/s during the April 2013 storm event. Changes in the concentration of major anions (nitrate, chloride, and sulfate) were variable during the three storm events, typical of karst springs; during larger events dilution because of rainfall appeared to decrease anion concentrations, whereas during the smaller January 2013 event (0.32 m³/s), nitrate and sulfate concentrations increased throughout the storm event. The variable response in spring water geochemistry may be because of antecedent conditions in the recharge area, especially because of the prolonged drought during the summer and fall of 2013. The suburban land use near the cave, including septic systems, may be a source of bacteria, nitrate, chloride, and sulfate to the spring, although the recharge boundaries for the spring have not been identified.