QUANTIFYING THE VARIABILITY IN ESCHERICHIA COLI (E. COLI) AT A KARST SPRING IN NORTHWESTERN ARKANSAS

Blowing Spring discharges from a cave in the city of Bella Vista in northwestern Arkansas. The spring has experienced progressive degradation of water quality since the 1990’s, including increased concentrations of nitrate, chloride, and Escherichia coli (E. coli). This research characterized spring-water quality by monitoring E. coli concentrations across a range of hydrologic conditions (base flow and storm events) from January 2007 to May 2013. A weir was constructed at Blowing Spring in January 2012 to develop a stage-discharge rating. To extend the record of E. coli data back through 2007, discharge at Blowing Spring prior to 2012 was estimated using a correlation with discharge data from U.S. Geological Survey gaging station 07188838, Little Sugar Creek near Pineville, Missouri (approximately 20 km from the spring). Blowing Spring and Little Sugar Creek experience similar changes in discharge during the period of 2012 to 2013 (r² = 0.80). Geographic Information Systems were used to quantify changes in land use since 2007 for the city of Bella Vista, Arkansas in the approximate recharge area of the spring, which is a mix of forest and suburban land use. The recharge area, which has not been identified using dye tracing, was estimated to be between 2.9 and 6.1 km² using the normalized base flow method and a base flow of 0.01 m³/s at the spring. During storm events, discharge at the spring increased from a mean base flow of 0.01 m³/s to a maximum flow of 4.2 m³/s. From March 2012 to May 2013, the median E. coli concentration at base flow was 15 MPN/100 mL. During three storm events (March 2012, January 2013, and April 2013), E. coli concentration increased with increasing discharge, exceeding the maximum quantitation limit of 2,420 MPN/100 mL at a discharge of 1.9 m³/s during the April 2013 storm event. Suburban land use near the cave, including septic systems, may be a source of E. coli, nitrate, and chloride to the spring.