Quantifying the variability in Escherichia coli (E. coli) throughout storm events at a karst spring in northwestern Arkansas, United States

Abstract

In northwestern Arkansas, karst aquifers have experienced degraded water quality due to impacts of land-use changes, such as increased urbanization or intensification of agriculture. Water quality at a karst spring was characterized by quantifying the variability of fecal-indicator bacteria, specifically Escherichia coli (E. coli), throughout base-flow periods and storm events. Water-quality change over time was assessed using an intermittent record of nitrate and chloride data since the 1990s. A Geographic Information System was used to quantify changes in land use over time and identify housing subdivisions using on-site septic systems. E. coli was significantly greater during storm events (649 CFU/100 mL or MP/100 mL) than base-flow periods (41 CFU/100 mL or MPN/100 mL). The increase in E. coli at the spring following storm events was interpreted to result from flushing of bacteria from the land surface, through the epikarst and karst conduits, and into groundwater. Nitrate and chloride have increased significantly since the 1990s, following the general pattern of increased urbanization in the recharge area. Septic-tank effluent may be degrading the water quality of the karst spring based on the dominance of on-site septic tank usage in the recharge area, unsuitable topography and soil type for septic tank absorption fields, increased nitrate and chloride concentrations concomitant with increased urbanization, and increase of the fecal-indicator bacteria E. coli following storm events.