MONITORING STORM-WATER QUALITY IN THE ARROWHEAD DRAINAGE BASIN, RAPID CITY, SD 2008-2009

The City of Rapid City, South Dakota, has implemented programs to improve storm-water quality in response to the “Phase II Final Rule” issued by the U.S. Environmental Protection Agency. Applicable State of South Dakota guidelines for beneficial uses on Rapid Creek include a maximum total suspended solids (TSS) concentration of 53 mg/L in any one sample and a maximum fecal coliform concentration of 400 colony forming units (CFU)/100 mL in any one sample. To better understand urban stormwater runoff in the Rapid City area, the U.S. Geological Survey, in cooperation with the city of Rapid City, began storm-water monitoring in 2008 in the Arrowhead drainage basin (a tributary to Rapid Creek), which contains a mix of residential communities, forest and rangeland, construction sites, and many septic systems. Previous sampling efforts had indicated high concentrations of fecal coliform and suspended sediment in this basin.

Two automatic water-quality sampling stations were installed within Arrowhead basin, one in July 2008 at the center of the basin and a second in July 2009 upstream from the confluence with Rapid Creek. The stations performed automated sampling during storm events and continuous monitoring of stage, along with air and water temperature. Precipitation data were collected at the first site. Thirteen storms were sampled during the spring, summer, and early fall months of 2008 and 2009. Storms ranged from long-duration, low-intensity storms to short-duration, high-intensity thunderstorms. Event mean concentrations (EMCs) averaged over the basin for the storm events were: 355 mg/L for TSS; 16800 CFU/100 mL for fecal coliform; and 2200 most probable number/100 mL for E. coli. EMCs of TSS ranged from 9 to 1900 mg/L, and fecal coliform counts ranged from 24 to 154,000 CFU/100 mL. E. coli to fecal coliform ratios ranged from 0.19 to 1.08 across the basin indicating that E. coli may be a major component of fecal coliform counts. Spatial patterns of the data indicate that TSS concentrations increase throughout the center of the basin then decrease through golf courses in the lower part of the basin to the outlet. Fecal coliforms increase throughout the basin to the outlet with no noticeable effect of surface conditions on the fecal coliform concentrations.