WATER-QUALITY IN THE LOWER RUSSIAN RIVER BASIN, SONOMA COUNTY, CALIFORNIA: IMPLICATIONS FOR BACTERIAL CONTAMINATION TO THE RUSSIAN RIVER DURING SUMMER FLOWS

Water-quality samples were collected from 7 sites along the lower Russian River and 11 tributaries between 2003 and 2005 to investigate whether tributary discharge is a potential source of bacterial contamination to the Russian River during summer flows. Standard bacterial indicators (total coliforms, E. coli, and enterococci) were detected in all Russian River samples, although no E. coli concentrations exceeded the 235 cfu/100 mL single-sample maximum allowable density for recreational waters. In contrast, the bacterial concentrations in samples from several tributary sites exceeded the E. coli single-sample maximum allowable density for recreational waters. A narrow range of stable isotope values in samples (~1 per mil δ¹⁸O) from the Russian River indicates that tributary discharge is not a significant source of inflow within this section of the river during the study period; however, these tributary discharges might be a potential source of bacteria if flows in the Russian River are reduced. Tributaries that maintained continuous flow generally were characterized by lower specific conductance, higher dissolved oxygen, and higher bacteria concentrations than tributaries that had intermittent flow. These results suggest stream-hyporheic exchange of tributary discharge might reduce the potential impact of bacterial contamination to the Russian River. To better understand how tributary discharge influences the water quality of the Russian River, samples will be collected from the lower segment of the Russian River and 7 tributaries during 2006: in early and late summer to represent the range in conditions during summer flow. All samples will be analyzed for the same constituents as previous years. In addition, the detection by PCR of a putative virulence factor, the enterococcal surface protein in Enterococcus faecium, will be used as an indicator of human fecal contamination. These analyses will indicate whether tributary discharges are a significant factor influencing bacterial concentrations in the Russian River during the late spring and early summer months.