Variations of Indicator Bacteria in a Large Urban Watershed

Abstract

The contamination of water resources by nonpoint-source fecal pollution is a major concern to human health and water quality throughout the world. The Santa Ana River (SAR) in southern California is an impaired stream with historically high fecal coliform counts. This study evaluated the presence of indicator bacteria at 13 sites in the middle Santa Ana River watershed (MSAR). The objectives of this study were to: (1) examine spatial and temporal characteristics of fecal bacteria loading during dry weather (low or baseline) flow, wet weather (storm) flow, and recessional flow (72 h after storm) along two creeks, at two wastewater treatment plant (WWTP) outlets, and at a control site in the MSAR; and (2) determine how the various sampling locations affected indicator bacteria concentrations in the watershed. Total coliform (TC), fecal coliform (FC), E. coli, enterococci, and total bacterial concentrations were characterized at 13 locations in the watershed over a two-year period. Analysis of covariance (ANOCOVA) was used to test each specific set of bacteria counts, site effects, water flow conditions, and the four water quality covariate effects. Our results showed that the control site (S1) and WWTP estimates always showed significantly lower indicator bacteria than the channels influenced by urban runoff and agricultural activities. The water flow effects indicated that the recessional flow transported significantly lower bacterial counts into the watershed than either the dry weather flow or the storm or wet weather flow. In addition, bacterial count estimates changed far more significantly across different sites in comparison to estimates across seasons or time. These results imply that total TC, FC, E. coli, and enterococci bacterial counts in the MSAR watershed were strongly influenced by spatial location effects, with contamination due to local agricultural and/or urban runoff, in contrast to elevated upstream contamination and/or discharge contamination associated with the two WWTPs. Therefore, this study has provided data for evaluation of the Santa Ana River watershed's total maximum daily load (TMDL) management plans that could also be applicable to other large watersheds with different nonpoint-source pollutants.