Mass Balance and Water Quality Modeling for Load Allocation of Escherichia coli in an Urban Watershed

A modeling approach based on mass balance coupled with the Hydrologic Simulation Program in FORTRAN (HSPF) is developed in this study to evaluate bacteria reduction strategies and their impact on water quality for an urban watershed in Texas. This approach allowed for spatial and temporal representation of bacteria loading from the adjacent watershed that was balanced with in-stream loading. This decreased the uncertainty in source loading estimates and reduced the number of variables requiring calibration. The resulting model is used to investigate whether reduction goals calculated by using the mass balance data alone would actually achieve water quality standards. Results indicate that 100% elimination of runoff loads would be required to achieve the applicable standards, which is a somewhat unrealistic and costly endeavor. The utility of the developed model is further demonstrated by investigating spatial reduction strategies targeting the various bacteria sources in the watershed. Results indicate that a 99% elimination of the upstream dry weather loading led to water quality compliance in the downstream subbasins. The modeling approach also indicates that the water body had a higher probability of achieving the single-sample standard than the geometric mean standard. DOI: 10.1061/(ASCE)WR.1943-5452.0000129. (C) 2011 American Society of Civil Engineers.