Spatial and temporal modeling of Escherichia coli sources and load allocations in an urban watershed

Bacteria impairments in Buffalo Bayou have numerous sources emanating from the watershed. These sources include wastewater treatment plants, sanitary sewer overflows (SSOs), septic systems, wet and dry nonpoint source discharges through municipal separate storm sewer system (MS4) pipes, direct deposition and sediment. This work focused on developing a two-tiered modeling approach for evaluation of Escherichia coli sources and loads in Buffalo Bayou, (i) the Bacteria Load Estimator Spreadsheet Tool (BLEST) and (ii) a dynamic HSPF water quality model. BLEST estimates bacteria loading on a subwatershed and segment basis for three flow conditions, low, median and wet weather flows. Results from BLEST in the upper watershed indicated that the largest source of E. coli loading in low flow was septic systems, while dry weather discharges from storm sewers and SSOs were the primary source of loading in the lower watershed. Under median flow and wet weather conditions, the largest loads were from wet weather nonpoint source loads draining through MS4s. The HSPF water quality model was set-up, calibrated and validated for January 1, 2001 through September 30, 2003. The model calibration was achieved using data collected from two pilot watersheds to supply in-situ decay rates and model verification data. The models errors between simulated and total volumes were generally less than 20% and errors for the overall E. coli geometric means were typically less than 50%. E. coli load reductions determined using BLEST ranged from 0% to 57% for low flows and 95% to 99% for wet weather. When the load reductions were implemented, however, HSPF modeling results indicated that the water quality standards were often not achieved. Wet weather nonpoint source loads discharges draining through MS4 pipes were shown to be responsible for the water quality exceedances. A major finding of this research is that non-point sources of bacteria exert influence long after a rainfall event and demonstrates the necessity to control wet weather sources of bacteria as they impact E. coli concentrations under all flow conditions in Buffalo Bayou.