Sources of Indicator Bacteria in the Houston Metropolitan Region, Houston, Texas

Bayous in the Houston Metro area, a relatively large urban watershed covering almost 600 square miles, exhibit elevated concentrations of Escherichia coli that exceed the allowable standards. On average, Houston Metro bayous exhibit exceedances of the single sample standard 78% and 75% of the time for E. coli and fecal coliform respectively (using data from the period 1981 to 2006 for approximately 74 water quality gages). Intensive sampling was conducted in the metro area in summer of 2006 to understand the effect of rainfall-runoff on E. coli concentrations. E. coli concentrations were significantly correlated to rainfall intensity on the antecedent day (R2:0.84) as well as flow in the bayou at select stations. From the intensive surveys, it was evident that E. coli concentrations increased to beyond their background levels during and after a rainfall event, although it was unclear if the increase was due to urban runoff or sediment resuspension or other wet weather driven processes or sources. Exceedances during low flow conditions indicated that point sources contributed significant loads of indicator bacteria, supporting the need to examine wastewater treatment plants, dry weather discharges of storm sewers and septic tank inputs.A modeling study using Hydrologic Simulation Program – FORTRAN (HSPF) was undertaken for Whiteoak bayou to assess the various sources and their contributions to water quality in this Houston metro watershed. The model was calibrated/validated for hydrology and water quality for the period of 2001 – 2003 at a total of six locations in the watershed. The hydrology calibration results indicated a relatively good agreement between observed and modeled data. The longitudinal plots revealed that the modeled and observed E. coli geomeans are not statistically different at a 95% confidence interval. The model results indicate that contributions from WWTPs and storm sewer discharges are important under dry weather conditions while nonpoint source runoff, septic systems, and sediment sources represent the key wet weather sources of E.coli to the Whiteoak bayou watershed.Current research is underway to investigate the underlying causes of water quality impairments in the other Houston Metro watershed and possibly correlate the observed E. coli concentrations to prevailing hydrologic conditions.