Modeling Onsite Wastewater Treatment Systems in a Coastal Texas Watershed

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Onsite wastewater treatment systems (OWTSs) are commonly used to treat domestic wastewater in the Dickinson Bayou watershed, located between Houston and Galveston. The Dickinson Bayou is classified as “impaired” by the Texas Commission on Environmental Quality due to high levels of indicator bacterium, Escherichia coli. Failing OWTSs in the watershed are possible sources for the impairment of the bayou. Nearly all of the watershed is at risk to failing OWTSs due to high water table and clay content in the soil. The HYDRUS modeling software for water and solute flow through variably saturated media was used to simulate the performance of (1) conventional OWTSs, (2) aerobic treatment units (ATUs) with spray distribution, and (3) mounded OWTSs under conditions indicative of the Dickinson Bayou watershed. The purpose of the study was to simulate system performance under existing conditions. Simulation results indicated that both the conventional and ATU systems fail due to effluent ponding and E. coli transport to the land surface due to high water tables and clay soils in the watershed. Simulations indicated that conventional and ATU systems failed when rainfall intensity was greater than 0.25 cm/h. However, the model simulations indicate mound systems did not fail under existing conditions as they did not allow E. coli to reach the surface or ponding to occur. Consequently, mound systems can be considered as better systems in this watershed to minimize bacterial loadings.