Optimization of Wastewater Management At a Watershed Level.

Dickinson Bayou, a coastal stream on the southern fringes of Houston sprawl, does not meet water quality standards for bacteria. On-site sewage facilities (OSSFs) contribute substantially to the total bacterial load of the stream. The watershed is dominated by soils with severe limitations for on-site systems: seasonal saturation and greater than 35% clay in the upper four feet. Standard gravity drainfield OSSFs were the default on-site systems until 1997, when a soil evaluation system was mandated in Texas. Thereafter, aerobic spray systems gradually became the default system. A Total Maximum Daily Load (TMDL) process is nearing completion for Dickinson Bayou. A TMDL Implementation Plan will likely point to some combination of OSSF upgrades, OSSF maintenance districts, and/or connection to centralized wastewater treatment plants (WWTPs) as potential ways to decrease OSSF bacterial loading to the Bayou.

None of these solutions come cheaply. We devised an OSSF Optimization Geographic Information System (GIS) to assist local managers to determine the best combination of upgrades etc based on cost and other mitigation scenarios. The entire population of OSSFs, permitted and non-permitted, was obtained for the watershed. The OSSFs were mapped by date and type, and overlain on a soil layer showing saturation and %clay, along with proximity to the bayou and its tributary and drainage system. A variety of scenarios based on potential OSSF risk (e.g, standard systems on limiting soils, aerobic treatment units without maintenance contracts, etc) and costs of potential solutions were explored. With this GIS and an estimation of the “willingness to pay” on the part of target populations and their political representatives, managers can present a series of feasible policy options.