De Facto Reuse in the Trinity River Basin
The Trinity River in Texas is an example of an effluent-dominated surface water system where de facto potable water reuse occurs. The section of the river south of Dallas/Forth Worth consists almost entirely of wastewater effluent under base flow conditions (Fono et al., 2006; TRA, 2010). In response to concerns about nutrients, the wastewater treatment plants in Dallas/Fort Worth that collectively discharge about 500 million gallons per day (MGD; 2 million m3/d) of effluent employ nutrient removal processes (Fono et al., 2006). Little dilution of the effluent-dominated waters occurs as the water travels from Dallas/Fort Worth to Lake Livingston, which is one of the main drinking water reservoirs for Houston (see figure below). Once the water reaches Lake Livingston, it is subjected to conventional drinking water treatment prior to delivery to consumers in Houston.
Results from hydrological models and contaminant monitoring indicate that contaminant attenuation takes place in the river and reservoir. During the estimated 2-week travel time between Dallas/Fort Worth and Lake Livingston, many of the trace organic contaminants undergo transformation by microbial and photochemical processes (Fono et al., 2006). Additional contaminant attenuation and pathogen inactivation also may occur during the water’s residence time in the reservoir. On an annual basis, about half of the water flowing into Lake Livingston is derived from precipitation. Therefore, water entering the drinking water treatment plant consists of approximately 50 percent wastewater effluent that has spent approximately 2 weeks in the Trinity River and up to a year in the reservoir before it becomes a potable water supply. The potable water from the Trinity River meets all of the Environmental Protection Agency’s water quality regulations and this de facto potable reuse system is an important element in the region’s water resource planning.
Trinity River Basin, showing Dallas/Fort Worth in the headwaters of the water supply for the city of Houston.
Improved integration of hydrological data and better watershed models make it possible to estimate the fraction of wastewater effluent in surface waters under a range of conditions. For example, Andrew Johnson and Richard Williams (Centre for Ecology and Hydrology, personal communication, 2009) used readily available data on river flows and volumes of wastewater effluent discharged by individual treatment plants to develop a hydrological model that predicts the fraction of wastewater effluent in different surface waters in and around Cambridge, UK, under base-flow conditions (Figure 2-2). Such hydrological data are available in
significance of de facto reuse. The existing regulatory structure for drinking water addresses this issue through requirements for periodic monitoring. For chemicals where the risk is based on lifetime exposure, average concentrations of contaminants are used. For pathogens and chemicals where risks are based on shorter exposures, low-flow measures might be appropriate, although it is beyond the committee’s charge to evaluate.