The Trinity River, Texas: A Case Study of the Limits of Waste-Load Evaluation

The Trinity River flows through the Dallas-Fort Worth metroplex and the agricultural downstream. The river is dominated by surface run-off. The semiarid North Texas climate changes from seasons of intense convective precipitation to virtual drought, with the result that river flows range over four orders of magnitude. The river in the metroplex has been channelized and leveed for flood protection and receives copious volumes of urban run-off. It also receives wastewater discharges, and at low flow over 99% of the river flow is treated effluent. This paper reviews water quality problems in the Trinity and the need for employing mathematical models in relating response of the river quality to external controls. Results of numerical modeling of the river and verification against field data are presented, and the use of the model in a succession of waste load evaluations (WLEs) is described. These WLEs have dictated advances in waste treatment to the point that the low-flow regime exhibits good, even excellent, water quality. Yet the Trinity continues to be plagued with critical situations and extensive fish kills. Implications on the relative merits of point-source controls when other factors are dominant and the difficulties of setting valid water-quality standards are discussed.