Waste Load Allocation Study. The WLA is the process that the Covered Parties use to calculate the Trinity River’s assimilative capacity for constituents (i.e., the river’s ability to absorb nutrients or biochemical oxygen demand loading without exceeding water quality standards), set discharge limits, and allocate that load out among the Compact members. These loadings are then codified in TPDES permit limits issued to the Covered Parties by TCEQ with overview by the EPA. The WLA is updated19, if needed, when the Compact’s WWTPs discharge permits are modified to account for population growth and increases in wastewater discharges. The most recent WLA was completed in 2021 (Xxxxxxx Associates, Inc. 2021) using the EPA-approved modeling software QUAL2K and excerpts are included below: Two models were set up: one for 2013 and one for 2017 and were set up to represent warm, effluent-dominated conditions in the Upper Trinity River. The 19 The original version of the WLA was completed in the 1970s and updated in 1986, 1998, 2011 and 2019-2021. There are no current plans to update the model. goal is to derive a consistent set of algal growth and nutrient reaction rates that are representative of effluent-dominated conditions. The sequence by which water quality constituents in the model were calibrated is as follows, 1) flow, 2) temperature, 3) total suspended solids, and 4) nutrients (chlorophyll-a, total phosphorous, and total nitrogen). Once an acceptable set of model parameters was identified, that set of parameters was incorporated into a final nutrient model. The final nutrient model was used to simulate instream conditions for eight TP limit scenarios. The model predicts that, within each discharge flow condition, none of the four TP effluent concentrations assessed produces a significant difference in the algal populations in the Upper Trinity River. The high turbidity of the river blocks a significant portion of the light needed for algae to grow. However, with less algal growth, less nutrients are assimilated; and the majority of the TP load discharged by Metroplex WWTPs is carried downstream and into the Middle Trinity River. The model also predicts that if the major WWTPs are discharging at the full flow volumes authorized by their permits there are lower algal concentrations in the Upper Trinity River than when the major WWTPs are discharging at their 2017 flows. This is due to the lower retention time in the river produced by the higher permitted flows. With a lower retention...
