Monday, September 9, 2024
11:30 AM - 12:00 PM
Location Name
Using CE-QUAL-W2 to Evaluate the Murfreesboro Water Resources Recovery Facility Expansion

The Murfreesboro Water Resources Recovery Facility (MWRRF) is currently rated to treat 24 MGD of primary wastewater; however, MWRRF would need to expand its treatment capacity to accommodate the rapid growth that is occurring within its service area. The MWRRF is currently permitted to discharge into the West Fork Stones River (WFSR) 11 miles upstream of J. Percy Priest reservoir. The reach that receives the discharge is currently designated as not fulfilling its designated uses because of exceedances of the DO criteria. Therefore, it is incumbent on MWRRF to demonstrate through modeling that expanded discharge will not result in a measurable decrease of the DO concentration in the WFSR. Hazen and Sawyer, on behalf of the Murfreesboro Water Resources Department (MWRD), developed a CE-QUAL-W2 model of the WFSR to evaluate the impact of expanded discharge into the river. CE-QUAL-W2 is a two-dimensional, laterally averaged, hydrodynamic and water quality model, and it was selected over other water quality models because of its ability to model the hydrodynamics and water quality in long and narrow river systems. CE-QUAL-W2 was originally developed by the US Army Corps of Engineers and is actively maintained by Dr. Scott Wells of Portland State University. The model has been used widely for riverine modeling and is an accepted quality model by the US Army Corps of Engineers, US Geological Survey, , Tennessee Valley Authority, and more. Continuous monitoring, physical and biological survey data, and water quality sampling data were collected throughout the summer of 2023 to develop a calibration dataset to support the development of the model. Temperature, DO, and water depth were continuously measured at six points along the river, flow measurements and cross sections were collected to characterize the river channel, surveys of macrophyte and periphyton abundance were conducted, and water quality grab samples measuring nutrients, chlorophyll, and CBOD were collected under low flow conditions. Collectively, these data supported the development of a well-calibrated model that simulates the DO dynamics of a 5 mile stretch of the WFSR downstream of MWRRF. The calibrated model predicted temperature with less than 1 degree C RMSE and predicted daily minimum and maximum DO concentrations with less than 0.3 mg/L RMSE. Results of this effort demonstrated that DO within this stretch of the WFSR is dominated by benthic photosynthesis and respiration. Measured periphyton biomass was as high as 25 grams per square meter, and daily swings of DO concentration were as high as 6.5 mg/L during low flow conditions (<13 cfs). Alternately, CBOD5 concentrations were less than 2 mg/L and had 5-day decay rate constants of k<0.12 per day. A modeling scenario at critical low flow and temperature conditions with the MWRRF discharge expanded from 16 MGD to 32 MGD demonstrated that the increased flow rate from the MWRRF resulted in smaller diurnal swings in DO, and that the minimum DO concentrations were elevated throughout the entire model. The results from this model will be used to support a permit application for expansion of the MWRRF.