1. An Introduction To Modeling of Surface Waters For TPDES PermitsMark A. Rudolph, P.E.TCEQ Water Quality Division

2. What Purpose Does Modeling Serve in the TPDES Permitting Process? • Develop permit limitations for the protection of surface water dissolved oxygen levels • Develop mixing estimates for discharges using diffusers

3. Why Model Dissolved Oxygen? • It’s a good general measure of water quality • It’s essential for the survival of aquatic life • The science is reasonably well understood • Oxygen demanding substances are commonly found in wastewaters • There’s a long regulatory history of dissolved oxygen modeling

4. Why Model Diffusers? • Most water quality standards apply at the edge of mixing zones, so effluent fraction at these distances needs to be estimated. • Processes controlling mixing are complex. • Enhanced mixing characteristics of diffusers can be predicted quantitatively with models. • Better site-specific predictions of necessary permit limits for Toxics can be derived.

5. Dissolved Oxygen Models: What are They Really? A collection of mathematical equations meant to describe interrelated chemical and physical processes controlling DO in natural waters. Or, more simply A dissolved oxygen accounting program that considers the major inputs and losses of DO.

7. Simple Model Equation(Streeter-Phelps) D is the saturation deficit, (D = DOsat- DO) k1is the deoxygenationrate k2is the reaeration rate Lais the initial oxygen demand of organic matter in the water, also called the ultimate BOD (BOD at time t=infinity). Dais the initial oxygen t is the elapsed time

8. Dissolved Oxygen Response to a Waste Load Input Waste Load Input *Permit limits for CBOD5/Ammonia-N/Dissolved Oxygen

9. Diffuser Modeling: What Factors are Considered? Factors affecting mixing: Diffuser geometry Single/multi-port Position/orientation in the water column Boundary interactions Shore/bottom hugging plumes Mass/Volume flux Flow Rate Pollutant concentration Mixing Processes Jet mixing Buoyant spreading Density Currents Diffusion Advection

10. Models: What are Their Strong Points? • Results are repeatable and consistent with scientific principles • Environmental conditions resulting from discharges can be evaluated quickly • Can be set up to run conservative (protective) scenarios without the need to collect a large amount of site-specific data

11. Models: What are Their Limitations? • Natural systems are complex. Even the most sophisticated models are simplifications. • Can be difficult to learn to use properly. Specialized training required. • Results are sensitive to assumptions and data quality (garbage in = garbage out). • No one model is appropriate for all situations. • Regulatory use requires the use of guidance documents to ensure consistency.

12. Models are Simplifications ModelReality

13. Models Routinely Used by the Water Quality Division • CORMIX • Diffusers • QUAL-TX • Streams, rivers, estuaries, other linear water bodies • Continuously Stirred Tank Reactor (CSTR) • Ponds, small lakes/reservoirs, coves of larger lakes • Water Quality Analysis Simulation Program (WASP) • Lakes, reservoirs, estuaries, can also be used for rivers

14. Role of Modeling Regulatory Guidance Documents • Ensures technical legitimacy • Consistency – level playing field • Helps with remembering all steps in a complex process • Public transparency

17. Example Modeling AnalysisProblem Statement TPDES Application Details: • 1.0 MGD domestic discharge for the City of Friendly, TX • Discharge proposed into Lake Slough thence into the San Antonio River • Applicant proposing limits consistent with secondary treatment; 20 mg/L BOD5, 20 mg/L TSS, 2 mg/L DO

19. Example Modeling AnalysisOther Key Information • Receiving water dissolved oxygen criteria; Lake Slough 3 mg/L, San Antonio River 5 mg/L • Regulatory base flow of 0.0 cfs for Lake Slough and 132 cfs for the San Antonio River • Other discharges to consider in the analysis; none • Site-specific hydraulic information available for these water bodies; none

20. Example Modeling AnalysisModel Choice and Rationale Since the water bodies receiving this discharge are streams and modeling procedures are well established for this scenario, use a QUAL-TX model. Due to the lack of site-specific hydraulic information, use statewide default hydraulic equations in the analysis.

21. Example Modeling AnalysisResults

22. Conclusions • Two types of modeling performed by the Water Quality Division: dissolved oxygen and mixing from diffusers. • Domestic limits from dissolved oxygen modeling include 5-day Carbonaceous Biochemical Oxygen Demand, Ammonia Nitrogen, and minimum dissolved oxygen.

23. More Conclusions • Mixing estimates from diffuser modeling are used to develop limits for toxics in certain industrial permits. • Models are valuable tools for developing protective permit limits but require proper use to provide useful results. • Contact information: Mark A. Rudolph, P.E. 512-239-4534 Mark.rudolph@tceq.texas.gov • Questions?