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

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

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

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.

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.

Simple Model Equation (Streeter-Phelps) D is the saturation deficit, (D = DOsat - DO) k 1 is the deoxygenation rate k 2 is the reaeration rate L a is the initial oxygen demand of organic matter in the water, also called the ultimate BOD (BOD at time t=infinity). D a is the initial oxygen t is the elapsed time

Dissolved Oxygen Response to a Waste Load Input Waste Load Input *Permit limits for CBOD 5 /Ammonia-N/Dissolved Oxygen

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

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

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. Models: What are Their Limitations?

Model Reality Models are Simplifications

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

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

Example Modeling Analysis Problem 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 BOD 5, 20 mg/L TSS, 2 mg/L DO

Example Modeling Analysis Other 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

Example Modeling Analysis Model 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.

Example Modeling Analysis Results Treatment Level (mg/L) (BOD 5 /Ammonia-N/DO) DO Criteria (mg/L) (Lake Slough/San Antonio River) Minimum Predicted DO (mg/L) (Lake Slough/San Antonio River) 20/12/23 / / /12/63 / / /12/63 / / /3/43 / / 5.98

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.

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 Questions?