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Peggy W. Glass, Ph.D P. Jonathan Young, Ph.D., P.E., Alan H. Plummer, Jr., P.E., BCEE of

SEEKING A VARIANCE TO CHANGE SURFACE WATER QUALITY STANDARDS FOR DISSOLVED OXYGEN IN THE BACKWATERS AND COVES OF LAKE GRAPEVINE. Peggy W. Glass, Ph.D P. Jonathan Young, Ph.D., P.E., Alan H. Plummer, Jr., P.E., BCEE of Alan Plummer Associates, Inc. TACWA November 14, 2007.

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Peggy W. Glass, Ph.D P. Jonathan Young, Ph.D., P.E., Alan H. Plummer, Jr., P.E., BCEE of

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  1. SEEKING A VARIANCE TO CHANGE SURFACE WATER QUALITY STANDARDS FOR DISSOLVED OXYGEN IN THE BACKWATERS AND COVES OF LAKE GRAPEVINE Peggy W. Glass, Ph.D P. Jonathan Young, Ph.D., P.E., Alan H. Plummer, Jr., P.E., BCEE of Alan Plummer Associates, Inc. TACWA November 14, 2007

  2. Presentation Outline • Introduction to TRA Denton Creek Regional Wastewater System (DCRWS) • Issues regarding WWTP Discharges to Lake Coves and Backwaters • TRA Approach to this Problem • Suggestions for Alternative TCEQ Approaches

  3. Denton Creek Regional Wastewater System

  4. 26 North Lake Argyle Justin Flower Mound DCRWS Plant Texas Motor Speedway 170 Marshall Creek Fort Worth Roanoke Grapevine Lake Alliance Airport Trophy Club 114 114 377 Denton Co. Tarrant Co. Westlake DCRWS Pressure System Service Area Haslet Keller 35W Southlake DFW International Airport Big Bear Creek Denton Creek Regional Wastewater System Collection System and Service Area

  5. DCRWS Treatment PlantEffluent Limits

  6. DCRWS Projected Flows and Treatment Capacity

  7. Overview of Issues • Historically • TCEQ evaluated receiving streams and open waters of lakes • ignored narrow coves and backwaters • Mid to Late 90s • TCEQ decided they needed to include backwaters and narrow coves in their water quality models used for wastewater permits • Began evaluating backwaters using QUAL-TX • Coordinated approach with EPA

  8. Discharge Going into Lake Backwater Open Lake Waters Backwater Free Flowing Stream Width Wastewater Treatment Plant Lake Surface Depth

  9. Discharge Going into Narrow Cove Open Lake Waters Narrow Cove Free Flowing Stream Width Wastewater Treatment Plant Lake Surface Depth

  10. Causes of Permitting Problems • Cove and Backwater Dynamics Provide Little Assimilative Capacity • Debris from stormwater runoff often ends up in backwaters • Free flowing stream suddenly slows down dramatically in backwater areas • Oxygen uptake increases and available oxygen decreases

  11. Causes of Permitting Problems (continued) • The characteristics of coves are different than the open waters of lakes • The 5 mg/L dissolved oxygen (DO) criterion assigned to open water is being applied to coves • But coves can have naturally occurring, relatively low concentrations of DO

  12. DCRWS Dilemma • Expansion to 11.5 MGD needed immediately. Annual average flow in September 2007 was 4.3 MGD. • Expansion to 16.5 MGD needed by 2012. • The model indicates the existing permitted flow of 5 MGD cannot be discharged at the existing outfall location and maintain DO water quality standards of 5.0 mg/L even with the following limits: CBOD5 = 5 mg/L NH3-N = 1 mg/L DO = 6 mg/L

  13. Solution • Secure Permit Variance Based on Loads • Secure DO Standard Change • Develop Second Outfall Location

  14. Permit Variance Based on Load

  15. Site Specific DO Standard Change Limitations • 5-to 10 Years Before it Takes Effects • A site-specific DO standard of 4 mg/L would only allow a discharge of about 10 MGD.

  16. Alternate Discharge at Whites Branch DCRWS Plant

  17. Other Examples of WWTP Permit Renewals Denied • City of Corsicana to Richland-Chambers Reservoir • City of Grapevine to Grapevine Reservoir • City of Jacksboro to Lake Johnson

  18. Discussion of Alternative Approaches • In an urbanized area like the D/FW metroplex, it isvery difficult to avoid discharging into tributaries to reservoir coves and backwaters • The current regulatory procedures are: - causing cancellation of previously approved permits - costing stakeholders millions of dollars - causing what many think are unnecessary changes to treatment processes and discharge locations

  19. Discussion of Alternative Approaches (continued) • Need a fundamental change in the regulatory procedures to recognize the unique characteristics of backwaters and narrow coves • The ongoing study by TCEQ / Baylor / Parsons Engineering is intended to define this transition area between stream and lake • One approach would be to apply the lake dissolved oxygen standards only to open lake waters

  20. Discussion of Alternative Approaches (continued) • Lower dissolved oxygen concentrations could be allowed in backwaters to recognize their unique ability to assimilate wastewaters • Previously approved permits could be renewed, as is, if no water quality problems have occurred

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