SEEKING A VARIANCE TO CHANGE SURFACE WATER QUALITY STANDARDS FOR DISSOLVED OXYGEN IN THE BACKWATERS
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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

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


Presentation outline

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


Denton creek regional wastewater system

Denton Creek Regional Wastewater System


Peggy w glass ph d p jonathan young ph d p e alan h plummer jr p e bcee of

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


Dcrws treatment plant effluent limits

DCRWS Treatment PlantEffluent Limits


Dcrws projected flows and treatment capacity

DCRWS Projected Flows and Treatment Capacity


Overview of issues

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


Discharge going into lake backwater

Discharge Going into Lake Backwater

Open Lake Waters

Backwater

Free Flowing Stream

Width

Wastewater Treatment Plant

Lake Surface

Depth


Discharge going into narrow cove

Discharge Going into Narrow Cove

Open Lake Waters

Narrow Cove

Free Flowing Stream

Width

Wastewater Treatment Plant

Lake Surface

Depth


Causes of permitting problems

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


Causes of permitting problems continued

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


Dcrws dilemma

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


Solution

Solution

  • Secure Permit Variance Based on Loads

  • Secure DO Standard Change

  • Develop Second Outfall Location


Permit variance based on load

Permit Variance Based on Load


Site specific do standard change limitations

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.


Peggy w glass ph d p jonathan young ph d p e alan h plummer jr p e bcee of

Alternate Discharge at Whites Branch

DCRWS Plant


Other examples of wwtp permit renewals denied

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


Peggy w glass ph d p jonathan young ph d p e alan h plummer jr p e bcee of

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


Discussion of alternative approaches continued

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


Discussion of alternative approaches continued1

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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