1. Critical Conditions Review
Nancy Vignali │ Water Quality Assessment Team

2. Where We Fit in the Permitting Process
Applications
Standards
Critical Conditions
DO Modeling
Biomonitoring
Permit Writers

3. Mixing Zone Regulations
EPA allows states to adopt their own mixing zone regulations as part of the states water quality standards (40 CFR §131.13).
TCEQ’s mixing zone regulation has been approved by EPA and can be found outlined in the Texas Surface Water Quality Standards (TSWQS)
standards/eq_swqs.html

4. Which Applications Receive Critical Condition Reviews?
All Industrial wastewater permits
All Municipal wastewater permits with biomonitoring requirements
Municipal wastewater permits with design flows of ≥ 1.0 MGD

5. Critical Conditions Workflow
Outfall Mapping
TEXTOX Menu Determination
Mixing Zone Determination
7Q2_HM Or
Effluent % Calculation

6. TEXTOX Menu Determination
Standards Assessment Worksheet
Details the characteristics of all receiving water bodies within 3 miles of the discharge point using TSWQS
Determine receiving waters uses and associated criteria; aquatic life use, toxicity (acute/chronic) and human health criteria – incidental, sustainable, public water supply
Capture any change in human health criteria for downstream receiving waters within three miles of the point of discharge

7. Immediate Receiving Water Water Body Within 3 Miles
Menu #
Immediate Receiving Water
Water Body Within 3 Miles 1
Intermittent water body
──── 2
Perennial ditch, stream, or river 3 4
Lake or lake-like water body 5
Wide tidal water body or narrow tidal with no upstream flow data 6
Narrow tidal water body with upstream flow data 7
Intermittent stream w/perennial pools 8 9
Narrow tidal water body w/ upstream flow 10

8. What is a Regulatory Mixing Zone?
A mixing zone is an area where wastewater discharged from a permitted facility enters and mixes with a stream or water body
A mixing zone is an established area where water quality standards may be exceeded as long as acute toxic conditions are prevented and all beneficial uses, such as drinking water, fish habitat, recreation human health and other uses are protected

9. Types of Mixing Zones
Zone of Initial Dilution (ZID) – Acute aquatic life criteria applies at the edge of the ZID
Mixing Zone (MZ) – Chronic aquatic life protection applies at the edge of the MZ
Human Health Mixing Zone (HH) – Chronic human health protection applies at the edge of the HH

10. Aquatic Life Mixing Zone (MZ)
Human Health
Mixing Zone
(HH)
400 ft.
Aquatic Life Mixing Zone (MZ)
ZID
200 ft.
50 ft.
Point of Discharge

11. Mixing Zone & Human Health MZ ZID Point of Discharge Direction of Flow
20 ft.
60 ft.
ZID
100 ft.
300 ft.
Point of Discharge

12. Critical Conditions – Final Result
In flowing waters, the river or stream’s:
Low-flow metric (7Q2) is the critical condition for receiving-water flow which is based on a consecutive Seven-day, Two-year Low Flow recurrence interval.
Harmonic Mean (HM) is calculated to establish human health limits
In pooled water bodies, freshwater and saltwater:
Percentages of effluent are calculated for water bodies at the edge of the MZ, ZID, and HH in the receiving water body by using the jet plume equation. These percentages are often referred to as the “critical dilution”.

13. Critical Conditions Contact Information
Jeff Borski
x5599
cubicle 200
Nancy Vignali
x1303
cubicle 197
June 2010 Procedures to Implement the Texas Surface Water Quality Standards
2014 Texas Surface Water Quality Standards

14. Questions?