1. Critical Conditions Review
Jeff Borski │ Water Quality Assessment Team

2. Where We Fit in the Permitting Process
Applications
Standards
Critical Conditions
DO Modeling
Biomonitoring
Permit Writers
Critical conditions reviews permits after the Applications and Standards teams, and before the D O modelers and biomonitoring staff conduct their reviews.

3. What Do We Do? Critical Conditions Reviews
Determine mixing zones
Calculate critical conditions (7Q2, effluent percentages)
Outfall Mapping (GIS and Paper Maps)
Coastal Zone Determinations
3-mile Requests
Pretreatment Reports
Critical Conditions Reviews
Determine mixing zones
Calculate critical conditions (7Q2, effluent percentages)
Outfall Mapping (GIS and Paper Maps)
Coastal Zone Determinations
3-mile Requests
Pretreatment Reports

4. Which Applications Do We Review?
All industrial wastewater permits
All municipal permits with biomonitoring
Municipal permits with design flows of ≥1.0 MGD
All industrial wastewater permits
All municipal permits with biomonitoring
Municipal permits with design flows of ≥1.0 MGD

5. What are Critical Mixing Conditions?
In flowing waters, the river or stream’s low-flow metric (7Q2) is the critical condition for receiving-water flow
In non-flowing waters, the width of the water body is the critical condition for receiving-water dilution
In flowing waters, the river or stream’s low-flow metric (7Q2) is the critical condition for receiving-water flow
In non-flowing waters, the width of the water body is the critical condition for receiving-water dilution

6. EPA Guidance on Critical Conditions
“States may, at their discretion, include in their State standards, policies generally affecting their application and implementation, such as mixing zones, low flows and variances. Such policies are subject to EPA review and approval.” (40 CFR §131.13)
Our guidance is outlined in the TSWQS
“States may, at their discretion, include in their State standards, policies generally affecting their application and implementation, such as mixing zones, low flows and variances. Such policies are subject to EPA review and approval.” (40 CFR §131.13)
Our guidance is outlined in the TSWQS

7. TEXTOX Menu Determination
Assess menu numbers for each type of water body within 3 miles of the discharge point
Assess menu numbers for downstream receiving waters with higher human health criteria than the initial receiving waters
— Human health screenings as needed for downstream water bodies
Assess menu numbers for each type of water body within 3 miles of the discharge point
Assess menu numbers for downstream receiving waters with higher human health criteria than the initial receiving waters
— Human health screenings as needed for downstream water bodies

8. 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
This is a table describing the criteria governing the assignment of TexTox menu numbers for water toxics screenings. Menu numbers range from 1 to 10, and are assigned based on the receiving water characteristics within 3 miles of the point of discharge.

9. What is a Regulatory Mixing Zone?
“The area contiguous to a permitted discharge where mixing with receiving waters takes place and where specified criteria, as listed in §307.8(b)(1) of this title, can be exceeded.” – TSWQS
“The area contiguous to a permitted discharge where mixing with receiving waters takes place and where specified criteria, as listed in §307.8(b)(1) of this title, can be exceeded.”
– TSWQS

10. 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 – Chronic human health protection at the edge of the HHMZ
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 – Chronic human health protection at the edge of the HHMZ

11. Aquatic Life Mixing Zone (MZ)
Human Health
Mixing Zone
(HH)
400 ft.
Aquatic Life Mixing Zone (MZ)
This is a photo overlay showing the basic layout of the three types of mixing zones typically assigned in TPDES permits for non-flowing water bodies.
ZID
200 ft.
50 ft.
Point of Discharge

12. Mixing Zone & Human Health MZ ZID Point of Discharge Direction of Flow
20 ft.
60 ft.
ZID
This is a photo overlay showing the basic layout of the three types of mixing zones typically assigned in TPDES permits for flowing water bodies.
100 ft.
300 ft.
Point of Discharge

13. Zone of Initial Dilution
The small area immediately adjacent to the point of discharge (PoD)
Numerical acute aquatic life criteria do not apply inside it, but narrative criteria do apply
No lethality to aquatic organisms
Discharge may not impede migration of aquatic organisms
The small area immediately adjacent to the point of discharge (PoD)
Numerical acute aquatic life criteria do not apply inside it, but narrative criteria do apply
No lethality to aquatic organisms
Discharge may not impede migration of aquatic organisms

14. Mixing Zone
MZs may not preclude passage of aquatic life to the extent that aquatic life use is significantly affected
Acute aquatic life criteria apply within the MZ
Chronic aquatic life criteria apply at the edge of the MZ
MZs may not preclude passage of aquatic life to the extent that aquatic life use is significantly affected
Acute aquatic life criteria apply within the MZ
Chronic aquatic life criteria apply at the edge of the MZ

15. Human Health Mixing Zone
Lakes – up to a 200-foot radius from the point of discharge
Estuaries – up to a 400-foot radius
Rivers – 100 feet upstream and 300 feet downstream
Chronic aquatic life criteria apply within the HHMZ
Human health criteria apply at the edge of the HHMZ
Lakes – up to a 200-foot radius from the point of discharge
Estuaries – up to a 400-foot radius
Rivers – 100 feet upstream and 300 feet downstream
Chronic aquatic life criteria apply within the HHMZ
Human health criteria apply at the edge of the HHMZ

16. Undefined Mixing Zones
Receiving waters that do not have defined mixing zones
Intermittent streams
Intermittent streams with perennial pools (ZID and MZ is not defined, but the human health mixing zone is defined)
Wetlands, marshes, and tidal mudflats
Receiving waters that do not have defined mixing zones
Intermittent streams
Intermittent streams with perennial pools (ZID and MZ is not defined, but the human health mixing zone is defined)
Wetlands, marshes, and tidal mudflats

17. Diffuser Reviews
For outfalls with diffusers, the DO modelers determine mixing-zone dimensions and effluent percentages
It’s possible to get effluent percentages that are lower than default levels because of increased mixing efficiency
Mixing zone shape may also differ from gravity-driven outfalls
Round, square, or irregular shape based on diffuser characteristics
For outfalls with diffusers, the DO modelers determine mixing-zone dimensions and effluent percentages
It’s possible to get effluent percentages that are lower than default levels because of increased mixing efficiency
Mixing zone shape may also differ from gravity-driven outfalls
Round, square, or irregular shape based on diffuser characteristics

18. What is the 7Q2?
The lowest average discharge (Q) over a period of 7 days with a recurrence interval of 2 years
Used to calculate effluent limits for aquatic life protection
Another way to think about the 7Q2
There is a 50% chance in any given year that the in-stream flow will be less than the 7Q2
For comparison, there is a 10% chance in any given year than the in-stream flow will be less than the 7Q10
The lowest average discharge (Q) over a period of 7 days with a recurrence interval of 2 years
Used to calculate effluent limits for aquatic life protection
Another way to think about the 7Q2
There is a 50% chance in any given year that the in-stream flow will be less than the 7Q2
For comparison, there is a 10% chance in any given year than the in-stream flow will be less than the 7Q10

19. How Do We Calculate the 7Q2?
Rank score of the moving 7-day average Q for each year
Calculator for Low Flows (CaLF)
Excel-based app created by UT researchers
Calculates values using 30 years of continuous USGS gaging data (ideally)
In areas with limited flow data, the 10th percentile of available flow data will be analyzed
Default value for perennial streams: 0.10 cfs (TSWQS)
Rank score of the moving 7-day average Q for each year
Calculator for Low Flows (CaLF)
Excel-based app created by UT researchers
Calculates values using 30 years of continuous USGS gaging data (ideally)
In areas with limited flow data, the 10th percentile of available flow data will be analyzed
Default value for perennial streams: 0.10 cfs (TSWQS)

20. How Do We Calculate the 7Q2?
Special guidelines for springflow-dominated segments (30 TAC §307.8(a)(2))
For such segments with listed species, take the 0.1th percentile of the lognormal distribution of flow data
For such segments without listed species, take the 5th percentile of the flow data
Listed in Appendix D of TSWQS
Special guidelines for springflow- dominated segments (30 TAC §307.8(a)(2))
For such segments with listed species, take the 0.1th percentile of the lognormal distribution of flow data
For such segments without listed species, take the 5th percentile of the flow data
Listed in Appendix D of TSWQS

21. What is the Harmonic Mean?
The reciprocal of the average of the reciprocals
Yields a truer average for rates, e.g. streamflows
Typically much lower than the arithmetic mean because it is less influenced by large outliers
Used to establish human health limits in flowing water bodies
Default value – 0.20 cfs
The reciprocal of the average of the reciprocals
Yields a truer average for rates, e.g. streamflows
Typically much lower than the arithmetic mean because it is less influenced by large outliers
Used to establish human health limits in flowing water bodies
Default value – 0.20 cfs

22. The Challenge of Critical Conditions
There are 3,700 named streams in Texas
Countless other unnamed tributaries & ditches
191,000 named stream miles
…Yet there are only about 800 USGS usuable gages in Texas
There are 3,700 named streams in Texas
Countless other unnamed tributaries & ditches
191,000 named stream miles
…Yet there are only about 800 USGS usuable gages in Texas

23. Determining the 7Q2 and Harmonic Mean
USGS gage upstream of the point of discharge
USGS gage downstream of the point of discharge
Upstream dischargers
SWQM station (10th percentile)
Drainage area ratio
Receiving Water Assessment (RWA)
Waste Load Evaluation (WLE)
Other sources of info (e.g. applicant model)
USGS gage upstream of the point of discharge
USGS gage downstream of the point of discharge
Upstream dischargers
SWQM station (10th percentile)
Drainage area ratio
Receiving Water Assessment (RWA)
Waste Load Evaluation (WLE)
Other sources of info (e.g. applicant model)

24. Intervening DischargersÅ
USGS Gage M M
Intervening Dischargers
Direction of flow M
A map showing the critical conditions methodology for calculating the 7Q2 and harmonic mean where there are intervening dischargers.
Point of Discharge

25. Determining the 7Q2 and Harmonic Mean
Calculate critical conditions for USGS gage
Calculate critical conditions for each intervening discharger using the 2 years of self-reporting data
7Q2 – minimum 2-year flow
HM – Harmonic mean of 2-year discharge
Gage + Intervening Dischargers = Critical Conditions for PoD
Calculate critical conditions for USGS gage
Calculate critical conditions for each intervening discharger using the 2 years of self-reporting data
7Q2 – minimum 2-year flow
HM – Harmonic mean of 2-year discharge
Gage + Intervening Dischargers = Critical Conditions for PoD

26. Critical Conditions for non-flowing water bodies
Effluent percentages are based on receiving water width
Jet Plume Equation
Effluent % = 2.8𝐷 𝜋 𝑅 ×100
All discharges ≥100 MGD are assessed 100% for all MZs
Effluent percentages are based on receiving water width
Jet Plume Equation
Effluent % = 2.8𝐷 𝜋 𝑅 ×100
All discharges ≥100 MGD are assessed 100% for all MZs

27. 1010 ft.
A photo overlay of a tidal water body showing the critical conditions methodology for measuring width and assigning mixing zone dimensions and dilution percentages.
Point of Discharge

28. Point of Discharge Human Health Mixing Zone 4% Mixing Zone 8% ZID 30%
400 ft.
Mixing Zone 8%
A photo overlay of a tidal water body showing the mixing zone dimensions and dilution percentages for a water body with a width of more than 400 feet.
ZID 30%
200 ft.
50 ft.
Point of Discharge

29. Critical Conditions Contact Info
Jeff Borski
x5599
Nancy Vignali
x1303
Jeff Borski
x5599
Nancy Vignali
x1303

30. Questions?
Questions?