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Mixing Zones

March 8, 2023

WIFI INFO NETWORK: ID: PASSWORD:

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Agenda

  • Clean Water Framework
  • Mixing Zones
    • Regulatory language and examples
    • Mixing zone permitting process
  • Downstream Use Protection
  • Questions

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CLEAN WATER FRAMEWORK

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Clean Water Framework

Colorado Water Quality Control Act/Clean Water Act

    • Beneficial uses (present and future)
    • Water quality standards
    • Antidegradation

Water Quality Control Commission

    • Statewide, river basin and other regulations
    • CDPHE – Water Quality Control Division
    • Permittees and public

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  • Discussion purposes.

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MIXING ZONES

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Mixing Zones

Potential Problem

Incomplete mixing near point sources of discharge may lead to exceedance of acute or chronic standards

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Mixing Zones

Potential Problem

Incomplete mixing near point sources of discharge may lead to exceedance of acute or chronic standards

Regulatory Solution

Limit exceedance of standards that might occur

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Mixing Zones

Potential Problem

Incomplete mixing near point sources of discharge may lead to exceedance of acute or chronic standards

Regulatory Solution

Limit exceedance of standards that might occur

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Considerations

Quality and quantity of effluent and receiving water under critical biological flow conditions

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Mixing Zones

Reg. 31 Definition

“…means that area of a waterbody designated on a case-by-case basis by the Division which is contiguous to a point source and in which certain standards may not apply.”

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Mixing Zones

Reg. 31 Definition

“…means that area of a waterbody designated on a case-by-case basis by the Division which is contiguous to a point source and in which certain standards may not apply.”

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Regulatory Framework

  • 31.10 Mixing Zones
  • MOU with EPA, U.S. Fish & Wildlife, WQCD
  • 2002 Mixing Zone Implementation Guidance

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REGULATORY LANGUAGE AND EXAMPLES

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Physical Mixing Zone (PMZ)

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Flow

Point Source

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Physical Mixing Zone (PMZ)

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Flow

Point Source

At boundary of PMZ mixing of effluent and receiving water is complete

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Physical Mixing Zone (PMZ)

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Flow

Point Source

At boundary of PMZ mixing of effluent and receiving water is complete

  • Exceedance Zone
  • Regulatory Mixing Zone
  • Permit

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Exceedance Zone (EZ) – Larger

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Flow

Point Source

EZ > PMZ

Regulatory action likely needed

Physical Mixing Zone

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Exceedance Zone (EZ) – Smaller

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Flow

Point Source

EZ < PMZ

Regulatory action may be needed

Physical Mixing Zone

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Regulatory Mixing Zone (RMZ) – Smaller

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Flow

Point Source

RMZ < EZ

Regulatory action needed to avoid standard(s) exceedance

EZ

RMZ

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Regulatory Mixing Zone (RMZ) – Smaller

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Flow

Point Source

RMZ < EZ

Regulatory action needed to avoid standard(s) exceedance

EZ

RMZ

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Regulatory Mixing Zone (RMZ) – Larger

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Flow

Point Source

Exceedance

Zone

RMZ > EZ

Permit set at EZ

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Regulatory Mixing Zone (RMZ) – Larger

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Flow

Point Source

Exceedance

Zone

RMZ > EZ

Permit set at EZ

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Permit Limits & Mixing Zone(s)

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Flow

Point Source

The permit limits the EZ to be equal to or smaller than the RMZ

Exceedance

Zone

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Permit Limits & Mixing Zone(s)

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Flow

Point Source

The permit limits the EZ to be equal to or smaller than the RMZ

  • Acute and/or chronic RMZ
  • Likely different for each parameter

Exceedance

Zone

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MIXING ZONE PERMITTING PROCESS

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Additional Constraints

  • Not overlap and harm beneficial uses
  • Comply with narrative standards (except sessiles in MZ)
  • Zone of passage for aquatic life
  • Bioaccumulation of toxins in fish or wildlife
  • Special habitat (spawning/nursery, threaten/endangered)
  • Human exposure through drinking water or recreation

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Additional Constraints (cont.)

  • Possibility of attracting aquatic life to effluent plume
  • Potential for adverse effects on groundwater
  • Toxicity or persistence of substance discharged

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  • NOT applicable for whole effluent toxicity-based limits

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Stream vs Lake Comparison

Streams

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Stream vs Lake Comparison

Streams

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Lakes (lake/arm/bay)

  • Chronic MZ ≤3% area
  • All chronic MZs <10% area
  • Acute, 10–25% chronic MZ
  • Artificial lake supplied principally by potable water are different

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Stream Specific Procedures

Effluent >> Receiving

  • ≥2:1 flow ratio
  • Use fully mixed condition

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Effluent << Receiving

  • ≥1:20 flow ratio
  • Based only on receiving water flow

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Stream Specific Procedures

Effluent >> Receiving

  • ≥2:1 flow ratio
  • Use fully mixed condition

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Effluent << Receiving

  • ≥1:20 flow ratio
  • Based only on receiving water flow

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Exclusion Tables – (width, mean depth, ≤15% low flow)

  • Determine if PMZ < RMZ and if MZ permit limits needed
  • May eliminate the need for field data on mixing

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Mixing Zone Studies

Reg. 31 Definition

“…means that area of a waterbody designated on a case-by-case basis by the Division which is contiguous to a point source and in which certain standards may not apply.”

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Regulatory Framework

  • Field data
  • Site-specific
  • Acute/chronic flows
  • 2002 Mixing Zone Implementation Guidance for procedures

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Contact Information

Andrew Sayers-Fay

Permits Section Manager

andrew.sayers-fay@state.co.us

303.692.3419

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QUESTIONS?

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Protecting Downstream Users

May 16, 2019

WIFI INFO NETWORK: ID: PASSWORD:

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Topics

  • Legal basis
  • Regulation 31.3
  • When to apply
  • Questions

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Legal Basis to Protect Downstream Uses

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Regulation 61.8(1)(b): “The Division shall not issue a permit...

(iii) When the imposition of conditions cannot ensure compliance with the applicable water quality requirements of all affected States.”

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Legal Basis to Protect Downstream Uses

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Regulation 61.8(1)(e)

“No permit shall be issued which allows a discharge that by itself or in combination with other pollution will result in pollution of the receiving waters in excess of the pollution permitted by an applicable water quality standard or applicable antidegradation requirement unless …”

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What About Reg. 31.3?

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“In assigning classifications and standards, the Commission shall take into consideration the water quality classifications and standards of downstream waters and shall ensure that as implemented through its policies, the water quality classifications and standards of downstream waters will be attained and maintained.”

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What About Reg. 31.3? (cont.)

In-stream water at the stated standards will not compromise downstream uses

  • Segmentation doesn't limit the fate and transport�of pollutants
  • Added due to EPA's 2015 water quality standards rule
  • Not intended to change policy or practice
  • Standards and permits long worked together
  • Not focused on effluent discharges

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When to Apply in Permit?

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Whether effluent flows to the downstream segments (e.g., drainage study)

  • Overall flow of facility
  • Flow of immediate segment
  • Assimilative capacity of the immediate segment
  • Distance to downstream segments

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When to Apply in Permit? (cont.)

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Potential impacts downstream

  • Whether the downstream segment has a water supply classification
  • Whether standards or hardness downstream are more or less stringent than the receiving water
  • Whether downstream has multiple discharges where assimilative capacity is divided reducing WQBELs

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Discharger Actions/Tools

If working on standards action consider downstream sampling and proposing action for downstream segments

  • Example with biotic ligand model (BLM) based copper standards for Upper Thompson:
    • Dischargers sampling numerous points downstream
    • Able to develop site-specific standards that were protective, but varied longitudinally downstream until other tributaries flowed came in

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Discharger Actions/Tools (cont.)

Conduct flow studies to determine if effluent reaches downstream waters (assess year-round/seasonal protection of downstream use)

  • Generally only for zero low flow/ephemeral streams
  • Examples:
    • XTO’s coal bed methane well discharge permits on�the Purgatoire River
    • Trapper Mine drainage study

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QUESTIONS?

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