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Mixing Zones, Reasonable Potential Analysis, and Permit Limits

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1. Mixing Zones, Reasonable Potential Analysis, and Permit Limits. A Quick Overview. Steve Schnurbusch Oregon Department of Environmental Quality [email protected] (503) 378-8240 x284. 2. Overview. Brief discussion of mixing zones Mixing Zone definitions and terminology

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Mixing Zones, Reasonable Potential Analysis, and Permit Limits

A Quick Overview

Steve Schnurbusch

Oregon Department of Environmental Quality

[email protected]

(503) 378-8240 x284

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Overview

  • Brief discussion of mixing zones
    • Mixing Zone definitions and terminology
    • Application of mixing zones
    • Types of mixing zone evaluations
  • Performing a reasonable potential analysis
  • Developing water quality-based effluent limits (WQBELs)
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Atmospheric Plumes

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Ambient density stratification causes boundary interaction in the form of terminal level formation (Image: Hotler, ETH).

(Photo: I. Wood, Univ. of Canterbury)

Surface Water Plumes

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Alien Waste Management

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What is a Mixing Zone?

A small area around an outfall pipe where water quality criteria are allowed to be violated

  • The Clean Water Act allows the use of mixing zones.
  • EPA does not have any mixing zone rules. Each state must adopt their own rules subject to EPA approval.
  • Mixing zones are allowed for the initial mixing of waste but not for treatment.
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Some Definitions

Zone of Initial Dilution (ZID) – small area in the immediate vicinity of the outfall where acute toxicity is allowed (also called the “acute” mixing zone)

Mixing Zone – small area in the vicinity of the outfall where chronic toxicity is allowed (also called the “chronic” mixing zone)

Dilution – the combined flow of the effluent and stream flow divided by the effluent flow

Acute Toxicity – The ability of a substance to cause severe biological harm or death soon after a single exposure or dose.

Chronic Toxicity – The capacity of a substance to cause long-term or delayed adverse health effects.

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Dilution

Dilution Factor =

(Used by Cormix and Visual Plumes)

Dilution Ratio =

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Some More Definitions

Acute Criterion – the highest instream concentration of a toxicant or an effluent to which organisms can be exposed for a brief period of time without causing an acute effect.

Chronic Criterion – the highest instream concentration of a toxicant or an effluent to which organisms can be exposed indefinitely without causing unacceptable effect.

Human Health Criterion – the highest concentration of a pollutant in water that is not expected to pose a significant risk to human health.

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Two-Number Aquatic Life Criteria

Acute Mixing Zone - must be limited in size to prevent lethality to drifting organisms – acute criteria must be met at the edge of this zone (ZID).

Chronic Mixing Zone - sized to protect the integrity of the waterbody as a whole – chronic criteria must be met at the edge of this zone (Mixing Zone).

Human Health - allows complete mix with the waterbody

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Mixing Zone Considerations

  • Prevent lethality to drifting organisms
  • Protect overall integrity of the water body
  • Allow for fish passage
  • Avoid shore hugging plumes
  • Avoid bottom attachment
  • Avoid overlap with critical habitat
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Mixing Zone Examples

  • The allowable mixing zone is that portion of the Sandy River from the point of discharge to 100 feet downstream of the point of discharge. The zone of initial dilution (ZID) shall be defined as that portion of the allowable mixing zone that is within 10 feet of the point of discharge.
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Mixing Zone Examples

  • The allowable mixing zone is that portion of the Pacific Ocean contained within a radius of two hundred fifty (250) feet from the outfall. The Zone of Immediate Dilution (ZID) shall be defined as that portion of the allowable mixing zone that is within twenty five (25) feet of the point of discharge.
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Mixing Zone Examples

  • That portion of the Willamette River extending from 10 feet upstream to 500 feet downstream and no more than ¼ the stream width at the point that the City of Eugene Millrace Stormwater System discharges to the Willamette River. The Zone of Immediate Dilution (ZID) shall be defined as that portion of the allowable mixing zone that is within fifty (50) feet from the point of discharge.
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Types of MZ Evaluations

Dye Studies

MZ Modeling

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CORMIX Simulation

Available Models

CORMIX

Applicable for riverine, estuary, and ocean outfalls

Simulates boundary interactions, bottom attachments, and unstable discharges

Visual Plumes

Applicable for “unbounded” water bodies only

Simulates stable discharges with no boundary interactions

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MZ Analysis Results

1. Dilution at Edge of ZID

2. Dilution at Edge of Mixing Zone

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Dilution Results

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Reasonable Potential Analysis (RPA)

Statistical method used to determine if a discharge causes, has the reasonable potential to cause, or contribute to an in-stream excursion above a water quality standard

  • The EPA methodology takes into account:
  • The variability of the pollutant parameter
  • The available dilution
  • Background concentration of pollutant upstream
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RPA Example

Lead: n = 20 Max = 25 ug/L

Mean = 15 ug/L Stddev = 6

Available dilution at MZ = 50

  • RPA Steps:
  • CV = stddev/mean = 6/15 = 0.4
  • Multiplying factor based on 20 samples and CV of 0.4 = 1.4
  • Maximum probable value = 25 x 1.4 = 32.5 ug/L
  • Instream concentration at MZ = 32.5/50 = 0.65 ug/L
  • Chronic lead criterion = 0.60 mg/L
  • 0.65 > 0.60 mg/L so there is a reasonable potential to violate lead
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Water Quality-Based Effluent Limits

CFR 122.44(d)(1)(ii) states that if the permitting authority determines a pollutant has the reasonable potential to cause or contribute to an excursion above a water quality criteria, the permit must contain effluent limits for that pollutant.

  • EPA has developed a permit limit derivation methodology that takes into account:
    • Background concentration of pollutant
    • Available dilution
    • Variability of the effluent data
    • Water quality criteria
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Calculating Permit Limits

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Summary

  • Brief overview of mixing zones
  • Applied results of a mixing zone analysis to a Reasonable Potential Analysis
  • Developed WQBELs that will be used in the Local Limits Evaluation
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