1 mixing zones, reasonable potential analysis, and permit limits a quick overview steve schnurbusch...
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Mixing Zones, Reasonable Potential Analysis, and Permit
LimitsA Quick Overview
Steve Schnurbusch
Oregon Department of Environmental Quality
(503) 378-8240 x284
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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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Ambient density stratification causes boundary interaction in the form of terminal level formation (Image: Hotler, ETH).
(Photo: I. Wood, Univ. of Canterbury)
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Alien Waste Management
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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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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)
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.
Mixing Zone – small area in the vicinity of the outfall where chronic toxicity is allowed (also called the “chronic” mixing zone)
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Dilution Factor = e
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Q
Dilution Ratio = e
r
Q
Q
(Used by Cormix and Visual Plumes)
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Q 1
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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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Outfall
Acute criteriamet
Chronic criteriamet
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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Prevent lethality to drifting organisms
Protect overall integrity of the water bodyAllow for fish passage
Avoid shore hugging plumes
Avoid bottom attachment
Avoid overlap with critical habitat
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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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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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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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Dye Studies
MZ Modeling
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CORMIXApplicable for riverine, estuary, and ocean outfalls
Simulates boundary interactions, bottom attachments, and unstable discharges
Visual PlumesApplicable for “unbounded” water bodies only
Simulates stable discharges with no boundary interactions
CORMIX Simulation
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2. Dilution at Edge of Mixing Zone
1. Dilution at Edge of ZID
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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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Lead: n = 20 Max = 25 ug/L
Mean = 15 ug/L Stddev = 6
Available dilution at MZ = 50
RPA Steps:
1. CV = stddev/mean = 6/15 = 0.4
2. Multiplying factor based on 20 samples and CV of 0.4 = 1.4
3. Maximum probable value = 25 x 1.4 = 32.5 ug/L
4. Instream concentration at MZ = 32.5/50 = 0.65 ug/L
5. Chronic lead criterion = 0.60 mg/L
6. 0.65 > 0.60 mg/L so there is a reasonable potential to violate lead
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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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USEPA Technical Support Document recommended permit limit procedureEPA/ 505/ 2-90-001(p. 100)Source Name Parameters
Toxic parameters Z - 95th % 1.645Lead Zinc Cadmium Parameter Z - 99th % 2.326fresh fresh fresh fresh CV 0.4
Background 0 0 0 0 sigma 0.38525Acute Criteria 14.28 na na na sigma4 0.19804Chronic Criteria 0.56 na na na sigman 0.11509
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ZID Dilution 10 na na na EquationsMZ Dilution 50 na na na WLAa = ZID * Acute criteria
WLAc = MZ * Chronic criteriaWLAa 142.80 na na naWLAc 28.00 na na na LTAa = WLAa * exp(0.5sigma 2̂ - Z * sigma)
LTAc = WLAc * exp(0.5sigma4 2̂ - Z * sigma4)LTAa 62.77 na na na * based on 99th %LTAc 18.01 na na na * based on 99th % LTA = min (LTAc, LTAc)
LTA 18.01 na na na MDL = LTA * exp(Z * sigma - 0.5sigma2)AML = LTA * exp(Z * sigman - 0.5sigman
2)
MDL 40.98 na na na * based on 99th %AML 21.63 na na na * based on 95th %
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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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