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Reasonable Potential

January 14, 2021

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What is “Reasonable Potential”?

Problem: Streams have standards for hundreds of pollutants

Which pollutant should be monitored for in a specific permit?
Which pollutants should have limits in a specific permit?

The Reasonable Potential Policy establishes a way to figure this out

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Is RP the whole story?

Nope! A permit may also include:

Tech-based limits

like ELGs and Reg. 62 limits

Antidegradation limits

for reviewable streams

Practice-based controls
TMDL waste load or concentration limits
Additional monitoring
Special studies
Requirements to implement the narrative standard (like TDS or PFAS)

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Legal Basis for Using Reasonable Potential to Set Limits

We can’t write a permit with limits that will lead to exceedances of stream standards

We must include permit limitations for any pollutant that causes or contributes to, or that has the reasonable potential to cause or contribute to, an exceedance of water quality standards , including narrative standards

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Regulation 61.8(2)(b)(i)(A) requires that “[l]imitations must control all pollutants or pollutant parameters which the Division determines are or may be discharged at a level which will cause, have the reasonable potential to cause, or measurably contribute to an excursion above any water quality standard.” (Emphasis added).

61.8(1)(b) The Division shall not issue a permit under the following circumstances...

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

Regulation 61.8(1)(e)

Subject to the provisions of subsection 31.14(15)(b), 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 the permit contains effluent limitations and a schedule of compliance specifying treatment requirements or the Division has granted a variance from the water quality standard.

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Example - Slate River Standards

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Different Kinds of Reasonable Potential Determinations

Quantitative
Qualitative

including RP processes specific to a General Permit

WET Testing

addressed in WET Testing Policy

CW1, WQCD Reasonable Potential Policy

https://cdphe.colorado.gov/clean-water-policies

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Quantitative Reasonable Potential and Effluent Limits

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Use at least 10 data points collected over at least a year (but more is better)

each quarter should be represented

Compare

how much pollution would be allowed before exceeding the standard (MAPC); and
the maximum this outfall has discharged (MEPC), sometimes with a small multiplier (like 1.1 or 1.2)

Is the MEPC bigger than the MAPC?

If yes, add a limit for the pollutant in the permit

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Example

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Qualitative Reasonable Potential and Effluent Limits

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Why are limits based on qualitative RP necessary to protect state waters?

There may not be enough data to do the quantitative method
Quantitative method was developed specifically for toxics and may not be appropriate for non-toxics.
Quantitative method assumes a stable and consistent discharge

Many discharges aren’t stable or consistent
Many pollutants may be in the effluent only sporadically

Where pollutants are specifically added to the discharge, like chemicals
Monitoring data may reflect treatment, and limits are needed to insure that treatment continues
Monitoring may reflect source control, and limits are needed to insure that source control continues
When we know that the category of discharger is likely to produce that pollutant

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Reasonable Potential and General Permit Effluent Limits

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General permits may have their own method for establishing RP

A kind of qualitative RP
Based on the amount of data we have, what we know about the category of discharge, and how protective the general permit is
Permittees can (and do!) comment and propose changes to this quantitative RP during the general permit’s public comment period

Ex: COG318000 (remediation dewatering)

Above 50% of the pollutants in your source water sample = RP for a limit

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Monitoring and RP

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The state’s job around monitoring and permits:

All permits shall specify required monitoring including type, intervals, and frequency sufficient to yield data which are representative of the monitored activity

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When do we require monitoring?

Some examples:

When there is quantitative RP, per the RP policy

If the MEPC (that maximum effluent, potentially with a multiplier) is 50% or more of the MAPC (the maximum before the stream standard is exceeded)

When there is not enough data to make a RP determination
So that we will have enough data to conduct a quantitative reasonable potential analysis for the pollutant in the next permit term
When monitoring is needed for development of a TMDL
When monitoring is needed to assess the effectiveness of practice-based controls

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More examples

When there is qualitative RP. This could include:

Intermittent changes in pollutant concentrations in amounts that could affect the outcome of the RP determination
Foreseeable conditions are expected to increase pollutant concentrations.
The maximum allowable pollutant concentration is below the PQL.
When we know that the category of discharger is likely to produce that pollutant

Ex: metals and cyanide are pollutants of concern for all major domestic WWTFs
Ex: if identified as a pollutant of concern by an ELG

Other instances when monitoring is needed to be representative of the activity

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Monitoring frequency

Generally governed by WQP20, Baseline Monitoring Frequency Policy
May also be governed by your general permit
Policy:

https://cdphe.colorado.gov/clean-water-policies
Allows you to request reductions in monitoring frequency in specific circumstances

We are open to doing a permit webinar around monitoring frequency if there is interest

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For more information...

The best information about RP and YOUR permit can be found in your permit’s fact sheet

There is usually a section dedicated to RP determinations

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Breakout Session: Monitoring and Reasonable Potential

Why we have used qualitative RP to require monitoring
Why we have required monitoring so that there is sufficient data for quanitative RP calculations in the next permit term

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Specific Questions

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