Reasonable Potential Analysis 

Process for Toxic Pollutants 

Version 3.1

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Internal Management Directive 

Last Updated: 02/13/2012 

By: Spencer Bohaboy 

DEQ 11-WQ-020-IMD 

Water Quality Division 

NPDES Permit 

Development 

811 SW 6th Avenue 

Portland, OR 97204 

Phone: (503) 229-5696 

 (800) 452-4011 

Fax: (503) 229-6762 

Contact: Spencer Bohaboy 

www.oregon.gov/DEQ 

DEQ is a leader in 

restoring, maintaining 

and enhancing the quality 

of Oregon’s air, land and 

water. 
Oregon DEQ Page ii 

Reasonable Potential Analysis IMD 

Disclaimer 

This internal management directive (IMD) represents the Department of 

Environmental Quality’s (DEQ) current directions to staff on how to conduct a 

Reasonable Potential Analysis for point source water quality permitting. This 

IMD is not final agency action and does not create any rights, duties, obligations, 

or defenses, implied or otherwise, in any third parties. This directive should not be 

construed as rule, although some of it describes existing state and federal laws. 

The recommendations contained in this directive should not be construed as a 

requirement of rule or statute. DEQ anticipates revising this document from time 

to time as conditions warrant. 

Contact Information 

Oregon Department of Environmental Quality 

Water Quality Division 

811 SW 6th Ave. 

Portland, OR 97204 

For more information: 

Spencer Bohaboy, (503) 229-5415 

Alternative formats (Braille, large type) of this document can be made available. 

Contact DEQ’s Office of Communications & Outreach, Portland, at (503) 229-

5696, or toll-free in Oregon at 1-800-452-4011, ext. 5696.

Document Development 

Prepared by: Spencer Bohaboy, Policy Development Specialist, Surface 

Water Management 

Developed by: Spencer Bohaboy, Policy Development Specialist 

Lyle Christensen, Permit Writer Northwest Region 

 Steve Schnurbusch, Permit Writer Western Region 

 Heidi Williams, PE, Plan Review Engineer Eastern Region 

Reviewed by: Dennis Ades, Manager Surface Water Management 

Greg Aldrich, Interim Administrator, Water Quality Division 

 Sonja Biorn-Hansen, Senior Water Quality Permit Specialist, 

Water Quality Division 

 Jane Hickman, JJD, Policy Analyst, Water Quality Division 

Approved by: Greg Aldrich, Interim Water Quality Administrator 

 ________________________________________ _________ Signed Copy on File at DEQ
Oregon DEQ Page iii 

Reasonable Potential Analysis IMD 

Table of Contents 

Section 1: Introduction 1 

1.1 Purpose of Directive 

1.2 Directive Applicability 

1.3 Overview of Permit Process 

1.4 Key Definitions 

1.5 Overview of the Reasonable Potential Analysis 

1.6 Sources of Information 

1.7 Questions and Contact Information 

Section 2: Tier 1 Monitoring 7 

2.1 Tier 1 Monitoring Overview 

2.2 Determination of Pollutants Requiring Evaluation 

2.2.1 Discussion of State and Federal Authority to Require Monitoring

2.2.2 Domestic Facilities: Evaluation and Monitoring Requirements 

2.2.3 Domestic Facility Pollutant Tables 

2.2.4 Industrial Facilities: Evaluation and Monitoring Requirements 

2.2.5 Industrial Facility Pollutant Tables 

2.3 Pollutant Parameters With-out Numeric Water Quality Criteria 

2.4 Reporting Procedures 

2.5 Schedule B: Suggested Monitoring Requirement Language 

Section 3: Identify Pollutants of Concern 27

3.1 Process Overview 

3.2 Entering General Facility Information 

3.3 Analysis of Identified Parameters 

3.4 Interpretation of Results 

3.5 Additional Information: Narrative Criteria and Whole Effluent Testing 

3.6 Next Step 

Section 4: Tier 2 Monitoring 33

4.1. Tier 2 Monitoring Overview 

4.2. Monitoring Action Letter 

4.3. Sampling Plan 

4.3.1. Characterization of Receiving Water Body 

4.3.2. Additional Effluent Characterization 

4.3.3. Source Investigation 

4.4. Next Steps 

Section 5: Reasonable Potential Analysis 39 

5.1 Reasonable Potential Analysis Overview

5.2 General Facility Information

5.2.1 Flow Rates

5.2.2 In-Stream Dilution
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Reasonable Potential Analysis IMD 

5.2.3 Fresh/Marine Water Status

5.2.4 Hardness

5.2.5 Statistical Calculation Values

5.3 Revised Effluent Characterization Data

5.4 Data Interpretation

5.5 Ambient Concentration

5.5.1 Data Entry Instructions

5.6 Comparison of Maximum Concentration to Water Quality Concentration

5.7 Troubleshooting

5.8 Conclusions

5.9 Next Steps

Section 6: Effluent Limit Calculation and Determination 53 

6.1 Effluent Limit Determination Process Overview

6.2 WLA and WQBEL Calculation

6.3 Compare WQBELs with TBELs

6.4 Next Steps

Section 7: Permit Applicant Consultation 59 

7.1 Overview

7.2 Permittee Notification of Results and Compliance Evaluation

7.3 Alternatives

7.4 Selected Management Option

7.5 Next Step

Section 8: Finalization of RPA and Effluent Limits and Documentation 

of Decision Rational 63 

8.1 Overview 

8.2 Final RPA and Effluent Limit Evaluation 

8.3 Documentation 

8.4 Outcome 1: Evaluation Report components for “no monitoring req’d” 

8.5 Outcome 2: Evaluation Report components where “monitoring is required” 

but “reasonable potential” is not determined 

8.6 Outcome 3: Evaluation Report components where “monitoring is required” 

and “reasonable potential” is determined, and “effluent limits” are calculated 

Appendix A: Revision History 67 

Appendix B: General Overview of RPA and WQBEL Calculation 69 

Appendix C: Analytic Methods, Limits, and Implementation Guidance 71 

Appendix D: RPA Workbook Walk-though and Example 73 

Appendix E: Whole Effluent Toxicity Guidance 83 

Appendix F: Intake Credit Guidance 103
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Section 1: Introduction 

1.1 Purpose of Directive 

The purposes of this Internal Management Directive (IMD) are to provide guidance for 

Department of Environmental Quality (DEQ) staff in determining whether an individual 

point source discharge (point source) contains toxic pollutants of concern (POCs) that

might cause an exceedance of the water quality standard in the receiving water body, and 

how to calculate effluent limits. The current water quality criteria for aquatic toxicity are 

listed in OAR 340-41 pollutant tables 20, 33A and 33B, and for human health water 

quality criteria in OAR 340-41 pollutant table 40. The IMD provides step-by-step 

guidance for identifying POCs, conducting the Reasonable Potential Analysis (RPA), 

calculating Water Quality Based Effluent Limits (WQBELs), and discusses various 

technical and policy issues. 

All of the RPA steps and calculations have been consolidated into a series of spreadsheets 

(RPA Workbook). When conducting the RPA, the permit writer uses these spreadsheets 

to determine the discharger’s monitoring requirements, identify POCs, calculate 

reasonable potential and if necessary, develop effluent limits. This document follows 

along with the spreadsheets, and provides basic instruction for each step and, where 

necessary, offers detailed technical and regulatory guidance. 

It is recommended that readers should first familiarize themselves with the RPA 

Workbook and refer to it while reading through this document. This includes reading 

through the Appendix D: RPA Workbook Walk-Through and Example. 

1.2 Directive Applicability 

State and Federal regulation require that a RPA must be conducted for all proposed and 

existing industrial and domestic NPDES individual point source dischargers. The 

number and type of pollutants for which the RPA must be conducted will vary with the 

size, type and potential hazard of the facility. This process is explained in detail in 

Section 2. 

1.3 Overview of Permit Process 

For most major domestic and industrial point sources, DEQ permit writers review permit 

applications, perform water quality-based modeling, review laboratory test results, 

develop effluent limits (where applicable) and incorporate their findings into permits to 

ensure that the discharge of pollutants (i.e. toxic, nutrients, pH, BOD, etc.) do not result 

in violations of water quality standards. Some minor facilities require an equivalent 

degree of analysis, but typically for a greatly reduced number of toxic pollutants. These 

processes are usually repeated every five years upon permit renewal. The RPA for toxic 

pollutants is often the most technically involved portion of the permit development 
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DEQ 11-WQ-020-IMD 2/13/2012 

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process. 

The Department has developed the following guidance documents that describe the parts 

of the permit development process that relate to the subjects covered in this document: 

• Anti-Degradation Policy Implementation IMD

• Compliance Schedules in NPDES Permits IMD

• Regulatory Mixing Zone IMD: Part 1 Allocating Regulatory Mixing Zones

• Regulatory Mixing Zone IMD: Part 2 Reviewing Mixing Zone Studies

1.4 Key Definitions 

• Reasonable Potential Analysis (RPA) is the process for determining "whether a discharge 

causes, has reasonable potential (RP) to cause, or contributes to an excursion above" 

Oregon's water quality criteria for toxic pollutants (in the receiving water body). 

• A Pollutant of Concern (POC) is a toxic pollutant that has been statistically evaluated and 

identified as having a “reasonable potential” to exceed the state water quality criteria (at 

the end of pipe). 

• Water Quality Based Effluent Limit (WQBEL) is an effluent limitation determined by 

selecting the most stringent of the effluent limits calculated using all applicable water 

quality criteria (i.e. human health or aquatic toxicity) for a specific point source to a 

specific receiving water. 

• Permit Evaluation Report (PER) is a document that summarizes the principal facts and 

the significant factual, legal, methodological, and policy question considered in preparing 

the draft permit and documents the decisions. The PER is also referred to as a “Fact 

Sheet” and the various requirements are covered under 40 CFR 124.8 and 124.56.

• Tier 1 Monitoring is a series of at least four sampling events designed to characterize a 

facility’s effluent and identify POCs. Each sample collected is subjected to laboratory 

analysis for a broad spectrum of pollutant parameters as determined by the permit writer. 

• Tier 2 Monitoring is a second round of sampling (for identified POCs only) necessary to 

characterize the receiving water and provide additional effluent characterization. Tier 2 

monitoring will enable the permit writer to model the impacts of a discharge upon the 

receiving waters, and complete the RPA. Generally, if the results of Tier 1 monitoring 

indicate that there is no reasonable potential for the discharge to exceed water quality 

standards at the end of the pipe, then Tier 2 monitoring is not needed. 

1.5 Overview of the Reasonable Potential Analysis 

To paraphrase EPA's Technical Support Document for Water Quality-based Toxics 

Control, a RPA is the process for determining "whether a discharge causes, has 

reasonable potential to cause, or contributes to an excursion above" Oregon's water 

quality criteria for toxic pollutants. Before conducting a RPA, a permit writer should first 

review any applicable Total Maximum Daily Loads (TMDLs) to determine if there are 

already facility-specific waste load allocations (WLA)1. The permit writer would then 

 

1 If indicated, the permit writer should confer with the TMDL development section or Basin 

Coordinator to ensure that the TMDL is sufficiently protective and that the underlying assumptions 

have not changed. 
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continue with the RPA process described in Figure 1-1 and 1-2 for any pollutants not 

specifically addressed in a TMDL. 

 

Figure 1-1 

Permit Development and RPA Process Overview 

• Step 1: As part of the preceding permit development, the permit writer 

develops a list of effluent monitoring requirements based upon the facility and 

receiving water classifications. Section 2 (RPA IMD)

• Step 2: During the first 24 months of the new permit term, the permittee 

conducts the required pollutant scan (Tier 1 Monitoring). Section 2

• Step 3: Using the Tier 1 data, the permit writer initially characterizes the 

effluent and identifies POCs using the Aquatic Toxicity RPA and Human 

Health RPA spreadsheets of the RPA Workbook to conduct an “end of pipe” 

reasonable potential (RP) determination. Section 3

• Step 4: The permit writer will report the identified POCs, associated 

monitoring requirements and recommendations to the permittee via 

Monitoring Action Letter (MAL). By the end of the month 36, for each POC the 

permittee will characterize the receiving water body, and (optionally) collect 

additional effluent or source investigation data (Tier 2 monitoring). Section 4

• Step 5: The permit writer will use the Tier 1 and 2 data, along with other 

information, to model the potential in-stream water quality impacts and 

complete the “in-stream” RP determination. This may include the use of an 

intake credit analysis as part of the determination. Section 5

• Step 6: For each POC found to have RP (in-stream), information will be 

transferred from the RPA spreadsheets to the Aquatic Limits and Human 

Health Limits spreadsheets to calculate a Water Quality Based Effluent Limit 

(WQBEL). This may include the use of rule-based intake credits in the limit 

calculation. The permit writer would normally include this limit in the permit 

unless a more stringent Technology Based Effluent Limit (TBEL) is 

applicable. Section 6

• Step 7: The permit writers will discuss the results of the RPA with the 

applicants. If the facility is not capable of initially meeting the effluent limit, 

the permittee may use the remainder of the permit term (~24 months) to 

collect additional information, attempt source reduction, develop treatment 

options, or justify a variance request or standards adjustment. In some 

cases, a Compliance Schedule may be granted, allowing the permittee 

additional time to implement source reduction or treatment options to meet 

the final effluent limit. Section 7

• Step 8: After receiving the EPA permit application forms (month 54), the 

permit writer would use the remaining time (six months) to finalize the RPA 

and WQBEL calculations, and then document their findings, interim 

calculations and management decisions in the Permit Evaluation Report. The 

permit would then be ready for re-issuance. Section 8 
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For new permit applications or permits where adequate monitoring data are not available 

to conduct a quantitative RPA, permit writers should use their best professional judgment 

to determine the most appropriate ways to characterize the facility and model the instream impacts. This might include the use of representative data or the use of an 

alternative qualitative RPA methodology as described in EPA guidance2. Please refer to 

Sections 4.2.1 and 5.4 of this document for more information. 

Once the RPA and calculation of WQBELs is complete, a summary of the analysis and 

results is documented in the PER. To complete the permit development process, the 

permit writer will conduct a Whole Effluent Toxicity (WET) review, and where 

necessary, increase the stringency of the effluent limits or permit conditions. A summary 

of the RPA process is included in Figure 1-2 below. 

Figure 1-2 

RPA Process w/Timeline and Implementation Options 

 

 

2 Technical Support Document for Water Quality-based Toxics Control, Section 3.2, P. 50.

Issue 

Permit 

Run RPA & 

Calculate WQBEL

• Optional: Intake 

Credit / Background 

Criteria 

• WET review 

• “Can permittee meet

WQBEL?” Y/N

Now What? 

• Source Reduction 

• Engineering Study 

• Compliance 

Schedule 

• Variance 

• Standards Change 

No

Identify POCs

• “End of pipe 

RPA” to identify 

Pollutants of 

Concern (POCs)

• Monitoring 

Action Letter 

(MAL) 

Tier 2 Monitoring

• Ambient 

characterization 

for POCs 

• Optional: More 

effluent data 

• Optional: Source 

Investigation 

Tier 1 

Monitoring 

• Characterize: 

o All Tables 1 & 

2 parameters 

o Select Table 3

parameters 

• Min. 4 samples

Year 0Year 2Year 3

Year 5

Select Option 

• Work with 

permittee to select 

most appropriate 

implementation 

option 

*Implementation options and related actions are colored in RED 

Yes 

Write Permit

• Select appropriate limits (lowest) 

• Det. Monitoring Requirements 

• Special Conditions (if necessary) 

o Compliance Schedule 

o Additional Monitoring 

o Req’d BMPs or Benchmarks 

• WET review 

• Document decisions in PER 
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1.6 Sources of Information 

The following is the typical information a permit writer must review to characterize the 

facility, effluent and the receiving waters. Most of the following are typically submitted 

by the permit applicant. In some cases, the permit writer will need to request or collect 

additional information. 

Domestic Facilities: 

• Permit application forms

o EPA Forms 1 and 2A (new permit) 

o EPA Forms 2A and OR DEQ Form NPDES-R (permit renewal) 

o Bio-solids Plan 

• Characterization and monitoring data as required by application form, permit and/or 

permit writer request 

• Discharge Monitoring Reports (DMRs), with a focus on the adequacy of detection 

limits for non-detectable values3

• Pretreatment Industrial Surveys or required information per 40 CFR 122.21 (j)(6).

• Total Maximum Daily Load Reports: Many facilities that discharge to 303(d) listed 

stream segments will be described and receive waste load allocations in TMDL 

Reports 

• 303(d) listings for receiving stream segment 

• Oregon DEQ LASAR ambient data for the applicable waterbody segment 

Industrial Facilities: 

• Permit application forms4

o EPA Form 1 & 2D (new permit) 

o EPA Form 2C and OR DEQ Form NPDES-R (permit renewal) 

• Characterization and monitoring data as required by application form, permit or 

permit writer request 

• Discharge Monitoring Reports (DMRs), with a focus on the adequacy of detection 

limits for non-detectable values 

• Total Maximum Daily Load Reports (TMDL): Many facilities that discharge to 

303(d) listed stream segments will be described and receive waste load allocations in 

TMDL Reports 

• 303(d) listings for receiving stream segment 

• Oregon DEQ LASAR ambient data for the applicable receiving stream segment 

• Design and process flow data 

• Hazardous material inventories (SARA Title III, RCRA, State Fire Marshal 

records). 

If the facility has a Regulatory Mixing Zone (RMZ), then a detailed mixing zone analysis 

should have been conducted. At minimum, the study should contain the dilutions values 

for the receiving water body and mixing zones under critical conditions. Please refer to 

 

3

 Daily and weekly data for major facilities, in electronic form, is available to the permit writer 

through the Discharge Monitoring System (DMS). Often monthly, annual and sporadic data are 

also available. 

4 Other EPA application forms include 2E (facilities which do not discharge process wastewater), 

2B (CAFOs and Aquatic Animal Production wastewater) and 2F (industrial stormwater only). 
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the Regulatory Mixing Zone Internal Management Directive for more information. 

The “dilution values” should be used instead of the effluent and ambient stream flow 

information in the RPA calculation. The permit writer can also contact the regional Plan 

Review Engineer for additional information regarding the appropriate flow value to use 

in the RPA that is consistent with critical conditions. . If no mixing zone analysis is 

available, it will be necessary to use the facility’s Design Flow5, along with critical 

effluent and ambient flow rates to calculate the available dilution using the RPA 

Workbook. 

Additionally, the following documents provide in-depth discussions of the permitting 

process and methods used in modeling reasonable potential and calculating effluent 

limits: 

• Technical Support Document for Water Quality-based Toxic Control, USEPA, 

March 1991 (TSD) 

• U.S. EPA NPDES Permit Writers’ Manual, USEPA, September 2010 

Finally, the Department maintains a website that contains various guidance documents, 

analytical and modeling tools, state rules, pertinent forms and other permit writer 

resources at: http://deq05/wq/wqpermits/PermitWritersCorner.htm. 

1.7 Questions and Contact Information 

For questions about this guidance, contact the Surface Water Management Section in the 

Water Quality Division. At the time of release, the point of contact for this guidance is 

Spencer Bohaboy at (503) 229-5415 or mailto:bohaboy.spencer@deq.state.or.us. 

 

5

 The flow that the facility was built to handle 
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Section 2: Tier 1 Monitoring 

2.1 Tier 1 Monitoring Overview 

The RPA process begins when the permit writer evaluates a facility to determine the 

spectrum of pollutant parameters that must be included in the permit monitoring 

requirements (Step 1). This pollutant scan is intended as a periodic check to determine if 

any of the pollutants historically associated with domestic or industrial facilities or for 

which the receiving water has been listed as water quality limited are present, and 

quantify the resulting concentrations6. Typically, this evaluation would occur during the 

preceding permit term so that the effluent monitoring requirements can be included in the 

Schedule B permit conditions7. 

For domestic facilities, the number and type of pollutants that require an RPA increases 

along with incremental (<0.1, 0.1 – 1.0, > 1.0 MGD) changes in the facility “design 

flow”. There might also be additional monitoring requirements for facilities with special 

conditions such as the discharge of PCB’s, pesticides or mercury. This is described in 

Section 2.2.1 of this document. 

For industrial facilities, the number and type of pollutants that require evaluation are 

determined by the facilities’ design flow rates, industrial categories, hazardous 

production materials, receiving water’s status and permitting history. This is described in 

Section 2.2.2 of this document. 

Upon permit renewal, the facility would implement their Tier 1 monitoring requirements 

by collecting at least four effluent samples that reflect the seasonal discharge 

characteristics (i.e. wet and dry) and have them analyzed for the specified pollutants. 

These actions must be completed by the 24th month of the permit term (Step 2). 

Once collected and analyzed, the permit writer would then evaluate the resultant data and 

identify any POCs in the facility’s effluent (Step 3) as described in Section 3. 

2.2 Determination of Pollutants Requiring 

Evaluation 

The process to evaluate a facility and identify monitoring requirements is complex and 

depends upon the type of facility. Figures 2-3, 2-4, 2-5 and the accompanying Domestic 

Facility Pollutant Table and Industrial Facility Pollutants Table walk the permit 

writer through the process to ensure that all federally and state mandated pollutant 

parameters are identified. 

 

6

 This scan is in addition to any compliance-based effluent monitoring 

7

 As part of the application for a new facility or extraordinary circumstances, a facility might be 

required by Permit Action Letter to conduct Tier 1 monitoring 
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Regardless of other factors, the following pollutant parameters must be analyzed in the 

RPA: 

• Pollutant parameters with effluent limits in the preceding permit 

• Pollutant parameters with monitoring requirements in the preceding permit8

• Pollutant parameters for which the receiving water body has been listed as “water 

quality limited” on the 303d list 

• Pollutant parameters that are “known9

” to be present in significant concentrations in a 

facilities’ source/intake water 

• Pollutant parameters that are “known” or otherwise expected to be present in significant 

concentrations in a facilities effluent 

• Pollutant parameters identified through the permit application process and described in 

the rest of this Section 

The RPA Workbook includes all the pollutant parameters that might need to be 

evaluated for a domestic or industrial facility, based upon state and federal guidelines. 

The permit writer will evaluate each parameter for inclusion in the analysis and record 

the result (“Yes” or “No”) in the “Monitoring Required?” column of the Monitoring 

Requirements spreadsheet (See Figure 2-1)

10. The permit writer should also document 

the facility’s Average Dry Weather Design Flow (ADWDF) rate in the appropriate box of 

the same spreadsheet. Once the evaluation is complete, the permit writer can supply a 

copy of the worksheet to the permit applicant and include a summary of the Tier 1 

monitoring requirements in Schedule B of the permit. 

For a new permit application, the permit writer will identify the pollutant parameters that 

require monitoring during a pre-application conference, and the results will be submitted 

as part of the facility’s application. 

For the first round of permit renewals after implementation of this IMD, the permit writer 

will develop the permit based upon the available data and include the new monitoring and 

evaluation procedures into the permit’s monitoring requirements (Schedule B). 

At various points of the permit cycle (month 24 and permit renewal), the permit writer 

will have the opportunity to add additional monitoring requirements to reflect newly 

authorized water quality criteria or recent waterbody listings (303d). This is discussed in 

Section 4.2. 

If WET testing is required, the permit writer should coordinate the testing to coincide 

with the Tier 1 monitoring. In the event of “toxicity” finding during the WET analysis, 

the simultaneous sampling will help with the required source investigation. Please refer 

to Appendix E (Whole Effluent Toxicity Guidance) or contact the Department’s WET 

 

8

 In some instances, such as a change in water quality criteria, change in treatment method (e.g. 

Cl-

 to UV) or de-listing of a water body, a permit writer might determine that it is appropriate to not 

renew a permit monitoring requirement. 

9

 For example intake water is from a 303(d) listed water body or contaminated groundwater. 10 For some pollutant parameters, total data may be collected in lieu of species specific data for 

Tier 1 monitoring. In the event that the pollutant is identified as a POC, species specific may be 

collected as part of the Tier 2 monitoring. 
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coordinator for more information. 

Figure 2-1 

Example of Monitoring Requirements Determination Worksheet 
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Figure 2-2 

Example of the RPA Workbook 

 

Figure 2-1 Explanation

Determine Monitoring Requirements 

1. The permit writer (PW) evaluates each facility to determine which pollutant 

parameters should be included in Tier One monitoring. 

2. On the Monitoring Required spreadsheet, the PW records facility identification 

information and the ADWDF rate used for the evaluation. 

3. The PW indicates with a “Yes” or “No” in the “Monitoring Required” column if 

monitoring is required as shown in Figure 2-1. 

4. The information entered on the “Monitoring Required?” spreadsheet will 

automatically be forwarded to the Aquatic Toxicity and the Human Health

Spreadsheets as shown in Figure 2-2. 

5. The PW checks to ensure that all pollutant parameters listed on the spreadsheets 

have been evaluated and have either a “Yes” or “No” in the column. 
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2.2.1 Discussion of State and Federal Authority to Require Monitoring 

The purpose of the RPA IMD is to direct DEQ permit writers on how to implement 

statutory and regulatory requirements for NPDES permitting. This IMD does not create 

any new authority but only implements existing authority. Following is a detailed 

discussion of these authorities. 

Per 40 CFR 123.1, the Department has been delegated (by the EPA) to administer the 

Clean Water Act (CWA) and implement the NPDES program in Oregon (except for tribal 

lands). The Environmental Quality Commission has adopted water quality 

standards/criteria to protect the beneficial uses of the State’s water bodies, as authorized 

by ORS 468B.048. These water quality standards/criteria become effective upon 

approval by the EPA. 

OAR 340-045-0035(1) requires DEQ to develop permits “in accordance with provisions 

of all applicable statutes, rules, regulations, and effluent guidelines of the State of Oregon 

and the U.S. EPA.” Per OAR 340-041-0033(2), the “levels of toxic substances in waters 

of the state may not exceed the applicable criteria...”. Note, there are both federal and 

state water quality standards that permittees must meet. 

To ensure that water quality standards applicable to the receiving water are met, the 

Department conducts a water quality based assessment11 to determine if there is a 

reasonable potential for a discharge to cause or contribute to an exceedance, and where 

appropriate, define effluent limitations. The permittee must “comply with the 

Department’s requirements for ..., reporting, monitoring, ... and sampling” (OAR 340-

045-0015 (5)(d)). 

Finally, per 40 CFR 123.25 (Requirements for permitting) and OAR 340-045-0065, the 

department must follow federal permit development and implementation procedures, 

including those described in 40 CFR 122.21 (Application for a permit) and 122.41(3)(h) 

(Duty to provide information). 40 CFR 122.21 directs the permittee to monitor for 

pollutant parameters, many of which are not reflected in Oregon’s water quality criteria12. 

40 CFR 122.41(3)(h) gives the Department the authority to request from the permittee 

any information to determine if “cause exists for modifying, revoking and reissuing, or 

terminating this permit or to determine compliance with this permit.” When requesting a 

water quality variance, per OAR 340-041-0059(4)(c) the permittee must provide 

“sufficient water quality data and analyses to characterize ambient and discharge water 

pollutant concentrations”. 

To summarize, the department has been delegated to implement the NPDES program and 

develop state water quality standards. The department must ensure that permittees 

comply with these standards. In order to develop appropriate effluent limits in permits, 

 

11 Based upon EPA’s TSD

12 Conversely, there are state water quality criteria w/no federal monitoring requirements. This is 

shown in the “No Fed. Req’s” column of the “RPA Workbook, Monitoring Required?” page. 
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the department requires permittees to sample their effluent and, in some cases, the 

receiving water body. The department also incorporates monitoring requirements into the 

permit to verify that the permittees are meeting the applicable effluent limitations. 

2.2.2 Domestic Facilities: Evaluation and Monitoring Requirements 

Per 40 CFR 122.21(j)(4) and DEQ guidance13, domestic facilities are required to monitor 

for specific pollutant parameters with a minimum frequency. The type of pollutants and 

the frequency varies based upon the facility’s Average Dry Weather Design Flow 

(ADWDF)14 in the following manner: 

• For all domestic facilities with an ADWDF of at least 1 MGD, the permit writer must 

conduct an RPA for all identified toxic pollutants. 

• For all domestic facilities with ADWDFs from 0.1 to less than 1 MGD, the permit writer 

must, at minimum, perform a RPA for chlorine and ammonia. 

• A permit writer should conduct a RPA for all 303(d) listed toxic pollutants in the 

receiving stream for a domestic facility of less than 1 MGD, when the pollutants are 

“known” or are reasonably expected to present in the effluent15. 

The guidance in Figure 2-3 (and Domestic Facility Pollutant Tables) walks the permit 

writer through the process of identifying the facility’s monitoring requirements to meet 

both the requirements included in EPA Application Form 2A, and any additional state 

or federal requirements. The Domestic Facility Pollutant Tables in Section 2.2.3 are 

taken from 40 CFR 122, App. J and EPA Application form 2A, and are augmented with 

any state water quality criteria16 not reflected in these sources. The missing state water 

quality criteria have been inserted into the appropriate chemical group within Tables 1A, 

1 and 2, and would only require monitoring when the chemical group is indicated. The 

remaining water quality criteria have been consolidated into Table 3, where only those 

individual pollutants known to be present in the effluent or for-which the receiving water 

body has been listed (303d) as “water quality limited”, would require monitoring. 

The pollutant scan must occur within the first two years of a permit term and each sample 

must be taken between four and eight months apart in order to be representative of 

seasonal variation (i.e. wet and dry). When possible, monitoring should be coordinated 

with other requirements such as local-limit evaluation, compliance monitoring or WET 

testing, to increase the overall relevance of the analytic results and maximize efficiency. 

For a new permit application it might be necessary for the permit writer to conduct a 

qualitative assessment process (Qualitative RPA) when facility specific effluent 

monitoring data is not available. In these circumstances the permit writer should refer to 

guidance in the USEPA NPDES Permit Writers Manual17 and EPA’s TSD, p. 49. 

 

13 Letters entitled “Toxic Monitoring Requirements for Domestic Wastewater Treatment Plants 

with NPDES Permits” dated 1/6/2006. 

14 if necessary, the permit writer should consider the use of an alternative “critical” effluent flow for 

pollutants or discharge scenarios where there might be wet weather impacts. 15 I.e., source water from listed water body, potential industrial pollutant sources w/in the 

collection area or contaminated ground water via I&I. Previously collected data may be used. 16 State water quality criteria are from Federal recommended criteria (section 304 A of CWA). 17 September 2010 revision, Section 6.3.3, Page 6-30. 
Reasonable Potential Analysis IMD Oregon Department of Environmental Quality 

DEQ 11-WQ-020-IMD 2/13/2012 

Version 3.1 Page 13 of 117 

Figure 2-3 

Domestic Tier 1 Monitoring Requirements Determination Process 

All facilities must sample for all 

Table 1A pollutants 

• Includes BOD/CBOD, TSS, Fecal 

Coliform, Flow, Temp & pH 

• Min. of 4 analyses 

• Additional parameters: Alkalinity 

In addition, all facilities w/design 

flow >0.1 MGD must sample for 

all Table 1 pollutants 

• Includes NH3, Cl-, DO, 

nitrate/nitrite, TKN, oil & grease, 

phosphorous and TDS 

• Min. of 4 analyses 

Start 

(40 CFR 122.21(j)) 

In addition, all facilities w/design 

flow >1.0 MGD must sample for all 

Table 2 pollutants 

• Metals [total recoverable], 

cyanide and total phenols 

o Min. of 4 analyses 

• VOCs, acid-extractable 

compounds and base-neutral 

compounds 

o Min. of 4 analyses 

• Additional parameters: 

Hardness & Whole Effluent 

Toxicity 

Required Tier 1 

Monitoring Parameters 

TMDL required monitoring: 

For example >1.0 MGD facilities in 

Willamette Basin must sample for 

mercury • See TMDL requirements 

• Includes pesticides, PCBs or 

other pollutants parameters with 

state water quality criteria 

• Min. of 4 analyses for applicable 

pollutant 

In addition, all facilities w/design flow 

>1.0 MGD must sample for any 

Table 3 parameters: 

• described as part of Dept.-

approved pretreatment program 

• for which the receiving water has 

been listed (303(d)) 

All facilities must sample for any 

pollutant parameter: 

• with current effluent limits or 

permit monitoring requirements 

• “known” or otherwise expected to 

be present in significant conc.s in 

a facility’s effluent 

• “known” to be present in 

significant conc.s in a facility’s 

source water (i.e. 303 d listed) 

• Min. of 4 analyses 
Reasonable Potential Analysis IMD Oregon Department of Environmental Quality 

DEQ 11-WQ-020-IMD 2/13/2012 

Version 3.1 Page 14 of 117 

2.2.3 Domestic Facility Pollutant Tables 

Tables 1A, 1 and 2 below are taken from Appendix J of 40 CFR, Part 122-NPDES 

permit Testing Requirements for Publicly Owned Treatment Works (POTW). State water 

quality criteria not addressed by these documents are added to the tables in the 

appropriate chemical group18, with the remaining criteria consolidated into Table 3. 

Please refer to Figure 2-3 for assistance in determining applicable monitoring 

requirements. 

Table 1A – Effluent Parameters for All POTWS 

Alkalinity as CaCO3 * pH

Biochemical oxygen demand (BOD-5 or 

CBOD-5) Temperature 

Bacteria: 

(Fecal Coliform, E. Coli &/or Enterococci) Total suspended solids 

Design Flow Rate 

Table 1-Effluent Parameters for All POTWS w/a Flow > 0.1 MGD 

Ammonia (as N) Kjeldahl nitrogen (TKN) 

Chlorine (total residual, TRC) Oil and grease 

Dissolved oxygen Phosphorus (Total) 

Nitrate/Nitrite Total dissolved solids (TDS) 

Table 2--Effluent Parameters for Selected POTWS 

Hardness (as CaCO3) 

Metals (total recoverable), cyanide and total phenols 

Antimony Lead 

Arsenic Mercury 

Arsenic (Inorganic) * Nickel 

Arsenic (III) * Selenium 

Beryllium Silver 

Cadmium Thallium 

Chromium Zinc 

Chromium (III) * Cyanide (Free)* 

Chromium (VI) * Cyanide (Total) 

Copper Total phenolic compounds 

Iron (Total) * 

 

 

18 Please note that all parameters listed with a “*” are state water quality criteria not listed in 40 CFR 122 or 

in the EPA application forms. 
Reasonable Potential Analysis IMD Oregon Department of Environmental Quality 

DEQ 11-WQ-020-IMD 2/13/2012 

Version 3.1 Page 15 of 117 

Table 2--Effluent Parameters for Selected POTWS (Continued) 

Volatile organic compounds 

Acrolein 1,1-dichloroethylene 

Acrylonitrile 1,2-dichloropropane 

Benzene 1,3-dichloropropylene 

Bromoform Ethylbenzene 

Carbon tetrachloride Methyl bromide 

Chlorobenzene Methyl chloride 

Chlorodibromomethane Methylene chloride 

Chloroethane 1,1,2,2-tetrachloroethane 

2-chloroethylvinyl ether Tetrachloroethylene 

Chloroform Toluene 

Dichlorobromomethane 1,1,1-trichloroethane 

1,1-dichloroethane 1,1,2-trichloroethane 

1,2-dichloroethane Trichloroethylene 

Trans-1,2-dichloroethylene Vinyl chloride 

Acid-extractable compounds 

P-chloro-m-cresol 2-nitrophenol 

2-chlorophenol 4-nitrophenol 

2,4-dichlorophenol Pentachlorophenol 

2,4-dimethylphenol Phenol 

4,6-dinitro-o-cresol 2,4,5-trichlorophenol * 

Dinitrophenols * 2,4,6-trichlorophenol 

2,4-dinitrophenol 

Base-neutral compounds 

Acenaphthene 3,3-dichlorobenzidine 

Acenaphthylene Diethyl phthalate 

Anthracene Dimethyl phthalate 

Benzidine 2,4-dinitrotoluene 

Benzo(a)anthracene 2,6-dinitrotoluene 

Benzo(a)pyrene 1,2-diphenylhydrazine 

3,4 benzofluoranthene Fluoranthene 

Benzo(ghi)perylene Fluorene 

Benzo(k)fluoranthene Hexachlorobenzene 

Bis (2-chloroethoxy) methane Hexachlorobutadiene 

Bis (2-chloroethyl) ether Hexachlorocyclo-pentadiene 

Bis (2-chloroisopropyl) ether Hexachloroethane 

Bis (2-ethylhexyl) phthalate Indeno(1,2,3-cd)pyrene 

Bis (Chloromethly) ether * Isophorone 

4-bromophenyl phenyl ether Naphthalene 

Butyl benzyl phthalate Nitrobenzene 

2-chloronaphthalene N-nitrosodi-n-propylamine 

4-chlorophenyl phenyl ether N-nitrosodimethylamine 

Chrysene N-nitrosodiphenylamine 

Di-n-butyl phthalate Pentachlorobenzene * 

Di-n-octyl phthalate Phenanthrene 

Dibenzo(a,h)anthracene Pyrene 

1,2-dichlorobenzene 1,2,4-trichlorobenzene 

1,3-dichlorobenzene 1,2,4,5-Tetrachlorobenzene * 

1,4-dichlorobenzene 

 
Reasonable Potential Analysis IMD Oregon Department of Environmental Quality 

DEQ 11-WQ-020-IMD 2/13/2012 

Version 3.1 Page 16 of 117 

Table 3 – Pesticides, PCBs and Other Parameters w/Water Quality Criteria

Organochlorine Pesticides* 

Aldrin Endosulfan I (Alpha) 

Technical-BHC (Hexachlorcyclo-hexane)* Endosulfan II (Beta) 

alpha-BHC Endosulfan * 

beta-BHC Endosulfan sulfate 

delta-BHC Endrin 

gamma-BHC (Lindane) Endrin aldehyde 

Chlordane (tech) Heptachlor 

Chloropyrifos Heptachlor epoxide 

Dementon Malathion 

4,4 ́-DDD Methoxychlor 

4,4 ́-DDE Mirex 

4,4 ́-DDT Parathion 

Dieldrin Toxaphene 

PCBs* 

Total PCBs ** 

**A Total PCBs analysis is determined by monitoring for PCB Aroclors 1016,1221,1232,1242, 

1248, 1254 and 1260 in one sample event and totaling the values. 

Other Parameters with Sate Water Quality Criteria* 

Barium, Total Guthion 

Manganese, Total Nitrosamines 

Sulfide-Hydrogen Sulfide N-Nitrosodibutylamine 

2,4,5-TP [2-(2,4,5-Trichlorophenoxy) propanoic 

acid] N-Nitrosodiethylamine 

2,4-D (2,4-Dichlorophenoxy acetic acid) N-Nitrosopyrrolidine 

Dioxin 2,3,7,8-TCDD Phosphorus, Elemental 

2.2.4 Industrial Facilities: Evaluation and Monitoring Requirements 

Per 40 CFR 122.21 and DEQ guidance19, industrial facilities are required to monitor for 

specific parameters with a minimum frequency. The monitoring requirements at a 

specific facility are determined based upon industrial category, pre-existing permit status, 

hazardous material present, new source performance standards or permit writer 

discretion20. The guidance in Figures 2-4 and 2-5 (and Industrial Facility Pollutant 

Tables I, II, III and IV21) walks the permit writer through the process of identifying the 

facility’s monitoring requirements to address both the requirements included in EPA 

Forms 2C22, 2D23 and 2E24, and any additional state or federal requirements. Figure 2-4

 

19 Letters entitled “Toxic Monitoring Requirements for Industrial Wastewater Treatment Plants 

with NPDES Permits” dated January 6, 2006. 

20 e.g., discretion might include known environmental concerns in the receiving waters or 

industrial category that could impact, or be impacted by the facility’s activity. 21 Tables II, III, IV contains pollutant parameters that require monitoring and analysis if required or 

expected to be present. Table V contains pollutant parameters that would require identification 

only (no monitoring) if they are expected to be present. 

22 Existing manufacturing, commercial, mining and silvicultural dischargers, 40 CFR 122.21(g) 23 New Sources and New Dischargers per 40 CFR 122.21(k) 24 Facilities Which Do Not Discharge Process Wastewater per 40 CFR 122.21(h)(4) 
Reasonable Potential Analysis IMD Oregon Department of Environmental Quality 

DEQ 11-WQ-020-IMD 2/13/2012 

Version 3.1 Page 17 of 117 

addresses industries identified as “Primary Industry25”, and Figure 2-5 addresses “NonPrimary Industry”. The Industrial Facility Pollutants Tables in Section 2.2.5 are taken 

from 40 CFR 122, App. D and EPA Application form 2A, and are augmented with any 

state water quality criteria not reflected in these sources. The missing state water quality 

criteria have been inserted into the appropriate chemical group within the tables and 

would only require monitoring when the pollutant parameters are known to be present in 

the effluent. 

Due to the complexity of determining the appropriate industrial monitoring requirements, 

the permit writer should work directly with the industrial facility to ensure that all 

reporting requirements are included in the facility evaluation. This might include review 

of hazardous material inventories (SARA Title III, RCRA or State Fire Marshal records) 

combined with assessment of facility processes to determine if those pollutants have the 

potential to enter the effluent. Another good source of information for the permit writer is 

the Hazardous Substances Used, Stored, Produced of Transferred at a Facility that 

Indicate Probability of Toxicity Table located in 40 CFR 302.4. 

Typically, the pollutant scan monitoring will occur within the first two years of a permit 

term and each sample must be taken between four and eight months apart, as to be 

representative of seasonal variation. For a new permit application it might be necessary 

for the permit writer to conduct a Qualitative RPA when facility specific effluent 

monitoring data is not available. In these circumstances the permit writer should refer to 

guidance in the USEPA NPDES Permit Writers Manual and TSD. The permit writer 

should be aware of the small business and intake exemptions indicated in 40 CFR 122 

(g)(8), but in almost all cases these are not applicable or do not result in a meaningful 

reduction of required monitoring parameters. 

Federal application rules require a minimum of one analysis (grab sample or composite 

sample) for each pollutant parameter. To ensure a robust effluent characterization, the 

department requires facilities to collect a minimum of four composite samples (or grab 

samples for pH, temperature, cyanide, total phenols, residual chlorine, oil and grease and 

fecal coliform) for pollutant parameters with water quality criteria. Where it can be 

demonstrated that a facility has no history of pollutant discharge, the permit writer may 

introduce a sunset provision or reduce the monitoring requirements (min. of one analysis) 

in the permit, documenting the action in the permit evaluation report. 

 

 

25 Facilities with activities described in Table I of this IMD or Table 2c-2 of the EPA Form 2-C. 
Reasonable Potential Analysis IMD Oregon Department of Environmental Quality 

DEQ 11-WQ-020-IMD 2/13/2012 

Version 3.1 Page 18 of 117 

Figure 2-4 

Primary Industry Monitoring Requirements Determination Process 

All facilities must sample for all

“General Pollutants” 

Follow procedure (right) to identify 

which “Organic Toxic 

Pollutants” to monitor for 

1. Select industrial category, Table I

2. Determine pollutant fractions, Table I

3. List corresponding pollutants for 

selected fractions, Table II

4. List all other “known” Table II pollutants 

o If <10 & 100 ppb = quantitative 

data or “reason for discharge” 

• Min. of 4 analyses w/quantitative data 

• Min. of 1 analysis w/out WQ crit. 

• Part 122.21(g)(7)(v)(A)

All facilities must sample for all

“Toxic Metals, Cyanide & Total 

Phenols”

• Includes BOD, COD, TOC, TSS, NH3, 

Flow, Temp (w/s) and pH 

• As listed in Part 122.21 (g)(7)(iii) 

• Additional parameter: Alkalinity

• As listed in Table III 

• Min. of 4 analyses w/WQ crit., rec. 10 

• Additional parameter: Hardness 

• Part 122.21 (g)(7)(v)(B) 

All facilities must identify known 

“Conventional and NonConventional Pollutants” and 

provide information as directed 

and in 40 CFR 122.21 

• Identify known pollutants from Table IV

• All identified pollutants w/effluent limits 

(i.e. ELG, WQBEL, TBEL) 

o Provide quantitative data 

• All other identified pollutants, provide 

one of the following: 

o Quantitative data 

o Reason for discharge 

• Part 122.21 (g)(7)(vi)(A)

All facilities must identify known 

Certain Hazardous Substances 

& Asbestos and provide 

information as directed • Identify known pollutants from Table V

• Report reason for discharge and source 

• Report any available quantitative data 

• Part 122.21 (g)(7)(vii)

Start 

(40 CFR 122.21(g))

Required Tier 1 

Monitoring Parameters 

• See TMDL requirements 

TMDL required monitoring: 

For example >1.0 MGD facilities 

in Willamette Basin must sample 

for mercury

All facilities must sample for 

any pollutant parameter: 

• with current effluent limits 

• “known” or otherwise 

expected to be present in 

significant concentrations in a 

facility’s effluent 

• “known” to be present in 

significant concentrations in a 

facility’s source/intake water 

• Min. of 4 analyses for parameters 

w/WQ criteria 

• Min. of 1 analysis w/out WQ crit. 
Reasonable Potential Analysis IMD Oregon Department of Environmental Quality 

DEQ 11-WQ-020-IMD 2/13/2012 

Version 3.1 Page 19 of 117 

Figure 2-5 

Non-Primary Industry Monitoring Requirements Determination Process 

All facilities must sample for all

“General Pollutants” 

Follow procedure (right) to identify 

which “Organic Toxic 

Pollutants” to monitor for 

• List all “known” Table II pollutants 

o If <10 & 100 ppb = quantitative 

data or “reason for discharge” 

• Min. of 4 analyses w/quantitative data 

• Min. of 1 analysis w/out WQ crit. 

• Part 122.21(g)(7)(v)(A)

All facilities must sample for all

known “Toxic Metals, Cyanide & 

Total Phenols”

• Includes BOD, COD, TOC, TSS, NH3, 

Flow, Temp (w/s) and pH 

• As listed in Part 122.21 (g)(7)(iii) 

• Additional parameter: Alkalinity

• Identify all “known” Table III pollutants 

• Min. of 4 analyses w/quantitative data, 

rec. 10 

o If <10 & 100 ppb = quantitative 

data or “reason for discharge” 

• Min. of 1 analysis w/out WQ crit. 

• Additional parameter: Hardness, for 

Cd, Cr III, Cu, Pb, Ni, Ag or Zn 

• Part 122.21(g)(7)(v)(B)

All facilities must identify known 

“Conventional and NonConventional Pollutants” and 

provide information as directed 

and in 40 CFR 122.21 

• Identify all “known” Table IV pollutants 

• All identified pollutants w/effluent limits 

(i.e. ELG, WQBEL, TBEL) 

o Provide quantitative data 

• All other identified pollutants, provide 

one of the following 

o Quantitative data 

o Reason for discharge 

• Part 122.21 (g)(7)(vi)(A) All facilities must identify known 

“Certain Hazardous Substances 

& Asbestos” and provide 

information as directed • Identify known pollutants from Table V

• Report reason for discharge and source 

• Report any available quantitative data 

• Part 122.21 (g)(7)(vii)

Start 

(40 CFR 122.21(g))

Required Tier 1 

Monitoring Requirements

• See TMDL requirements 

TMDL required monitoring: 

For example >1.0 MGD facilities in 

Willamette Basin must sample for 

mercury

All facilities must sample for 

any pollutant parameter: 

• with current effluent limits 

• “known” or otherwise expected to 

be present in significant 

concentrations in a facility’s 

effluent 

• “known” to be present in 

significant concentrations in a 

facility’s source/intake water 

• Min. of 4 analyses for parameters 

w/WQ criteria 

• Min. of 1 analysis w/out WQ crit. 
Reasonable Potential Analysis IMD Oregon Department of Environmental Quality 

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Version 3.1 Page 20 of 117 

2.2.5 Industrial Facility Pollutant Tables 

The tables below are taken from Appendix D to 40 CFR, Part 122-NPDES Permit 

Application Testing Requirements.

Table I—Testing Requirements for Organic Toxic Pollutants 

by Industry Category

Industry category GC/MS fraction1

Volatile Acid Neutral Pesticide 

Adhesives and sealants X X X 

Aluminum forming X X X 

Auto and other (coin operated) laundries X X X X 

Battery manufacturing X X 

Coal mining 

Coil coating X X X 

Copper forming X X X 

Electric and electronic components X X X X 

Electroplating X X X 

Explosives manufacturing X X 

Foundries X X X 

Gum and wood (all subparts except D and F) X X 

Subpart D—tall oil rosin X X X 

Subpart F—rosin-based derivatives X X X 

Inorganic chemicals manufacturing X X X 

Iron and steel manufacturing X X X 

Leather tanning and finishing X X X 

Mechanical products manufacturing X X X 

Nonferrous metals manufacturing X X X X 

Ore mining (applies to the base and precious 

metals/Subpart B) 

 X 

Organic chemicals manufacturing X X X X 

Paint and ink formulation X X X 

Pesticides X X X X 

Petroleum refining X 

Pharmaceutical preparations X X X 

Photographic equipment and supplies X X X 

Plastic and synthetic materials manufacturing X X X X 

Plastic processing X 

Porcelain enameling 

Printing and publishing X X X X 

Pulp and paperboard mills See Pulp and Paperboard Mills Table

Rubber processing X X X 

Soap and detergent manufacturing X X X 

Steam electric power plants X X 

Textile mills (Subpart C—Greige Mills are 

exempt from this table) 

X X X 

Timber products processing X X X X 

X = Testing required 1The pollutants in each fraction are listed in Table II, 
Reasonable Potential Analysis IMD Oregon Department of Environmental Quality 

DEQ 11-WQ-020-IMD 2/13/2012 

Version 3.1 Page 21 of 117 

Pulp and Paperboard Mills Table 

40 CFR 430, 

Subpart*

GS/MS fractions

VOA Acid Base/neutral Pesticides 

A 2 1 2 1 

B 2 1 2 2 

C 2 1 2 2 

D 2 1 2 2 

E 1 1 2 1 

F 1 1 2 2 

G 1 1 2 2 

H 1 1 2 2 

I 1 1 2 2 

J 1 1 1 2 

K 1 1 2 2 

L 1 1 2 2 

1 = Must test 

2 = Do not test unless “reason to believe” it is discharged 

*Subparts are defined in 40 CFR Part 430

Table II—Organic Toxic Pollutants in Each of Four Fractions in Analysis by 

Gas Chromatography/Mass Spectroscopy (GS/MS) 

Volatiles

1V acrolein 2V acrylonitrile

3V benzene 5V bromoform

6V carbon tetrachloride 7V chlorobenzene

8V chlorodibromomethane 9V chloroethane

10V 2-chloroethylvinyl ether 11V chloroform

12V dichlorobromomethane 14V 1,1-dichloroethane

15V 1,2-dichloroethane 16V 1,1-dichloroethylene

17V 1,2-dichloropropane 18V 1,3-dichloropropylene

19V ethylbenzene 20V methyl bromide

21V methyl chloride 22V methylene chloride

23V 1,1,2,2-tetrachloroethane 24V tetrachloroethylene

25V toluene 26V 1,2-trans-dichloroethylene

27V 1,1,1-trichloroethane 28V 1,1,2-trichloroethane

29V trichloroethylene 31V vinyl chloride

Acid Compounds

1A 2-chlorophenol 2A 2,4-dichlorophenol

3A 2,4-dimethylphenol 4A 4,6-dinitro-o-cresol

5A 2,4-dinitrophenol 6A 2-nitrophenol

7A 4-nitrophenol 8A p-chloro-m-cresol

9A pentachlorophenol 10A phenol

11A 2,4,6-trichlorophenol
Reasonable Potential Analysis IMD Oregon Department of Environmental Quality 

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Version 3.1 Page 22 of 117 

Table II—Organic Toxic Pollutants (continued)

Base/Neutral

1B acenaphthene 2B acenaphthylene

3B anthracene 4B benzidine

5B benzo(a)anthracene 6B benzo(a)pyrene

7B 3,4-benzofluoranthene 8B benzo(ghi)perylene

9B benzo(k)fluoranthene 10B bis(2-chloroethoxy)methane

11B bis(2-chloroethyl)ether 12B bis(2-chloroisopropyl)ether

13B bis (2-ethylhexyl)phthalate 14B 4-bromophenyl phenyl ether

15B butylbenzyl phthalate 16B 2-chloronaphthalene

17B 4-chlorophenyl phenyl ether 18B chrysene

19B dibenzo(a,h)anthracene 20B 1,2-dichlorobenzene

21B 1,3-dichlorobenzene 22B 1,4-dichlorobenzene

23B 3,3′-dichlorobenzidine 24B diethyl phthalate

25B dimethyl phthalate 26B di-n-butyl phthalate

27B 2,4-dinitrotoluene 28B 2,6-dinitrotoluene

29B di-n-octyl phthalate 30B 1,2-diphenylhydrazine (azobenzene)

31B fluroranthene 32B fluorene

33B hexachlorobenzene 34B hexachlorobutadiene

35B hexachlorocyclopentadiene 36B hexachloroethane

37B indeno(1,2,3-cd)pyrene 38B isophorone

39B napthalene 40B nitrobenzene

41B N-nitrosodimethylamine 42B N-nitrosodi-n-propylamine

43B N-nitrosodiphenylamine 44B phenanthrene

45B pyrene 46B 1,2,4-trichlorobenzene

Pesticides

1P aldrin 2P alpha-BHC 

3P beta-BHC 4P gamma-BHC 

5P delta-BHC 6P chlordane 

7P 4,4′-DDT 8P 4,4′-DDE 

9P 4,4′-DDD 10P dieldrin 

11P endosulfan I (alpha) 12P endosulfan II (beta) 

13P endosulfan sulfate 14P endrin 

15P endrin aldehyde 16P heptachlor 

17P heptachlor epoxide 18P PCB- Aroclor 1242 

19P PCB- Aroclor 1254 20P PCB- Aroclor 1221 

21P PCB- Aroclor 1232 22P PCB- Aroclor 1248 

23P PCB- Aroclor 1260 24P PCB- Aroclor 1016 

25P toxaphene 

Table III – Other Toxic Pollutants (Metals and Cyanide) and Total Phenols

Antimony Nickel

Arsenic Selenium

Beryllium Silver

Cadmium Thallium

Chromium Zinc

Copper Cyanide (Free & Total)

Lead Phenols

Mercury Hardness (required to det. WQ criteria) 
Reasonable Potential Analysis IMD Oregon Department of Environmental Quality 

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Version 3.1 Page 23 of 117 

Table IV – Conventional and Nonconventional Pollutants Required To Be 

Tested by Existing Dischargers if Expected to be Present (in the effluent)

Aluminum, Total Malathion * 

Arsenic (III) * Manganese, Total 

Arsenic, Inorganic * Methoxychlor * 

Barium, Total Mirex *

BHC – Technical * Molybdenum, Total 

Boron, Total Nitrate-Nitrite 

Bromide Nitrogen, Total Organic 

Chloride Nitrosamines 

Chlorine, Total Residual N-Nitrosodibutylamine 

Chloromethly Ether, Bis * N-Nitrosodiethylamine 

Cobalt, Total N-Nitrosopyrrolidine 

Color Oil and Grease 

Chlorophenoxy Herbicide (2,4,5-TP) * Parathion * 

Chloropyrifos * PCBs, Total * 

Chromium (III) * Pentachlorobenzene * 

Chromium (VI) * Phosphorus, Elemental * &Total 

Dementon * Radioactivity 

2,4-D (2,4-Dichlorophenoxy acetic acid) * Sulfate 

Dioxin 2,3,7,8-TCDD * Sulfide 

Dinitrophenols * Sulfide-Hydrogen Sulfide 

Endosulfan * Sulfite 

Fecal Coliform Surfactants 

Fluoride Tetrachlorobenzene,1,2,4,5 * 

Guthion * Tin, Total 

Iron, Total * Trichlorophenol, 2,4,5 * 

Magnesium, Total Titanium, Total 

Please note that all parameters with a “*” are state water quality criteria not 

listed in 40 CFR 122 or in the EPA application forms (i.e. 2A, 2C)

Table V - Toxic Pollutants and Hazardous Substances Required To Be 

Identified by Existing Dischargers if Expected To Be Present

Toxic Pollutants

Asbestos 

Hazardous Substances

Acetaldehyde Crotonaldehyde 

Allyl alcohol 2,4-D (2,4-Dichlorophenoxy acetic acid) 

Allyl chloride Diazinon 

Amyl acetate Dicamba 

Aniline Dichlobenil 

Benzonitrile Dichlone 

Benzyl chloride 2,2-Dichloropropionic acid 

Butyl acetate Dichlorvos 

Butylamine Diethyl amine 

Captan Dimethyl amine 

Carbaryl Dintrobenzene 

Carbofuran Diquat 

Carbon disulfide Disulfoton 
Reasonable Potential Analysis IMD Oregon Department of Environmental Quality 

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Version 3.1 Page 24 of 117 

Table V – Continued: Hazardous Substances

Chlorpyrifos Diuron 

Coumaphos Epichlorohydrin 

Cresol Ethion 

Ethylene diamine Phenolsulfanate 

Ethylene dibromide Phosgene 

Formaldehyde Propargite 

Furfural Propylene oxide 

Guthion Pyrethrins 

Isoprene Quinoline 

Isopropanolamine 

Dodecylbenzenesulfonate Resorcinol 

Kelthane Strontium 

Kepone Strychnine 

Malathion Styrene 

Mercaptodimethur 2,4,5-T (2,4,5-Trichlorophenoxy acetic acid) 

Methoxychlor TDE (Tetrachlorodiphenylethane) 

Methyl mercaptan Trichlorofan 

Methyl methacrylate Triethanolamine dodecylbenzenesulfonate 

Methyl parathion Triethylamine 

Mevinphos Trimethylamine 

Mexacarbate Uranium 

Monoethyl amine Vanadium 

Monomethyl amine Vinyl acetate 

Naled Xylene 

Napthenic acid Xylenol 

Nitrotoluene Zirconium 

Parathion 

2.3 Pollutant Parameters Without Numeric Water 

Quality Criteria 

Following the preceding procedures will typically result in the selection of a number of 

pollutant parameters that do not have corresponding numeric water quality criteria26. 

These pollutants will be required to be included in the permit monitoring conditions. 

The RPA Workbook includes these pollutant parameters so that the permit writer may 

use the individual spreadsheet to identify all applicable monitoring requirements. Where 

visible on the Monitoring Required spreadsheet, these parameters are typically hidden

on the remaining spreadsheets. To unhide these pollutant parameters the permit writer 

should highlight the rows above and below the hidden row, right click the mouse and 

select “Unhide”. 

 

 

26 For example, Beryllium, Kjeldahl nitrogen, total chrome, etc. 
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2.4 Reporting Procedures 

The permit must require the permittee to submit a Summary Report (both electronic and 

paper versions) and copies of the laboratory analytic reports for all Tier 1 and subsequent 

Tier 2 effluent monitoring with a certification statement pursuant to 40 CFR 122.22(d). 

The Department will provide an electronic template to the permittee for developing the 

Summary Report, and the analytic report can be generated by either the analytical 

laboratory or the permittee as long as it meets the requirements described in this section. 

Typically, effluent data collected for Tier 1 or 2 monitoring will be submitted to the 

Department’s Inspector at the regional office for compliance purposes, and a duplicate 

data set sent to DEQ’s WQD OIS group to be entered into the Department’s reporting 

database, Discharge Monitoring System (DMS). The Summary Report must be submitted 

to the Department by the permittee within 30 calendar days after receipt of the analytical 

report for the final round of sampling. 

The permittee may use a letter with the certification statement to cover the submittal of 

the Summary Report and laboratory analytical reports. The letter should include: 

• Facility Identification (facility legal name, DEQ File and Permit Numbers). 

• Outfall identification. 

• Reference to the specific analytic report. 

• Signature from a responsible corporate officer (industrial), principal executive 

officer (domestic) or proprietor. 

• The following statement: 

I certify under penalty of law that this document and all attachments were 

prepared under my direction or supervision in accordance with a system 

designed to assure that qualified personnel properly gather and evaluate the 

information submitted. Based on my inquiry of the person or persons who 

manage the system, or those persons directly responsible for gathering the 

information, the information submitted is, to the best of my knowledge and 

belief, true, accurate, and complete. I am aware that there are significant 

penalties for submitting false information, including the possibility of fine and 

imprisonment for knowing violations. 

At a minimum, the analytic report should include: 

• Facility identification information and date of analysis. 

• Each pollutant parameters stated with exactly the same spelling as in the permit. 

• Corresponding Chemical Abstract Service number for each pollutant parameter. 

• The analytic method used for each pollutant parameter. 

• The Method Detection and Quantitation Limit for each pollutant parameter. 

• For all toxics, the reported analytic values in the units of micrograms per liter 

(ug/l). 

• If a sample result is at or below the detection level, report the result as less than 

the specified detection level. For example, if the detection level is 1.0 ug/L and 

the result is non-detect, report “<1.0 ug/L” on the analytic report. To calculate 

the mass load from this result, use the detection level. Report the mass load as 
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less than the calculated mass load. For example, if flow is 2 MGD and the 

reported sample result is <1.0 ug/L, report “<0.017 lb/day” for mass load on the 

analytic report (1.0 ug/L x 2 MGD x conversion factor = 0.017 lb/day). 

• If a sample result is above the detection level but below the quantitation level, 

report the result as the detection level preceded by the Department’s data code 

“e”. This code identifies the result as being between the detection level and 

quantitation level. For example, if the detection level is 1 ug/l and the 

quantitation level is 5 ug/L and the sample result is 4 ug/L, report “e1 ug/l” on 

the analytic report. To calculate the mass load from this result, use the detection 

level. Report the mass load as less than the calculated mass load preceded by 

“e”. For example, if flow is 2 MGD and the reported sample result is e1.0 ug/L, 

report “e0.017 lb/day” for mass load on the analytic report (1.0 ug/L x 2 MGD x 

conversion factor = 0.017 lb/day). 

• Quality Assurance/Quality Control information. 

The permittee is already required to maintain the sampling and analysis records 

including chain of custody and quality assurance/quality control information and make 

monitoring records available for inspection. 

2.5 Schedule B: Suggested Monitoring 

Requirement Language 

The department has developed suggested Schedule B permit language that includes 

recommended monitoring frequencies for most pollutant parameters. Various versions of 

this language applicable to different types of industrial and domestic facilities are located 

on the Permit Writers’ Corner of QNET under the “Permit Template” heading. 
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Section 3: Identify Pollutants of 

Concern 

3.1 Process Overview 

The next step of the RPA process (Step 3) is use the Tier 1 monitoring data to identify 

POCs in the effluent. The POCs are identified by statistically evaluating the data 

collected in the Tier 1 monitoring scan. The permit writer uses the Aquatic and Human 

Health RPA spreadsheets to project27 a maximum potential effluent concentration value 

(at the “end of pipe”). The calculated maximum concentration is then compared to the 

State Water Quality Criterion28. Any pollutant exceeding the water quality criterion is 

identified as a POC and will require additional analysis in the Steps 4 and 5, where the 

receiving water body is characterized and (optionally) additional effluent or collection 

system data is collected to determine in-stream reasonable potential. No further action is 

required for the remaining pollutant parameters, although they would be included in the 

monitoring requirements for the next permit cycle. Please refer to Figure 3-1 for an 

example of this step. 

3.2 Entering General Facility Information 

To begin using the RPA Workbook, enter the facility’s effluent flow and hardness data 

into the General Facility Information box at the top of the Aquatic RPA spreadsheet. 

Based upon the answer to Question 1 in the box, the spreadsheet is designed to either use 

a “dilution value” taken from a mixing zone analysis (in Question 4), or calculate a 

dilution value using general effluent and stream flow information (in Question 3). Please 

refer to the areas indicated by arrows on Figure 3-1. 

When calculating a dilution value in Question 3, refer to the guidance29 in the RMZ IMD 

Part 2 when determining the appropriate effluent values to enter into the “Eff. Flow Rate” 

portion of the information box. Although not necessary for this step, it is recommended 

that the balance of the ambient stream flow and dilution information (if available) also be 

entered at this time. 

 

 

27 The spreadsheets project a normal distribution curve about the available data points and the 

maximum projected value reflect a 95th or 99th percentile. 28 OAR 340-041-0033(2) Levels of toxic substances in waters of the state may not exceed the 

applicable (Aquatic Toxicity) criteria listed in Tables 20, 33A, and 33B. Additionally, 

concentrations may not exceed the currently effective Human Health Water Quality Criteria. 29 Section 4.4, Discharge Characteristics, p. 30. 
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Figure 3-1 

Example of the RPA Workbook 

Figure 3-1 Explanation 

Identify Pollutants of Concern

6 The PW enters the effluent flow rate and hardness values into the “General 

Facility Information” portion of the spreadsheet. 

7 The PW enters the “# of samples” collected, “Highest Effluent Conc.” (or 

“Effluent Conc.”) and “Coefficient of Variation” into the spreadsheet. 

8 The spreadsheet will calculate and report a “Calculated Max. Eff. Conc.” and 

compare it to the water quality criterion. 

9 If the calculated concentration exceeds the water quality criterion, then a “Yes” is 

reported in the “RP WQ at End of Pipe?” column. 

10 If the calculated concentration does not exceed the water quality criterion, then a 

“No” is reported in the “RP at End of Pipe?” column. 

11 The PW should check to ensure that there is a reported value for every pollutant 

parameter that is indicated as requiring evaluation or where monitoring data (“# of 

Samples” or “Highest Conc.”) has been entered.

?1 

?3 

?4 
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3.3 Analysis of Identified Parameters 

For each pollutant parameter with a “Yes” in the “Evaluation Required?” column, the 

permit writer will perform a preliminary analysis and fill in effluent monitoring data for 

the applicable parameter located under the heading “Identify Pollutants of Concern” in 

the workbook. The permit writer enters the number of samples taken during the 

monitoring period into the “# of Samples” column30. For the aquatic toxicity and noncarcinogen human health evaluations, the permit writer then enters the value (in ug/l) of 

the highest reported concentration during the monitoring period into the “Effluent 

Conc.” column. For the carcinogenic human health evaluations, the permit writer then 

enters the value of the geometric mean31 of the data into the “Effluent Conc.” column. 

In cases where all analytic results are “non-detect”, the permit writer should enter an 

“nd” into the “Effluent Conc.” column. Values should be entered into the spreadsheets 

according to the department’s significant figures guidance (Significant Figures IMD). 

Finally, the permit writer enters a Coefficient of Variance (CV) into the Column entitled 

“Coefficient of Variance”. If the number of samples of the data set is less than “10,” the 

permit writer should enter a default of “0.6”. If the number of samples is greater than or 

equal to “10,” the permit writer should calculate the CV using the following method. 

 

30 In addition to copies of the analytic reports, the permittee will submit a Summary Report of the 

analytic results using an electronic template. This will help to simplify the data entry process. 31 In the case of geomeans, values of “0” cannot be used. It is recommended that the permit 

writer use their professional judgment and either use a geometric mean where “ND = 1⁄2 of DL”, or 

take the mean of the values and multiply by a safety factor of 0.9. (ref. Nehls & Akland, 1973)

Example: How to calculate a CV from a data set

Given: I have the following values from the analytical reports 

1.0, 1.0, 1.0, 2.0, 1.0, 3.0, 2.0, 1.0, 3.0, 4.0, 5.0, 1.0 ug/l 

Answer: There are 12 data points, with a mean (or average) value of 2.1

(1.0+1.0+1.0+2.0+1.0+3.0+2.0+1.0+3.0+4.0+5.0+1.0) / 12 = 2.1 

To calculate the standard deviation, we compute the difference of each data point 

from the mean, and square the result: 

(1.0-2.1)2 = 1.2 (1.0-2.1)2 = 1.2 (3.0-2.1)2 = 0.8 

(1.0-2.1)2

 = 1.2 (3.0-2.1)2 = 0.8 (4.0-2.1)2 = 3.6 

(1.0-2.1)2

 = 1.2 (2.0-2.1)2 = 0.0 (5.0-2.1)2 = 8.4 

(2.0-2.1)2 = 0.0 (1.0-2.1)2 = 1.2 (1.0-2.1)2 = 1.2 

Next we divide the sum of these values by the “degrees of freedom” (n-1), and take 

the square root, which gives the standard deviation: 

[(1.2+1.2+1.2+0.0+1.2+0.8+0.0+1.2+0.8+3.6+8.4+1.2) / (12-1)] 1/2 = 1.4 

Therefore, the standard deviation is 1.4. Finally we divide the standard deviation by 

the mean, and round to arrive at a coefficient of variation of 0.7. 

CV = 1.3 / 2.1 = 0.6666... rounds to 0.7 

Note: When using a spreadsheet, there might be computational differences in the results 

when compared to rounding each calculation step. 
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3.4 Interpretation of Results 

The workbook will auto-calculate a statistical maximum effluent concentration and 

compare the result to each water quality criterion32. This is reported in the column 

entitled “Estimated Max. Eff. Conc.” If the estimated effluent concentration is equal to 

or exceeds the criteria, then a “Yes” will be entered into the column entitled “RP at end 

of Pipe?”. Pollutant parameters that exceed the criteria at the end of pipe will require 

ambient water characterization and further analysis in Step 3 of the RPA. This is 

illustrated in Figure 3-2. 

Pollutant parameters with a “No” entered into the “Exceed WQ” column do not exceed 

the water quality criteria at the end of pipe and do not require additional reasonable 

potential analysis. In instances where Tier 1 analytic results are “non-detect”, additional 

analyses are not typically required although the permit writer should review the analytic 

limits and consider additionally monitoring during the Tier 2 monitoring. The permit 

writer should document the evaluation of both the affirmative and negative results by 

placing a copy of the applicable workbook pages into the PER with a brief discussion. 

3.5 Additional Information: Narrative Criteria and 

Whole Effluent Testing 

It should be noted that a permittee will typically be required by 40 CFR 136 and EPA 

application forms to monitor for more pollutants than there are State Water Quality 

Criteria. Based on this data, a permit writer might use their discretion to require WET 

analysis (if it is not already required). The data might also be used as part of a Toxic 

Identification Reduction Evaluation (TIRE) that is mandated when an “affirmative” WET 

finding is made. Finally, the data might be used in the investigation of a beneficial use 

impairment not addressed through the RPA nor WET analysis. 

When required33, the WET analysis is conducted after the RPA and identifies instances of 

aquatic toxicity (not identified in the RPA) or synergistic effects. Whereas the RPA is 

designed to protect water quality using the State Water Quality Criteria, the WET 

analysis is the primary mechanism to protect water quality using the state’s Narrative 

Toxic Criteria34. In instances where there is evidence of beneficial use impairment, the 

 

32 In the instance of a RPA for Aquatic Toxicity, the maximum effluent concentration will be 

compared to water criterion for both the criterion continuous concentration (CCC) (chronic) and 

the criterion maximum concentration (CMC)(acute) and indicate a “Yes” if either are exceeded. 

In the instance of a RPA for Human Health, the maximum effluent concentration will be compared 

to the water criterion for both the Water + Fish Consumption and Fish Consumption criterion. 

33 For example, facilities with a design flow greater than 1 MGD are required to conduct WET 

testing. When possible, WET sampling should occur in conjunction with priority pollutant 

sampling. 

34 OAR 340-041-0033(1) Toxic substances may not be introduced above natural background 

levels in waters of the state in amounts, concentrations, or combinations that may be harmful, 

may chemically change to harmful forms in the environment, or may accumulate in sediments or 
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permit writer may use best professional judgment and the guidance values listed in Table 

33C to apply the Narrative Toxic Criteria. For additional information regarding WET, 

please contact the WET coordinator (located at the DEQ Lab) or Surface Water 

Management Technical Staff. 

Figure 3-2 

Example of Result Interpretation 

3.6 Next Step 

After the permit writer has completed the identification of the POCs, the permit writer 

will report the results, (Tier 2) monitoring requirements and recommendations to the 

permittee (Step 4). The permittee must then develop and implement a sampling plan to 

collect the necessary data during the following year. This is presented in Section 4 of the 

IMD. 

 

 

bioaccumulate in aquatic life or wildlife to levels that adversely affect public health, safety, or 

welfare or aquatic life, wildlife, or other designated beneficial uses.
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Section 4: Tier 2 Monitoring 

4.1. Tier 2 Monitoring Overview 

By the end of the second year (month 24) of the permit term, the permit writer should 

have identified the pollutant parameters subject to evaluation and then performed an endof-pipe reasonable potential screening to identify the POCs (Step 3). In Step 4, the 

permittee conducts Tier 2 monitoring to collect/submit the ambient water quality data and 

any additional effluent data for each identified POC that will enable the permit writer to 

model the impacts upon the receiving waters and complete the RPA in Step 5. 

Depending upon the potential source35 of the pollutant, a Source Investigation (SI) should 

be considered to support a later request for compliance alternatives such as an intake 

credit36, site specific background criterion and/or a compliance schedule or variance. The 

nature and scope of the SI should be a reflection of environmental issues and the type of 

compliance alternative being considered. (The role of the source investigation as it 

relates to these compliance tools is outlined in Section 4.3.3 and relevant IMDs.) 

Additionally, Tier 2 monitoring will be used to address cases where new water quality 

criteria have been recently promulgated or the receiving water body has recently been 

listed as “water quality limited.” This affords two opportunities (Tier 1 and 2) during a 

permit term to introduce new criterion or address recently listed water bodies into the 

permit development process. 

Finally, Tier 2 monitoring may be used to require the collection of “dissolved” or 

“fraction” pollutant concentration information for those cases where Tier 1 screening 

indicated significant concentrations of the “total” counterparts of the same pollutant 

parameters37. This is intended to allow use of low cost screening methods to identify the 

need for higher cost methods for select pollutant parameters. 

After evaluation of the Tier 1 data and identification of the POCs, the permit writer will 

issue a Monitoring Action Letter (MAL) reporting the POCs and requiring the permittee 

to develop a Sampling Plan and collect the data necessary to complete the RPA or 

support a compliance alternative (see Section 4.2). In cases where this data is already 

available or is not necessary38, the permit writer may decide to continue with the analysis 

without additional monitoring requirements. The permittee must submit the completed 

Sampling Plan to the permit writer for review, and upon approval, begin implementation 

in a timely manner. The permit writer should ensure that Sampling Plan meets the permit 

 

35 For example: Intake water, I&I, ground water etc. 36 Please refer to Appendix F for additional information on the use of Intake Credits 37 For example, cyanide, arsenic, chrome III and VI.

38 For example: Discharges to 303d listed waters where mixing zones are not permitted. Where 

data will be collected as part of a collaborative effort with other permittees. Where existing data is 

already available and is adequate for characterization. 
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requirements and adequately characterizes the facility’s effluent and the receiving water 

body. 

The timeline of the Tier 1 and 2 monitoring is designed to identify potential 

environmental issues early in the permit development process so that treatment, reduction 

or compliance alternatives can be explored, developed and approved by the end of the 

permit term. If the required or recommended Tier 2 monitoring data is not collected, then 

the Department may not have sufficient data to approve the use of the compliance 

alternatives. 

The permittee will report all effluent data using the procedures described in Section 2.4

of this document. For all other Tier 2 data (ambient, intake concentrations, etc.), the 

permittee will submit copies of the laboratory analytic reports with a certification 

statement to the Department’s Inspector at the regional office as described in Section 2.4. 

4.2. Monitoring Action Letter 

To initiate the Tier 2 monitoring, the permit writer will issue a MAL to the permittee 

reporting the results of the “end of pipe analysis” and identified POCs, and the 

requirement for the permittee to develop a Sampling Plan that addresses the flowing 

requirements and recommendations: 

Requirements 

• ambient characterization of the receiving water body for each identified POC 

o For aquatic toxicity pollutant evaluations, data containing a minimum 

of 4, 24-hour composite samples taken during the critical low flow 

period (i.e. dry period) must be provided. 

o For human health pollutant evaluations, data containing a minimum of 

4, 24-hour composite samples taken to characterize the annual average 

condition must be provided. 39

• if necessary, effluent and ambient characterization for pollutant fractions 

identified by screening analysis for total pollutants40

• if necessary, effluent and ambient characterization for any recently41

promulgated water quality criteria that are applicable to the permittee 

• if necessary, effluent and ambient characterization for any pollutant parameter 

for which the receiving water body has been recently listed as “Category 5, 

Water Quality Limited” on the 303 d list 

• all required monitoring and/or data submittal must be completed by the end of 

permit year three (month 36)42

 

39 Two samples should be taken in the wet season (Feb.) and two in the dry season (Sept.). 40 i.e. Inorganic As and As III, for total As 41 Since the issuance of the permit. 42 This date may be extended up to month 48, as long as there is available time to investigate and 

pursue environmental compliance options prior to the permit expiration date. 
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• the permittee must submit a data summary report and copies of all analytical 

laboratory reports to the permit writer by a specified date (~month 36-37). 

Recommendations 

• if necessary, additional effluent characterization for identified POCs 

• if necessary, a Source Investigation to identify pollutant sources and quantify 

mass loading in preparation for the use of an intake credit or site specific 

background pollutant criterion 

The permit writer will typically develop the MAL in consultation with the permittee to 

identify characterization or investigation recommendations that address any site-specific 

considerations. Template language for the MAL and Schedule B permit language is 

posted on the Permit Writer’s Corner on the QNET. 

4.3. Sampling Plan 

The main purpose of the Sampling Plan is to allow the permittee to efficiently consolidate 

the various monitoring requirements (see Section 4.3.1 and 4.3.2) into a site-specific 

monitoring plan. The secondary purpose is to integrate, when necessary, the elements of 

a source investigation (see Section 4.3.3) into the characterization process so as to make 

both efforts more robust and cost efficient. As described in the MAL, the permittee 

develops the plan in consultation with the permit writer. 

The requirement to develop a Sampling Plan, and to provide ambient data for POCs and 

effluent and ambient data for pollutant fractions is per a narrative statement in the 

Schedule B permit language and stems from the authority under 40 CFR 122.41(3)(h) 

(“duty to provide information”); 40 CFR 122.21, (“application requirements”); and state 

monitoring requirements per OAR 340-045-0015(5)(c) and (d). The permit writer also 

has the legal authorities for changing the frequency of the Tier 1 monitoring or reporting 

requirements per minor modification under 40 CFR 122.63 and OAR 340-045-

0055(2)(a)(B). Therefore, the permit writer should treat a permittee’s failure to submit a 

Sampling Plan or provide data per the approved plan, as a permit violation. Additionally, 

the failure to provide the required monitoring data may result in a more stringent RPA 

analysis and a limitation of available compliance options to address identified 

environmental issues. The permit writer will review the plan and provide written 

comments or approve within a timely manner. The permit writer should include the 

approved Sampling Plan and Departmental approval letter in the file. 

The sampling dates in the plan should ensure the completion of the required monitoring 

described in the plan by the end of year three (month 36)43. Accordingly, the plan must 

be developed, approved and the monitoring completed within one year after the POCs are 

identified. In cases where additional data is not required or already available to complete 

the RPA, a Sampling Plan and additional monitoring are not necessary. In these cases, 

 

43 This date may be extended up to month 48, as long as there is available time to investigate and 

pursue environmental compliance options prior to the permit expiration date. 
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the permit writer must document in the PER that all data necessary to complete the RPA 

has been collected and a Sampling Plan is unnecessary. 

4.3.1. Characterization of Receiving Water Body 

The permit writer should first determine if it is necessary to characterize the receiving 

water body for each POC. For waters that are listed as “water quality limited” (Cat 5) on 

the 303(d) list, the use of mixing zones and dilution values are generally not permitted for 

the listed pollutant parameters. Please refer to Section 5.5 and the RMZ IMD for 

additional information. If the water body is listed as having a completed TMDL (Cat 3), 

the TMDL will assign waste load allocations. Check the TMDL webpage for more 

information at http://www.deq.state.or.us/WQ/TMDLs/basinlist.htm. In these instances, 

the collection of ambient characterization data for the specific pollutant parameters is 

generally not required44. The permit writer should note any pollutant parameters for 

which ambient characterization is not necessary in the PER. 

For receiving water bodies not on the 303(d) list, it is necessary to characterize the 

ambient conditions for each POC during the permitted discharge periods (i.e., year round, 

summer-only). This data is used in conjunction with the effluent characterization data to 

conduct the RPA. The ideal is to use a robust historic dataset45 (see below) to 

characterize the water body, but when existing data is unavailable additional monitoring 

will be required. Per the monitoring requirements in the MAL, a minimum of 4 data 

points per flow period would be required. In cases where there are multiple POCs in 

different flow periods (i.e. aquatic toxicity and human health), the monitoring could be 

consolidated to a total of 6 samples with 4 in the critical period and 2 in the non-critical 

period. 

Upon permit writer approval, the permittee may submit alternative ambient monitoring 

data in lieu of conducting sampling, as long as the submitted data is geographically 

relevant and current, and possesses appropriate analytical limits. Alternative data might 

include ambient data from U.S. EPA’s STORET database, DEQ’s LASAR database, 

Washington Dept. of Ecology database or any other source of numeric or narrative data 

for the POC for the receiving water in the vicinity of the point of discharge. If there are 

several applicants discharging to the same reach of a receiving water body, chemical 

monitoring data may be derived from other applicants’ studies, or may be generated in a 

group monitoring study performed by multiple applicants discharging in the same 

receiving water environment. Please refer to Table 4-1 for data sources and internet 

links. The permit writer should describe the basis of alternative methodologies in the 

PER. 

 

44 The Permit Writer might consider requiring ambient monitoring data if they believe that the 

particular river segment is not exceeding water quality criteria. In these cases, the data could be 

submitted to the LASAR data base and included in the next triennial state water quality 

assessment. This might allow for further segmentation of the water body, and the potential that a 

mixing zone might be permitted in the future if assimilative capacity is available. 

45 For example, if a TMDL has been issued or updated, or DEQ has conducted a watershed 

assessment since the last permit renewal, there may be more current data available. 
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Table 4-1 

Water Quality and Stream Flow Databases 

Data 

Source 

Data Type Internet Link 

OR. DEQ Misc. WQ 

Data http://deq12.deq.state.or.us/lasar2/

OR DEQ 303d List / 

TMDL List http://www.deq.state.or.us/wq/assessment/assessment.htm

NOAA, 

NW River 

Forecast 

Center 

Stream 

Flow Data, 

Forecasting 

http://www.nwrfc.noaa.gov/index.shtml

USACE, 

Columbia 

Basin 

Temp., 

TDG and 

Flow data 

http://www.nwdwc.usace.army.mil/TMT/wqwebpage/mainpage.htm

USEPA, 

STORET 

Misc. WQ 

Data http://www.epa.gov/storet/index.html

USGS Misc. WQ 

Data http://waterdata.usgs.gov/nwis

WA. DOE Misc WQ 

Data http://www.ecy.wa.gov/databases/wq.html

For new permit applications, it might be necessary for the permit writer to use 

representative data to conduct the RPA. This can include projected ambient data using a 

representative sample location, data from other, similar water bodies, partial monitoring 

data sets, etc. If the permit writer can’t make a reasonable projection, it might be 

necessary to proceed with the permit using an expedited monitoring plan and re-opener 

clause, or a final effluent limit paired with a compliance schedule. 

4.3.2. Additional Effluent Characterization 

The permit writer should conduct the RPA based upon a robust data set with a minimum 

of ten sample points (4 (Tier 1) + 6 (Tier 2) = 10). This will allow better characterization 

of the effluent and the ability to calculate a facility-specific coefficient of variation, 

resulting in a more representative RPA. Since the Tier 1 monitoring only requires a 

minimum of four samples per pollutant, additional monitoring will be required to provide 

a robust data set for the universe of RPAs identified by Tier 1 monitoring. 

Depending upon the results from the Tier 1 monitoring, the permit writer may send a 

permit action letter (as described in Section 4.2) requiring the permittee to collect 

additional effluent monitoring data (a minimum of 6 data points) for each POC. The 

permit writer might also require additional effluent sampling for any pollutant if there 

were issues meeting the Quantitation Limits, concerns regarding “false positives,” or any 

other implementation issue encountered during the Tier 1 monitoring. If it is determined 
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that the Tier 1 monitoring was adequately robust46 to conduct the RPA, the permit writer 

might allow additional effluent monitoring to be omitted from the sample plan. 

4.3.3. Source Investigation 

Depending upon knowledge of the permittee’s facility and the identified POCs, the 

permit writer may recommend the permittee to conduct a source investigation of the POC 

to identify the pollutant’s source, quantify the mass transported and determine the 

ultimate fate. This information would be used to support a request for compliance 

alternatives such as intake credits, variance, site specific water quality criteria, etc. 

Without this source investigation data being collected in a timely manner, the permit 

writer will have limited options for calculating WQBELs into the next permit renewal 

that the permittee can comply with. 

A Source Investigation should, at minimum, identify the source (or sources) and quantify 

the mass loading rates for the pollutant of concern. The scope of the source investigation 

should depend upon the type and magnitude of the potential environmental impact. The 

permittee and permit writer should review the requirements for the use of an intake credit 

described in Appendix F or for the calculation of a Site Specific Background Pollutant 

Criterion (described in future IMD). The source investigation can be developed in an 

iterative manner that relies upon periodic findings and milestones to determine the need 

for additional monitoring. 

The permit writer should encourage permittees to identify and use any existing ambient 

or intake monitoring databases as part of the source investigation. Examples include 

municipal pretreatment program information and the Department of Health Services

(DHS) drinking water monitoring databases. 

4.4. Next Steps 

By the end of year three, the permit writer will use the results from the Tier 1 & 2 

Monitoring in calculating the RPA in the Step 5 and, if applicable, determining effluent 

limits in Step 6. At the end of year five, the RPA and WQBELs (if applicable) will be 

finalized for incorporation into the renewed permit. 

 

 

46 ie. More than one sample detected above the Detection Limit 
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Section 5: Reasonable Potential 

Analysis 

5.1 Reasonable Potential Analysis Overview 

Once the Sampling Plan (Tier 2 monitoring) has been implemented and the appropriate 

effluent and ambient characterization data for each identified POC have been collected in 

Step 4, the permit writer should be ready to enter the information into the RPA 

Workbook and conduct the initial RPA in Step 5. At the end of the permit term and as 

part of the permit renewal, the permit writer will conduct a final RPA using any 

additional characterization data. 

To conduct the initial RPA, the permit writer will use the partially completed RPA 

Workbook that was last used to identify POCs in Step 3. The permit writer would then 

use the Summary Report of analytic results or retrieve collected effluent and ambient 

characterization data from the Department’s DMS47 or LASAR databases to complete the 

RPA Workbook. The permit writer will then enter any additional General Facility 

Information, revised effluent characterization values48 and calculated Coefficients of 

Variation (if applicable). Finally, they will then enter the Ambient Concentration values 

and the RPA workbook will process the calculations and report the RP status. This is 

illustrated in Figure 5-1. 

The permit writer should proof check all data entries and ensure that the RPA Workbook

made all required calculations. The RPA spreadsheet pages are linked to the 

corresponding Aquatic Toxicity and Human Health Limit spreadsheet pages and data 

will automatically be transferred to the limit calculation page of the workbook in Step 6. 

A summary of the values used in the various RPA calculations is described in Table 5-1

and each is discussed through the rest of this section. In the event that a facility has a 

non-typical discharge49, the permit writer should consult the discharge characterization 

guidance described in Section 4.4 of the RMZ IMD Part 2. 

Where adequate data is available and the minimum conditions have been met, the permit 

writer may consider conducting an intake credit analysis. This may change the outcome 

of the RPA or adjust any calculated WQBELs to reflect intake pollutant concentrations. 

Guidance is available in Appendix F and an analysis module is included on the Aquatic 

Toxicity and Human Health RPA spreadsheet pages of the RPA Workbook. 

 

 

47 DMS and tutorial information are available on the Qnet: http://deqapp1/dms/default.aspx. 48 Ie. Number of samples, highest/mean concentration, and coefficient of variance. 49 i.e. Wet Season only discharge, marine/estuarine discharges or facilities operating between 

85% and 100% of capacity. 
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Figure 5-1 

Example of the RPA Workbook 

 

Figure 5-1 Explanation 

12 The PW enters any remaining values into the “General Facility Information” 

portion of the spreadsheet. 

13 Where additional effluent characterization was required in the Sampling Plan, the 

PW enters the updated “# of samples” collected, “Highest (sample) Conc.” and 

calculated “Coefficient of Variation” into the spreadsheet. 

14 The spreadsheet will re-calculate and report a “Calculated Max. Eff. Conc.” and 

compare it to the water quality criterion. 

15 The PW should check each revised parameter to determine if there is still a 

potential to exceed WQ criteria (at end of pipe). 

16 If there is no longer a potential to exceed the WQ criteria, the “Yes” reported in the 

“Exceed WQ at End of Pipe?” column would have changed to “No”, and no further 

action is required for that pollutant parameter.

General Facility Information 

Revised 

Data 
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5.2 General Facility Information 

The General Facility Information box (Figure 5-2) contains the general facility and 

receiving water characterization information such as fresh/marine status, flow rates, 

available dilution, hardness and the statistical values used in the calculations. The permit 

writer needs to complete this information to conduct the RPA. Please note that the boxes 

for the Aquatic Toxicity and Human Health RPAs have slight differences in ambient 

flow rates used and the need for harness information. 

Figure 5-2 

General Facility Information 

5.2.1 Flow Rates 

Depending upon the type of RP analysis being conducted (i.e. aquatic toxicity vs human 

health), various ambient and facility low flow conditions are used in the analysis. The 

flow conditions used are: 

• 1Q10: The minimum 1-day flow which occurs once in 10 years on average 

• 7Q10: The minimum 7-day flow which occurs once in 10 years on average 

• 30Q5: The minimum 30-day flow which occurs once in 5 years on average 

• Harmonic Mean Flow: This is the number of daily flow measurements divided by the 

sum of the reciprocals of the flows (that is, the reciprocal of the mean of reciprocals). 

• Geometric Mean: is a type of mean or average, which indicates the central tendency of a 

set of numbers where the numbers are multiplied and then the nth root of the resulting 

product is taken. The geometric mean of a data set is given by: 

• Average Dry-Weather Design Flow: (ADWDF) This is considered a “base flow” 

condition for a sewer collection area that is exclusive of additions due to inflow and 

infiltration or storm water additions. 
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In some cases, adequate data to calculate these flow conditions might not be readily 

available. There are a variety of accepted hydrologic and statistical approaches that might be 

used to project, interpolate, correlate or represent a data set to calculate these conditions. 

When used, a description of the hydrologic/statistical approach used should be included 

in the PER and, if necessary, requirements to collect additional data in the permit 

condition. 

Table 5-1 

Summary of RPA Variables 

RPA 

Type

Effluent 

Conc. Ambient Conc. Effluent Flow Rate

Ambient 

Flow Rate

%Confidence

/ %Probability

Aquatic 

Life: 

Acute 

Max Conc. 

• 1-3 samples: 

Max. Conc. 

• >4 samples: 

90th percentile 

See RMZ IMD 

Part 2, Sec. 4.4 1Q10 99% / 95% 

Aquatic 

Life: 

Chronic 

Max Conc. 

• 1-3 samples:

Max. Conc. 

• >4 samples: 

90th 

percentile 

See RMZ IMD 

Part 2, Sec. 4.4 7Q10 99% / 95% 

Human 

Health 

• Carcinogens

• Geometric 

MeanNonCarcin.

Max Conc. 

Geometric Mean 

• Carcinogens

Average 

Annual Flow 

• Non-Carcin.

ADWDF 

• Carcinogens

Harmonic 

Mean Flow 

• Non-Carcin.

30Q550

NA 

5.2.2 In-Stream Dilution 

The RPA is essentially a simple in-stream water quality model that estimates the impacts 

of an effluent upon a receiving water body. One of the key variables in this model is the 

available dilution. The RPA Workbook allows for two methods of incorporating the 

available dilution. The first is to use a factor derived from a Mixing Zone Analysis per 

departmental guidance (Regulatory Mixing Zone IMD). The second is to use a default 

factor calculated directly from the ratio of discharge and stream flow rates, with an 

allowance fraction51. The General Facility Box has a series of questions and answers 

that, depending on how the permit writer answers, can facilitate either method. The 

permit writer must place a “y” or “n” in the appropriate box (Question #1) to tell the 

spreadsheet which method to employ. 

Most major, and many minor facilities, should have conducted a mixing zone study to 

determine the available dilutions at the edge of the zone of initial dilution and the edge of 

the mixing zone. This analysis must be conducted in accordance to the Department’s 

guidance and will result in the calculation of a series of dilution factors specifically for 

 

50 If the effects from certain non-carcinogens are manifested after a lifetime of exposure, then 

harmonic mean flow may be appropriate, (EPA TSD, p. 89) 51 In both cases a mixing zone is established, but the difference lies in how the dilution factors are 

determined. 
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use in Aquatic Toxicity and Human Health RPAs. Permit writer should enter mixing 

zone factors into the indicted spaces (Question #4). These factors will typically be listed 

in the Mixing Zone Analysis Report. 

If no mixing zone analysis is available, the RPA Workbook has the ability to calculate 

available dilution using the Design Flow52 of the facility and various stream flow 

conditions (Question #3). A summary of the various flow rates and conditions used for 

the RPAs is included in Table 5-1. The permit writer should refer to the department’s 

mixing zone guidance for an explanation of how to calculate the various flow values. 

The databases listed in Table 4-1 are sources of stream flow data that can the basis of 

flow value calculations. The permit writer might also contact the regional Plan Review 

Engineer for additional information regarding facility flow performance. 

 

 

52 The flow that the facility was built to handle 

Primer on Regulated Mixing Zones 

A regulatory mixing zone (RMZ) is an area defined in an NPDES permit where: 

1. A discharge undergoes initial dilution and mixing in the receiving stream; 

2. Water quality standards can be suspended for a short distance downstream of a 

discharge provided several conditions are met (see RMZ IMD Part 1); and 

3. Mixing zones are designed to be protective of human health, aquatic habitat and 

the water body as a whole. 

A RMZ is comprised of a both a chronic and acute mixing zone. The chronic mixing 

zone is the area encompassed by the entire RMZ and the water quality criteria for 

both aquatic toxicity (chronic) and human health must be met at the boundary. 

The acute mixing zone or “Zone of Immediate Dilution” (ZID) is the area immediately 

surrounding the outfall and within the RMZ. In this area, the acute and chronic aquatic 

life criteria may be exceeded as long as lethal impacts prevented. 

Example of Regulatory Mixing Zone (RMZ) for a River 

 

Zone of immediate dilution (ZID) 

Discharge Plume 

RMZ length 

RMZ width 
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5.2.3 Fresh/Marine Water Status 

The state toxic water quality criteria vary based upon the type of water body; marine or 

freshwater. The permit writer must place a “y” (for fresh water) or “n” (for marine water) 

in the appropriate box (Question #2) to tell the spreadsheet which set of water quality 

criteria to employ. The permit writer should request technical assistance when the 

salinity of the receiving water can not readily be determined (ie. the receiving water is 

between 1 and 10 ppt).53 

5.2.4 Hardness 

For most metals, the state aquatic toxicity water quality criteria are dependent upon the 

hardness of the receiving water. Therefore, it is necessary to enter effluent and ambient 

hardness information to calculate the water quality criteria and model the effluent’s 

impact upon the receiving waters. Effluent hardness data must be included with the 

applicants monitoring data. Ambient hardness data can usually be found on the 

Department’s LASAR or other governmental database (see Table 4-1). Data entered 

should be reflective of the average effluent and ambient flow conditions being modeled. 

For example, a permit writer should calculate the average ambient hardness from data 

taken during the dry season, low flow condition or other appropriate seasonal condition 

based on the mixing zone study critical time period. 

 

 

53 It should be noted that EPA’s WET testing protocols are also salinity based but used different 

salinity levels than state water quality critieria. 

Fresh and Marine Waters Determination Process 

The following salinity values are used to determine the status of the receiving water. 

Geographic salinity data is available on the Department’s LASAR database and on the 

QNET: Permit Writers Corner

• All waters with a salinity less than 1 parts per thousand (PPT) of salinity are 

considered fresh waters and use the fresh water criteria. 

• All waters with a salinity greater than 10 PPT of salinity are considered marine 

waters and use the marine water criteria 

• All waters with salinity between 1 and 10 PPT of salinity must meet the more 

conservative of the fresh or marine water quality criteria 
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5.2.5 Statistical Calculation Values 

The RPA calculates a log-normal distribution curve from the effluent data and then 

projects an “Estimated Maximum Effluent Concentration”

 54 at a specified Probability 

Basis and Confidence Level (i.e. Figure 5-2). The Probability Basis reflects the upper 

boundary of the effluent distribution curve and the Confidence Level indicates the 

reliability of the estimated maximum effluent concentration. The statistical values 

recommended by the Department are included, by default, in the RPA Workbook. A 

permit writer may use best professional judgment to determine alternative values and the 

decision criteria should be included in the PER. 

The values used in the Aquatic Toxicity and Human Health Analysis are summarized in 

Table 5-1. For a more detailed discussion of the statistics used in the RPA, please refer 

to Section 3.3 of the TSD. 

Figure 5-2 

Example of a Log-Normal Distribution of Effluent Data 

5.3 Revised Effluent Characterization Data 

Working from the partially completed RPA spreadsheets from Step 3, the permit writer 

should revise the data entered for those pollutant parameters identified as a POC and 

where Tier 2 monitoring information was collected or located. Typical revisions include 

an update to the “# of Samples”, reevaluation of the Maximum Effluent Concentration

(Aquatic Toxicity), re-calculation of the Effluent Concentration (Human Health) and the 

calculation of a Coefficient of Variation55. 

 

54 “The estimated maximum concentration is the upper bound of the expected lognormal 

distribution of effluent concentrations at a high confidence level. 55 See Section 3.3 of this IMD for information in calculation of a CV 

Est. Max 

Conc. 
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Once completed, the spreadsheets will automatically re-calculate the Estimated Maximum 

Effluent Concentration. If the estimated effluent concentration is equal to or exceeds the 

water quality criteria, then a “Yes” will be entered into the column entitled “Exceed WQ 

at end of Pipe?”. Pollutant parameters that exceed the criteria at the end of pipe require 

ambient water characterization. Note: The Permit Writer should maintain electronic and 

paper copies of both the partially completed RPA Workbook and revised version for the 

record. 

5.4 Data Interpretation 

Since the RPA is a statistical analysis that requires multiple data points, it is subject to 

greater uncertainty when based upon a single or relatively few data points. Due to the 

large number of water quality criteria that are below available analytic limits, this can be 

a relatively common occurrence. This is part of the reason that additional monitoring is 

typically required for identified POCs. Even after additional data is collected in the Tier 

2 monitoring, there might only be one, two or three detects out of a sample set of ten. In 

these instances, the permit writer might consider contacting Surface Water Management 

for technical assistance in using one of the following alternative RPA processes: 

• Single data point: Since a distribution curve cannot be developed around a single 

data point, the permit writer should utilize a “Qualitative RPA” according to the 

guidelines discussed in Section 6.3.3 of the USEPA’s NPDES Permit Writers 

Manual (Sept. 2010) and Section 3.2 of the TSD. 

• Two to three data points: Based upon permit writer discretion, an alternative 

statistical analysis technique may be considered. For example, the TSD (p. E-10) 

recommends the use of a Delta-Lognormal Distribution analysis in cases when the 

data contain a mixture of non-detect values and values above the detection limit. 

Other methods include Maximum Likelihood Estimation (MLE) or Regression on 

Order Statistics (ROS) 

The permit writer should also consider the potential of “false positives” due to sampling 

error or upset facility condition when evaluating characterization data. 

• False positive: If there is conclusive information that a data point was a false 

positive due to laboratory error, the permit writer may remove the data point from 

the analysis and document the decision in the PER. 

• Upset condition: If there is conclusive information that a data point was a false 

positive due to an upset condition, the permit writer should work with the 

permittee to develop and implement mitigation or operational practices to ensure 

that the upset condition does not re-occur. The terms of this should be included in 

the PER. The data point may then be removed from the analysis. Later, if 

another monitoring event detects a similar upset condition, even after the 

implementation of the mitigation or operation practices, it should be considered 

representative of the effluent and used in subsequent analysis. 
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5.5 Ambient Concentration 

The goal of the ambient monitoring is to determine the condition of the receiving water 

body and identify the available assimilative capacity for each POC. Assimilative 

Capacity is the capacity of a natural body of water to receive wastewaters or toxic 

materials without exceeding water quality criteria. In cases where there is assimilative 

capacity, Departmental regulations may allow for the designation of a Regulatory Mixing 

Zone (RMZ). The RMZ is a small area around the outfall where suspension of water 

quality criteria are allowed as long as lethality is prevented and the integrity of the water 

body as a whole is protected. When a RMZ is permitted, the RPA is calculated at the 

regulatory boundary of the RMZ. 

In cases where the ambient pollutant concentrations exceed the corresponding water 

quality criteria, no assimilative capacity is available and a RMZ is not permitted for that 

particular pollutant. In cases where the ambient pollutant concentrations are below the 

corresponding water quality criterion, assimilative capacity is available and a RMZ might 

be allowed per the Department’s mixing zone guidance (Regulatory Mixing Zone IMD). 

In cases where the stream segment of the receiving water body has been listed on the 

303(d) list as “Water Quality Limited” (Cat 5), even if data indicates that assimilative 

capacity is available, RMZs may not be permitted56. The implications of this is that any 

POC discharged to a water body listed as water quality limited for the same pollutant will 

have reasonable potential and require an effluent limit calculated to meet the water 

quality criterion at the end of pipe. 

Once the background concentrations are put into the RPA spreadsheet under the “ambient 

concentrations” column, the spreadsheet automatically calculates whether assimilative 

capacity is available and, if allowed, factors in dilution data to determine the Maximum 

Effluent Concentration at the acute (ZID) and chronic (RMZ) mixing zone boundaries. 

This maximum concentration is essentially the total concentration value when the 

ambient pollutant concentration and effluent concentrations are combined and allowed to 

mix within the ZID and mixing zone. 

 

 

56 In these instances where assimilative capacity is available, the permit writer should consider 

supplying ambient monitoring data to the DEQ Lab for inclusion into the LASAR database. This 

data would subsequently be evaluated as part of the Sate-wide Water Quality Assessment and 

might lead to a segmentation of the listed water body. This might result in the “Water Quality 

Limited” status being limited to only the segments where the criteria are actually being exceeded.
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5.5.1 Data Entry Instructions 

The permit writer would enter the data collected in the Tier 2 Monitoring into the RPA 

Workbook column entitled “Ambient Conc.” using the following guidelines: 

In cases where the receiving water body is listed on the 303d list as “Water Quality 

Limited” for a POC, the permit writer should set the “Ambient Conc.” to equal either the 

highest water quality criteria value or, an even higher recorded ambient concentration. 

This will effectively remove all dilution from the calculation. The permit writer should 

make a note of the actual ambient value in the “RPA Run Notes:”

57. This is demonstrated 

in Figure 5-3. 

 

 

57 To set the ambient concentration equal to the water quality criteria, the permit writer should 

enter a “=” followed by the cell number of the highest criterion in the “Ambient Conc.” cell. 

Ambient Data guidelines for use in RPA Calculation 

• Aquatic Toxicity Analysis 

3⁄4 For 4 or more data points, the permit writer would enter 

the 90th percentile of the data range 

3⁄4 For less than 4 data points, the permit writer would enter 

the most conservative value

• Human Health Analysis 

3⁄4 The permit writer should calculate and enter the 

geometric mean
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Figure 5-3 

Example of 303(d) Procedure 

 

Figure 5-3 Explanation 

17 For each remaining POC, the PW enters the appropriate value for ambient 

pollutant concentration into the “Ambient Conc.” Column. 

18 If the receiving stream segment is 303(d) listed for an POC then no mixing zone 

is permitted. The ambient value must be set to a value equal to or greater than 

the highest water quality criteria. See example. 

19 The PW should describe any changes due to listing status in the RPA Run Notes 

of the spreadsheet and PER. 

20 The spreadsheet will then calculated the pollutant concentrations at the various 

mixing zone boundaries (if allowed) and RP status. 
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5.6 Comparison of Maximum Concentration to 

Water Quality Concentration 

In the case of the human health toxicity calculation, a maximum in-stream concentration 

will be calculated. If there is available assimilative capacity and a mixing zone is 

allowed, the calculated maximum concentration will reflect dilution and be calculated to 

reflect conditions at the boundary of the mixing zone (RMZ). The aquatic toxicity 

spreadsheet will employ a similar calculation, with the addition of a calculation to 

determine the concentration at the boundary of the acute mixing zone (ZID). The 

spreadsheet will then compare each calculated maximum concentration to the applicable 

water quality criteria and indicate the results in the “Is there Reasonable Potential to 

Exceed” column. A report of “yes” indicates that there is reasonable potential and a 

report of “no” indicates that there is not reasonable potential. An example of an aquatic 

toxicity calculation is illustrated in Figure 5-4, below. 

Figure 5-4 

Example of RPA analysis 
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5.7 Troubleshooting 

Since all the calculation variables have already been entered, there is little to do in this 

step other than trouble-shoot the process and evaluate the results. When receiving water 

body information is entered into the General Facility Information section, the applicable 

water quality criteria will be calculated and reported in the “WQ Criteria” columns. If 

the word “data” appears in the columns, either dilution or hardness data is missing. If a 

“0” appears in the column, a value less than zero was calculated and the field is not set to 

report to the appropriate decimal point. If there is not an acute or chronic water quality 

criterion for a particular pollutant parameter, a “na” will be reported in the applicable 

field. 

5.8 Conclusions 

Before completing the RPA and calculating effluent limits, the permit writer should 

evaluate the results to ensure accuracy. Probably the most relevant question is “do the 

results make sense?” Line 1 in Figure 5-5 below is a good example. Here the maximum 

concentration at the boundary of the mixing zone (RMZ) was reported as 0.00 ug/l as 

compared to the reported concentration of 0.0 ug/l, resulting in “yes” for reasonable 

potential to exceed the chronic criteria. In this case it would be prudent to check the 

calculated water quality criteria value by expanding the number of decimal points 

reported in both fields. This would result in a maximum concentration of 0.0013 ug/l vs. 

the water quality criteria of 0.0010 ug/l58. 

Figure 5-5 

Example of RPA Evaluation 

 

58 Although the calculated maximum concentration (0.0013 ug/l) was greater than the calculated 

water quality criteria (0.0010 ug/l), these results might be within the range of analytic variability 

and the permit writer should consult the department’s policy on rounding and significant digits 

when ultimately determining whether there is reasonable potential. 

Line 1
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In some cases where RP is indicates, the permit writer may consider conducting an intake 

credit analysis. As long as a number of statutory conditions are met, the analysis allows 

the permit writer to consider pollutant concentrations originating from the municipalities 

or industries source water as a credit when determining RP or determining compliance 

with a WQBEL. Please refer to Appendix F for guidance on the use and calculation of 

an intake credit. The data required for this analysis is similar in scope to the 

requirements for calculating a Site Specific Background Pollutant Criterion. At this 

point, the permit writer should keep this option in mind, but the actual application of the 

criterion approach should not be undertaken until after a WQBEL has been calculated in 

Step 6. The criterion approach would then determine an alternative criterion, upon which 

a new RPA and WQBEL calculation would be conducted. 

5.9 Next Steps 

Once the permit writer has evaluated and confirmed the results of the RPA, they should 

determine the applicable effluent limit as described in the next Section. The RPA 

Workbook has been designed to link the RPA spreadsheet with the effluent limit 

calculation spreadsheet. The general facility information and ambient/effluent water 

quality data will be automatically transferred to the effluent limit calculation spreadsheet. 
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Section 6: Effluent Limit Calculation 

and Determination 

6.1 Effluent Limit Determination Process 

Overview 

Once the permit writer has conducted the RPA and determines that a facility has 

reasonable potential to exceed the in-stream water quality criteria, Step 6 is to calculate 

the Waste Load Allocations59 (WLA) and derive Water Quality Effluent Limits 

(WQBELs). Much of this process occurs automatically as relevant data is transferred 

from the RP spreadsheets to the Effluent Limits spreadsheets of the RPA Workbook60. 

Because of the statistical variability of the effluent concentration and the lack of a 

probability basis, it is necessary to translate the WLAs into WQBELs to ensure proper 

implementation and enforcement. In 303-d listed stream segments where a formal 

TMDL has been completed and WLA’s assigned, the permit writer must translate the 

TMDL’s WLAs into WQBELs. In some cases, it might be possible to use an intake 

credit in determining compliance with the calculated WQBEL. 

Once calculated, the permit writer would compare the derived limits (WQBEL or TMDL 

based) with any applicable Technology Based Effluent Limits (TBEL), and select the 

value that is most protective of the receiving water body. The elements of this process 

are presented in Figures 6-1. 

6.2 WLA and WQBEL Calculation 

The Department has developed methodology from EPA’s 1991 Technical Support 

Document (TSD) to calculate WLAs and WQBELs to be protective of applicable water 

quality criteria in the receiving water. This methodology accounts for the allowable 

dilution, background concentration, effluent variability, and sampling frequency to 

calculate a WQBEL. Table 5-1 summarizes the critical conditions (effluent 

concentration, background concentration, effluent flow, etc.) that are used in effluent 

limit calculation to be protective of aquatic life and human health. 

As previously mentioned, data and results are automatically transferred from the RPA 

spreadsheet to the WQBEL Calculation spreadsheet. Of particular interest will be the 

RPA results that trigger the calculation of effluent limits. The only additional 

information the spreadsheet will require to be entered by the permit writer is the 

 

59 WLAs are the portion of the receiving water’s TMDL that is allocated on one of its existing or 

future point sources of pollution so that surface water quality is protected at all flow conditions. 60 The detailed guidance for calculating a permit effluent limit used in the workbook is described in 

depth in Section 5 (Permit Requirements) of EPA’s TSD. 
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monitoring requirement frequency (#/mo) that will be required in the permit (as shown in 

Figure 6-1, boxed areas). These applicable areas are highlighted in orange on the 

spreadsheet61. 

Once the applicable data has been entered into the WQBEL spreadsheets, the WLAs and 

WQBELs for the applicable pollutant parameters are calculated. A summary and list of 

formulas used in the calculations is included in the boxes below. When available, an 

intake credit may be used in determining compliance with the WQBEL. Please refer to 

Appendix F for guidance in the use of intake credits and permit language. 

Figure 6-1 

Example of WQBEL Calculation 

 

61 When calculating the Average Monthly Limit (AML) as part of the Aquatic Toxicity WQBEL, it is 

necessary to use a value of greater than one sample per month to prevent the calculation of a 

value could allow an exceedance of a water quality criterion. The applicable Aquatic Toxicity 

WQBEL calculation spreadsheet has been set up to automatically enter a value of “2 

samples/month” where the entered value is less than “2”. This is not applicable to Human Health 

WQBEL calculations since the AML is equal to the WLA and does not factor in monitoring 

frequency. Please refer to the TSD p. 107 & 110 for additional guidance. 
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Summary of the Aquatic Toxicity WQBEL Calculation Process 

Step 1: The first step is to calculate Waste Load Allocations (WLA) based on 

the acute and chronic aquatic criteria. The WLAs are calculated based on 

available dilution and the background concentration. 

The permit writer will calculate WLAs for both chronic and acute aquatic life 

impacts from respective water quality criteria (WQC’s). The formulas for 

calculating WLAs (chronic & acute) are: 

eff receivingwater eff

MZ chr background background c

whereDilution Q Q Q

WLA Dilution x WQC C C

( )/

( )

= +

= − +

eff receivingwater eff

a ZID acute background background

whereDilution Q Q Q

WLA Dilution x WQC C C

( ) /

( )

= +

= − +

Step 2: A corresponding Long-Term Average (LTA) is then calculated from 

each of the WLAs by multiplying the LTA by a statistical factor derived from 

the Coefficient of Variation (CV) of the monitoring data.

[ σ zσ ] a a LTA WLA e − = × 2 0.5

[ ] 2

4

2 5 4 0. σ zσ

c c LTA WLA e − = ×

Step 3: The permit writer then selects the most limiting (or lower) of the acute 

aquatic life and chronic aquatic life LTA’s to be protective of the most sensitive 

beneficial use. 

LTA = min (LTAchr, LTAacute) 

Step 4: The most limiting LTA is then expressed as the Maximum Daily Limit 

(MDL) and/or Average Monthly Limits (AML) in the permit by multiplying the 

LTA by a statistical factor derived from CV, Confidence Interval and 

monitoring frequency. [ ] 2 σ −0.5σ = × z MDL LTA e

[ ] 2

n 0.5 n z AML LTA e σ − σ = ×

Figure 6-1 Explanation 

27 For each remaining pollutant parameter where a reasonable potential 

determination was made, the results will be transferred from the RPA page to 

the effluent limit calculation page. 

28 For each pollutant parameter where reasonable potential is indicated, the 

spreadsheet will automatically calculate the Waste Load Allocations using data 

from the RPA pages. 

29 The permit writer will need to enter the number of monthly compliance 

monitoring events to calculate the effluent limits. 
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6.3 Compare WQBELs with TBELs 

Once the permit writer has calculated the Aquatic Toxicity and Human Health WQBELs, 

(both AMLs and MDLs) the next step is to compare the results to any applicable TBELs 

for toxic pollutants. The permit writer identifies the TBELs based upon the permitted 

facility’s Standard Industrial Classification (SIC) code or EPA’s industrial classification 

scheme. For each category and sub-category of process identified, the EPA has 

published a series of Effluent limit Guidelines (ELGs) and performance standards. These 

guidelines appear in 40 CFR Parts 405-499. A list of the existing ELGs with hyperlinks 

is provided in Table 6-2. 

The WQBEL or TBEL with the lowest value for each criteria (Human Health and 

Aquatic Toxicity) and each loading type (AML and MDL) should be included in the final 

permit. In most cases, it will be the WQBEL that is more environmentally protective 

than the corresponding TBEL. 

Summary of the Human Health WQBEL Calculation Process 

Setting limits based on human health criteria is slightly different from the procedure 

for aquatic toxicity because the exposure periods the criteria are based on are 

generally longer than a month and may extend up to 70 years. EPAs TSD 

recommends a slightly different methodology to account for this exposure period. 

Step 1: First the WLA is calculated for both the Water + Fish Consumption and 

Fish Consumption water quality criteria (WQC). Depending on the carcinogen 

status of the pollutant parameter the dilution used for calculation varies from the 

30Q5 to Harmonic Mean. If the back ground concentration of the receiving water 

body exceeds the WQC, then the WLA = WQC. 

`

( )/

( )

eff receivingwater eff

background background

whereDilution Q Q Q

WLA Dilutionx WQC C C

= +

= − +

Step 2: The Average Monthly Limit (AML) is set equal to the minimum WLA 

calculated for Water + Fish Consumption and Fish Consumption. 

AML = min (WLAWater + Fish, WLAFish)

Step 3: The Maximum Daily Limit (MDL) is then calculated based on the 

relationship between the AML and a multiplication factor statistically derived from 

the relationship between the confidence level, coefficient of variation and proposed 

monitoring frequency. 

MDL = AML x Mfactor 

Mfactor= [ ] 2 zσ −0.5σ e / [ ] 2

n 0.5 n z e σ − σ

*a table of the Mfactors is included on page 106 of the TSD
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Table 6-2 

Existing Effluent Limit Guidelines 

Industry Category 40 CFR 

Part 

First 

Promulgated 

Aluminum Forming 467 1983 

Asbestos Manufacturing 427 1974 

Battery Manufacturing 461 1984 

Canned and Preserved Fruits and Vegetable Processing 407 1974 

Canned and Preserved Seafood (Seafood Processing) 408 1974 

Carbon Black Manufacturing 458 1978 

Cement Manufacturing 411 1974 

Centralized Waste Treatment 437 2000 

Coal Mining 434 1985 

Coil Coating 465 1983 

Concentrated Animal Feeding Operations (CAFO) 412 1974 

Concentrated Aquatic Animal Production (Aquaculture) 451 2004 

Copper Forming 468 1983 

Dairy Products Processing 405 1974 

Electrical and Electronic Components 469 1983 

Electroplating 413 1981 

Explosives Manufacturing 457 1976 

Ferroalloy Manufacturing 424 1974 

Fertilizer Manufacturing 418 1974 

Glass Manufacturing 426 1974 

Grain Mills Manufacturing 406 1974 

Gum and Wood Chemicals 454 1976 

Hospitals 460 1976 

Ink Formulating 447 1975 

Inorganic Chemicals 415 1982 

Iron and Steel Manufacturing 420 1982 

Landfills 445 2000 

Leather Tanning and Finishing 425 1982 

Meat and Poultry Products 432 1974 

Metal Finishing 433 1983 

Metal Molding and Casting (Foundries) 464 1985 

Metal Products and Machinery 438 2003 

Mineral Mining and Processing 436 1975 

Nonferrous Metals Forming and Metal Powders 471 1985 

Nonferrous Metals Manufacturing 421 1984 

Oil and Gas Extraction 435 1979 

Ore Mining and Dressing (Hard Rock Mining) 440 1982 

Organic Chemicals, Plastics  and Synthetic Fibers (OCPSF) 414 1987 

Paint Formulating 446 1975 

Paving and Roofing Materials (Tars and Asphalt) 443 1975 

Pesticide Chemicals Manufacturing, Formulating and Packaging 455 1978 

Petroleum Refining 419 1982 
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Table 6-2 Continued 

Existing Effluent Limit Guidelines, 

Industry Category 40 CFR 

Part 

First 

Promulgated 

Pharmaceutical Manufacturing 439 1983 

Phosphate Manufacturing 422 1974 

Photographic 459 1976 

Plastic Molding and Forming 463 1984 

Porcelain Enameling 466 1982 

Pulp, Paper and Paperboard 430 1998 

Rubber Manufacturing 428 1974 

Soaps and Detergents Manufacturing 417 1974 

Steam Electric Power Generating 423 1982 

Sugar Processing 409 1974 

Textile Mills 410 1982 

Timber Products Processing 429 1981 

Transportation Equipment Cleaning 442 2000 

Waste Combustors 444 2000 

6.4 Next Steps 

After calculating and confirming the effluent limits, permit writers should ensure that all 

raw data, spreadsheets, mixing zone models and notes detailing any decision rationale are 

preserved in the permit file. This will help the permit writer to communicate the findings 

to the permit applicant and the public at the end of year three, and ensure that all required 

information to write the PER at the end of year five is available. 

In addition to the requirements to determine if individual pollutants will have reasonable 

potential to exceed water quality criteria, there is also the narrative requirement (OAR 

340-041-0033(1)) to “prevent the discharge of toxic substances into Oregon’s waters 

above natural background levels in amounts that are toxic to aquatic life”. The 

Department employs WET analysis to determine if the effluent in its entirety causes 

toxicity in aquatic organisms. The permit writer should refer to any current Departmental 

guidance or the Department’s WET Testing Coordinator located at the DEQ Laboratory. 
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Section 7: Permit Applicant 

Consultation 

7.1 Overview 

By the end of the third year of the permit term, the permit writer has used Tier 1 and 2

monitoring results to characterize the facility’s effluent and receiving water body, and 

conduct a RPA. Where pollutant parameters are found to possess RP, they then 

calculated a WQBEL and identified any applicable TBELS, selecting the most 

conservative for the final permit. At this point in the permit development process (Step 

7), the permit writer is ready to inform the permit applicant of the results and work with 

them to develop a management strategy for achieving regulatory compliance during the 

remaining two years of the permit term. 

7.2 Permittee Notification of Results and 

Compliance Evaluation 

Once the RPA and effluent limit determination process has been completed, the permit 

writer should notify the permittee of the results via memorandum along with a copy of 

the applicable spreadsheets from the RPA Workbook. Shortly thereafter, the permit 

writer should meet with the permit applicant to discuss the results and whether the facility 

is currently able to meet the proposed effluent limits. 

The permit applicant should perform a technical evaluation of their current or proposed 

treatment facility to determine if the proposed effluent limits can be met. Given the fact 

that the RPA was based upon facility performance data, in most cases the applicant will 

need to implement a source control effort, operational change or infra-structure 

improvement in order to meet the proposed effluent limit. The permit writer should 

discuss plans with the permit applicant to identify and select the best option to ensure a 

pathway to compliance. 

7.3 Alternatives 

In some cases, a readily available management option might not exist that will ensure 

compliance upon renewal of the permit. The permit writer should discuss with the permit 

applicants the various options that are available. 

In cases where a management option is feasible, the permittee may not be able to comply 

with a newly applicable WQBEL immediately upon permit issuance, a Compliance 

Schedule allowing for the use of interim effluent limits may be allowed. A permit 

developed incorporating a compliance schedule will typically have a series of milestones 

and corresponding interim effluent limits, with the final effluent limit being implemented 
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as soon as practicable. Please refer to the Department’s guidance, Compliance 

Schedules in NPDES Permits for more information. 

 

In cases where a management option is not feasible in the remainder of the permit term, 

other Standards-based approaches are available. These options typically require a 

minimum of quantitative data to demonstrate the origin and mobility of the pollutant in 

both the effluent and/or receiving waters. Typically, the permit writer will have required 

an initial look at collecting this data as part of the Tier 2 monitoring. The management 

options include: 

• Development of site specific water quality criteria: The Department may apply site 

specific water quality criteria in the Basin Descriptions in OAR Division 340-41-

033(b)(6) to reflect naturally occurring conditions. Please refer to the Site Specific 

Background Pollutant Criterion IMD (future) for guidance in the calculation and use 

of a criterion or contact the Surface Water Management Section for technical assistance. 

• Variance from water quality criteria: A variance is a short-term exemption from meeting 

water quality standards which would otherwise be applicable to an individual discharger. 

It is granted for a specific pollutant(s) and does not otherwise modify the standards. It 

does not exempt the discharger from compliance with applicable technology-based limits 

(TBELs) or water quality-based limits for other pollutants. Underlying water quality 

standards remain in effect for all other purposes (e.g., impaired water listings, TMDL 

development, etc.). It is granted for a specific period of time (length of time varies by 

state). The discharger must either meet the standard upon the expiration of this time 

period or must make a renewed demonstration of "un-attainability.” Often, a variance is 

used as a bridging mechanism to a TMDL, providing the permittee and permitting 

authority the time to better characterize the watershed, identify pollutant sources, and 

ultimately develop achievable waste load allocations. 

• Use Attainability Analysis: The Department may remove a designated beneficial use if it 

can demonstrate that attaining the use is not feasible due to one of several conditions. The 

process by which a use is removed is a Use Attainability Analysis (UAA). This option 

would most typically be used where there is a naturally occurring pollutant or a legacy 

pollutant present in the intake and receiving waters preventing the attainment of use. 

Please refer to the following link for additional information.

• (Future Implementation Tools go here)

7.4 Selected Management Option 

Once the permit applicant and the permit writer have agreed upon either a pathway to 

compliance by the end of the (current) permit term, compliance schedule or standardsbased management option, the details should then be documented (immediately) in 

correspondence to the permit applicant with copies to the file, and ultimately in the 

permit terms and conditions of the renewed permit. The permit applicant will have the 

remainder of the permit term (and perhaps longer depending upon the option selected) to 

begin implementation the terms of the selected option and ensure that they are able to 

meet the requirements of the new permit without delay. 

 
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7.5 Next Step 

A management option should have been selected and implementation begun within six 

months of the notification of results. To complete the permit renewal, the permit writer 

would only need to evaluate any additional characterization data mandated by the 

selected management option and make any necessary adjustments to the final effluent 

limits before issuing the final permit. 

 
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Section 8: Finalization of RPA and 

Effluent Limits and Documentation of 

Decision Rational 

8.1 Overview 

Near the end of the permit term (Step 8), the permit writer will finalize the RPA using 

any additional characterization data collected since the initial RPA, and adjust the 

effluent limits accordingly. They will then document the decision making processes, 

calculations and any approved management options employed. The results of the RPA 

and effluent limit calculation process are then included in the permit (either new or 

renewed). 

8.2 Final RPA and Effluent Limit Evaluation 

The permit writer will use the completed RPA Workbook from Steps 5 & 6 at the end of 

year 3 and update the information entered to reflect any additional characterization data62. 

This might result in a change to the previously calculated effluent limits. In this case, the 

permit writer should revaluate the calculated WQBELs relative to TBELs (if applicable). 

If there is an effluent limit in the existing permit, then Anti-Backsliding requirements 

normally prevent any changes that would result in a less-conservative limit. Pursuant to 

the conditions in 40 CFR 122.44 (k)(2)(i), a less stringent limit may be allowed. Please 

contact the Surface Water Management Section for technical assistance on this topic. 

8.3 Documentation 

The RPA Workbook has been designed to record all the decisions and calculations made 

in the course of the analysis from identifying monitoring requirements though effluent 

limit calculation. The permit writer should include the final RPA Workbook pages in the 

PER. Additionally, the permit writer should include additional supportive documentation 

based upon one of the three possible outcomes of RPA in the WQBEL section of the 

evaluation report: Finally, the RPA Workbook pages from the identification of POC 

(Step 3), the initial RPA (Step 5) and initial effluent calculation (Step 6) should be 

included in the administrative record. The three outcomes are: 

• Outcome 1: Monitoring is not required for toxic pollutants 

 

62 For example, there might be more detailed information regarding season variation in ambient 

pollutant concentration that might affect the outcome. At least, an increase number of sample 

events will reduce the amount of uncertainty in the calculation and result in a less conservative 

CV and Multiplier.
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• Outcome 2: Monitoring is required for some or all toxic pollutants, and there is 

not a reasonable potential to exceed water quality criteria for the monitored 

pollutants 

• Outcome 3: Monitoring is required for some or all toxic pollutants, and there is a 

reasonable potential to exceed water quality criteria. Accordingly, effluent 

limitations are calculated to prevent the exceedance of water quality criteria. 

Each of these outcomes and required documentation are discussed in detail in Sections 

8.4 through 8.6. Additionally, the department has developed a Fact Sheet Evaluation 

Matrix located on the QNET to assist the permit writer in the documentation of the 

permit development process and provide ready-made permit evaluation report language. 

8.4 Outcome 1: Evaluation Report Components 

for “no monitoring required” 

The following applicants will not have to analyze their effluent for toxic pollutant 

parameters (excluding NH3 and Cl-) as described in Section 2 of this document, and thus 

will have neither, effluent limits nor monitoring requirements in their permit: 

• Domestic plants with an average dry weather design flow (ADWDF) of less than 

1 MGD , and do not meet any of the following criteria: 

o those with a significant industrial user that discharges into the treatment 

plant that may be a potential source of pollutants 

o any other conditions that the individual permit writer feels would warrant 

such level of effort 

o those discharging to water body listed as Category 5 on the 303 d list 

o those “known” to contain Table 2 or 3 pollutants in source water or 

effluent 

• Non-Primary Industrial facilities that have no “known” pollutants from Table II, 

III, IV and V, and with no identifiable concern for such pollutants. 

The components outlined below need to be addressed in the PER to provide the basis of 

why monitoring for toxic pollutants was not required: 

• Attached RPA spreadsheet with the “Monitoring Required” Column completed 

• Verification of ADWDF less than 1 MGD for domestics 

• Verification of the finding of “no effluent limit monitoring required” as described 

in process set forth in Section 2

• Identification of any State Water Quality Criteria pollutants that are water quality 

limited for the receiving water body and if a TMDL has been completed for the 

pollutant 

• Description of any potential source of State Water Quality Criteria pollutants in 

the influent load to the facility, including the results from an industrial waste 

survey if the facility has performed one 

• Verification of the type of treatment technology, and whether the permit writer 

would expect a greater pass through of toxic pollutants 
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• Any miscellaneous factors that may warrant a detailed evaluation of potential risk 

of source to receiving water for toxic pollutants. 

8.5 Outcome 2: Evaluation Report Components 

Where “reasonable potential” is Not 

Determined 

The components outlined below need to be addressed in the evaluation report: 

1. The completed RPA spreadsheet with all columns completed 

2. Identification of any State Water Quality Criteria pollutants that are water quality 

limited for the receiving water body and if a TMDL has been completed for the 

pollutant 

3. In addition to the summary statistics presented in the spreadsheets, the permit 

writer might opt to discuss standard deviation and presence of “non-detects” in 

effluent and ambient sampling in the narrative summary 

4. Rationale or source of information for dilution ratios and background 

concentrations 

5. Rationale or source of information used in an intake credit analysis or in 

calculating a site-specific background pollutant analysis 

8.6 Outcome 3: Evaluation Report Components 

Where “reasonable potential” is Determined, 

and “effluent limits” are Calculated 

The components outlined below need to be addressed in the evaluation report: 

1. The completed RPA and Effluent Limit Calculation spreadsheets with all columns 

completed 

2. Items 2, 3, 4 and 5 from Outcome 2 (Section 7.3), above 

3. Rationale for the “probability basis” and the “confidence level” used in the 

effluent limit calculation. 

4. Comparison of the calculated water quality-based effluent to the TMDL/WLAdriven limit, Best Available Technology limit, and existing permit limit, if 

applicable for specific scenario 

5. If applicable, discussion of the use of an intake credit in determining compliance 

with a WQBEL 
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Appendix A: Revision History 

Revision Date Changes Editor 

1.0 09/2005 Initial Publication MF 

2.0 12/2006 Updated revision MF & JN 

3.0 8/2011 

Extensive revisions to reflect changes 

in environmental regulation, new 

Departmental policies regarding timing 

of the permit process, inclusion of new 

RPA and WQBEL spreadsheets and 

revision of Quantitation Limit values. 

SRB 

3.1 2/13/2012 

Addition of Intake Credit guidance in 

App. F, removal of App. C Quantitation 

Limits to a stand-alone IMD, 

discussion of recently adopted water 

quality criteria and implementation 

options, and numerous minor 

corrections and clarifications 

SRB 

 
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Note: Human Health Criteria listed here are incorrect. Please refer to most current g

Appendix C: Analytic Methods, Limits, and ImpGuidance 

Guidance has been moved to own IMD entitled Analytical Methods and Limits for NPDES Permit 
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Appendix D: RPA Workbook Walkthough and Example 

The following is a walkthrough on how to conduct a Reasonable Potential Analysis (RPA) for 

toxics using the Department’s RPA workbook. Please note that the Steps described here-in are 

slightly different than the steps discussed in the body of the IMD. Each workbook (Domestic 

and Industrial) is comprised of the following spreadsheets: 

• Monitoring Required: Contains all pollutant parameters that the facility might potentially 

be subject to and is used to record the facility’s monitoring requirements for the RPA 

process 

• Aquatic Toxicity RPA: Used to identify Pollutants of Concern, model in-stream impacts 

and determine Reasonable Potential

• Aquatic Toxicity Limits: Takes information from Aquatic Toxicity RPA spreadsheet along 

with ambient characterization data to calculate a WQBEL

• Human Health RPA: Used to identify Pollutants of Concern, model in-stream impacts 

and determine Reasonable Potential

• Human Health Limits: Takes information from Human Health RPA spreadsheet along 

with ambient characterization data to calculate a WQBEL

• Misc. Pollutant Parameters: Spreadsheets used for the RPA and limit calculations for 

pH, Ammonia, and Chlorine, that are not coved in the RPA IMD. 

 
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The spreadsheets are linked in order to forward information and calculation results between 

them. The case study is of a hypothetical Domestic waste water treatment plant (WWTP) that 

discharges to a freshwater body within the Willamette River Basin listed as water quality limited 

for 4’4’ DDT, Aldrin and Dieldrin. The primary municipal water supply is the same water body 

approximately 3 miles upstream from the outfall. The average dry weather discharge flow rate is 

10.1 MGD. Additionally, PCBs and copper have been identified in the receiving water body, 

and are presumed to be drawn into the municipal water supply. Of the two, only PCB’s have 

been recorded at concentrations near or greater than water quality criteria. The WWTP currently 

has an effluent limit for Inorganic Arsenic. A mixing zone analysis has been completed by the 

facility and the results are included with the permit application. 

Step 1: Determining which pollutant parameters require monitoring and analysis 

As part of the preceding permit renewal or new permit application process, the permit writer 

would begin by using the guidance in Section 2 and the flowchart (Figure 2-3) illustrating the 

domestic monitoring requirements determination process to identify the facility’s Tier 1 effluent 

monitoring requirements. The following pollutant groups or specifically indicated pollutant 

parameters should be included: 

• Inorganic Arsenic due to current effluent limits. Compliance monitoring data may be submitted 

in-lieu of characterization data. 

• PCB’s due to knowledge that the pollutant is present in the significant concentrations in the 

primary municipal water supply (ie. intake from Willamette River). Since they are in the source 

water, they are most likely also present in the effluent. The permit writer would need to evaluate 

the potential for the concentration of the intake pollutants (ie. copper) to determine if it might 

reach a threshold of significance (near or greater than the water quality criteria).

• 4’4’ DDT, Aldrin and Dieldrin due to the knowledge that the pollutant is present in the significant 

concentrations in the primary municipal water supply. Even if facility had a separate water 

supply, these pollutant parameters would still be flagged for monitoring due to the design flow 

(>1.0 MGD) and the listing status of the receiving waterbody.

• Table 1 Effluent parameters for all POTWs w/flow > 0.1 MGD

• Table 2 Effluent Parameters for Selected POTWs including metals, volatile organic 

compounds, acid-extractable compounds and base-neutral compounds pollutant groups. This 

would include copper. 

• Table 3 Departmental Pesticides and PCB Monitoring Requirements: Under this provision, 

only Aldrin, 4’4’ DDT and Dieldrin would require monitoring because of the listing status of the 

receiving water body. 

• Methyl Mercury due to Willamette Basin Requirements.

The permit writer would record all identified monitoring requirements on the Monitoring 

Required spreadsheet by toggling a “yes” or “no” in the “Monitoring Required?” column. The 

number of samples required should be recorded in the “# of Samples” column. Portions of the 

completed spreadsheet may be cut and pasted into the Schedule B of the permit language 

(monitoring requirements). A screenshot of the completed Monitoring Required Spreadsheet is 

presented below: 
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Upon issuance, an electronic version of the spreadsheet that includes an Analytic Summary 

Report template will be provided to the permittee. The permittee will use the template to report 

the results of their Tier 1 monitoring in both electronic and physical copies. This will allow the 

permit writer to efficiently review and import analytic data into the RPA Workbook. A screen 

shot of the monitoring spreadsheet and report template is presented below: 

# of 

Samples 

Column

Monitoring 

Required? 

Column
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Step 2: General Facility Information 

The next step is to fill in the RPA Run and General Facility Information sections of the Aquatic 

Toxicity RPA spreadsheets. This includes facility identification, effluent characterization, 

dilution rates (from RMZ analysis), whether the water body is fresh and the hardness of the 

effluent and receiving water body. If a mixing zone analysis where not available, there is a 

provision to enter facility and stream flow information instead. The spreadsheet will cross 

reference the “Monitoring Required?” information from the Monitoring Required spreadsheet. 

The section also has default confidence and probability basis for the calculations. Many of these 

elements are presented in the illustration below: 

Analytic Summary 

Report: 

Tier 1 Results
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The permit writer would conclude the step by repeating the process for the human health criteria 

using the Human Health spreadsheet. It should be noted that the Aquatic Toxicity and Human 

Health spreadsheets will only have the pollutant parameters listed for which there are state water 

quality criteria, and can look very different. Additionally, the spreadsheets might potentially 

have differing mixing zone dilution numbers. 

 

Information forwarded 

from the Monitoring 

Required spreadsheet

Alternative 

dilution 

calculation

RP at End 

of Pipe?
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Step 3: Identifying the Pollutants of Concern in the Effluent 

By the second year (Month 24) of a permit term, the Tier 1 Monitoring data should have been 

collected and is ready for analysis. For all pollutant parameters that require monitoring and 

analysis as denoted by a “Yes” in the “Evaluation Required” column, the permit writer will enter 

the number of samples and the coefficient of variation into the appropriate columns. For the 

Aquatic Toxicity Spreadsheet, the highest concentration (of effluent) will be entered, but for the 

human health spreadsheet the geometric mean or maximum concentration will be entered 

depending upon the carcinogen status indicated in the “Carcinogen” Column. 

Once entered, the spreadsheet calculates the maximum effluent concentration at the end of pipe 

and then compares it to the water quality criteria. Pollutants that exceed the water quality criteria 

are Pollutants of Concern (POC) and are subject to further characterization and RPA analysis. 

For each POC, a “Yes” will be indicated in the “RP at end of pipe?” column. In the example 

below, cadmium, lead and silver have been identified as a POCs. It should be noted that if there 

are errors in the calcuation process (as with mercury) neither a “yes” or “No” will appear in the 

column. In this case, a CV value was not entered resulting in the calculation not occuring. 

Please refer to the screen shot below where the applicable area is magnified. 

 
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Step 4: Sampling Plan 

The permit writer would use their knowledge of the facility, receiving water body, potential for 

the use of an intake credit or site specific background pollutant criterion, and the type of 

environmental issues suggested by the Tier 1 monitoring to develop the Tier 2 monitoring 

requirements (ambient data, additional effleunt data, source investigation, etc.). The permit 

writer would then send the permit applicant a Monitoring Action Letter describing the identified 

POCs and Tier 2 monitoring requirements, with the direction to develop a Sampling Plan. 

Typically, the permit writer would advise the permit applicant during the Sampling Plan 

development to ensure that the necessary data is gathered in the most time and cost efficent 

manner. The permit applicant would then submit the completed plan to the permit writer for 

review and approval. Once approved, the plan would be implemented over the course of the 

following year (Month 24 – 36). 

The importance of a developing a robust data set that more accuractly characterizes the pollutant 

concentration of the effluent is explified in the screen shot below. Here the permit writer has reevaluated the previosly identified POCs using a more robust data set. The most notable result of 

this can be seen with lead where previously an exceedance at the end of pipe was indidcates, but 

using 10 data points as opposed to 4, no exceedance was projected. 

 

RP at End 

of Pipe? 
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Step 5: Determining In-stream Concentration After Mixing / Determining RP 

Further analysis will be required for only those POCs identified as exceeding water quality 

criteria at the end of pipe. Accordingly the Sampling Plan and Tier 2 monitoring was designed 

to collected ambient data for the POCs, and where necessary develop a more robust data set. 

Once the TIER 2 monitoring data has been received, the permit writer should update the 

“Identify Pollutants of Concern” section of the spreadsheet with the more robust data sets. As 

seen in the previous example, this resulted in a pollutant parameter (lead) as being determined 

that there was not potential to exceed water quality criteria at the point of discharge, and no 

further analysis is required. 

For the Aquatic Toxicity Spreadsheet, the permit writer will enter the maximum or 90th

percentile ambient concentration (depending on the number of data points) in to the column 

entitled “Ambient Concentration”. For the Human Health spreadsheet the geometric mean of the 

ambient data will be entered. The spreadsheet will then calculate the in-stream concentrations at 

the boundaries of the various mixing zones. 

The spreadsheet then compares the projected in-stream concentrations to the water quality 

criteria. If the criteria are exceeded, then a “Yes” will appear in the appropriate portion of the “Is 

there Reasonable Potential to Exceed?” column. Please note how reasonable potential is 

determined for both acute and chronic conditions. 

If the necessary data is available, the permit writer may conduct an intake credit analysis and 

determine if RP is still present. 

Step 6: Calculation of Effluent Limits 

For all pollutant parameters where reasonable potential is determined, the information will 

automatically be transferred to the Aquatic Toxicity or Human Health effluent limit calculation 

spreadsheets. The permit writer need only insert the anticipated amount of monthly monitoring 

for the pollutant parameter. This number is important since it affects the how conservative the 

final calculated effluent will be. Both the Monthly Average and Maximum Daily effluent limits 

will be calculated in the final, “Effluent Limit” columns of the spreadsheets. 

RP at End 

of Pipe? 
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In some instances, the permit writer might consider varying the Confidence Intervals to affect the 

effluent limit calculation outcome. For any changes from the default settings in the spreadsheet, 

the permit writer should document the basis behind the changes in the Permit Evaluation Report 

(PER). 

The analysis indicated that there was reasonable potential for cadmium and silver resulting in 

water quality based effluent limits (WQBELs). Once the permit writer has completed the human 

health portion of the analysis, any resulting limits would be compared and the most conservative 

(for each pollutant parameter) selected. The selected WQBELS would then be compared to any 

applicable TBELS, and the most conservative placed into the permit. Since this case study is of 

a domestic WWTP there would be no TBELS. If allowed, the permit writer might opt to use the 

intake concentration of the source water when determining compliance with the selected 

WQBEL. Copies of the RPA and limit calculation spreadsheets should be included in the PER. 

 
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Step 7: Quantitation Limits and Compliance Levels 

An additional feature of the RPA Workbook is a field that compares calculated effluent limits to 

the Department’s Quantitation Limits. In cases where an effluent limit is below the analytic 

range of available methods the Quantitation Limit becomes the default compliance level. In this 

example, for silver a compliance level of 1.00 ug/l would be used in-lieu of the calculated 

WQBEL of 0.10 ug/l. 

This field can be seen on the electronic spreadsheet, although it is hidden on printed report. An 

example is presented in the screen shot below: 
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Appendix E: Whole Effluent Toxicity 

Guidance 

The Department’s Whole Effluent Toxicity (WET) guidance was not updated as part of this RPA 

IMD revision. Permit writer should refer to the guidance and example permit language 

presented on the WET Section of the Permit Writer’s Corner located on the QNET. 

For additional information or technical assistance, please contact the WET Coordinator located 

at the DEQ Laboratory (Lori Pillsbury (503) 693-5735). 

The following language has been taken (unchanged) from the September, 2005 revision of the 

RPA IMD. Permit Writers may refer to this guidance, although they should their discretion and 

defer to the updated guidance from the Permit Writer’s Corner or from the WET Coordinator 

when appropriate. 
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7.1 Background 

What is 

“Whole 

Effluent 

Toxicity” 

testing? 

“Whole Effluent Toxicity” (WET) testing measures whether an effluent in its 

entirety (with its mixture of various chemicals) causes toxicity in aquatic 

organisms. WET testing is usually conducted in controlled laboratory 

experiments in which aquatic organisms are exposed to samples of effluent at 

different dilutions. 

Why is it 

required? 

WET testing addresses the OAR 340-041-0033(1) requirement to prevent the 

discharge of toxic substances into Oregon’s waters above natural background 

levels in amounts that are toxic to aquatic life. Although DEQ has adopted 

numeric water quality criteria in Tables 20, 33A, and 33B for the protection of 

aquatic life for a number of chemicals that might be found in wastewater 

discharges, other chemicals do not have criteria because information on 

toxicity is limited and the data requirements for deriving criteria are rigorous. 

In addition, the numeric water quality criteria are derived for individual 

chemicals, whereas some wastewater discharges contain mixtures of chemicals 

that might have synergistic toxic effects even though the amount of any 

individual chemical in the mixture is below toxicity levels. WET testing 

results are used to address these information gaps and ensure that discharges 

are not toxic. 

Acute toxicity What is it?

OAR 340-041-0053(2)(a)(A) defines acute toxicity as lethality to aquatic life 

as measured by a significant difference in lethal concentration between the 

control and 100 percent effluent in an acute bioassay test 

What is the performance standard (rule requirement)?

OAR 340-041-0053(2)(a)(A) prohibits acutely toxic discharges; however, 

lethality in 100% effluent may be allowed if there is a mixing zone with 

immediate dilution of the effluent (“zone of immediate dilution” or “ZID”). 

How is compliance with the standard determined?

In permits where a mixing zone is allowed (e.g. discharge to a river with a 

large flow) and a ZID exists, acute toxicity would be indicated when a 

statistically significant difference in survival between the tested dilutions and 

the control occurs at dilutions greater than that found at the edge of the ZID. If 

there is no ZID, acute toxicity would be indicated when a statistically 

significant difference in survival occurs between 100% effluent and the 

control. 

To comply with the rule definition of acute toxicity, “hypothesis testing” must 

be conducted to evaluate acute toxicity. This means that there should be no 

statistical difference between the WET test results from the effluent sample 

and control groups at a significance level of α=0.05. Often, the laboratory 

conducting the bioassays will test a series of dilutions of effluent (e.g. 100%, 
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50%, 25%, 12% and 6% effluent) to determine the No-Observable-EffectConcentration (NOEC) of the effluent in order to meet the requirement for no 

statistical difference between effluent (at some dilution) groups and control 

groups. For more information on evaluation WET test results, see the EPA 

methods documents cited in the Test Methods section below. 

Chronic 

toxicity 

What is it?

OAR 340-041-0053(2)(b)(A) characterizes measurement of chronic toxicity as 

the concentration that causes long-term sublethal effects, such as significantly 

impaired growth or reproduction in aquatic organisms, during a testing period 

based on the test species life cycle. 

What is the performance standard (rule requirement)?

OAR 340-041-0053(2)(b)(A) does not allow chronic toxicity outside of a 

mixing zone. 

How is compliance with the standard determined?

In permits with a mixing zone, chronic toxicity is assessed by examining the 

effect of effluent at various doses on organisms in bioassays, making sure that 

the doses of effluent tested bracket the concentration at the edge of the mixing 

zone. If no mixing zone exists, chronic toxicity would still be assessed using a 

series of effluent doses (including 100%); however, the point of compliance 

would be at end of pipe. 

To determine statistical significance in chronic toxicity testing, the Department 

uses EPA’s Technical Support Document for Water Quality-based Toxics 

Control, March 1991 (TSD), which recommends that the concentration of 

effluent that results in 25% inhibition of the parameter used to determine a 

chronic effect (e.g. growth) be the benchmark for determining whether the 

results of chronic tests indicate toxicity. (This 25% inhibition concentration is 

also referred to as the IC25.) For example, when testing a series of dilutions of 

effluent on the growth of fathead minnows, a dilution yielding 12% inhibition 

of growth will not be considered as a significant chronic effect, but 25% 

inhibition of growth would be significant. 

The IC25 is based on a point estimate technique, which assumes a continuous 

dose-response relationship (often represented by a sigmoid curve describing 

the relation between amount of inhibition or effect and the dose of compound 

or mixture). 
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Since the relationship is continuous, a choice must be made as to some level of 

effect that will be considered “safe” and statistically significant. According to 

EPA (TSD), “point estimation techniques are the preferred statistical methods 

in calculating endpoints for effluent toxicity tests” for chronic toxicity. This is 

due to the inability to estimate test precision when using hypothesis testing for 

deriving a No Observed Effect Concentration (NOEC), thus making the 

inherent variability of bioassay results difficult to address statistically. EPA 

compared data from the point estimate and NOEC approaches and concluded 

that the “IC25 is approximately the analogue of an NOEC derived using 

hypothesis testing.” 

For more information on evaluation WET test results, see the EPA methods 

documents cited in the Test Methods section below. 

Test methods EPA has published methods for conducting and evaluating WET testing (the 

latest are listed below). 

• “Methods for measuring the acute toxicity of effluents and receiving 

waters to freshwater and marine organisms” (Fifth Edition, October 

2002) (EPA 821-R-02-012; http://www.epa.gov/OST/WET/). 

• “Short-term methods for estimating the chronic toxicity of effluents 

and receiving waters to freshwater organisms” (Fourth Edition, October 

2002) (EPA 821-R-02-013). 

• “Short-term methods for estimating the chronic toxicity of effluents 

and receiving waters to marine and estuarine organisms” (Third 

Edition, October 2002) (EPA 821-R-02-014) 

and such methods should be used for meeting WET testing 

requirements. 

• “Short-term methods for estimating the chronic toxicity of effluents 

and receiving waters to west coast marine and estuarine organisms” 

(August 1995) (EPA/600/R-95/136) 

Use Table 6 below to determine whether to use a freshwater or saltwater 

method. 

Effluent Concentration 

Percent Inhibitio

n

100% 

100% 

0% 

0% 
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Table 6. Choosing Freshwater or Saltwater Method 

IF Receiving 

Stream is 

AND Intake or Influent is THEN Species for WET 

Test is 

Freshwater1 Freshwater Freshwater 

Saltwater Freshwater 

Saltwater2 

Freshwater Freshwater for acute test 

Saltwater for chronic test 

Saltwater Saltwater 

1 Freshwater is water in which the salinity is equal to or less than 1 part per 

thousand 95% or more of the time. 

2 Saltwater is water in which the salinity is equal to or greater than 10 parts 

per thousand 95% or more of the time. 

Note: Water between 1 and 10 parts per thousand salinity should be considered of 

mixed salinity and WET determined for both fresh and salt water. 

Test species Acute toxicity testing requires exposure of multiple species (which at a 

minimum will include a fish and an invertebrate to effluent for 48 (for an 

invertebrate) or 96 hours (for a fish). Chronic toxicity testing also requires 

exposure of multiple species (which at a minimum will include a fish, an 

invertebrate, and a plant) to effluent for various durations depending on the 

experimental endpoint specified in the methods. The species appropriate for 

acute and chronic testing are listed in Table 7. 
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Table 7. WET Test Species 

FRESHWATER 

IF 

appropriat

e WET 

Test is 

AND 

Species 

Type 

required is 

THEN Species for WET test should include one of the following 

per Species Type 

Acute 

invertebrate Ceriodaphnia dubia (daphnid) 

Daphnia pulex and D. magna (daphnids) 

fish Pimephales promelas (fathead minnow) 

Oncorhynchus mykiss (rainbow trout) 

Chronic 

invertebrate Ceriodaphnia dubia (daphnid) 

fish Pimephales promelas (fathead minnow) 

plant Raphidocelis subcapitata (green alga also known as Selanastrum 

capricornutum) 

ESTUARINE & MARINE 

IF 

appropriat

e WET 

Test is 

AND 

Species 

Type 

required is 

THEN Species for WET test should include one of the following 

per Species Type 

Acute 

invertebrate 

Mysidopsis bahia (mysid shrimp) 

Holmesimysis costata (mysid shrimp), specific to Pacific Coast 

waters 

fish 

Cyprinodon varieganus (sheepshead minnow) 

Menida beryllina (inland silverside) 

M. menidia (Atlantic silverside) 

M peninsulae (tidewater silverside) 

Chronic 

invertebrate 

Mysidopsis bahia (mysid shrimp) 

Arbacia punctulata (sea urchin) 

Haliotis rufescens (red abalone), specific to Pacific Coast waters 

Crassostrea gigas (Pacific oyster), specific to Pacific Coast waters 

Mytilus spp. (mussel), specific to Pacific Coast waters 

Holmesimysis costata (mysid shrimp), specific to Pacific Coast 

waters 

Strongylocentrotus purpuratus (sea urchin), specific to Pacific 

Coast waters 

Dendraster excentricus (sand dollar), specific to Pacific Coast 

waters 

fish 

Cyprinodon variegatus (sheepshead minnow) Menidia beryllina

(inland silverside) 

Atherinops affinis (topsmelt), specific to Pacific Coast waters 

plant Champia parvula (red macroalga) 

Macrocystis pyrifera (giant kelp), specific to Pacific Coast waters 

Species used Several of the species listed in the above table can be used for both acute and 
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for both acute 

and chronic 

toxicity testing 

chronic toxicity testing using what is called a “dual endpoint” laboratory test. 

Dual endpoint test are those in which the protocols for chronic toxicity testing 

are carried out with additional information collected on survival at 48 and 96 

hours to check for acute toxicity. These tests are desirable because analytical 

costs are lower and the permit writer may allow the submission of dual 

endpoint test results in lieu of individual acute and chronic test results. 

For freshwater, these include: 

• Ceriodaphnia dubia (water flea), and 

• Pimephales promelas (fathead minnow). 

In this example, the acute test endpoints would be survival at 48 hours for 

water flea and at 96 hours for fathead minnow and the chronic endpoints 

would be survival and reproduction after 8 days for water flea and body 

weight after 7 days for fathead minnow. 

When both intake and receiving stream are saltwater, species with dual 

endpoints include: 

• Cyprinodon variegatus (sheepshead minnow) 

• Menida beryllina (inland silverside) 

• Holmesimysis costata (mysid shrimp) 

• Mysidopsis bahia (mysid shrimp) 

7.2 WET Review in the Permitting Process 

Where WET 

review fits in 

the permit 

process 

Once an application for a discharge permit has been subjected to reasonable 

potential analysis for the pollutants listed in Tables 20, 33A, and 33B of OAR 

340-041 (see previous chapters), then the analysis turns to determining 

whether there is reasonable potential for the discharge to cause toxicity due to 

chemicals or combinations in the effluent that have not been addressed in the 

tables listed above. 

What does the 

permit writer 

need to do? 

The permit writer must: 

1. Conduct a WET review as part of the application review process. 

2. If necessary, include appropriate WET requirements in Schedule D 

Special Conditions of the NPDES permit. 

3. Document decision making process. 
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Overview of 

WET review 

process 

The WET review process consists of two major steps: 

1. Determining if the discharge has a risk of causing aquatic toxicity. (Figure 

5) 

2. If there is a risk, determining if WET testing is required in the permit. 

(Figure 6) 

Figures 5 and 6 provide summaries of these processes in flow charts, 

expressing each process as a series of yes and no questions and findings. In 

some instances, information for answering the questions can be taken directly 

from the application materials; in others, answers result from analyzing 

information from other sources (e.g. federal regulations specifying which 

chemicals are considered toxic to aquatic life). A more complete explanation 

of the steps within each flowchart is given in Section 7.3 for Figure 5 and 

Section 7.4 for Figure 6. 

Compliance 

with WET 

requirements 

If WET requirements are included in the permit in Schedule D, compliance 

with these requirements must be evaluated. Section 7.6 and Figure 7 describe 

the necessary steps staff must take to perform this evaluation. At this time, the 

Department recommends this Special Conditions approach rather than numeric 

limits; however, if WET requirements are included in the permit as numeric 

limits, then the permit writer should consult the TSD for possible approaches. 
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7.3 Step A: Determining if there is a risk of aquatic toxicity 

Is there a risk 

of aquatic 

toxicity (Figure 

5)? 

A risk of aquatic toxicity in the discharge exists if the facility meets at least 

one of the following conditions (see Figure 5 for the decision flow chart): 

1. The facility uses, stores, produces as a product or waste, or transfers 

any hazardous substance listed in 40 CFR §302.4 with a statutory code 

of 1 or 2 [referring to Sections 311 (b)(4) or 307(a) of the Clean Water 

Act] unless permittee demonstrates to DEQ’s satisfaction that there is 

no possibility of these items being discharged. Appendix F contains a 

list of these substances based on the July 2005 electronic code of 

federal regulations (http://ecfr.pgoaccess.gov, select “Title 40 – 

Protection of Environment” and then navigate to 40 CFR 302.4). 

(Figure 5, Step 1) 

2. The facility’s discharge contains any toxic pollutant listed in Appendix 

D of 40 CFR Part 122 for which there are no water quality criteria for 

aquatic life protection listed in 40 CFR 131.36(b)(1). Appendix G 

contains a list of these substances. (Figure 5, Step 2) 

3. The facility belongs to an industry category identified in 40 CFR Part 

122, Appendix A (NPDES Primary Industry categories), listed in the 

Table 8. (Figure 5, Step 3) 

4. The facility is a municipal sewage collection and treatment system with 

a flow equal to or greater than 1 million gallons per day (MGD). 

(Figure 5, Step 4) 

5. The facility is a municipal sewage collection and treatment system that 

receives a discharge from any industry category identified in 40 CFR 

Part 403, Appendix C (currently not specified). (Figure 5, Step 5) 

6. The facility exceeded the acute or chronic whole effluent toxicity 

performance standard within the last five years (unless that facility 

already has whole effluent toxicity limits or the Department has made a 

finding that such limits should be removed because of attainment of 

WET performance standard). (Figure 5, Step 6) 

7. The facility’s discharge is suspected to be toxic because of apparent 

damage to aquatic life in the receiving water. (Figure 5, Step 7) 

8. The Department determines that the facility has potential to discharge 

toxics (including those chemicals listed in Table 33C Water Quality 

Guidance Values in OAR 340-041) in toxic amounts. (Figure 5, Step 

8) 

What’s next? If the discharge from the facility does not meet any of the above conditions, 

then there is no risk of aquatic toxicity and there are no WET requirements for 

the permit. If it does, the permit writer must determine if WET requirements 

are needed in the permit (see Section 7.3 and Figure 6). 

 
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2. Discharge any toxic pollutant listed in 

40 CFR Part 122 Appendix D that do not 

have aquatic life criteria? 

Yes 

No

No

No

No

No

No

Yes 

Yes 

Yes 

Yes 

Yes 

Yes 

No

No

1. Use, store, produce as a product or 

waste, or transfer any hazardous substance 

listed in 40 CFR 302.4 with a statutory 

code of 1 or 2 (See App. F)? 

Does the facility... 

3. Belong to an industry category listed in 40 

CFR Part 122 Appendix A (See Table 8)? 

5. Constitute a municipal sewage collection & treatment 

system which receives a discharge from any industry listed in 

40 CFR Part 403 Appendix C? 

6. Have at least one instance of 

exceeding the acute or chronic WET test 

performance standard in the last 5 years? 

8. Have the potential to discharge 

toxics in toxic amounts? 

7. Have suspected toxicity because of 

apparent damage to aquatic biota? 

9. Proceed to worksheet for 

determining WET compliance 

requirements (Figure 6) 

4. Constitute a domestic sewage collection & treatment 

system with a flow ≥ 1 MGD? 

Yes 

10. Off-Ramp: No WET 

testing requirement in 

permit

Figure 5. WET Testing: Is there a Risk of Aquatic Toxicity. 
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Table 8. NPDES Primary Industry Categories from 40 CFR 122 Appendix A 

Adhesives & sealants Inorganic chemicals mfg. Photographic 

equipment/supplies 

Aluminum forming Iron & steel mfg. Plastics processing 

Auto & other laundries Leather tanning & finishing Plastic/synthetic materials 

mfg. 

Battery mfg. Mechanical products mfg. Porcelain enameling 

Coal mining Nonferrous metals mfg. Printing & publishing 

Coil coating Ore mining Pulp & paper mills 

Copper forming Organic chemicals mfg. Rubber processing 

Electrical/electronic 

components 

Paint & ink formulation Soap & detergent mfg. 

Electroplating Pesticides Steam electric power plants 

Explosives mfg. Petroleum refining Textile mills 

Foundries Pharmaceutical preparations Timber products processing 

Gum & wood chemicals 

7.4 Step B: Determining WET requirements 

Are WET 

requirements 

needed (Figure 

6)? 

If a risk of aquatic toxicity exists, then the permit writer must determine 

whether the permit should include WET requirements. This process involves 

evaluating WET testing data (if available) to assess the reasonable potential for 

the effluent to cause toxicity and, therefore, exceed the narrative toxics criteria. 

The following steps, corresponding to Figure 6, detail key decision steps for 

the permit writer: 

Step 1. Does the applicant have existing, adequate WET test results? 

• At least 10 test results from quarterly tests conducted within 

the past permit cycle or, for smaller sources (at the discretion 

of the permit writer), results from 4 tests performed in the past 

41⁄2 years. Permit writer has the option of requiring WET 

testing on only the most sensitive species (as established in 

previous tests) once 4 tests using all 3 species have been 

adequately performed. 

• Testing must have been done on the EPA-approved species for 

both acute and chronic toxicity using EPA-approved methods. 
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• Depending on the degree of dilution at the edge of the zone of 

immediate dilution (ZID) and mixing zone, results should be 

from testing that focused on bracketing the relevant dilution to 

increase the likelihood of identifying the lowest observed 

effect level (LOEL) and no observed effect level (NOEL). 

If yes, go to Step 2. 

If no, require testing as detailed in the next section, WET Testing 

Requirements. 

Step 2. Do the test results indicate that the discharge meets the WET 

performance standard? See Section 7.5, Evaluating WET data. 

If yes, require yearly WET test in permit and monitor for changes. 

If source has less than 10 test results, then permit writer should 

require additional WET tests to allow for 10 test results to be 

submitted for the next renewal application. 

If no, specify WET requirements in permit as detailed in the next 

section, WET Testing Requirements. If source has less than 10 test 

results, then permit writer should require additional WET tests to 

allow for 10 test results to be submitted for the next renewal 

application. 

WET testing 

requirements 

See Table 9 to determine the WET testing requirements. 

 
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Table 9. WET Testing Requirements 

IF there are AND the results Then 

No data 

NA 1) require semiannual 

(yearly for smaller 

sources) WET testing 

for duration of permit; 

2) may reopen permit 

if discharge fails 

WET test 

performance standard 

in first two years of 

permit; and 

3) Go to Figure 7

Partial data 

Meets performance 

standard 

No WET 

requirements; monitor 

for changes in 

process; require 

yearly WET tests in 

permit. 

Fails performance 

standard 

1) 1) require 

semiannual (yearly for 

smaller sources) WET 

testing for duration of 

permit; 

2) Specify WET 

compliance 

requirements in 

permit and 

compliance 

monitoring; and 3) Go 

to Figure 7.

Sufficient data 

Meets performance 

standard 

No WET 

requirements; monitor 

for changes in 

process; require 

yearly WET tests in 

permit. 

Fails performance 

standard 

1) Specify WET 

compliance 

requirements in 

permit and 

compliance 

monitoring; and 2) Go 

to Figure 7.
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Additional 

testing: bioassessment 

studies 

Although following the EPA WET testing methodology using a fish, an 

invertebrate, and a plant significantly increases the likelihood of detecting 

aquatic toxicity from the whole effluent if it exists, there remains a chance that 

such tests will fail to predict toxicity to local biota. Therefore, the use of 

bioassessment methods could add important information for evaluation of an 

application and should be considered when appropriate. Surveys to identify 

and enumerate macroinvertebrates (the Department has adopted a modified 

version of Water Quality Monitoring Guide as the appropriate methodology; 

contact the Watershed Assessment section of the DEQ Laboratory Division for 

details) above and below a discharge outfall can be used in conjunction with 

laboratory WET tests to more fully characterize the chance that an effluent 

poses a risk of aquatic toxicity. 

At this time, the Department will not require such bioassessment studies for 

fulfilling the WET requirements for the purposes of issuing a permit. 

However, permit writers should consider including bioassessment monitoring 

when a permittee fails to meet a performance standard during routine 

monitoring. Such bioassessment monitoring should be conducted in parallel 

with traditional laboratory WET testing so that results can be compared to give 

a more complete assessment of potential effects of the discharge on the 

receiving water. In the case of conflicting results between the laboratory WET 

test and the bioassessment monitoring, the laboratory WET test results should 

carry precedence (until such time that the Department’s experience with 

bioassessment monitoring indicates a need to change this approach). 

 
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1. Does applicant have 10 

WET test results prior to 

application or results 

from 4 tests performed in 

the last 4.5 years for 

smaller sources? 

2. Has the discharge 

met the WET test 

performance 

standard? See 

Section 7.X, p. X.

Require: 

1) semiannual (yearly for 

smaller sources) WET testing for 

duration of permit; 

2) reopening permit if discharge 

fails WET test performance 

standard in first two years of 

Require yearly WET tests in permit (or 

additional tests to reach 10 tests for 

renewal) and monitor for changes. 

No

Yes 

No

Yes 

Specify WET requirements in 

permit. 

Go to Figure 7. 

See Section 7.1 

Background, for 

proper WET testing 

method and species 

selection.

Figure 6. WET Testing: Determining Limits. 
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7.5 Evaluating WET data and determining next steps 

Evaluating 

WET data 

The permit writer must evaluate WET test results to determine if the 

performance standard is being met. Figure 7 provides an overview of the 

decision process for evaluating WET data and determining what actions the 

permittee must take in response to this evaluation. 

Step 1. If the discharge is in compliance, then WET monitoring is continued 

according to the frequency and conditions specified in the permit. If 

the discharge is not in compliance then further evaluation is 

necessary. 

• Acute toxicity: If there is no statistical difference between the 

WET test results from the effluent sample dilution as allowed by 

a ZID (100% effluent sample if no ZID) and control groups at a 

significance level of α=0.05 

• Chronic toxicity: If there is less than 25% inhibition of growth or 

other chronic effect (e.g., reproduction), there is no indication of 

chronic toxicity and the discharge complies. 

Evaluating WET test results is not an easy task. EPA provides 

several guidance documents on this issue (see Test Methods in 

Section 7.1), but the permit writer is not expected to be an expert on 

the subject. 

What to do 

when WET 

tests indicate 

toxicity 

If WET testing indicates a problem with ongoing toxicity, the permit writer 

must determine if further monitoring is required or a toxicity 

identification/reduction evaluation plan should be prepared. Which 

determination to make depends on whether the permittee acknowledges the 

toxicity of the effluent or believes the WET testing results are inaccurate. 

Permittee does not acknowledge toxicity of effluent

Step 2. If the permittee does not acknowledge that the effluent is toxic (e.g. 

claims that WET monitoring tests are erroneous), then permit writer 

should require: 

a. Submission of results from three (3) monthly acute/chronic tests 

that repeat the monitoring tests that returned indications of 

toxicity, and 

b. Possibly at least one (1) bio-assessment test to be done 

simultaneously with one of the acute/chronic tests. 

These increased monitoring requirements should resolve any 

uncertainty regarding the initial results and prevent delay of the steps 

necessary to come into compliance. 

Step 3a. Further testing indicates no impact: If the result of the increased 
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WET monitoring reveals no further occurrence of toxicity and the 

bio-assessment indicates no adverse impact, then the permit writer 

requests and reviews the permittee’s transient toxicity report that 

documents the possible causes and prevention of the transient 

toxicity. See Transient toxicity report later in this section for 

information on what needs to be in this report. 

Note: If the bio-assessment indicates an adverse impact, then further 

bio-assessment monitoring should be required in support of the 

subsequent permit application. After filing the acceptable transient 

toxicity report, the permittee conducts further monitoring according 

to the frequency and conditions specified in the permit. For more 

information, see When WET test and bio-assessments differ, p. 51. 

Further testing indicates impact: If the results of increased WET test 

monitoring indicate further occurrence of toxicity, then the process of 

developing a toxicity identification/reduction evaluation plan is 

initiated regardless of the bio-assessment monitoring results. 

Step 4. Evaluate if the reduction effort is successful. 

• If the reduction effort is unsuccessful, then the permittee must 

continue to carry out the TI/RE plan until the cause of the toxicity 

is clearly identified through the TI/RE, or toxicity is eliminated. 

• If the reduction effort is successful, then the permittee conducts 

further monitoring according to the frequency and conditions 

specified in the permit. 

Permittee acknowledges toxicity of effluent

Step 2. If the permittee acknowledges that the effluent is toxic, then the 

permit writer requests a toxicity identification/reduction evaluation 

plan from the permittee for Department review and approval within 

60 days of receipt of the final test results. See Toxicity 

Identification/Reduction Evaluation Plan, for more information on 

this plan requirement. 

Step 3b. Evaluate results of the TI/RE plan to determine whether the source of 

the toxicity is apparent. 

1. If “no,” then the permittee continues to carry out the TI/RE plan 

until the source of the toxicity is identified or toxicity is 

eliminated through other actions. 

2. If “yes,” then the permit writer should allow the permittee 2 to 6 

months to confirm the identity of and eliminate the source of 

toxicity. The Department may allow up to six months before the 

permittee initiates the investigation outlined in the EPA manuals 

for facility personnel to attempt to control the most likely sources 

of toxicity through efforts such as changes in plant operation, 
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replacement of a toxic material used in the facility, or 

improvement of best management practices. 

Step 4. Evaluate if the reduction effort is successful. 

• If the reduction effort is unsuccessful, then the permittee must 

continue to carry out the TI/RE plan until the cause of the toxicity 

is clearly identified through the TI/RE, or toxicity is eliminated. 

• If the reduction effort is successful, then the permittee conducts 

further monitoring according to the frequency and conditions 

specified in the permit. 

When WET 

results and 

bioassessments 

differ 

If the results from the laboratory WET tests conflict with those of the bioassessment study, then the results of the laboratory WET tests should be used 

to decide whether to file a transient toxicity report or develop a toxicity 

identification/reduction evaluation plan. In the case where results from 

laboratory WET tests are unclear (i.e. some low level toxicity is suggested but 

is not definitive), then the bio-assessment results can be used to decide on 

which path to proceed. 

Interpretation of the possible outcomes of laboratory WET tests and bioassessment monitoring is shown in the following table: 

If WET test 

results indicate: 

and bio-assement 

results indicate: 

Then require: 

Toxicity 

Adverse impact Toxicity Identification/Reduction 

Evaluation Plan 

No impact Toxicity Identification/Reduction 

Evaluation Plan 

No Toxicity 

Adverse impact 

Transient toxicity report & 

additional bio-assessment 

monitoring 

No impact Transient toxicity report 

Transient 

toxicity report 

The transient toxicity report must include: 

1. The results of the initial failed WET test, 

2. The results of the follow up WET tests/bio-assessment studies, and 

3. An explanation for the initial failed WET test, e.g. WET test result is 

an outlier due to high rainfall or ammonia value. 

Toxicity 

Identification/ 

Reduction 

Evaluation 

This plan should follow the steps described in the latest versions of EPA 

manuals (and references therein), such as (but not limited to): 

• “Toxicity reduction evaluation guidance for municipal wastewater 

treatment plants” (EPA 833/B-99-002; 
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Plan http://www.epa.gov/npdes/pubs/tre.pdf) 

• “Generalized methodology for conducting industrial toxicity reduction 

evaluations” (EPA 600/2-88-070) 

• “Toxicity Identification Evaluations: Characterization of Chronically 

Toxic Effluents, Phase I” (EPA 600/6-91-005F) 

Removing 

WET 

requirement 

A specific requirement in the permit to comply with the WET performance 

standard may be removed upon permit renewal if: 

1. The permittee has demonstrated compliance with the WET 

performance standard associated with that limit for at least the last 3 

consecutive test years following effluent characterization or for an 

entire subsequent permit term, and 

2. There have been no changes made to operations or effluent quality 

within the last three years that would otherwise require additional 

effluent characterization. 
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3a. Does 

Toxicity 

occur 

again? 

Implement TI/RE 

plan 

1. Does monitoring 

indicate 

compliance with 

WET standard? 

2. Does 

permittee 

acknowledge 

effluent is 

toxic? 

Require increased WET 

testing to 3 monthly 

chronic/acute and 

possibly 1 bio-assessment 

Request 

TI/RE plan 

for DEQ 

approval 

3b. Is 

source of 

toxicity 

apparent?

Request transient 

toxicity report or 

TI/RE report for 

review 

Continue monitoring 

specified in permit 

Yes

No

Yes 

No Yes

Allow 2 to 6 months to 

confirm & eliminate 

source 

Continue 

monitoring 

specified in 

permit 

No

Yes

Yes 

No

4. Is 

reduction 

effort 

successful? 

No

Figure 7. WET Testing: Compliance and Enforcement. 
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Appendix F: Intake Credits 

Introduction 

On June 16th Oregon Environmental Quality Commission adopted into rule (OAR 340-045-

0105) the use of Intake Credits for determining reasonable potential (RP) and establishing water 

quality based effluent limits (WQBELs). The intake credit provision allows a permit writer to 

quantify the amount of a pollutant (intake pollutant63) in a facility’s source water and deduct a 

corresponding amount from the facility’s estimated discharge when evaluating reasonable 

potential or demonstrating compliance with an effluent limit. The impact of this provision is that 

a facility is only held accountable for their direct impacts to the source water and water quality. 

Previously, a facility would have been responsible for any pollutants contained in the source 

water plus their direct impacts. 

A permit writer might initially consider the use of an intake credit after reviewing Tier 1

monitoring results. If there are indications that a significant portion of an identified Pollutant of 

Concern (POC) can be attributed to the source water, the permit writer should require the facility 

to include appropriate source investigation steps in their Sampling Plan and Tier 2 monitoring. 

In implementing an intake credit, the permit writer will need to assemble adequate data to 

quantify both the pollutant concentration and volume of the source water withdrawn, 

demonstrate that the a series of conditions has been met, and use the resultant findings in the RP 

and WQBEL calculations. Finally, the permit writer might need to make special considerations 

(i.e. on-going monitoring requirements, or treatment performance measures) attributed to intake 

credits when finalizing the permit. 

Reasonable Potential Threshold Conditions 

The Intake Credit rules have a series of threshold conditions that must be met in order for the use 

of an intake credit to determine RP. There are also a similar set of conditions that must be met 

for the use of an intake credit to establish WQBELs. The permittee must satisfy all the following 

five conditions to qualify for the “reasonable potential” use of the procedure: 

1. The facility withdraws 100 percent of the intake water containing the pollutant from the same 

body of water into which the discharge is made; 

An intake pollutant is considered to be from the “same body of water” as the discharge if the 

intake pollutant would have reached the vicinity of the outfall point in the receiving water within a 

reasonable period had it not been removed by the permittee. This finding may be deemed 

established if: 

• There is a direct hydrological connection between the intake and discharge points; 

• The background concentration of the pollutant in the receiving water (excluding any 

amount of the pollutant in the facility's discharge) is similar to that in the intake water; and 

• Water quality characteristics (e.g., temperature, pH, hardness) are similar in the intake 

and receiving waters. 

An intake pollutant from groundwater may also be considered to be from the “same body of 

water” if the aforementioned conditions are met and the pollutants are not a result of human 

 

63 An “intake pollutant” is the amount of a pollutant that is present in public waters (including groundwater) at the time 

it is withdrawn from such waters by the discharger or other facility supplying the discharger with intake water.
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activity such as industrial, commercial, or municipal operations, disposal actions, or treatment 

processes. 

2. The facility does not contribute any additional mass of the identified intake pollutant to its 

wastewater Mass Conditions 

Any facilities (or municipalities) introducing a measurable mass of the pollutant into an effluent 

stream would not be allowed an intake credit for that pollutant when determining RP. 

3. The facility does not alter the identified intake pollutant chemically or physically in a manner 

that would cause adverse water quality impacts to occur that would not occur if the 

pollutants were left in-stream; 

Of particular concern are shifts in pollutant speciation that might result in increased toxicity (e.g. 

Chrome III to Chrome VI) 

4. The facility does not increase the identified intake pollutant concentration at the edge of the 

mixing zone, or at the point of discharge if a mixing zone is not allowed, as compared to the 

pollutant concentration in the intake water, unless the increased concentration does not 

cause or contribute to an excursion above an applicable water quality standard; and 

In most cases the receiving water body will exceed the water quality criterion, although 

in rare instances a small amount of assimilative capacity might be available and a intake 

credit in conjunction with an intake credit would be allowed. 

5. The timing and location of the discharge would not cause adverse water quality impacts to 

occur that would not occur if the identified intake pollutant were left in-stream. 

WQBEL Threshold Conditions 

The Intake Credit rules have a series of threshold conditions that must be met in order for the use 

of an intake credit to establish WQBELs. A summary and brief explanation of the conditions is 

presented below: (please note that these conditions are numbered 6 - 10 so they can be uniquely 

referred to later in the discussion) 

6. The facility withdraws 100 percent of the intake water containing the pollutant from the same 

body of water into which the discharge is made; 

Please refer to the guidance described in Condition #1 

7. The observed maximum ambient background concentration and the intake water 

concentration of the pollutant exceeds the most stringent applicable water quality criterion 

for that pollutant; 

The intent of the condition is to limit the use of intake credit based WQBELs to bodies of water 

that are determined, or have the potential to be determined as “water quality limited”. 

8. The facility does not alter the identified intake pollutant chemically or physically in a manner 

that would cause adverse water quality impacts to occur that would not occur if the 

pollutants were left in-stream; 

Please refer to the guidance described in Condition #3 

9. The facility does not increase the identified intake pollutant concentration, as defined by the 

department, at the point of discharge as compared to the pollutant concentration in the 

intake water; and 

10. The timing and location of the discharge would not cause adverse water quality impacts to 

occur that would not occur if the identified intake pollutant were left in-stream. 
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Intake Credit Procedure 

The following is a discussion of the steps the permit writer should pursue to evaluate each of the 

threshold conditions. The permit writer should document this process and include an intake 

credit analysis summary in the Permit Evaluation Report. 

Reasonable Potential Threshold Evaluation 

Step 1: The permit writer should collect available data with the intension of evaluating the RP 

and WQBEL threshold conditions. Typically, the data required to perform all of these 

evaluations (especially Condition #’s 3, 4 & 5) will not be readily available, and can be resolved 

later in the facility’s Sampling Plan described in Step 2. This data will also be sufficient to 

evaluate the WQBEL Condition #’s 8, 9 & 10. 

To evaluate Condition #1, the permit writer should establish the location of the source water 

intake points, relative to the location of the outfall. The following are likely sources of 

information that can be used for performing these evaluations: 

• DEQ’s drinking water program for well locations and logs: 

http://www.deq.state.or.us/wq/dwp/contacts.htm

• DEQ’s LASAR and USGS’s Water Data for Oregon databases for ambient surface and ground 

water quality data 

• Oregon’s municipal Drinking Water Data Online database for municipal intake concentrations, 

source type and well locations 

• Facility permit renewal application data summary (facility file) and Discharge Monitoring Reports

• Municipal drinking water and waste water treatment plant flow and pollutant concentration records 

The permit writer should develop a location map showing the outfall and various intake locations 

for inclusion in the Permit Evaluation Report (see Figure 1). The permit writer should also 

develop a process diagram showing the identified intake water sources, process use, treatment 

and discharge (see Figure 2). 

Only those sources “upstream” of the outfall location would be permitted for consideration of an 

intake credit. With ground water sources (well or I&I), the permit writer need to establish the 

direct hydrologic connection with the receiving water body and how the pollutant would have 

reached the vicinity of the outfall within a reasonable period. The Drinking Water Data Online 

database identifies those municipal wells that are under direct influence of surface water or are 

located close enough to nearby surface water to receive direct surface water recharge (under 

Source Type). For example, the wells identified in Figure 1 are listed as “GU - Under direct 

influence of surface water” in the database. 

The condition does not require that a 100% of a facility’s source water come from the same body 

of water. Rather, the condition requires that 100% of the intake pollutant credited be from the 

same body of water. For facilities with multiple intakes from different bodies of water, the 

intake concentration should be flow weighted (see below) to reflect only those sources that meet 

the rule’s condition (i.e. be from the same body of water). 
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If Condition #1 cannot be met, the facility may not determine RP or establish WQBELs using an 

intake credit. 

Figure 1 

Example of an Intake Credit Evaluation Map 

To evaluate Condition #2, the permit writer should investigate the facility to determine if there is 

the potential for additional contributions of a pollutant resulting in a measurable increase in 

mass. For industrial facilities, a review of the production process description, hazardous material 

data bases and handling records64 may identify likely sources of pollutant contributions. For 

domestic facilities, the municipalities pretreatment program (if present) is the most ready source 

 

64 For example, Toxic Release Inventory, Oregon Hazardous Substance Information Survey forms, Material Safety 

Data Sheets and Hazardous Waste (RCRA) Records. 

Intake Concentration Flow Weighted Concentration Calculation 

Itotal= [(I1*Q1)+(I2*Q2)+(...)+(In*Qn)]/Qtotal

Itotal = Total allowable Intake concentration 

Ii= pollutant concentration from intake source i 

Qi= Flow rate from intake source i

Outfall 

Location

Intake 

Locations
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of information, although it might be necessary to review likely sources with-in the collection 

area followed up by further investigation. In any case (domestic or industrial) where pollutant 

contribution is suspected, the permit writer should request point source monitoring to confirm 

presence and quantify the discharge65. 

If Condition #2 cannot be met, the facility may not determine RP using an intake credit, although 

there is still the potential of establishing WQBELs. Typically, the data necessary to evaluate 

Conditions #3, #4 and #5 is not readily available, and must be collected as part of the Sampling 

Plan. 

Step 2: To evaluate Condition #3 and #4, the permit writer will need further characterization of 

the identified intake waters and discharged effluent relative to each other during the same 

discharge period. Unless the facility already has the necessary data, a series of parallel 

monitoring events and appropriate laboratory analysis must be included in the facility’s Sampling 

Plan. 

The Sampling Plan should quantify the mass and concentration of the subject pollutant in the 

intake waters and effluent through a series of parallel monitoring events. Ideally, flow rates 

should be recorded at all sample locations and, if required, the average hydrologic residence time 

(HRT) between intake and discharge should be estimated. A subset of the collected samples 

should be split and analyzed to determine if there are any physical or chemical66 changes that 

would result in additional adverse water quality impacts. Where an engineering study or 

variance request might later be necessary, the facility might consider monitoring the ambient 

source water (pre-filtration) or treatment facility influent as to better characterize the process and 

determine treatment efficiencies (see Figure 2). 

 

 

65 Even if this action disqualifies the ability to conduct a RP, a WQBEL is still available and the data will assist the 

facility’s source reduction efforts. 

66 i.e. change in speciation or pH that results in greater toxicity 
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Figure 2 

Example of a Simple Process Diagram for a Domestic Facility w/Sample Locations 

An example of the various elements described in the Sampling Plan is presented below: 
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Using the collected data, the permit writer will evaluate the facility for Conditions #3 and #4. 

When evaluating Condition #3, the permit writer should be aware of potential changes in 

pollutant speciation or water chemistry that might result in an increased pollutant toxicity, 

mobility or bio-availability. The permit writer should also consider potential changes in 

temperature, flow regime, dissolved oxygen, nutrient content, etc. that might result in an adverse 

water quality impact. Please consult with staff in the Department’s Laboratory Division or 

TMDL Sections when determining monitoring requirements or evaluating resultant data. 

In order to evaluate Condition #4, the permit writer essentially needs to conduct the RPA to 

determine if there is an “increase the identified intake pollutant concentration at the edge of the 

mixing zone67, or at the point of discharge if a mixing zone is not allowed, as compared to the 

pollutant concentration in the intake water.” The RPA Workbook has been modified to perform 

an intake credit analysis and will indicate if an intake credit is not allowed as described in Step 3

in the next Section. 

To evaluate Condition #5, the permit writer should consider the timing and location impacts of 

the discharge upon the receiving waterbody. Of particular concern, would be those waterbodies 

that are effluent dominated, have a very low base flows or are subject to channelization. Where 

it can be demonstrated that the receiving water body would be adversely impacted, a intake 

credit would not be permitted in determining RP. 

 

67 In rare cases where a receiving water body’s ambient concentration is just below the water quality 

criteria, a mixing zone might be permitted.

Example of Sampling Plan Elements for Inorganic Arsenic in a 

POTW 

Required Elements 

• 7 consecutive 24-hour composite samples for: 

o Intake location (treated drinking water) 

o Effluent (may stagger sampling time from Intake by 2 days 

to account for system residence time) 

• Continuous flow monitoring for the intake and effluent location 

during the monitoring period 

• Analyze all samples for Inorganic Arsenic 

• Split 2 of the aforementioned sampling events (both intake and 

effluent) and analyze for Total Arsenic, Inorganic Arsenic, Arsenic 

III, Arsenic V, pH, temperature, DO and hardness. 

Optional Elements 

• 7 consecutive 24-hour composite samples for: 

o Ambient location (Raw water intake) 

o Influent (may stagger sampling time from Intake by 2 days 

to account for system residence time) 

• Analyze all samples for Inorganic Arsenic 

• Continuous flow monitoring for the influent location during the 

monitoring period 
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If it can be demonstrated conclusively that conditions #s 1, 3, 4, & 5, cannot be met, an intake 

credit (for both RP and WQBEL purposes) would not be allowed and no further action should be 

taken. If condition #2 (no addition of pollutant mass) cannot be met, the facility is disqualified 

from using an intake credit in determining RP, but is still allowed to use an intake credit in 

establishing WQBELs. 

Conducting a Reasonable Potential Analysis using an Intake Credit 

Step 3: To apply an intake credit, it is necessary of conduct an intake credit analysis by 

calculating a series of adjusted effluent and ambient pollutant concentrations using an intake 

concentration (IC) value. This The RPA Workbook has been modified (see below) to perform 

this analysis, although the permit writer will still need to calculate an appropriate IC value 

depending upon the type of RPA (aquatic toxicity vs. human health) and the carcinogen status of 

the pollutant of concern. 

For an Aquatic Toxicity RPA, the IC is calculated using the following instructions: 

For a human health RPA, the IC is calculated using the following instructions: 

Intake Concentration calculation instructions for Aquatic Toxicity 

RPA 

1. Calculate the Ratio of average intake concentration (I) to average 

effluent concentration (E) 

Intake Ratio = [(I1+I2+...+In)/n] / [(E1+E2+...+En)/n] 

*where necessary, use flow weighted individual intake values 

2. Multiply the maximum effluent concentration by intake ratio to 

determine intake concentration (IC) used in RPA 

IC = Emax x Intake Ratio 

Intake Credit Analysis

Enter appropriate 

Intake Concentration

Results 
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Once the appropriate IC values have 

been calculated and entered, the RPA 

Workbook checks to ensure that the IC 

is greater than or equal to the effluent 

concentration. If there is an increase 

in concentration, an “X” will be 

reported and the calculation ends per 

Condition #4 in Step 2. In this case, 

the determination of RP for the 

pollutant of concern stands and a 

WQBEL would be necessary. 

If there is not an increase in effluent 

concentration over intake, then a 

check mark (“√”) is reported and the 

intake credit calculations progresses 

and a final RP determination is 

reported in the “Is there Reasonable 

Potential to Exceed?” columns (see 

screen shot right). If a “No” is 

reported, there is not RP and no 

WQBEL is necessary. If a “Yes” is 

reported, there is still RP and a 

WQBEL is necessary in Step 4. 

Establishing Water Quality Effluent Limits Threshold Evaluation 

Step 4: If a permittee is found to have RP, the permit writer should evaluate the WQBEL 

Threshold Conditions, and if met, calculate WQBELs in Step 5. Since most of these threshold 

conditions (#6, #8 and #10) are the same as the RP Threshold conditions (#1, #3 and #5), they 

are already met. To evaluate Condition #7 the permit writer would review “the observed 

maximum ambient background concentration and the intake water concentration of the 

pollutant” and where these values “exceed(s) the most stringent applicable water quality 

criterion for that pollutant”, allow the development of a WQBEL for that parameter. This 

condition is to limit the establishment of WBELs using intake credits to those water bodies with 

ambient concentrations at or above water quality criteria. It should be noted that there are no 

limitations on the addition of additional mass of a pollutant to the effluent as described in 

Condition #268. Condition #9 will be met by the establishment of WQBELs that prevents the 

facility from increasing “the identified intake pollutant concentration, as defined by the 

 

68 Any additional mass must be removed prior to discharge as determined by Condition #9. 

Intake Concentration calculation instructions for Human Health RPA 

• For Carcinogens, use the geometric mean of intake data

• For Non-Carcinogens, follow instructions for Aquatic Toxicity 

RPA

*where necessary, use flow weighted intake values 

Intake Confirmation 
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department, at the point of discharge as compared to the pollutant concentration in the intake 

water”. 

Calculating Aquatic Toxicity Effluent Limits using an Intake Credit 

Step 5: If the facility has me the applicable WQBEL Threshold Conditions, a WQBEL may be 

established using an intake credit. The following language may be used in the permit: 

If the (compliance) effluent monitoring results indicate that the effluent concentration of 

the pollutant is equal to or less than the intake concentration, then the calculated effluent 

limits are not applicable, and therefore, the discharge is in compliance. Otherwise, the 

effluent must comply with the calculated effluent limitations. 

This will require that the facility monitor both the intake source and effluent in tandem, and 

make a rolling comparison to ensure the terms of the language is being met. The alternative 

effluent limit is calculated on the Effluent Limit page of the RPA Workbook. Upon entering the 

number of monthly compliance samples, the Effluent Limits page will automatically calculate 

the applicable effluent limits as seen below: 

Various Intake Credit Scenarios 

The following figures have been prepared to demonstrate a very simplistic intake credit scenario. 

Scenario 1 is a simplified example where a Facility is using surface water that ambient 

concentration is above water quality criteria. Based upon current rule, the evaluation only 

reflects the concentration at the point of discharge to address the application of current rules. 

Scenario 2 and 3 demonstrate the use of intake credit procedure using the same variables as in 

Scenario 1 for the RPA and WQBEL calculation processes, respectively. 

Enter # of 

Monthly Samples

Calculated 

Effluent Limits
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Rule Language 

The following is the applicable rule language from OAR, Division 45-0105. 

(1) General Provisions. The following provisions apply to the consideration of intake pollutants in 

determining reasonable potential under section (2) of this rule and the consideration of intake 

pollutants in establishing water quality based effluent limits under section (3) of this rule. 

These provisions do not alter the permitting authority's obligation under 40 CFR 

122.44(d)(vii)(B) to develop effluent limitations consistent with the assumptions and 

requirements of any available waste load allocations for the discharge, that is part of a TMDL 

prepared by the department and approved by EPA pursuant to 40 CFR 130.7, or prepared by 

EPA pursuant to 40 CFR 130.7(d). 

(a) An “intake pollutant” is the amount of a pollutant that is present in public waters 

(including groundwater as provided in subsection (d), below, at the time it is withdrawn 

from such waters by the discharger or other facility supplying the discharger with intake 

water. 

(b) An intake pollutant is considered to be from the “same body of water” as the 

discharge if the department finds that the intake pollutant would have reached the 

vicinity of the outfall point in the receiving water within a reasonable period had it not 

been removed by the permittee. This finding may be deemed established if: 

(A) The background concentration of the pollutant in the receiving water 

(excluding any amount of the pollutant in the facility's discharge) is similar to that 

in the intake water; 

(B) There is a direct hydrological connection between the intake and discharge 

points; and 
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(C) Water quality characteristics (e.g., temperature, pH, hardness) are similar in 

the intake and receiving waters. 

(c) The department may also consider other site-specific factors relevant to the transport 

and fate of the pollutant to make the finding in a particular case that a pollutant would or 

would not have reached the vicinity of the outfall point in the receiving water within a 

reasonable period had it not been removed by the permittee. 

(d) An intake pollutant from groundwater may be considered to be from the “same body 

of water” if the department determines that the pollutant would have reached the vicinity 

of the outfall point in the receiving water within a reasonable period had it not been 

removed by the permittee, except that such a pollutant is not from the same body of 

water if the groundwater contains the pollutant partially or entirely due to human activity, 

such as industrial, commercial, or municipal operations, disposal actions, or treatment 

processes. 

(e) The determinations made under Sections (2) and (3), below, will be made on a 

pollutant-by-pollutant and outfall-by-outfall basis. 

(2) Consideration of Intake Pollutants in Determining Reasonable Potential: 

(a) The department may determine that there is “no reasonable potential” for the 

discharge of an identified intake pollutant to cause or contribute to an excursion above a 

narrative or numeric water quality criterion contained in Oregon’s water quality standards 

where a discharger demonstrates to the satisfaction of the department (based upon 

information provided in the permit application or other information) that: 

(A) The facility withdraws 100 percent of the intake water containing the pollutant 

from the same body of water into which the discharge is made; 

(B) The facility does not contribute any additional mass of the identified intake 

pollutant to its wastewater; 

(C) The facility does not alter the identified intake pollutant chemically or 

physically in a manner that would cause adverse water quality impacts to occur 

that would not occur if the pollutants were left in-stream; 

(D) The facility does not increase the identified intake pollutant concentration at 

the edge of the mixing zone, or at the point of discharge if a mixing zone is not 

allowed, as compared to the pollutant concentration in the intake water, unless 

the increased concentration does not cause or contribute to an excursion above 

an applicable water quality standard; and 

(E) The timing and location of the discharge would not cause adverse water 

quality impacts to occur that would not occur if the identified intake pollutant were 

left in-stream. 

(b) Upon a finding under subsection (a) of this section that an intake pollutant in the 

discharge does not cause, have the reasonable potential to cause, or contribute to an 

excursion above an applicable water quality standard, the department is not required to 

include a water quality-based effluent limit for the identified intake pollutant in the 

facility's permit, provided: 

(A) The NPDES permit evaluation report includes a determination that there is no 

reasonable potential for the discharge of an identified intake pollutant to cause or 

contribute to an excursion above an applicable numeric water quality criterion 

and references appropriate supporting documentation included in the 

administrative record; 
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(B) The permit requires all influent, effluent, and ambient monitoring necessary to 

demonstrate that the conditions above in subsection (a) of this section are 

maintained during the permit term; and 

(C) The permit contains a re-opener clause authorizing modification or revocation 

and re-issuance of the permit if new information shows the discharger no longer 

meets the conditions in subsection (a) (A) through (E) of this section. 

(3) Consideration of Intake Pollutants in Establishing Water Quality Based Effluent Limits 

(WQBELs): 

(a) The department may consider pollutants in intake water as provided in section (3) 

when establishing water quality-based effluent limitations based on narrative or numeric 

criteria, provided that the discharger has demonstrated that the following conditions are 

met: 

(A) The facility withdraws 100 percent of the intake water containing the pollutant 

from the same body of water into which the discharge is made; 

(B) The observed maximum ambient background concentration and the intake 

water concentration of the pollutant exceeds the most stringent applicable water 

quality criterion for that pollutant; 

(C) The facility does not alter the identified intake pollutant chemically or 

physically in a manner that would cause adverse water quality impacts to occur 

that would not occur if the pollutants were left in-stream; 

(D) The facility does not increase the identified intake pollutant concentration, as 

defined by the department, at the point of discharge as compared to the pollutant 

concentration in the intake water; and 

(E) The timing and location of the discharge would not cause adverse water 

quality impacts to occur that would not occur if the identified intake pollutant were 

left in-stream. 

(b) Where the conditions in subsection (a) of this section are met, the department may 

establish a water quality-based effluent limitation allowing the facility to discharge a 

mass and concentration of the intake pollutant that are no greater than the mass and 

concentration found in the facility’s intake water. A discharger may add mass of the 

pollutant to its waste stream if an equal or greater mass is removed prior to discharge, 

so there is no net addition of the pollutant in the discharge compared to the intake water. 

(c) Where proper operation and maintenance of a facility’s treatment system results in 

the removal of an intake water pollutant, the department may establish limitations that 

reflect the lower mass and concentration of the pollutant achieved by such treatment. 

(d) Where intake water for a facility is provided by a municipal water supply system and 

the supplier provides treatment of the raw water that removes an intake water pollutant, 

the concentration of the intake water pollutant will be determined at the point where the 

water enters the water supplier’s distribution system. 

(e) Where a facility discharges intake pollutants from multiple sources that originate from 

the receiving water body and from other water bodies, the department may derive an 

effluent limitation reflecting the flow-weighted amount of each source of the pollutant 

provided that adequate monitoring to determine compliance can be established and is 

included in the permit. 

(f) The permit will specify how compliance with mass and concentration-based limitations 

for the intake water pollutant will be assessed. This may be done by basing the effluent 

limitation on background concentration data. Alternatively, the department may 

determine compliance by monitoring the pollutant concentrations in the intake water and 
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in the effluent. This monitoring may be supplemented by monitoring internal waste 

streams or by a department evaluation of the use of best management practices. 

(g) In addition to the above, effluent limitations must be established to comply with all 

other applicable State and Federal laws and regulations including technology-based 

requirements and anti-degradation policies. 

(h) When determining whether WQBELs are necessary, information from chemicalspecific, whole effluent toxicity and biological assessments will be considered 

independently. 

(i) Permits limits must be consistent with the assumptions and requirements of waste 

load allocations or other provisions in a TMDL that has been approved by the EPA.