Storm Water Panel Recommendations to the 

California State Water Resources Control Board

The Feasibility of Numeric Effluent Limits 

Applicable to Discharges of Storm Water 

Associated with Municipal, Industrial and 

Construction Activities

June 19, 2006
Storm Water Panel on Numeric Limits June 19, 2006

Page ii

Panelists:

Brian Currier, PE

Research Engineer, Office of Water 

Programs, California State University 

Sacramento, 

Sacramento, California

Gary Minton, Ph.D, PE

Resource Planning Associates, 

Seattle, Washington

Robert Pitt, Ph.D., PE

Cudworth Professor of Urban Water 

Systems, Dept. of Civil, Construction, 

and Environmental Engineering, 

University of Alabama, Tuscaloosa, 

Alabama

Larry A. Roesner, Ph.D., PE. 

Colorado State University, Fort 

Collins, Colorado

Ken Schiff

Deputy Director, Southern California 

Coastal Water Research Project, 

Westminster, California

Mike Stenstrom, Ph.D., PE

Dept. of Civil and Environmental 

Engineering, University of California 

at Los Angeles,

Los Angeles, California

“The opinions I express are my own and do not 

represent official US EPA policy.”

Eric Strassler

Senior Policy Analyst, Office of Water, 

US USEPA

Washington, District of Columbia

Eric Strecker, PE

Principal, GeoSyntec Consultants, 

Portland, Oregon
Storm Water Panel on Numeric Limits June 19, 2006

Page iii

Table of Contents 

Background....................................................................................................................1

California’s Permits ......................................................................................................1

Court Decisions ............................................................................................................2

Panel’s Findings on Feasibility of Numeric Effluent Limits Applicable 

to Municipal Activities................................................................................................4

Municipal Observations ...............................................................................................4

The Problem with Existing Effluent Limit Approaches........................................5

Technical Issues.......................................................................................................6

Municipal Recommendations .....................................................................................8

A Technically Sound and Pragmatically Enforceable BMP Design and the 

Permit Process .......................................................................................................11

Strategies for Stormwater Management to Protect Urban Water 

Environments ..........................................................................................................12

Panel’s Findings on Feasibility of Numeric Effluent Limits Applicable 

to Construction Activities........................................................................................15

Construction Observations .......................................................................................15

Construction Recommendations..............................................................................15

Panel’s Findings on Feasibility of Numeric Effluent Limits Applicable 

to Industrial Activities...............................................................................................19

Industrial Observations..............................................................................................19

Industrial Recommendations ....................................................................................21

Tables

Table 1 - Effects of Urbanization on Hydrologic Regime in Colorado and Georgia

...................................................................................................................................13

Table 2- Approach to Establish Numeric Limits or Action Levels at Existing or 

New Facilities..........................................................................................................20

Figures

Figure 1 - Exceedance Frequencies for Detention Basins in Fort Collins, 

Colorado ..................................................................................................................13
Storm Water Panel on Numeric Limits June 19, 2006

Page 1

Background

The NPDES storm water permit program came into being as a result of the 1987 

amendments to the federal Clean Water Act and its implementing regulations. In 

California, the State Water Resources Control Board (State Water Board) and 

the nine Regional Water Quality Control Boards (Regional Water Boards) 

implement the NPDES storm water program. 

The Clean Water Act amendments, Section 402(p) require that discharges of 

storm water from large and medium municipal separate storm sewer systems 

(MS4s) and discharges of storm water associated with industrial activities be in 

compliance with NPDES permits. MS4 permits require that the discharge of 

pollutants be reduced to the maximum extent practicable (MEP). Discharges 

associated with industrial activities, were required to meet the technology based 

standards of best available technology economically achievable (BAT) or best 

conventional pollutant control technology (BCT), and to meet water quality 

standards.

In 1990, USEPA promulgated regulations (40 CFR Part 122.26) for the NPDES 

storm water program. These regulations clarified what industrial activities were 

subject to storm water permit. Construction that resulted in a land disturbance of 

five or more acres was included as an industrial activity subject to NPDES storm 

water permit. The regulations also delineated what was to be included in permit 

applications and the programmatic elements that were to be in a permit and 

storm water management program for MS4s or storm water pollution prevention 

plan for industrial activities. 

California’s Permits

In 1990, MS4 permits were issued to Santa Clara County by the San Francisco 

Bay Regional Water Board and to Los Angeles County by the Los Angeles 

Regional Water Board. These permits were appealed to the State Water Board. 

The primary basis of the appeals was the lack of numeric limits in the permits. 

The entities that brought the appeals argued that the permits needed to include 

numeric limits, as the discharges of pollutants must not only be reduced to the 

MEP, but they must also meet water quality standards. The State Water Board, 

in hearing these appeals, determined that it was not feasible at the time to 

develop numeric limits for MS4 permits, and that water quality standards could 

and should be achieved through the implementation of best management 

practices (BMPs). Since this ruling, the Regional Water Boards have typically 

not included numeric limits in storm water permits. 

The State Water Board has adopted NPDES General Permits for the Discharge 

of Storm Water Associated with Industrial Activities and for the Discharge of 

Storm Water Associated with Construction Activities. Both of these permits 

contain language stating that developing numeric limitations is infeasible. 
Storm Water Panel on Numeric Limits June 19, 2006

Page 2

Court Decisions

In addition to these actions on MS4 permits at the State level, there have been a 

number of rulings from the federal courts regarding the NPDES Storm Water 

program.

One of the most significant is from the federal court, 9th District Court of Appeals 

from 1999. In its published opinion on Defenders of Wildlife vs. Browner, the 

Court held that MS4 permits need not require strict compliance with water quality 

standards. Rather, compliance was to be based upon the MEP standard. 

However, the permitting authority (the State Water Board/Regional Water Boards 

for California) could at their option require compliance with standards. The State 

Water Board through the permit and appeals process has in fact required that the 

discharges from MS4s meet water quality standards, but has stated that 

compliance with numeric standards can be achieved through the implementation 

of BMPs in an iterative fashion.

The Browner decision also found that discharges of storm water associated with 

industrial activities must be in strict compliance with water quality standards. 

In 2004 the State Water Board conducted a public hearing on a draft General 

Industrial Storm Water permit. This draft permit met with significant opposition 

from non-government or non-industrial organizations (NGOs) due to the absence 

of numeric limits. Staff revised the draft permit to include the benchmarks 

contained in the USEPA multi-sector general permit. This change resulted in 

strong opposition from the regulated community. 

The concerns that have been raised by the NGOs and the regulated community 

are similar, though they do not necessarily agree on the best way to address 

them. Both believe that permitting has become overly complex, and that it is 

extremely difficult, if not impossible to objectively determine if a facility, operation 

or municipality is in compliance with its permit requirements. The NGOs argue 

that requiring storm water permittees to comply with numeric effluent limits will 

result in an easier way to measure compliance. The regulated community 

agrees, to a degree, but they argue that it is not simply a matter of selecting a 

number that is suitable for a POTW or industrial waste discharge. Due to the 

unique nature of storm events and storm water discharges, any numeric limit that 

is placed in a storm water permit must take into consideration the episodic nature 

of storm events and be truly representative of storm water discharges. In 

addition, the regulated community has argued that there are going to be 

pollutants in storm water discharges that did not originate in the MS4 (run on) or 

that they do not have the means to control, and therefore should be given special 

consideration. 

In response to these arguments, State Water Board directed staff to convene a 

panel of storm water experts to examine the feasibility of developing numeric 
Storm Water Panel on Numeric Limits June 19, 2006

Page 3

limits for storm water permits. Specifically, this panel of experts was asked to 

consider the following:

“Is it technically feasible to establish numeric effluent limitations, or 

some other quantifiable limit, for inclusion in storm water permits? 

How would such limitations or criteria be established, and what 

information and data would be required?”

“The answers should address industrial general permits, construction 

general permits, and area-wide municipal permits. The answers 

should also address both technology-based limitations or criteria and 

water quality-based limitations or criteria. In evaluating establishment 

of any objective criteria, the panel should address all of the following: 

(1) The ability of the State Water Board to establish appropriate 

objective limitations or criteria; (2) how compliance determinations 

would be made; (3) the ability of dischargers and inspectors to monitor 

for compliance; and (4) the technical and financial ability of 

dischargers to comply with the limitations or criteria.” 

Staff invited 10 individuals from the academic and scientific community to 

participate on the panel. Of the 10, eight agreed to participate. These eight met 

in a public session on September 14, 2005 and heard presentations from the 

regulated and NGO communities. They also heard comments from the public at 

large. They met again on September 15, 2005 to discuss the public comments 

and to begin to formulate a response. It was also decided at this meeting that 

they would form sub-committees to address municipal (MS4), industrial and 

construction discharges separately. These sub-committees worked on drafts 

statements for each of these, circulating them over the course of a number of 

months.

The panel met again in private session on April 3 and 4, 2006. The purpose of 

these meetings was to address unresolved issues and to develop the final 

response to the State Water Board. It was also decided to combine the three 

working statements into one Statement of Findings. The following discussion is 

the panel’s findings and is broken into three program element areas: municipal, 

construction, and industrial.
Storm Water Panel on Numeric Limits June 19, 2006

Page 4

Panel’s Findings on Feasibility of Numeric Effluent Limits 

Applicable to Municipal Activities 

Municipal Observations

1. The current practice for permitting, designing, and maintaining 

municipal stormwater treatment facilities (called BMPs herein) on the 

urban landscape does not lend itself to reliable and efficient 

performance of the BMPs because: 

• Permitting agencies, including EPA, States, and local governments, 

have rarely developed BMP design requirements that consider the 

pollutants and/or parameters of concern, the form(s) that the 

pollutants or parameters are in, the hydrologic and hydraulic nature 

of how they pollutants and flow arrive, and then the resulting unit 

processes (treatment and/or flow management processes) that 

would be required to address these pollutants or parameters. 

• The permitting agencies generally are not accountable for the 

performance of the BMP, and thus give much leeway to the 

developer with respect to the type of BMPs to be constructed, and 

to the details of the design, although some states do have detailed 

design standards and have conducted performance tests to identify 

acceptable devices for their area.

• The developer is not responsible in most all cases for the 

performance of the BMP, so the treatment facilities are designed to 

minimize the cost and/or area of the facility and/or ease of 

permitting, not maximize the pollutant removal efficiency and/or 

flow management of the BMP

• Because BMPs are not held to any, or very few, long-term 

performance criteria, they are typically not maintained except for 

aesthetic purposes. Very few stormwater agencies are responsible 

for BMP maintenance on private property, and public facilities are 

maintained mostly in response to clogging and/or resultant 

drainage or aesthetic problems. Even for stormwater agency 

facilities, maintenance is often limited.

2. The principal reasons for the failure of BMP performance is improper 

BMP selection, design and/or lack of maintenance.

• The California BMP Handbooks and other local requirements leave 

too much of the BMP selection and design to the discretion of the 

designer, and thus do not address many if not all of the receiving 

water quality issues
Storm Water Panel on Numeric Limits June 19, 2006

Page 5

• BMPs need to be designed to facilitate maintenance; this is rarely 

done because it costs the developer money and the BMP designer 

is rarely responsible for the maintenance.

• Given the amount of debris in urban runoff, and the fact that the 

hydraulic capacity of many BMPs may be exceeded several to 

many times per year, BMPs require more maintenance than other 

types of stormwater control facilities. Since urban BMP 

maintenance is generally left to untrained homeowner associations 

and maintenance personnel for commercial properties, inadequate 

maintenance is a near certainty. Even stormwater agencies often 

do not have and/or apply the resources necessary to maintain 

agency owned BMPs.

3. Improvements in the design of municipal BMPs, including residential 

and commercial as well as municipally owned facilities are necessary 

to ensure better performance (i.e. sizing, geometry, inlet and outlet 

design, etc.) and to specifically target receiving water quality issues. 

The Problem with Existing Effluent Limit Approaches

Effluent limit approaches usually focus only on conventional water quality

constituents that may not be solely or at all responsible for the receiving water 

beneficial use impairments in urban receiving waters. The important stressors 

that affect many use impairments can include one or more of the following and 

may vary in importance from system to system:

• The effect of increased flows and/or volumes (i.e. 

hydromodification) that can lead to stream channel 

erosion/sedimentation with resulting habitat destruction

• Sediment contamination (such as enrichment of urban stream 

sediments with fine-grained heavily polluted particulates; large 

organic debris masses causing low sediment DO; settled bacteria 

causing large bacteria gradients with sediment depth etc.)

• Impaired aesthetic value (caused by gross floatables, noxious 

sediments, etc.)

• Unsafe conditions (caused by dangerous debris, highly fluctuating 

stream flows and stages, etc.)

• Dissolved and suspended pollutants that are bioavailable in the 

water column and/or result in downstream sediment contamination
Storm Water Panel on Numeric Limits June 19, 2006

Page 6

• Elevated temperatures from urban heating effects on runoff and on 

open conveyances and permanent pool BMPs

It is very difficult to determine specific causative agents or the level of control 

needed, for a specific beneficial use impairment in a receiving water body. The 

Stormwater Effects Handbook: A Tool Box for Watershed Managers, Scientists, 

and Engineers (Burton, G.A. Jr., and R. Pitt, ISBN 0-87371-924-7. CRC Press, 

Inc., Boca Raton, FL. 2002. 911 pages) was written to be used as a guide for 

stormwater managers to identify their local receiving water problems and to 

assist in identifying the causative factors. The methods described would need to 

be applied to a specific area or region to obtain an understanding of local 

conditions and problems. Although expensive, comprehensive investigations 

such as these should be considered an investment to help minimize wasteful 

expenditures due to the application of inappropriate control practices in a 

watershed. 

Monitoring for enforcement of numeric effluent limits would also be challenging. 

While spot checks could be made at some of the many outfalls in an area, there 

is wide variation in stormwater quality from place to place, facility to facility, and 

storm to storm. Coefficients of variation approaching 1 or higher are not 

uncommon and there are few factors that can be used to significantly reduce this 

variation. Analysis of the National Stormwater Quality Database indicates that 

geographical location and land use are the most important factors affecting 

stormwater quality for most constituents. Some are also affected by the 

antecedent dry period before the rain and more highly developed watersheds 

(containing large fractions of impervious areas) often show elevated “first-flush” 

concentrations in the first portion of the storms for some, but not all pollutants. 

Since the storm-to-storm variation at any outfall can be high, it may be 

unreasonable to expect all events to be below a numeric value. In a similar 

circumstance, there are a number of storms each year that are sufficiently large 

in volume and/or intensity, to exceed the design capacity volume or flow rates of 

most BMPs. Assessing compliance during these larger events represents yet 

another challenge to regulators and the regulated community.

Technical Issues

Even for conventional pollutants, there presently is no protocol that enables an 

engineer to design with certainty a BMP that will produce a desired outflow 

concentration for a constituent of concern. A possible exception is removal of 

Total Suspended Solids in extended detention basins, and some types of media 

filters. The typical approach for evaluating BMP pollutant removal efficiency has 

been percent removal; but observed removal efficiencies vary greatly from facility 

to facility and it has been demonstrated that percent removal varies directly with 

the inflow concentration. 

Few, if any, BMPs are designed using the first principles laws of physics, 

chemistry and/or biology for pollutant removal and/or flow-duration control. It will 
Storm Water Panel on Numeric Limits June 19, 2006

Page 7

take a substantial research effort, including data gathering on well-designed 

BMPs, to develop design criteria for the removal of pollutants with confidence 

intervals that enable us to make reliable estimates of the median and variance of 

the effluent concentrations to be expected from the various types of BMPs. Until 

this is done, it will be very difficult to assign legally enforceable numerical effluent 

limitations to any particular BMP.

Drawing upon the body of knowledge that currently exists regarding pollutant 

removal efficiency, it is possible to estimate mean effluent concentrations and 

variances for a number of constituents for different types of BMPs, albeit not in a 

legally enforceable sense. Effluent concentration distributions for a number of 

BMPs are available in the International BMP Database (www.bmpdatabase.org) 

from more then 250 studies throughout the US. The following outlines key issues 

that have been identified regarding the technical feasibility of setting objective 

criteria for both existing areas and new or redeveloping areas:

• Effluent concentration estimates could be made for a given 

constituent and a particular BMP from a larger number of BMPs 

than available in the BMP Database using literature values of 

percent removal and local or national data on stormwater runoff 

EMC data. However, the results from this work would be 

significantly less reliable then the BMP Database data as it could 

be biased if the influent concentrations for the studied BMP types 

did not match general urban runoff.

• Designing the facility more rigorously with respect to the physical, 

chemical and biological processes (e.g. unit processes) that are 

active in the BMP would give confidence that the BMP would 

perform at least as well, if not better than the average performance 

determined from the literature. A WEF/ASCE task force is currently 

updating their Urban Runoff Quality Management Manual of 

Practice; design guidance of BMPS will make better use of the 

physical, chemical, and biologic processes taking place in the BMP 

before, during and after a storm event. This manual will build upon 

recent research efforts employing a unit process based approach 

for BMP design and selection. These research efforts were 

supported by the Water Environment Research Foundation 

(WERF) and the National Cooperative Highway Research Program 

(NCHRP).

• A BMP designed and constructed according to a set of criteria 

described above, could be presumed to deliver an effluent with a 

mean constituent concentration and variance similar to the 

performance numbers developed from the literature if it is properly 

maintained. Enforcement would comprise periodic inspection of 

the facility using a checklist of items to be inspected. While not an 

effluent limit, this seems practical and quantifiable.
Storm Water Panel on Numeric Limits June 19, 2006

Page 8

• Most all existing development rely on non-structural control 

measures, making it difficult, if not impossible to set numeric 

effluent limits for these areas because little is known about the 

quantity and quality performance of non-structural controls. 

However, certain development characteristics in some existing 

development areas that minimize the amounts of impervious areas 

in a drainage area have been shown to be quite effective in 

reducing adverse hydromodifications in the receiving waters, and 

should be encouraged.

Municipal Recommendations

It is not feasible at this time to set enforceable numeric effluent criteria for 

municipal BMPs and in particular urban discharges. However, it is possible to 

select and design them much more rigorously with respect to the physical, 

chemical and/or biological processes that take place within them, providing more 

confidence that the estimated mean concentrations of constituents in the 

effluents will be close to the design target. Moreover, with this more rigorous 

design and an enforceable maintenance program, it can be presumed that these 

facilities will continue to deliver effluent qualities that are reasonably close to the 

design effluent concentrations over the life of the facility. And if proper 

maintenance is performed (enforced), the facilities can be expected to perform 

throughout their design life at the same or better efficiency as when newly 

constructed. Depending on the pollutants and parameters of concern and BMP 

choices, it is very likely that treatment trains of structural BMPs will be required in 

many cases.

For catchments not treated by a structural or treatment BMP, setting a numeric 

effluent limit is basically not possible. However, the approach of setting an 

“upset” value, which is clearly above the normal observed variability, may be an 

interim approach that would allow “bad actor” catchments to receive additional 

attention. For the purposes of this document, we are calling this “upset” value an 

Action Level because the water quality discharged from such locations are 

enough of a concern that most all could agree that some action should be taken. 

Action Levels could be developed using at least three different approaches. 

These approaches include: 1) consensus based approach; 2) ranked percentile 

distributions; 3) statistically-based population parameters. 

The consensus-based approach would be to agree upon effluent concentrations 

that all parties feel are not acceptable. For example, most parties would likely 

agree that an average concentration of dissolved copper above 100 ug/l from an 

urban catchment would not be acceptable. This would be an Action Level value 

that would trigger an appropriate management response. This approach may not 

directly address the issue of establishing numeric effluent criteria and achieving 

desired effluent quality, but the consensus-based approach would ensure that 

the “bad actor” watersheds received needed attention.
Storm Water Panel on Numeric Limits June 19, 2006

Page 9

The ranked percentile approach (also a statistical approach) relies on the 

average cumulative distribution of water quality data for each constituent 

developed from many water quality samples taken for many events at many 

locations. The Action Level would then be defined as those concentrations that 

consistently exceed some percentage of all water quality events (i.e. the 90th

percentile). In this case, action would be required at those locations that were 

consistently in the outer limit (i.e. uppermost 10th percentile) of the distribution of 

observed effluent qualities from urban runoff.

The statistically based population approach would once again rely on the 

average distribution of measured water quality values developed from many 

water quality samples taken for many events at many locations. In this case, 

however, the Action Level would be defined by the central tendency and variance 

estimates from the population of data. For example, the Action Level could be 

set as two standard deviations above the mean, i.e. if measured concentrations 

are consistently higher than two standard deviations above the mean, an Action 

situation would be triggered. Other population based estimators of central 

tendency could be used (i.e. geomean, median, etc.) or estimates of variance 

(i.e. prediction intervals, etc.). Regardless of which population-based estimators 

are used (or percentile from above), the idea would be to identify the [statisticallyderived] point at which managers feel concentrations are significantly beyond the 

norm.

The ranked percentile and population-based estimators are highly dependent 

upon the data sets used to calculate them. There are a number of options that 

were considered by the Panel, but ultimately they were broken into two distinct 

categories. The first category was for new development/redevelopment and the 

second was for built out urban environments. For new 

development/redevelopment, the panel recommends using the data set 

associated with the international BMP database (www.bmpdatabase.org). This 

data set represents the variety of water quality from the most up to date, best 

conducted and reported BMP studies. The database effort does not limit itself to 

BMPs types or designs; it focuses on technically sound monitoring studies and 

reporting information. Therefore there could be some screening of studies to 

those thought to be well designed BMPs to then develop effluent quality 

distributions and statistics on performance. Certainly, there is no expectation that 

urban stormwater managers could improve water quality beyond what would be 

reported in this dataset. 

In built-out urbanized environments, there are greater opportunities to examine 

various data sets for setting Action Levels. For the Panel, these opportunities 

were a function of spatial scale. The first opportunity would be at the local scale. 

Some urban stormwater monitoring programs have been in existence for 10 

years or longer. Examples include the Los Angeles County Department of Public 

Works, City of Sacramento, Orange County, San Diego County, amongst others. 

Using permit specific data sets may make sense if issues of climatic variability or 
Storm Water Panel on Numeric Limits June 19, 2006

Page 10

localized geomorphology are important. The next scale would be to combine 

these California municipal permit monitoring data sets, especially if lack of data 

for specific constituents of concern in any one location or region is an important 

issue. The largest scale would be the National Stormwater Quality Database 

(NSQD) from municipal monitoring programs across the nation 

(http://unix.eng.ua.edu/~rpitt/Research/ms4/Paper/Mainms4paper.html). This 

data set includes monitoring data from urban areas such as residential, 

commercial, industrial, freeway, institutional, and mixed use which is especially 

useful if small sample size limits the use of local data. One advantage of using 

smaller (and local), rather than larger, spatial scales is the ability to update data 

sets for revising Action Levels. The NSQD may not be updated for quite some 

time, but local data sets can be updated periodically (annual amendments, 10-

year rolling averages, every permit cycle, etc). Ultimately, Action Levels would 

be expected to become lower as outliers are removed from data sets and as 

improved water quality data are collected through targeted management actions. 

It may be appropriate to eliminate older data sets as well over time.

One element to consider when comparing monitoring data to Action Levels is the 

concept of a design volume for water quality (also known as the Water Quality 

Capture Volume – WQCV, WEF #23 and ASCE publication #87, 1998) or a 

design flow rate. The WERF and NCHRP efforts mentioned above include 

recommendations regarding design sizing using continuous simulation 

techniques for both volume-based and rate-based BMPs. The Panel 

acknowledged that several to more times each year, the runoff volume or flow 

rate from a storm will exceed the design volume or rate capacity of the BMP. 

Stormwater agencies should not be held accountable for pollutant removal from 

storms beyond the size for which a BMP is designed. 
Storm Water Panel on Numeric Limits June 19, 2006

Page 11

A Technically Sound and Pragmatically Enforceable BMP Design and the Permit 

Process

The diagram below provides guidance for determining what BMPs are required in 

a newly developing watershed. Under Condition 1 where the receiving water 

quality is not impaired, determination of the appropriate BMP would be by Best 

Professional Judgment (BPJ). Any of the “state approved” BMPs could be used. 

The permittee would be required to design the treatment facilities in accordance 

with the California BMP Handbook, which should be revised as a criteria 

manual, rather than a guidance 

manual and include more 

physiobiochemically based design 

criteria designed to address an agreed 

upon set of “Pollutants and 

Parameters of Concern” based upon 

knowledge of the pollutants and 

parameters that generally are of 

concern in urban runoff, with perhaps 

some differences on receiving water 

type.

A detailed maintenance plan and 

schedule would be required that 

includes:

1. Actions to be taken and when,

2. Designation of the party legally 

accountable for the facility 

maintenance, and 

3. A whole-life cost estimate for 

the facility that include 

maintenance. 

Compliance with the design criteria 

and the maintenance plan and 

schedule would constitute 

achievement of the design effluent 

criteria. In the event of failure by the 

responsible party to perform the 

required maintenance and/or to 

perform it to the required level of 

quality, the whole-life cost schedule 

could be used to determine the 

consideration that the defaulting 

responsible party would pay to the 

new responsible party that takes over 

the maintenance.

Identify 

Receiving Water 

Body

Water Body

303d listed?

Require

TechnologyBased BMPs

BPJ

Identify 

Constituents of 

Concern

Have

TMDLs

been set?

Require BMP(s)

 by BAT for 

Constituents of 

Concern

BMPs selection 

based on 

removal 

efficiency

yes

yes

no

no

Condition 1

Condition 2

Condition 3

Monitor BMP 

Maintenance for 

Compliance

Monitor BMP 

Maintenance for 

Compliance

Monitor BMP 

Maintenance for 

Compliance
Storm Water Panel on Numeric Limits June 19, 2006

Page 12

Under Condition 2 where water quality impairment exists but a TMDL has not 

yet been performed, BAT would be required, which means applying the BMPs 

that can practicably (to be defined) be employed to produce the lowest effluent 

concentrations (e.g. the lower grouping of BMP effluent quality) of the 

constituent(s) of concern. Several types of BMPs may fulfill the BAT standard if 

these BMPs have performance that is not statistically or practically differentiable. 

This case will allow flexibility in choosing among that sets of BMPs that 

demonstrate superior performance. As in the case of Condition 1, compliance 

with the maintenance plan and schedule would constitute compliance with the 

design effluent criteria.

Condition 3, which occurs when a TMDL has been specified for the BMP or for 

the tributary watershed, may (or may not be) actually be less stringent that 

Condition 2 if the TMDL allows for a higher effluent concentration of the 

constituents of concern than that discharged by a BAT facility. The same 

requirements would apply for the design criteria, and the maintenance plan and 

schedule would constitute the guarantee of design effluent concentrations from 

the BMP.

Strategies for Stormwater Management to Protect Urban Water Environments

Stormwater effluent limits can become very complex if all the issues are to be 

directly addressed. If complex, they are not likely to be workable. However, too 

much simplification can also lead to ineffective programs. Therefore, a 

reasonable first step is needed, based on local data. Compliance monitoring (e.g. 

BMP inspections) is also needed to ensure that the goals are likely to be met. 

Most likely goals will have to be revised over time. The overall strategy should 

contain these objectives:

• Effectiveness

• Affordability

• Enforceability, and

• Flexibility
Storm Water Panel on Numeric Limits June 19, 2006

Page 13

Table 1 - Effects of Urbanization on Hydrologic Regime in Colorado and Georgia

Annual 

Precipitation

Mean 

Storm 

Depth* Runoff Events per Year Annual Runoff (mm)

Location

Millimeters 

per Year Millimeters Undeveloped Developed Undeveloped Developed

Fort Collins, 

CO 335 11 27 47 12 124

Atlanta, GA 1262 18 48 78 36 500

* Values obtained from Fig. 5.3 ASCE MOP (1998)

Runoff volume and peak flows have been recognized as two of the most 

important stormwater factors needing control. Table 1 (Roesner and Nehrke) 

shows that urbanization dramatically changes the hydrologic regime of urban 

waterways. In both Atlanta (a higher rainfall area) and Fort Collins (a semiarid 

area), the number of runoff events per year on developed land increases by a 

factor of 2 times the number of runoff events that occur in the undeveloped state; 

and the runoff volume increases by a factor of ten! The peak flows also increase 

dramatically as shown in Figure 1 below, but as also seen on the figure, the 

peak flow frequency curve can be adjusted back to its predevelopment character 

by the proper application of runoff controls. But while these controls restore the 

peak flow frequency to its natural regime, the duration of flows at the low end (but 

still channel “working”) of the flow frequency curve is greatly increased, which 

raises potential for channel scour in stream channels with erosive soils.

Figure 1 - Exceedance Frequencies for Detention Basins in Fort Collins, Colorado

Exceedance Frequency for Detention Basins in Fort Collins, Colorado

0.001

0.010

0.100

1.000

10.000

100.00 10.00 1.00 0.10 0.01

Exceedances per Year

Uncontrolled

Undeveloped

100&Rmstr BMP for 

Hist.1.4yr Controls

1.4-yr Frequency
Storm Water Panel on Numeric Limits June 19, 2006

Page 14

Since many of the stormwater pollutants are strongly associated with 

particulates, stormwater particulate control is also often a component of 

stormwater control programs. Therefore, an effective stormwater control strategy 

that could be encouraged is a combination of several practices, listed below in 

the order of increasing events:

• On-site stormwater reuse, evapotranspiration and infiltration for the 

smallest storms and up to specific targeted events, depending on site 

limitations (soil characteristics and groundwater contamination 

potential) (usually by conservation design emphasizing infiltration, 

disconnecting paved areas, etc.) 

• Treatment of excess runoff that cannot be infiltrated, again, up to a 

specific targeted runoff volume (usually by sedimentation or filtration) 

For pollutants of concern, it should be demonstrated that the BMP(s) 

need to include the physical, biological, and/or chemical treatment 

processes that address the typical pollutants of concern and/or 

specific pollutants in the case of 303D listed water bodies or those 

with established TMDLs.

• Control of energy discharges for the channel forming events (such as 

through storage-release, focusing on flow-duration analyses and peak 

flow frequency analyses). To be most effective, this should to be 

completed under a watershed management plan and not site-by-site.

• Provide safe drainage for damaging events (conventional drainage, 

plus secondary drainage systems)

• In watersheds that are already experiencing damaging flow impacts to 

streams, it could be in many circumstances much more cost-effective 

(and effective period) to develop through a watershed plan a natural 

stream stabilization approach that could address both the existing 

development and the remaining smaller infill or otherwise smaller new 

development. In these cases, requiring the remaining new 

development to implement flow-duration control would not solve the 

issue in a measurable way and resources would be better spent 

restoring the functions of the creek with instream enhancements.
Storm Water Panel on Numeric Limits June 19, 2006

Page 15

Panel’s Findings on Feasibility of Numeric Effluent Limits 

Applicable to Construction Activities

Construction Observations

Regarding the question of the technical feasibility of Numeric Limits for 

stormwater discharges from construction activities, the Panel bases its 

recommendations on the following observations.

1. Limited field studies indicate that traditional erosion and sediment controls 

are highly variable in performance, resulting in highly variable turbidity 

levels in the site discharge.

2. Site-to-site variability in runoff turbidity from undeveloped sites can also be 

quite large in many areas of California, particularly in more arid regions 

with less natural vegetative cover and steep slopes.

3. Active treatment technologies involving the use of polymers with relatively 

large storage systems now exist that can provide much more consistent 

and very low discharge turbidity. However, these technologies have as yet 

only been applied to larger construction sites, generally five acres or 

greater. Furthermore, toxicity has been observed at some locations, 

although at the vast majority of sites, toxicity has not occurred. There is 

also the potential for an accidental large release of such chemicals with 

their use

4. To date most of the construction permits have focused on TSS and 

turbidity, but have not addressed other, potentially significant pollutants 

such as phosphorus and an assortment of chemicals used at construction 

sites.

5. Currently, there is no required training or certification program for 

contractors, preparers of soil erosion and sediment control Stormwater 

Pollution Prevention Plans, or field inspectors.

6. The quality of stormwater discharges from construction sites that 

effectively employ BMPs likely varies due to site conditions such as 

climate, soil, and topography. 

7. The States of Oregon and Washington have recently adopted similar 

concepts to the Action Levels described earlier.

Construction Recommendations

It is the consensus of the Panel that active treatment technologies make Numeric 

Limits technically feasible for pollutants commonly associated with stormwater 

discharges from construction sites (e.g. TSS and turbidity) for larger construction 

sites. Technical practicalities and cost-effectiveness may make these 

technologies less feasible for smaller sites, including small drainages within a 

larger site, as these technologies have seen limited use at small construction 

sites. If chemical addition is not permitted, then Numeric Limits are not likely 

feasible. Whether the use of Numeric Limits is prudent, practical or necessary to 

more effectively achieve nonpoint pollution control is a separate question that 
Storm Water Panel on Numeric Limits June 19, 2006

Page 16

needs to be answered, but is outside the scope of this Panel. However, Action 

Levels are likely to be more commonly feasible. For small sites or smaller 

drainages within larger sites, or where chemicals cannot be used, the Panel 

recommends that Action Levels be specified.

Advanced systems lend themselves to Numeric Limits because of historically 

reliable treatment, while non-active controls are less predictable. Advanced 

systems have been in use in some form since the mid-1990s. At this time, there 

are two general types of systems. With each general system the stormwater is 

retained on-site, treated, and released more slowly. One system employs 

polymer coagulation and sedimentation. The second system employs polymer 

coagulation with direct filtration. Both types of systems are considered reliable, 

and can consistently produce a discharge less than 10 NTU. These systems 

have been used successfully at many sites in several states since 1995 to 

reduce turbidity to very low levels. Non-active erosion and sediment control 

BMPs, while effective when applied and adequately maintained, produce more 

highly variable in effluent quality, making setting Numeric Limits difficult, if not 

impossible. 

An important consideration in setting Numeric Limits or Action Levels is that in 

many locations in California the natural background turbidity and/or TSS levels in 

stormwater runoff are quite high. This is particularly true in semi-arid or arid 

regions, which tend to have less vegetative cover. For example, natural runoff 

concentrations in Emerald Creek, on the Newport Coast, above any developed 

areas have been over 5,000 mg/l during runoff events. The Los Angeles County 

Monitoring Data sets included an open land use watershed that also showed 

TSS levels significantly above other types of urban land uses. Therefore, it is 

important to consider natural background levels of turbidity or TSS in setting 

Numerical Limits or Action Levels for construction activities. The difficulty in 

determining natural background concentrations/levels for all areas of the state 

could make the setting of Numeric Limits or Action Levels impractical from an 

agency resource perspective.

While the Panel concludes that Numeric Limits or Action Levels are technically 

feasible, the Panel has several reservations and concerns.

1. The active treatment systems have generally been employed on sites five 

acres or larger. While the systems are technically feasible for sites of any 

size, including sites or drainages as small as an acre or less, the cost may 

be prohibitive. The cost-effectiveness of active treatment systems is 

greatly enhanced for large drainage areas, at which construction occurs 

for an extended period of time, over one or more wet season. There is 

also a more “passive” active system that is employed in New Zealand that 

uses captured rainfall to release the chemical into flows entering a 

detention system that requires less instrumentation and flow measurement 

infrastructure. Even more passive systems such as the use of polymer 
Storm Water Panel on Numeric Limits June 19, 2006

Page 17

logs and filter bags are currently under development for small sites. 

Regardless, the Panel recommends that the Board give particular 

attention to improving the application of cost-effective source controls to 

small construction sites.

2. In considering widespread use of active treatment systems, full 

consideration must be given to whether issues related to toxicity or other 

environmental effects of the use of chemicals has been fully answered. 

Consideration should be given to longer-term effects of chemical use, 

including operational and equipment failures or other accidental excess 

releases.

3. Consideration should be given to the seasonality of applying Numerical 

Limits. There may be sites where summer only construction that complies 

with Action Levels may be preferred to year-round that sites that include 

winter construction that complies with Numeric Limits. In such cases, 

applying Numeric Limits to summer construction may be a disincentive to 

scheduling active grading during dry periods. Allowing summer only 

construction sites to comply with action levels would discourage winter 

construction activities.

4. Consideration should be given to whether Numeric Limits would apply to 

all construction sites or only those with significant disturbed soil areas 

(e.g. active grading, un-vegetated and/or un-stabilized soils). A site could 

meet certain conditions to be considered “Stabilized” for the runoff season. 

5. Where Numeric Limits are not feasible or where they would not apply 

during designated seasons or site conditions, the Panel recommends that 

the Board consider the concept of Action Levels for sites where only 

traditional erosion and sediment controls are applied or construction sites 

that are considered “stabilized” for the runoff season. An Action Level 

indicates a failure of BMPs (within some storm size limits). 

6. The Board should consider Numeric Limits or Action Levels for other 

pollutants of relevance to construction sites, but in particular pH. It is of 

particular concern where fresh concrete or wash water from cement 

mixers/equipment is exposed to stormwater. 

7. The Board should consider the phased implementation of Numeric Limits 

and Action Levels, commensurate with the capacity of the dischargers and 

support industry to respond. 

8. The Panel recommends that a Numeric Limit or Action Level should be 

compared to the average discharge concentration. The minimum number 

of individual samples required to represent the average discharge 

concentration for a storm will need to be defined.

9. The Board should set different Action Levels that consider the site’s 

climate region, soil condition, and slopes, and natural background 

conditions (e.g. vegetative cover) as appropriate and as data is available. 

With active treatment systems, discharge quality is relatively independent 

of these conditions. In fact, active treatment systems could result in 

turbidity and TSS levels well below natural levels, which can also be a 

problem for receiving waters.
Storm Water Panel on Numeric Limits June 19, 2006

Page 18

10. The Board should consider whether the Numeric Limits or Action Levels 

should differ between receiving waters that are water quality limited with 

respect to turbidity, sediment or other pollutants associated with 

construction, from those water bodies that are not water quality limited.

11. The Panel recommends that Numeric Limits and Action Levels not apply 

to storms of unusual event size and/or pattern (e.g. flood events). The 

determination of Water Quality Capture Volume should consider the 

differing climate regions to specify these events. 

12. The Board should set Numeric Limits and Action Levels to encourage 

loading reductions as appropriate as opposed to only numeric 

concentrations. Examples include phased construction (e.g. limited 

exposed soil areas or their duration), infiltration, and spraying captured 

runoff in vegetated areas as means to reduce loading.

13. The Panel is concerned that the monitoring of discharges to meet either 

the Action Levels or Numeric Limits may be costly. The Panel 

recommends that the Board consider this aspect.
Storm Water Panel on Numeric Limits June 19, 2006

Page 19

Panel’s Findings on Feasibility of Numeric Effluent Limits 

Applicable to Industrial Activities

Industrial Observations

The Panel believes that Numeric Limits are feasible for some industrial 

categories. Industries have control over their facilities. They control access, 

construction practices, product substitution to affect pollution prevention and the 

types of treatment systems to be used to mitigate stormwater runoff. There are 

many treatment systems or prevention practices that have been in place for 

lengthy periods, extending back to the 1980s in many cases. For example, there 

is much known today about construction materials, such as roofing materials 

(roofing composition, gutters, paints and coatings, products that abrade or tend 

to create solids or litter, etc). Other examples include development of pervious 

surfaces, or infiltration methods. 

The decision for the value of Numeric Limits should be made in one of two ways. 

When there is a TMDL that defines the permissible load for a watershed, the 

Numeric Limits should be set to meet the TMDL. Consideration mus t be given for 

both the pollutant concentration as well as the volume of runoff, since both 

contribute to the impacts that required the TMDL to be implemented. 

When there is no TMDL, the Numeric Limits should be based upon sound and 

established practices for storm water pollution prevention and treatment, using 

an approach analogous to that used in the NPDES wastewater process in the 

1970s. In this approach phased, Numeric Limits were first set that were based 

upon the use of best currently available technology, and permittees were given a 

defined period for compliance. Permits were established based upon industry 

types or categories, with the recognition that each industry has its own specific 

problems and financial viability. 

To establish Numeric Limits for industrial sites requires a reliable database, 

describing current emissions by industry types or categories, and performance of 

existing BMPs. The current industrial permit has not produced such a database 

for most industrial categories because of inconsistencies in monitoring or 

compliance with monitoring requirements. The Board needs to reexamine the 

existing data sources, collect new data as required and for additional water 

quality parameters (the current permit requires only pH, conductivity, total 

suspended solids, and either total organic carbon or oil and grease) to establish 

practical and achievable Numeric Limits. 

In cases where the industrial activity is similar to activities covered by the MS4 

permit (roofs, parking lots, etc), the approach or limits for industries should be the 

same as for MS4 permittees. In cases where the industrial activity is similar to 

land disturbance activities (e.g. landfills, gravel mines, etc.), there exists data and 

design experience with runoff control, capture and advanced treatments systems 

(e.g. systems using polymer to enhance total suspended solids removal – see 
Storm Water Panel on Numeric Limits June 19, 2006

Page 20

the construction section) that may make Numeric Limits feasible for new facilities, 

and the approach and limits should be the same as for construction permittees. 

The same conditions and issues related to active treatment discussed in the 

construction section apply here.

In cases where there is less certainty in the data for both stormwater 

characterization or BMP performance to establish Numeric Limits, there maybe 

sufficient data to establish Action Levels. Action Levels set for industrial sites 

that discharge to MS4s should not exceed those set for MS4 permittees.

The Panel recognizes that existing and new facilities may have to be treated 

differently and recommends the approach in Table 2. 

Table 2- Approach to Establish Numeric Limits or Action Levels at Existing or New 

Facilities

Numeric Limits Action Levels Notes

Indoor No Yes, similar to 

MS4

Existing 

Facility

Outdoor

Yes if data are 

adequate for the 

specific 

industrial activity 

and BMP

Yes, using 

industrial 

database

Action Levels 

should approach 

MS4 action 

levels.

Indoor Yes – BMP 

Database

Technology 

based, similar to 

MS4 New 

Development 

New 

Facility

Outdoor

No, unless 

sufficient data 

exist for the 

specific 

industrial activity 

and BMP

Yes when 

sufficient data are 

available
Storm Water Panel on Numeric Limits June 19, 2006

Page 21

Industrial Recommendations

The Panel has several reservations and concerns:

• The Panel recognizes the inadequacy of current monitoring data sets and 

recommends improved monitoring to collect data useful for establishing 

Numeric Limits and Action Levels. 

• Required parameters for future monitoring should be consistent with the 

type of industrial activity instead of the current parameters (i.e., monitor for 

heavy metals when there is reasonable expectation that the industrial 

activity will cause greater heavy metals concentrations in the storm water).

• Insofar as possible, the Panel prefers the use of California data (or 

National data if it can be shown to be applicable to CA) in setting Numeric 

Limits and Action Levels. 

• The Panel recognizes that economies of scale exist for large facilities and 

large groups of single facilities. 

• Industrial facilities that do not discharge to MS4s should have to 

implement BMPs for their non-industrial exposure (e.g., parking lots, roof 

runoff) similar to commercial facilities in MS4 jurisdictions.

• Regardless of Action Levels or Numeric Limits, the permittees should 

implement a suite of minimum BMPs – good housekeeping, employee 

training, preventing materials from exposure to rain, etc.

• SIC categories are not a satisfactory way of identifying industrial activities 

at any given site. The Board should develop a better method of 

characterizing industrial activities that can impact storm water. 

• The Panel recognizes this is a large task and recommends prioritizing the 

implementation of this approach to achieve the greatest reduction of 

pollutants statewide. 

• Increasingly, a number of industries have moved industrial activities 

indoors, preventing storm water pollution. The Panel recognizes that 

these facilities should be granted some sort of regulatory relief from 

industrial Numeric Limits or action levels, but should still be required to 

comply with MS4 permit requirements.

The Panel recognizes the need to make progress in monitoring and reducing 

storm water discharge from industrial facilities, but urges the Board to consider 

the total economic impact and not unduly penalize California industries with 

respect to industries outside of California.