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Stormwater Monitoring

MS4 permittees and regulatory agencies frequently evaluate program effectiveness through a combination of monitoring, tracking, and reporting. Requirements in MS4 permits are often written to enable iterative improvements as your program grows and learns how to best manage stormwater runoff in your community.

Stormwater monitoring refers to the gathering and analysis of information in order to understand water quality impacts from stormwater runoff and how those impacts change over time. This may include analyzing water sampling data, as well as other pertinent environmental data, but stormwater monitoring also includes the evaluation of programmatic processes and outcomes in order to revise and improve your stormwater program. It should be noted that Phase I MS4 Permits often require the permittee to implement a water quality sampling program to fulfill stormwater monitoring requirements. On the other hand, Phase II monitoring requirements are typically limited to non-sampling exercises such as conducting surveys, tracking progress toward measurable goals, and tracking visual observations in receiving waters.

The more information that your program has about water quality and stormwater runoff impacts, the more empowered your community (and your environmental regulatory agency) is to make well-informed decisions in regard to stormwater management.

Photo of stormwater discharging from a concrete drainage structure into a rock-lined drainage pathway.

Challenges in implementing monitoring programs.
Variations in monitoring approaches.
Evolution of monitoring programs.
Resources

Challenges in implementing monitoring programs

It can be challenging for local program managers to detect ambient water quality change due specifically to stormwater and correlate MS4 program actions with quantifiable outcomes. Also, in some instances, new requirements have been added to permits without carefully considering how to evaluate performance and adjust program actions over time. These adjustments sometimes increase the resources needed for monitoring and result in lengthy and intense reporting efforts for permittees with little perceived benefit to the permittee, regulator, or water quality.

It is important to note that not all municipal stormwater programs are required by their MS4 permits to implement formal stormwater monitoring programs. Because monitoring programs can be costly to implement, it might not be feasible for some municipal stormwater programs to exceed the requirements of their permit and use stormwater monitoring as a means to evaluate their success in attaining favorable water quality outcomes. However, all municipal stormwater programs should (and are typically required to) use some sort of objective metric(s) to evaluate program success at least once per year.

Monitoring, tracking, evaluation, and reporting refer to how MS4 programs:

Track activities
Evaluate progress and effects
Pose key questions to answer through monitoring
Sample stormwater runoff and/or receiving waters
Analyze results
Make program changes in response to observations
Report to permitting authorities

Variations in monitoring approaches

The U.S. EPA developed regulations for MS4s in two phases:

Phase I requires medium and large MS4s to obtain an NPDES permit coverage for their stormwater discharges.
Phase II requires small MS4s to do the same.

Both Phase I and Phase II regulations require permittees to assess their stormwater control measures (i.e., BMPs) and perform some level of reporting to regulatory authorities.

Most states are authorized to implement the NPDES program and therefore write MS4 permits. Because states attempt to write permits with clear, specific, and measurable requirements that consider their local conditions, there can be variations in the way different permittees carry out monitoring, tracking, evaluation, and reporting requirements. Differences also exist between Phase I and Phase II MS4 permits. For example, Phase I regulations require permittees to develop a water quality monitoring program; larger MS4s might have requirements that necessitate sophisticated sampling programs with annual expenditures of more than $1M. While the Phase II regulations allow monitoring, they often do not require it. As a result, some MS4 permittees do not have any monitoring programs at all.

The extent to which MS4 permittees are required to implement stormwater monitoring programs has progressed over time. Initially, programs attempted to broadly monitor receiving waters and wet weather and dry weather discharges. This progressed into attempting to measure the effectiveness of specific program elements, post-construction stormwater management and green infrastructure in particular, as a means to better manage stormwater in the long term. More recently, the focus has expanded to include effectiveness monitoring for specific BMPs and water quality-based total maximum daily load (TMDL) requirements. Permittees have taken different approaches to water quality monitoring at varying scales and with different technologies. These are explored in more depth below.

Evaluation Strategies

Other methods can be as useful as monitoring for evaluating the success of your program and making adjustments over time. These include:

Tracking progress toward measurable goals.
Tracking pollutants removed from the MS4 or from potentially entering the MS4.
Tracking visual conditions at receiving waters.
Conducting surveys to evaluate public awareness and satisfaction with the municipal stormwater program.

Evolution of monitoring programs

There have been many iterations of monitoring approaches and requirements in MS4 permits. These iterations have incorporated lessons learned as permittees developed differing approaches and as regulators and permittees worked to interpret the results. The following table documents some of these changes as exhibited in California Phase I MS4 permits. This table is intended to provide a general point of observation and does not attempt to capture status or changes in all programs nationwide.

Across the country, MS4 programs fall in different places along the continuum of program implementation and monitoring. There remains a need to improve monitoring, tracking, evaluation, and reporting approaches to better determine the effectiveness of program actions and allow for adaptive management. For smaller Phase II communities, it might be most effective to collaborate or build upon existing monitoring efforts with neighboring communities or organizations.

Though terminology varies and reflects differences in program requirements and approaches, general concepts can broadly be viewed as “monitoring, tracking, evaluation, and reporting” and can feed program effectiveness assessment efforts and program implementation adjustments. Stormwater monitoring programs should always be approached by seeking answers to specific questions. For example, does flow in the lower segment of the river, downstream from where stormwater from the industrial park has discharged, contain more or less oil and grease than the upper segment?

The diagram below illustrates a conceptual program framework model for fostering an integrated perspective between water quality monitoring and program implementation activities. This framework can help program managers manage and adjust their programs while enabling permitting authorities to evaluate compliance and adjust permits. To gain a more comprehensive view of a program’s effectiveness, activity tracking information (i.e., non-water quality data) needs to be tied to water quality data obtained through monitoring (e.g., routine monitoring, special studies). Over time, a permittee could use this integrated dataset to track and assess a program’s function and effects and report this information and lessons learned to their permitting authority.

Case Study: Los Angeles County, California

Through its principal permittee and a regional monitoring group, Los Angeles County MS4 permittees conduct monitoring in receiving waters and in-system locations during some storm events. Cause and effect connections are inferred to stormwater program actions taken upstream in the monitored watershed. Permittees use modeling (using effectiveness estimates for existing BMPs and accounting for anticipated load reductions for new BMPs) to estimate the likely overall effect of BMP implementation within the watershed and to assist with BMP targeting. The County also encourages stormwater capture and reuse to reduce runoff and pollutants into waterways.

Case Study: Washington, D.C.

Washington, D.C., has used geographically targeted BMP implementation and monitoring designed to detect “signals” in water quality change based on intensive implementation of green infrastructure BMPs in the targeted area. The efforts include interim measurable milestones so the evaluation timeline is constrained. It should be noted that this program is designed to address both MS4 and CSO LTCP/Consent Decree requirements and has come with significant investment from the community.

Figure showing Contractual Relationships Under Design-Bid-Build and Design Build

Conceptual Program Assessment Framework, Highlighting the Linkage Between Water Quality Monitoring and Evaluation of Implementation Activities to Inform Adjustments to Program Implementation.

Case Study: The Evolution of California’s Phase I Program

Early Generation Permits (1990s)	Middle Generation Permits (2000s)	Recent Generation Permits (2010s)
Programs had limited knowledge of system assets and there were few known water quality drivers to direct program implementation.	System assets were better known and there was increasing awareness of the need to address specific water quality issues and to begin iterative program improvements. Newer data management tools were starting to be used and some information on BMP effectiveness was becoming available.	There is a greater focus on specific pollutants of concern largely driven by TMDL provisions in MS4 permits. There is an increased concern about asset management and long-term maintenance of system assets. There is a broader focus on stormwater impacts and value beyond water quality (e.g., water supply augmentation, flood risk, urban amenities/climate impacts). There are new automated and sensor-based monitoring methods that can enable different monitoring designs.
Monitoring efforts were mainly focused on characterizing flows from the system and establishing baseline monitoring data for urban water quality conditions and trends. Sampling was required for a few storms per year, with little to no sampling during dry weather. Sampling was rarely conducted from MS4 outfalls; instead it was collected at convenient locations in the lower parts of watersheds to characterize “mass emissions” from all upstream MS4 discharges (often comingled with other sources and infiltration). Permittees typically designed their own monitoring programs. Paper reporting of water quality data and other program activity measures (e.g., inspections, street sweeping) through qualitative descriptions and/or semi-quantitative information.	More specific minimum control measure (MCM) requirements and narrative requirements to meet water quality standards (WQS) were included in permits. Some permits included numeric triggers or action levels for POCs and requirements for low impact development approaches for new/redevelopment. Some permits began to use surrogate indicators (e.g., flow retention, impervious cover) to reduce flows and pollutant loadings and protect receiving waters from geomorphic impacts. Monitoring efforts were mainly focused on receiving waters (rarely outfalls) to determine whether WQS were being met and whether MS4s were causing or contributing to exceedances. Permittees continued visual inspections of assets, BMPs, and dry weather flows and documented their occurrence in annual reports. There was rarely an analysis of their effectiveness.	Permits include more specific water quality-based requirements, often connected to TMDLs. Permit structure varies depending upon whether goals are expressed in terms of outcomes (numeric limits or triggers) or activities (BMP systems based on analysis of needs). Models are increasingly used to inform long-term program design and predict necessary control levels. Minimum control measures remain but, in some instances, focus on a subset that are viewed as more effective. Adoption of asset management allows for operations and maintenance activity reporting and a determination of optimal asset inspection and maintenance schedules. There is less of a focus on basic water quality trend monitoring in receiving waters and more of a focus on representative outfall monitoring to help evaluate causation. There is an increasing use of automated samplers, but in limited locations.
Monitoring program designs rarely enabled key management questions (including compliance questions) to be answered based on the collected data. Insufficient data was collected to detect pollutant trends in receiving water or distinguish among contributing land uses or geographical areas. There was insufficient evaluation and reporting to ensure that stormwater controls (e.g., post-construction BMPs) were installed and properly maintained. There was insufficient data or analysis to evaluate effectiveness of MCMs or other activities/BMPs in addressing specific water quality concerns.	There was still insufficient data collected to detect pollutant trends in receiving water or distinguish relative contributions from different land uses, geographical areas, or individual permittees. Monitoring designs did not support robust statistical analysis or provide a linkage between receiving water impacts and specific MS4 discharges (i.e., unable to answer the key question of whether the MS4 was causing or contributing to a WQS exceedance). There was still insufficient data and analysis regarding BMP effectiveness to determine whether installed BMPs were resulting in the intended benefits. Reporting and program evaluation still did not thoroughly address the effectiveness of MS4 programs in creating the desired water quality outcomes.	Improvement is still needed to evaluate the effectiveness of activities performed under the MCMs. In many cases, program implementation and monitoring requirements continue to mount while few are removed from permits. New sensor technologies are not widely being used in monitoring program design. Much receiving water and outfall monitoring still does not facilitate source analysis, compliance evaluation, or effectiveness evaluation. There is a need to better understand how increasing reliance on modeling affects monitoring and reporting needs. Modeling capacity and monitoring design will need to evolve to better account for non-water quality intended benefits (e.g., water supply augmentation through infiltration, reduced flood potential, heat island impact reductions).

Resources

The publications listed below provide more information about stormwater monitoring.

Article Description	Categories	categories_hfilter
Urban Stormwater BMP Performance Monitoring Author: Geosyntec Consultants and Wright Water Engineers, Inc. \| Developed/Updated on Date: October 2009 Web Link: http://www.bmpdatabase.org/Docs/2009%20Stormwater%20BMP%20Monitoring%20Manual.pdf This Manual provides guidance for all stages of BMP monitoring programs ranging from the early stages of study design to the end stages of data interpretation and reporting. Guidance is provided for monitoring a broad range of individual BMPs as well as overall site monitoring with multiple distributed BMPs, such as is the case with LID sites. This Manual focuses primarily on the collection, reporting, and analysis of water quantity and quality measurements at the heart of quantitative BMP efficiency projects. It does not address in detail sediment sampling methods and techniques, biological assessment, monitoring of receiving waters, monitoring of groundwater, streambank erosion, channel instability, channel morphology, or other activities that in many circumstances may be as, or more, useful for measuring and monitoring water quality for assessing BMP efficiency. In some cases, references for additional information on these subjects have been provided. The 2002 version of this monitoring manual was developed to promote collection of more useful and representative data associated with BMP studies, as well as more consistent reporting of monitoring results appropriate for inclusion in the International Stormwater BMP Database. Since that time, both the International Stormwater BMP Database and stormwater management practices have continued to evolve, prompting this second release of the manual.	Management of Post-Construction Site Runoff, Water Quality Outcomes: Stormwater Monitoring	management-of-post-construction-site-runoff stormwater-monitoring
Planning for and Assessing the Effectiveness of Stormwater Programs Author: CASQA \| Developed/Updated on Date: February 2015 Web Link: https://www.casqa.org/sites/default/files/effectiveness_assessment/final_casqa_planning_for_and_assessing_the_effectiveness_of_stormwater_programs_jun_20151.pdf The primary purpose of this Guidance Document is to establish specific “how to” guidance with examples for managers in planning and assessing their MS4 programs. It approaches effectiveness assessment as an integral part of a comprehensive strategic planning process. It is designed for use by MS4 program managers involved in developing and implementing all aspects of stormwater programs, but it should also be useful to a variety of dischargers under other stormwater permits and programs (e.g., construction and industrial), as well as other environmental managers with a need for guidance on management and assessment principles.	Water Quality Outcomes: Stormwater Monitoring	stormwater-monitoring
Monitoring to Demonstrate Environmental Results: Guidance to Develop Local Stormwater Monitoring Studies Using Six Example Study Designs Author: Center for Watershed Protection \| Developed/Updated on Date: August 2008 Web Link: https://www.epa.gov/sites/production/files/2015-11/documents/monitoring_guidance_full_report.pdf The central purpose of this manual is to provide guidance to MS4 communities on developing monitoring studies whose results can help improve their local stormwater programs by getting more pollutant reduction out of the total community stormwater investment. This manual presents six monitoring study designs that can be used by MS4 communities to assess their local stormwater programs. Many monitoring studies fail to produce useful data because of poor study design, quality control or data management issues, unforeseen field situations such as vandalism, droughts or floods, or because the study was over-scoped or under-budgeted. This manual is designed to navigate the stormwater manager through these complexities so they can be confident to develop a monitoring study and the results it produces to get the most out of their limited stormwater dollars.	Water Quality Outcomes: Stormwater Monitoring	stormwater-monitoring
Model Monitoring Program for Municipal Separate Storm Sewer Systems in Southern California Author: Stormwater Monitoring Coalition \| Developed/Updated on Date: August 2004 Web Link: https://www.waterboards.ca.gov/sandiego/water_issues/programs/stormwater/docs/wqip/2013-0001/J_References/J098.pdf This report describes a model monitoring program for receiving waters affected by urban runoff in both wet and dry weather. It provides a common design framework for municipal urban runoff programs and Regional Board staff to use in developing and/or revising program requirements for monitoring receiving waters for impacts, status and trends, toxicity, mass emissions, and source identification. This effort was funded in part by the State Water Resources Control Board (SWRCB), prompted by Senate Bill 72 (Kuehl), which addressed the standardization of sampling and analysis protocols in municipal stormwater monitoring programs. The development of the model monitoring program itself was organized through the Southern California Stormwater Monitoring Coalition (SMC).	Illicit Discharge Detection and Elimination, Water Quality Outcomes: Stormwater Monitoring	illicit-discharge-detection-and-elimination stormwater-monitoring
Interstate Technology Regulatory Council’s Stormwater Best Management Practices Performance Evaluation Author: ITRC \| Developed/Updated on Date: November 2018 Web Link: https://stormwater-1.itrcweb.org/ This guidance offers the user details on post-construction BMP lifecycle processes including contracting, cost considerations, installation factors including construction challenges, inspection checklists, quality control, and record drawings. It addresses long-term technology- and performance-based operational strategies, including aspects such as routine and non-routine maintenance. Data and information from existing publicly available BMP performance programs has been incorporated into an online BMP Screening Tool. Using site-specific pollutant treatment requirements and installation considerations, the BMP Screening Tool can assist the user by identifying a list of BMPs that may be appropriate for a given site. The BMP Screening Tool also provides users summarized information on the treatment efficiency, installation requirements, and maintenance issues regarding the identified BMPs, with links to access more detailed information.	Water Quality Outcomes: Stormwater Monitoring	stormwater-monitoring
Improving Stormwater Program Monitoring, Evaluation, Tracking, and Reporting: Workshop Report and Recommendations Author: U.S. EPA Region 9 \| Developed/Updated on Date: October 12, 2018 Web Link: https://www.epa.gov/sites/production/files/2018-10/documents/improving_stormwater_program_monitoring-10-12-2018.pdf In March 2018, EPA Region 9, with assistance from EPA Headquarters and in partnership with the State of California, invited 31 stormwater experts from across the country to Oakland, California, for a two-day workshop titled Improving Stormwater Permit Approaches to Monitoring, Tracking, Evaluation, and Reporting. Participants included representatives from EPA, state Clean Water Act permitting agencies, local stormwater programs, national associations, consulting firms, and nonprofit organizations. The workshop was designed to explore current requirements and practices for municipal stormwater program monitoring, evaluation, tracking, and reporting and identify opportunities for improvement that would support more effective program implementation. This report aims to provide a synthesis of participant ideas and contributions, along with other existing research, to identify the most promising opportunities for strengthening MS4 permits and program implementation. It includes an overview of the workshop discussions, specific recommended actions, case studies, summaries of known efforts related to the recommendations, and, where possible, some indication of commitment by stakeholder groups or organizations (e.g., trade associations, permittees, states, universities) to make progress in a given area. The table on the following page presents a brief synopsis of the recommendations.	Water Quality Outcomes: Stormwater Monitoring	stormwater-monitoring
BMP Performance Monitoring Data Compilation to Support Reasonable Assurance Analyses Author: U.S. EPA \| Developed/Updated on Date: Summer of 2002 Web Link: http://ftp.sccwrp.org/pub/download/ DOCUMENTS/TechnicalReports/1081_ BMPPerformanceRAA.pdf This project roughly doubled the existing data set for flow-through BMPs in California. Influent-effluent monitoring of 81 different flow-through structural BMPs was compiled, totaling 1,700 site-events (e.g., storm-BMP combinations). The most site-events were compiled for vegetated swales (22.4%), media filters (21.5%), and constructed wetlands (38.6%). However, the vast majority of constructed wetland site-events were from just two BMPs, hindering extrapolation and uncertainty analysis to other constructed wetlands. The dry pond (5.8%), wet pond (7.4%), permeable pavement (0.1%), and bioretention systems (4.2%) provided too few site-events for conducting uncertainty analysis.	Water Quality Outcomes: Stormwater Monitoring, Water Quality Outcomes: Watershed-Based Models	stormwater-monitoring watershed-based-models
A Strategic Approach to Planning for and Assessing the Effectiveness of Stormwater Programs Author: California Stormwater Quality Association \| Developed/Updated on Date: February 2015 Web Link: https://www.casqa.org/sites/default/files/effectiveness_assessment/final_casqa_planning_for_and_assessing_the_effectiveness_of_stormwater_programs_jun_20151.pdf The primary purpose of this guidance document is to establish specific “how to” guidance with examples for managers in planning and assessing their MS4 programs. It approaches effectiveness assessment as an integral part of a comprehensive strategic planning process. It is designed for use by MS4 program managers involved in developing and implementing all aspects of stormwater programs, but it should also be useful to a variety of dischargers regulated under other stormwater permits and programs (e.g., construction and industrial), as well as other environmental managers with a need for guidance on management and assessment principles. A structured approach to planning and assessing stormwater programs can help managers ensure that their programs are properly targeted, determine whether intended results are being efficiently and cost-effectively achieved, relate implementation results to conditions in urban runoff and receiving waters, and, ultimately, help guide managers toward implementation strategies with the greatest opportunity for long-term success.	Good Housekeeping in Municipal Operations, Illicit Discharge Detection and Elimination, Management of Construction Site Runoff, Management of Post-Construction Site Runoff, Program Goals and Management: Developing a Program, Program Goals and Management: Long-Term Planning, Program Goals and Management: Vision and Goals, Public Education and Outreach, Public Participation, Water Quality Outcomes: Stormwater Monitoring	good-housekeeping-in-municipal-operations illicit-discharge-detection-and-elimination management-of-construction-site-runoff management-of-post-construction-site-runoff developing-a-program long-term-planning vision-and-goals public-education-and-outreach public-participation stormwater-monitoring

Stormwater Monitoring

Contents

Challenges in implementing monitoring programs

Variations in monitoring approaches

Evolution of monitoring programs

Case Study: The Evolution of California’s Phase I Program

Resources

Urban Stormwater BMP Performance Monitoring

Planning for and Assessing the Effectiveness of Stormwater Programs

Monitoring to Demonstrate Environmental Results: Guidance to Develop Local Stormwater Monitoring Studies Using Six Example Study Designs

Model Monitoring Program for Municipal Separate Storm Sewer Systems in Southern California

Interstate Technology Regulatory Council’s Stormwater Best Management Practices Performance Evaluation

Improving Stormwater Program Monitoring, Evaluation, Tracking, and Reporting: Workshop Report and Recommendations

BMP Performance Monitoring Data Compilation to Support Reasonable Assurance Analyses

A Strategic Approach to Planning for and Assessing the Effectiveness of Stormwater Programs