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Minnesota Stormwater Manual

Recommendations and guidance for utilizing major stormwater outfall monitoring to meet TMDL permit requirements
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image outfall monitoring
example of outfall monitoring. Photo courtesy of MPCA's Flickr site.

The primary reasons for an MS4 to monitor water quality at major storm sewer outfalls (i.e., major outfalls) are to (a) evaluate compliance with MS4 wasteload allocations (WLAs) established in applicable TMDLs, and (b) evaluate compliance with water quality standards established in Minnesota Rule, Chapter 7050 (MINN. R. 7050, 2018). Demonstrating compliance with established WLAs or applicable water quality standards are two methods by which an MS4 can demonstrate it is meeting pollutant loading goals established within the applicable TMDL.

Note: established WLAs should be reviewed before attempting to evaluate compliance with applicable water quality standards, as the water quality standard applicable to the receiving water body may not be reasonably achieved at a stormsewer outfall. In these situations, the MS4 should evaluate compliance with the established WLA.

The following subsections outline the basics of developing an outfall monitoring program to meet goals established within the Guidance Manual for Assessing Quality of Minnesota Surface Waters for Determination of Impairment: 305(b) Report and 303(d) List (MPCA, 2018a), evaluating compliance with WLAs established in applicable TMDLs, and developing an outfall monitoring protocol standard operating procedure (SOP). Specifically, the Establishing a Monitoring Program section provides guidance on when, where, and what to sample, while the Establishing a Monitoring Protocol section provides guidance on how to sample (i.e., sampling SOP).

Establishing a Monitoring Program

Before developing or implementing an outfall monitoring program, the applicable TMDL(s) should be reviewed to determine (a) if an ongoing water quality monitoring program is or will be implemented and (b) what organization(s) are responsible for water quality monitoring (e.g., individual MS4s, the applicable Watershed District or Watershed Management Organization (WMO), etc.). Review of ongoing and planned water quality monitoring programs can help an individual MS4 permittee optimize sampling procedures to supplement ongoing monitoring efforts, and can help individual MS4 permittees identify potential partnering opportunities with other organizations (e.g., WDs, WMOs, Citizen-Assisted Monitoring Program (CAMP), Soil and Water Conservation Districts (SWCDs), Metropolitan Council, etc.).

In addition to review of planned or ongoing monitoring programs, the applicable TMDL(s) and drainage areas to major outfalls within the MS4 should be reviewed to determine the following information:

  • TMDL review:
    • What portion of the MS4 is included in the TMDL (Note: TMDL subwatersheds may be obtained from the MPCA)? The entire municipal boundary? A fraction of the MS4 area?
    • How is the MS4 wasteload allocation (WLA) structured (e.g. required pounds of reduction per year (lbs/yr); required areal loading rate (lbs/acre/year); etc.)?
  • MS4 outfall and drainage area review:
    • Is stormwater from the MS4 collected into major outfalls (i.e., outfalls which drain a large area and can be monitored), or are there many small, diffuse outfalls throughout the MS4 area?
    • What portion of the TMDL watershed area drains to major outfalls?
    • What is the total drainage area to major outfalls? Do drainage areas to major outfalls include area from other, neighboring MS4s?
  • Water quality standard review:
    • What water quality standard(s) (MINN. R. 7050, 2018) apply to the impaired waterbody?

The steps outlined in the following subsections can be used to determine if outlet monitoring is a viable strategy to demonstrate WLA compliance, and, if so, how to develop a monitoring program to evaluate WLA compliance.

Outfall monitoring feasibility

A critical first step to developing an outfall monitoring program is to evaluate the feasibility of outfall monitoring for evaluating WLA compliance. A detailed review of drainage areas to major outfalls should be conducted and compared to the impaired waterbody drainage area established in the TMDL.

Note: if the TMDL drainage area is not adequately outlined within TMDL documentation, TMDL drainage area documentation can be requested directly from the MPCA.

The following criteria can be used to determine if an MS4 is a good candidate for an outfall monitoring program:

  • Good candidate for outfall monitoring:
    • MS4 storm sewer is consolidated into a small number of large outfalls.
    • Drainage areas to large outfalls coincide well with the TMDL watershed area.
    • Outfalls are easily accessible for monitoring and data analysis.
    • Sufficient staff and resources (e.g., time, equipment, etc.) to conduct outflow monitoring.
  • Poor candidate for outfall monitoring:
    • MS4 storm sewer discharges to a large number of small outfalls.
    • Significant portions of the MS4 discharge via overland runoff (i.e., significant portions of the MS4 are not tributary to outfalls).
    • Outfalls are difficult or dangerous to access.
    • Insufficient staff and resources to conduct outflow monitoring.

If outfall monitoring is not feasible, monitoring receiving waters or monitoring BMPs may be used by MS4s to evaluate WLA compliance and impairment status of receiving waters. If outfall monitoring is feasible, this section provides monitoring program recommendations for evaluating compliance with areal loading WLAs (e.g., pounds of TSS per acre per year (lbs/acre/year) and this section provides recommendations for evaluating compliance with water quality standards applicable to the impaired receiving water.

Areal loading rate

Recommendations for outfall monitoring program development depend on how MS4 WLAs are established in the applicable TMDL(s). Outfall monitoring can be used to directly evaluate WLAs that are developed as a required areal loading rate (e.g., pounds of TSS loading per acre per year (lbs/acre/year)). The following list outlines specific outfall monitoring program recommendations to evaluate compliance with areal loading rate WLAs. Specific monitoring protocol recommendations are discussed here. Outfall Monitoring Program Recommendations for Evaluating Areal Loading Rate:

  • Monitored area:
    • Monitor all major outlets discharging to the impaired waterbody (see feasibility discussion above).
    • Calculate the total drainage area (acres) to each monitored outlet within the TMDL watershed area. These areas will be used to calculate the areal loading rate at each monitoring location.
  • Monitored parameters:
    • Minimum: flow rate (e.g., using depth-velocity monitoring) and the pollutant for which the WLA areal loading rate was developed (e.g., TP).
    • Recommended: flow rate, the pollutant for which the WLA areal loading rate was developed (e.g., TP), and any associated response variables used to define impairment (e.g., chlorophyll-a, BOD5, etc.).
  • Monitoring period:
    • Minimum: the seasonal duration used to define impairment for the given pollutant (e.g., April 1 through September 30 for TSS).
    • Recommended: year-round monitoring if sufficient baseflow to prevent freezing conditions at monitoring location. If freezing occurs at monitoring station, monitor April through November.
  • Sampling protocol:
  • Continual monitoring using continuous flow loggers and automated or in situ sampling is required to accurately evaluate areal loading rate (see discussion here).

Compliance with water quality standards

Outfall monitoring can be used to directly evaluate compliance with areal loading rate WLAs, but is less suited to evaluating WLAs if the form of annual loading (e.g., lbs/yr, lbs/day), required annual reduction (lbs/yr), and required percent reduction from baseline conditions. This is because (a) it is unlikely the outfall watershed(s) will match exactly to the MS4 watershed area established in the TMDL, complicating the comparison of outfall loading to WLA annual loading established in the TMDL, and (b) outlet monitoring alone cannot be used to determine pollutant reduction achieved by BMPs within the watershed (see [https://stormwater.pca.state.mn.us/index.php?title=Recommendations_and_guidance_for_utilizing_monitoring_to_meet_TMDL_permit_requirements#Monitoring_BMPs section on monitoring BMPs).

For this reason, rather than using outfall monitoring to directly evaluate WLA compliance, MS4s with non-areal loading rate based WLAs (e.g., annual loading, annual load reduction, etc.), can instead use outfall monitoring to evaluate compliance with water quality standards applicable to the impaired waterbody. Demonstrating that outflow from the MS4 is compliant with water quality standards applicable to the impaired waterbody demonstrates that the MS4 is not contributing to impairment. The following list outlines specific recommendations for the development of an outfall monitoring program to evaluate compliance with applicable water quality standards. Specific monitoring protocol recommendations are discussed here.

Establishing a Monitoring Protocol

The MPCA has specific requirements and protocols related to water quality data collection, laboratory processing of water quality samples, data processing of laboratory results, and final submittal of water quality data to the MPCA for review. Specifically, the MPCA uses a data processing system called EQuIS (Environmental Quality Information System) to store and process water quality data collected from over 17,000 sampling locations across the state, and requires that data be collected and processed in a manner which is compatible with EQuIS. A detailed description of the EQuIS system, including preparation of field monitoring results for submittal to the MCPA is presented here.

Although the MPCA has not developed SOP guidance specifically related to performing storm sewer outlet monitoring, much of the guidance in the Standard Operation Procedures: Intensive Watershed Monitoring – Stream Water Quality Component (MPCA, 2018d) is directly applicable to sampling from large storm sewer outfalls.

In addition to the general stream monitoring SOP (MPCA, 2018d), two additional outfall monitoring guidance documents are discussed, below: the Capitol Region Watershed District (CRWD) 2015 Stormwater Monitoring Report (CRWD, 2015) and the University of Minnesota (UMN) Optimizing Stormwater Treatment Practices (Erickson et al, 2013). These documents provide specific guidance related to outfall monitoring and outfall monitoring equipment, including automated sampling techniques and equipment, but do not provide detailed SOP instructions (e.g., pre-sampling requirements, equipment and supply checklists, on-site sampling procedures, health and safety protocols, etc.), which should supplement the general stream monitoring SOP (MPCA, 2018d).

It is recommended that the documents reference herein be used in conjunction with specific monitoring guidance outlined in the applicable TMDL when developing an outfall monitoring SOP. The following subsection provides an example of how outfall monitoring can be used to evaluate compliance with an areal loading rate WLA.

Click here to link to a case study.

Additional Resources

  • CRWD 2015 Stormwater Monitoring Report (CRWD, 2015): this document provides a detailed summary of outlet monitoring performed throughout the watershed district in 2015. The document describes how the CRWD established an outflow monitoring network, and highlights how continuously monitoring “full water quality stations” were established using area-velocity flow sensors and automated water samplers. Additionally, the document discusses monitoring QAQC procedures and describes data processing and algorithms used to partition flows (i.e., separate baseflow from event flow), calculate event pollutant loading, baseflow pollutant loading, flow-weighted average concentration calculations, and pollutant areal yield (lbs/acre).
  • Optimizing Stormwater Treatment Practices (Erickson et al, 2013): this document provides detailed information on water quality sampling approaches (discrete sampling, composite sampling, manual, time-weighted, and flow-weighted sampling), and information on “continuous” flow sampling with automated samplers and in situ monitoring equipment (i.e., monitoring probes capable of monitoring pollutants in real time).
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