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).
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:
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.
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:
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.
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:
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.
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.
TMDL MS4 permit guidance
TMDL toolkit for MS4 permit compliance
TMDL guidance and general information