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Recommendations and guidance for using monitoring to meet TMDL permit requirements

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monitoring image

Example of lake monitoring. Image courtesy MPCA Flickr site

Monitoring guidance presented below will assist Municipal Separate Storm Sewer System (MS4) permittees evaluate and demonstrate compliance with total suspended solid (TSS), total phosphorus (TP), bacteria, and chloride wasteload allocations (WLAs); and evaluate the impairment status of waterbodies and monitor the impact of total maximum daily load (TMDL) implementation strategies.

The guidance on in this section provides specific recommendations related to four (4) monitoring strategies.

The four (4) monitoring strategies were selected based on input from and coordination with the Minnesota Pollution Control Agency (MPCA) and response of MS4 permittees to a voluntary survey conducted by the MPCA. Throughout this document, “MS4s” refers specifically to National Pollutant Discharge Elimination System (NPDES) regulated MS4s (i.e., Phase I and Phase II MS4s required to obtain NPDES permit coverage for their stormwater discharges).

The following table provides a summary of how each of the four (4) monitoring strategies can be used to evaluate WLA compliance and evaluate waterbody impairment status. Before adopting monitoring strategies discussed below, the applicable TMDL(s) and MS4 WLA(s) should be reviewed and compared to recommendations in the table below. Additionally, 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.).

Note: regardless of whether an ongoing monitoring program has been established, individual MS4s may independently monitor water quality to supplement data collected by other organizations to help track and evaluate impairment status, while employing proper quality assurance/quality control (QA/QC) methods for data collection and reporting. 

MS4 TMDL compliance monitoring strategy summary
  Monitoring strategies
MS4 TMDL Compliance Strategy Monitoring receiving waters - lakes Monitoring receiving waters - streams Monitoring outfalls Monitoring BMPs
Monitor improving water quality and delist from 303(d) Impaired Waters List X X    
Upstream/downstream monitoring to identify “hot spots” (e.g., locations where intervening flow is causing impairment)   X    
Evaluating compliance with MS4 wasteload allocation (WLA)…
WLA in terms of mass reduction (e.g., lbs/year, org/day, etc.)     X X
WLA in terms of percent reduction from baseline condition (e.g., 40% reduction)     X X
WLA in terms of areal loading reduction (e.g., lbs/acre/year)     X  

Monitoring Survey

In 2018, the MPCA conducted a survey of MS4 permittees related to water quality modeling and water quality monitoring. Half of respondents (26 of 54) actively perform water quality monitoring or are otherwise involved in the collection of water quality monitoring data, with monitoring of receiving waters being the most commonly reported form of water quality monitoring. Responses from permittees were used to inform the selection of monitoring strategies and are summarized below.

Evaluating Impairment Status

A majority of TMDL Implementation Plans include recommendations related to ongoing monitoring of the impaired waterbody. Ongoing monitoring is critical to evaluating the impact of TMDL implementation strategies (e.g., BMP implementation) as they are enacted throughout the watershed. If ongoing monitoring shows that water quality has improved such that the water body is no longer impaired, that water body can be removed (i.e., delisted) from the 303(d) Impaired Waters List. Within the Guidance Manual for Assessing Quality of Minnesota Surface Waters for Determination of Impairment: 305(b) Report and 303(d) List (MPCA, 2018a), the MPCA has provided specific data and assessment requirements needed to consider removing a water body from the 303(d) Impaired Waters List. Requirements for the four (4) pollutants are included, below.

Note: Use links in the sidebar menu to view specific guidance related to monitoring of receiving lakes and streams. Additionally, the monitoring case studies provide examples of how TMDL implementation strategies and monitoring were used to delist two impaired waterbodies: Wirth Lake, originally listed for excess nutrient (i.e., TP) impairment, and Nine Mile Creek, originally listed for turbidity (i.e., TSS) impairment.

Total suspended solids (TSS) must have the following

  • At least 20 observations (i.e., water quality samples) (pre- and post-corrective action) in the most recent 10 years, of which at least 10 observations (pre- and post-corrective action) are in the most recent 5 years; or
  • At least 20 observations in the most recent 5 years, and evidence of corrective action (i.e., actions taken to improve water quality) in the watershed of sufficient dimension (i.e., sufficient load reduction to impact water quality in the receiving waterbody) to change impairment status. In either case, there must be fewer than 10% of samples exceeding the water quality standard.

Lake eutrophication (TP) must have the following

  • At least 8 paired (i.e., collected at the same time) TP, corrected (i.e., corrected for pheophytin) chlorophyll a (chl-a), and Secchi disk measurements (June to September) over a minimum of 2 years for the most recent 10 years.
  • If TP meets the standard, and either chl-a or Secchi meet the standard, the lake will be removed from the Impaired Waters List.
  • If TP exceeds the standard and corrected chl-a AND Secchi meet the standard, and an improving trend in TP is observed or management activities are in place to maintain improved chl-a or Secchi observations, the lake may be delisted.

Delisting based on these criteria will be evaluated by the MPCA on a case-by-case basis, and will require the local entity to provide information that details how the response conditions will be met over time.

Stream eutrophication (TP) must have the following.

  • Monitoring of the causative variable (i.e., TP) and the response variable(s) (i.e., chlorophyll-a, BOD5 (biological oxygen demand), pH, or diel DO (dissolved oxygen) flux) that were used to list the AUID (Assessment Unit identifier) meet the standard within the summer period (June 1 through September 30);
  • A minimum of 12 paired samples over a minimum of 2 years of the causative and response variable(s);
  • A minimum of 20 pH samples over a minimum of 2 years; and
  • A minimum of 2 DO sonde deployments; each with a length of 4 days and occurring in separate years.

E. coli bacteria must have the following.

  • A minimum of 15 observations over a two-year period in the most recent 10 years, with a minimum of 5 observations per month for at least 3 months when the standard is applicable (April – October).
  • Impairment is evaluated by calculating the geometric mean of data collected within each month (April – October) since corrective actions were taken in the watershed of sufficient dimension to change impairment status.
    • Note: if a sufficient number of observations have been aggregated within a 2-year time period since corrective actions were taken (i.e., 5 observations per month for 3 months, April – October), the most recent 2-year time period may be used to evaluate impairment.
    • Note: a “month” can be defined as any non-overlapping 30-day period from April – October (i.e., “month” does not need to be defined by the individual calendar months from April – October).
  • To change impairment status, there must be no exceedance of the monthly mean standard (126 organisms per liter) by the geometric mean of data collected within any month period when the standard applies (April – October). Additionally, no more than 10% of samples taken within any month period shall exceed the “maximum” standard (1,260 organisms per liter).

Chloride must have the following.

  • At least 5 observations (pre and post-corrective action) for any 3-year interval in the most recent 10 years; or
  • At least 5 observations for any 3-year interval in the most recent 5 years, and evidence of corrective action in the watershed of sufficient dimension to change impairment status.
  • In either case, no more than one exceedance of the chronic water quality standard in any 3-year interval (note: the chronic standard is a 4-day average concentration).

The water quality standards referred to in the delisting criteria, above, are specific to the pollutant (e.g., TSS), the water body type (e.g., stream), and the water use classification (e.g., Class 2A). The TMDL report for the impaired waterbody is required to outline applicable water quality standards for all water(s) included within the report. Additionally, a complete list of all water quality standards applicable to Minnesota waters can be found in the Minnesota Rule, Chapter 7050 (MINN. R. 7050, 2018). As noted in the Guidance Manual for Assessing Quality of Minnesota Surface Waters for Determination of Impairment: 305(b) Report and 303(d) List (MPCA, 2018a), all delisting decisions are subject to review by the appropriate watershed assessment and professional judgment teams. Because the review team will consider watershed conditions and the representativeness of collected data, watershed improvement information should be summarized and presented to the review team. Special effort should be taken to ensure that collected data are representative of a wide range of watershed conditions (e.g., variability of flow conditions, seasonal variability, etc.).

Monitoring receiving waters

The primary reasons for a permittee to perform monitoring of receiving waters are to (a) evaluate the impact of TMDL implementation strategies enacted throughout the watershed, and (b) evaluate impairment status to determine if the impaired waterbody can be removed (i.e., delisted) from the 303(d) Impaired Waters List. The following subsections outline the basics of developing a 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), and developing a 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).

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