Step 9: Sampling

When do the 2020-2025 sampling requirements begin?

Under the 2020 general permit, quarterly sampling began July 1, 2020. Sampling resumes for all permitted facilities, regardless of benchmark values and averages obtained during the 2015 permit cycle. 

Permittees are required to sample quarterly until results from four separate quarters average below the permit benchmark value(s).

  • Quarter 1: July, August, September 2020 (results are due prior to February 21, 2021)
  • Quarter 2: October, November, December 2020 (results are due prior to February 21, 2021)
  • Quarter 3: January, February, March 2021 (results are due prior to April 21, 2021)
  • Quarter 4: April, May, June 2021 (results are due prior to July 21, 2021)

If you were you unable to collect samples during Quarters 1-4, continue sampling: File 2020-2025 Industrial stormwater sampling worksheet (wq-strm3-69)

If you have collected samples but the averaged results are above the permit benchmark value(s), continue sampling (use industrial stormwater sampling worksheet).

New applicants will start sampling the first full calendar quarter after receiving permit coverage.

View guidance materials

Videos and guidance on industrial stormwater sampling

Monitoring guidance manual

Work with a testing laboratory

Permittees may use any Minn. Department of Health Certified Testing Laboratory to conduct analyses required by the permit:

Questions for the lab

  • Is the lab certified for the analyses that need to be sampled for?
  • Can this lab analyze the pollutant parameters I am sampling for?
  • What is the price of analyzing my samples?
  • What are the bottle delivery responsibilities?
  • What are the holding time/turnaround times?
  • What is the reporting methodology?
  • Can the laboratory help me with questions, such as how to take samples or help in getting sample bottles to me?
  • Where is the lab located and are they available to help me with my sampling needs?

Determine whether you have effluent monitoring requirements

Permittees with effluent limit monitoring requirements must collect an additional, separate stormwater sample.

Two-and-a-half percent (0.025) of permittees also have industrial stormwater Effluent Monitoring Requirements. Is this you? This is once-per-year, every year of the permit cycle. If you had no off-site surface water discharges, you are still required to submit the sampling form with a "no flow" explanation.

NOTE: Permittees who do not pass their effluent monitoring limits, it is a violation. Permittees who fail to submit results or a "no flow" form, it is a violation.

Determine when to sample

Permittees can take a stormwater sample anytime there is a measurable runoff event, which means any volume flowing past/through their monitoring location that allows you to collect a sample. You should take samples within 30 minutes of the start of a measurable discharge. If you aren't able to collect a sample within 30 minutes of flowing stormwater, still collect a sample and record on the SWMR why a sample could not be collected within the first 30 minutes of a measurable runoff event. Be sure it's been dry for at least 72 hours (3 days) since the last time you collected a sample.

Tip: If faced with collecting a sample after 30 minutes of discharge leaving your monitoring location or trying to sample another day, do not wait another day. Collect the sample now.

Winter thaw storm event

Because you are required to collect four benchmark samples in four separate quarters, winter thaw or winter rains will occur and collecting stormwater samples in the winter is often obtainable state-wide. Snow melt has the potential to carry significant accumulations of pollutants offsite.

Obtain equipment required for sampling

  • Flashlight (non-sparking)
  • Hard hat
  • Safety goggles
  • Reflective vests
  • Gloves (latex or nitrile)
  • Rain gear
  • Safety shoes
  • Traffic cones
  • Rain gauge
  • Eye wash bottle
  • Paper towels  
  • Ice cooler/shipping cooler
  • Field sampling notebook
  • Waterproof pens
  • pH meter
  • Sampling bottles
  • Preservatives   
  • Waterproof labels      
  • First aid kit


Averaged results exceeded? Modify/add new BMPs

If you collected four samples over four separate quarters and your averaged results were above the permit benchmark values, you are required to continue quarterly sampling until four results in a row average below the benchmark values.

First, determine which parameters you will be required to re-sample.Second, identify the significant material or industrial activity that the exceedance is connected to. Consider:

  • What BMPs you can modify and change at your facility?
  • Can you move that material or conduct that activity indoors or within a permanent, storm-resistant shelter?
  • If not, are there other BMPs you can add to minimize or eliminate the contaminants of concern?

What other areas may be causing you to exceed? Are there structural and/or non-structural BMPs that can help pass your benchmark monitoring requirements?

PDF icon Industrial Stormwater Best Management Practices Guidebook (wq-strm3-26)


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