Monitoring of stormwater outfalls is required under the new MS4 permit, however the approach is slightly different than under the 2004 permit.

Now, all MS4 outfalls that discharge directly to stormwater impaired waters should be sampled at least once during the 5-year term of the permit. Visit our MS4 Map viewer to find out what waters in your town are impaired by stormwater. Note that all MS4 outfalls must be inventoried and mapped as part of the IDDE requirements but for this requirement, those outfalls that discharge to stormwater impaired waters must also be sampled for that impairement.

If you already have wet weather sampling data, it can be used to meet this requirement.

Key instructions:

• Only sample for the existing stormwater impairment(s) (e.g. nitrogen, phosphorus, bacteria). If impairment is 'Other pollutant of concern', sample for turbidity.
• Collect samples during any rain event that results in a discharge from the outfall.
• Complete drainage area investigations where screening indicates high pollutant levels (see table below) and implement BMPs to address the impairment. As an example, if a waterbody exceeds the nitrogen limit, target residents in the watershed for outreach on reducing nitrogen.
• Once half of all impaired waters outfalls have been sampled, select 6 of the highest polluting outfalls for annual screening.

Download the flowchart 

Before you begin the dry weather sampling requirements in the IDDE section, each MS4 must develop an inventory of all outfalls in priority areas, sort those outfalls into 4 categories (problem, high priority, low priority, excluded), and rank all the outfalls in each category based on specific characteristics. See the IDDE implementation section of this website or Appendix B of the permit for more details.

The IDDE section requires dry weather sampling of outfalls in priority areas that are classified as high or low priority (it does NOT need to be done for catchments in priority areas that are classified as problem or excluded outfalls). Dry and wet weather sampling is to be used as part of the follow-up investigations to help identify where an illicit discharge is coming from.

What to test for

All the dry and wet weather samples may be done with test kits or meters (except bacteria) and need to be analyzed for the constituents listed below. Proper sampling procedures need to be followed - for more information, there's a recording of the IDDE Dry weather screening and sampling presentation given at the IDDE workshop on the IDDE page. Also, see EPA's guide to proper sampling procedures.

• ammonia
• chlorine
• conductivity
• salinity
• E. coli. (freshwater) or enterococcus (saline or brackish receiving water)
• surfactants
• temperature
• Pollutant(s) identified as causing the impairment of the receiving water, if it is impaired*

*Note that no test kits exist for total nitrogen or total phosphorus - these samples will likely need to be sent to a laboratory.

Samples that exceed any of the following criteria shall be ranked at the top of the High Priority catchments for further investigation:

• Ammonia > 0.5 mg/L
• Surfactants > 0.25 mg/L
• Bacteria levels greater than the criteria applicable to the receiving water (see table on Impaired waters monitoring factsheet) OR detectable levels of chlorine.

Download the flowchart

Download the flowchart

Example spreadsheet for collecting required data during water quality sampling.

And an example fieldwork checklist used for water quality monitoring at the UConn Storrs campus.

Download [5.1 MB]

Sampling Cost Estimates

We compiled example monitoring equipment that may be used to meet permit requirements and included costs for both in-house test kit/portable meter sampling vs. laboratory services. We've also created a map of the certified labs towns can use to analyze samples for bacteria, total N and total P.

NOTE: Bacteria sampling can’t be done in the field so requires lab services.

Test kit/Meter vs Lab Cost Estimates