Water Quality Monitoring


There are two provisions of the permit that call for water quality sampling/monitoring. The first requires wet weather sampling of all outfalls to impaired waters (page 41-44). The second is the water quality monitoring associated with your IDDE program (Appendix B, page 7-8). Details on each are provided below.

It should be noted that there is some overlap (and some differences) between sampling requirements under the two sections. As such, if you follow the requirements for the IDDE section you will also be satisfying the requirements under the impaired waters screening section. See the chart below for a comparison of the two sections.

  Outfalls to Impaired Waters IDDE
area covered all outfalls to impaired water outfalls in priority areas (urbanized area overlapped by impaired waters or DCIA>11%) that are categorized as either high or low priority catchments (see IDDE section of this website or appendix B of permit for more information)
type of sampling wet weather
  • dry weather for initial baseline screening
  • dry and/or wet weather screening as needed based on results of catchment investigations
pollutant(s) to screen for
  • listed impairment pollutant(s),
  • test for turbidity for waters impaired by pollutants other than nitrogen, phosphorus or bacteria
listed impairment pollutant(s) (if any), PLUS
  • ammonia
  • chlorine
  • conductivity
  • salinity
  • E. coli. (freshwater) or enterococcus (saline or brackish receiving water)
  • surfactants
  • temperature
  1. conduct drainage area investigation that includes:
    • land use/development patterns
    • business or commercial activities
    • industrial activities
    • DCIA
    • natural contributors
    • potential MS4 maintenance issues
    • residential activities
    • any other potentially related activities
  2. implement approriate control measures
  3. select 6 of the highest contributing outfalls for annual sampling
catchment investigation that includes:
  • review of mapping, historic plans and records
  • manhole inspection
  • procedures to isolate sources of illicit discharges
deadlines 6/30/19 (2004 MS4s)
6/30/20 (2017 MS4s)
begin annual monitoring once complete screening of half of outfalls
baseline sampling:
6/30/20 (2004 MS4s)
6/30/22 (2017 MS4s)

Outfall Decision Tree

Because of the different yet overlapping requirements of the two water quality monitoring sections of the permit, it can get very confusing which requirements apply to which outfalls. We put together this very fancy outfall monitoring decision tree to help simplify things for you (printable version available here). For each of your outfalls simply follow the questions below!

Screening & Sampling Outfalls to Impaired Waters

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

Under the 2017 Permit, all MS4 outfalls that discharge directly to impaired waters must be sampled at least once during the 5-year term of the permit. Visit our CT MS4 Towns & Data Map to find out what waters in your town are impaired. NOTE: All outfalls must be inventoried/mapped as part of the IDDE program. Only those that discharge to impaired waters must also be screened/sampled.

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

Key water quality monitoring instructions:

  • Only screen for the existing impairment(s) (e.g. nitrogen, phosphorus, bacteria). If impairment is something else, sample should be screened for turbidity.
  • Take samples during any rain event that results in a discharge from the outfall
  • Complete follow-up investigations where screening indicates high pollutant levels (see Screening Threshold table below)
  • Once half of all impaired waters outfalls have been sampled, select 6 of the highest contributing outfalls for annual screening

IDDE Screening & Sampling

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.

  • 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 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 OR detectable levels of chlorine

How are samples to be analyzed?

We compiled example monitoring equipment that may be used to meet permit requirements and included costs for both in-house sampling vs. laboratory services. Click here for EPA's guide to proper sampling procedures.

There are helpful videos developed for the Central Massachusetts Regional Stormwater Coalition including two covering outfall monitoring and water sampling and dry weather outfall screening that apply directly to the requirements in this section (Note: certain details, like the timeline, are different for the CT MS4 permit).

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

Portable Meter vs Lab Cost Estimates

Pollutant Test Kit/Meter Price Samples per kit Cost per sample Comments Commercial lab cost per sample (estimate)
Ammonia Hach N1-8 $88 100 $0.88 Reagent for this kit contains mercury $9.00
Surfactants Hach DE-2 $307 32 $9.59   $16.00
Chlorine Hach CN-66F $55 100 $0.55   $3.00
Chloride Hach 8-P $56 100 $0.56   $5.00
Turbidity Hach 2100Q $1180 n/a     TBD
  LaMotte 2020we $980 n/a      
  HF Scientific MicroTPW $799 n/a      
Nitrogen, phosphorus Lamotte Smart3 (Requires Heater Block for TN/TP) $999 + $779 (heater block) = $1778 $123 (TN)
$4.90 (TN)
$7.56 (TP)
  • Reagents also need to be purchased.
  • Samples need to be digested in heater block prior to analysis (can't be done in field)
  • *Per sample costs do not include cost of meter and heater block
$8 (TP)
$23 (TN)