New option for taking nitrogen samples

There wasn’t a cheap and simple way to take field measurements of Total Nitrogen (TN).  Samples had to be sent to a lab – until now!

To help reduce water quality testing costs, CT DEEP agreed to allow MS4 communities to use less expensive field tests for nitrate and ammonia to estimate Total Nitrogen.  If your TN estimate exceeds 2.5 mg/L then a sample should be brought to a lab to officially determine its Total Nitrogen value.  If the results are below 2.5 mg/L, you do NOT have to conduct additional nitrogen testing.

To estimate TN for your sample, plug in your values for nitrate (mg/L) and ammonia (mg/L) into this formula:

TN=1.94 x [(nitrate + ammonia) ^ 0.639]


When do I have to sample for Total Nitrogen again?

There are a few situations where the MS4 permit requires towns and institutions to sample for Total Nitrogen (TN):


  • Dry weather baseline screening:

If you see flow during dry weather baseline screening at an outfall that discharges directly to a waterbody impaired by Nitrogen (or ‘Nitrogen and Phosphorus’).

  • Catchment investigation procedure:

Wet weather sampling of outfalls during the catchment investigation procedure when the receiving waterbody is impaired by Nitrogen (or ‘Nitrogen and Phosphorus’).

Impaired waters monitoring:

  • If there is a waterbody impaired by Nitrogen (or ‘Nitrogen and Phosphorus’), you need to sample the wet weather discharge from any MS4 outfall that empties directly into that waterbody.

An easy way to see if there is a Nitrogen (or ‘Nitrogen and Phosphorus’) impaired waterbody in your town, go to the MS4 Map Viewer and click on any purple or red waterbody to see what’s listed as its Stormwater Pollutant of Concern in the pop-up window.