Water Quality Trading FAQ for Industrial Stormwater Facilities

Maryland Water Quality Trading


How is the retrofit requirement translated from impervious surface retrofits to nitrogen, phosphorus, and sediment reductions?

Consistent with Table D.1 of the August 2014 Accounting for Stormwater Wasteload Allocations and Impervious Acres Treated guidance, impervious acre equivalents can be translated into edge-of-stream (EoS) nitrogen and​ phosphorus loads based on the following equations:

  • 1 Impervious Acre Equivalent = Equivalent Nitrogen Reduction + Equivalent Phosphorus Reduction + Equivalent Sediment Reduction
  • Equivalent Nitrogen Reduction = # of Required Acres x 7.69 lbs of Nitrogen per year
  • Equivalent Phosphorus Reduction = # of Required Acres x 1.91 lbs of Phosphorus per year
  • Equivalent Sediment Reduction = # of Required Acres x 860 lbs of Sediment per year

To convert from EoS reduction rate into a required credit, an edge-of-tide (EoT) factor must be applied based on the location of the facility. EoT factors can be found on the Maryland Water Quality Trading Program EoT Factors ArcGIS webpage. The appropriate EoT factor for your facility can be found by inputting your facility location in the search bar above the map and then clocking on the location of your facility.

Example of Calculating Required Credit*:

Impervious area to be offset is 2.65 Impervious Acres, EoT factor for all constituents = 1

2.65 x [(7.69 lbs N/yr x 1 EoT) = 20.38 =21 TN Credits to be acquired
2.65 x [(1.91 lbs P/yr x 1 EoT)  = 5.06 =6 TP Credits to be acquired
2.65 x [(860 lbs Sediment/yr x 1 EoT) = 2279 =2279 Sediment Credits to be acquired

*It is important to note that after you complete your calculation for required credits, that you need to round up fractions to the next whole number. This is due to the regulations defining 1 credit as 1 lb of each constituent, which does not allow for trading of partial credits.

Can I purchase credits from any region?

Credits must be purchased from the Chesapeake Bay Segmentshed where your facility is located. For assistance in determining your segmentshed, you should consult Maryland's interactive map of Chesapeake Bay Segmentsheds.

Where can I find available credits?

Credits available will be posted on Maryland's registry. (A link will be provided here once credits have been registered.) If no credit is available in your watershed, please contact the trading administrator at mde.wqtrading@maryland gov. A request for credit can be created and posted on the registry and potential credit generators may submit proposals for generating credit.

How do I generate credit?

For an industrial discharger, credit can be generated either via the discharge permit, by reducing loads of nutrients and/or sediment in the effluent, or via the installation of stormwater practices (or alternative practices) on site. Stormwater and alternative practices are those that you would use to meet your restoration requirement. Instructions on generating credits is available on the Department's Generating Water Quality Credits webpage.

Additional Information and Related Links