MS4 Supplemental Environmental Project (SEP)

Walden implemented a Supplemental Environmental Project (SEP) to address a violation observed during USEPA’s MS4 inspection at a Highway Yard. At the time of this inspection, the Contractor installing underground storage tanks at the site discharged unfiltered turbid water into the storm drain, in violation of NPDES regulations. When USEPA notified the Town of the violation, Walden developed a proposal on behalf of the Town, detailing a SEP that would be implemented in order to offset this penalty. USEPA approved the proposed SEP, and Walden completed the project, saving the Town $27,000 in fines.

The approved SEP consisted of assisting the NYSDEC in developing Total Maximum Daily Loads (TMDLs) standards for nitrogen being discharged into the bay. NYSDEC delineated the area that contributes nitrogen load into the bay by the Hydrological Unit Codes (HUCs). Walden compiled available maps showing the locations of the MS4 structures and outfalls within the area delineated by the NYSDEC and divided the HUCs into sub-watersheds based on the subsurface soil characteristics. Based on these maps, Walden conducted field inspections in each sub-watershed to assess the characteristics and condition of the drainage structures and classify land use in the area. Walden used GPS technology to record location-specific information. Walden classified the structures as leaching or non-leaching, recorded the materials of construction, measured the dimensions and depth to bottom, identified the connections to each structure and flow direction of all pipes entering the structure, and recorded the invert elevations all piping.

Walden utilized GIS to analyze the field data and mapped information in order to calculate the watershed areas for each drainage structure. An Excel spreadsheet was created to calculate stormwater flow through MS4 structures and the associated nitrogen load contribution into the bay.


Posted on

July 13, 2021