Stormwater Discharge Investigation

The Client

Facilities that are at risk of higher levels of zinc in their stormwater discharge

The Challenge

Several manufacturing facilities recently detected zinc in their stormwater discharge above the benchmark value as established in the South Carolina Department of Environmental and Control’s National Pollutant Discharge Elimination System (NPDES) General Permit for Stormwater Discharges Associated with Industrial Activities (IGP).  According to the IGP, if zinc is detected in the facility’s stormwater discharge above the benchmark value for 4-quarterly sampling events, a facility must review the selection, design, installation, and implementation of current control measures to determine if modifications are necessary to meet zinc benchmark values.  A facility is then required to:

• Make the necessary modifications and continue quarterly monitoring until the facility has completed 4 additional quarters of monitoring for which the average does not exceed the benchmark or
• Make a determination that no further pollutant reductions are technologically available and economically practicable and achievable in light of best industry practice to meet the technology-based effluent limits or are necessary to meet the water-quality-based effluent limitations included in the IGP.

If the average concentration of a pollutant exceeds a benchmark value, and the manufacturing facility determines that exceedance of the benchmark is attributable solely to the presence of that pollutant in the natural background, the manufacturing facility is not required to perform corrective action or additional benchmark monitoring.  Rather, the facility must amend its SWPPP and retain supporting documentation for the natural background determination.

Gel Engineering Services Provided

• GEL prepared Sampling and Analysis Plans (SAPs) for submittal to the manufacturing facilities.  The SAPs included a summary of the project, the proposed number of samples for collection, the method of sample collection, additional detail regarding the sampling requirements for regulatory compliance, the constituents for analysis, and the analytical methods for analysis of the materials.  In addition, GEL researched and reviewed publicly available surface, groundwater, and geotechnical studies for the purposes of natural background determination and adequate number of samples. 
• GEL conducted extensive evaluations of the facilities’ stormwater to determine locations of potential zinc sources and if the zinc detected in the stormwater discharge was attributed to natural background concentrations in the surface soils and ground waters.  Stormwater samples were collected from various locations throughout each facility identified by the client as potentially contributing to the discharge of each outfall in addition to samples at each outfall.  The samples were collected, transported, and submitted  to GEL Laboratories, LLC for analysis.  GEL prepared reports that provided the characteristic profile for zinc in background surface and ground waters and that, depending on the findings, may be used to fulfill the IGP requirement for attributing zinc in the stormwater discharge to natural background levels.