• Prepared updated Stormwater Pollution Prevention Plan (SWPPP)
• Perform RIPDES reporting
• Evaluated stormwater treatment alternatives to meet facility stormwater discharge permit benchmarks
• Permit, design, and implement selected stormwater treatment alternative
Challenge
A private industrial client with stormwater discharge compliance obligations pertaining to their individual Rhode Island Pollutant Discharge Elimination System (RIPDES) permit retained GZA to provide engineering consulting services.
Solution
GZA prepared a Stormwater Pollution Prevention Plan (SWPPP), incorporating existing on-site stormwater treatment best management practices (BMPs), as well as numerous good housekeeping procedures, and helped facility operations personnel implement the SWPPP. GZA also performs the facility’s quarterly RIPDES reporting, including preparation of Discharge Monitoring Reports (DMRs). In response to exceedances of the facility’s RIPDES permit benchmarks, the Rhode Island Department of Environmental Management (RIDEM), mandated completion of an engineering evaluation for additional stormwater treatment BMPs. GZA performed it on the facility’s behalf, evaluating: bioretention, extended detention/retention ponds, a physical/chemical treatment plant, and an underground infiltration system. Based on stormwater quality analysis, the results of on-site BMP performance pilot testing, and space limitations, bioretention was selected as the preferred BMP. GZA conducted a hydrologic and hydraulic analysis and designed the bioretention swale, along with auxiliary storage and conveyance structures, including a 6,000 gpm pump station, 350,000-gallon aboveground storage tank (AST) with aeration, flow controls and monitoring system, a force main, and a sediment forebay. The system is designed to pump only the water quality volume (first one-inch of stormwater runoff) generated by the facility to the AST. The tank outlet was designed with an orifice plate to retain water in the tank for aeration and meter the tank discharge rate to the bioretention swale and corresponding pretreatment forebay for treatment. GZA prepared a cost estimate to construct the system; construction plans, details and specifications; and assisted with contractor selection. GZA also provided full time construction oversight. When the treatment system was successfully placed in service, stormwater effluent quality met all benchmarks and TSS and metals concentrations were reduced by more than 90%.
Benefit
GZA’s keen understanding of the regulatory requirements and extensive experience with stormwater BMPs led to successful engineering design and resolution of the client’s regulatory issues.