Avoiding Level 1 and 2 ERA – It All Starts with a Pollutant Source Assessment
Geosyntec was contracted by a small concrete batch plant facility located in the greater Santa Cruz area in early 2013 to support them with two violation notices, one from a non-governmental organization and the other from the Central Coast Regional Board. Stormwater discharges from the site did not meet established IGP benchmarks for aluminum, iron, total suspended solids (TSS), and pH. For IGP compliance, the facility had to implement best management practices (BMPs) and document their efforts to achieve best available technology economically achievable and best conventional pollutant control technology (BAT/BCT).
We first performed a detailed pollutant source assessment to characterize the stormwater runoff water quality to identify the most appropriate BMP technology for the facility. As a result of the storage and use of sand, aggregate, and Portland cement, the facility stormwater runoff contains very fine particulate, which is difficult to both settle or filter. Our initial approach was to conduct a column settling test to assess how quickly these fine particles would settle and see if we could achieve reductions in pollutant concentrations by settling alone. Using a stormwater runoff sample, the column test showed that the fine particulates tended to settle over a period of 48 hours and achieved significant reductions in concentrations of TSS, iron, and aluminum. The water pH also decreased. During the column test, we sampled the settled water at various times. This sampling showed that the iron and aluminum are in the particulate form and not in the dissolved form.
After developing a full understanding of the runoff pollutant characteristics and in consideration of the limited facility area and high rates of stormwater discharge, we recognized passive flow-through BMPs showed little promise for adequately reducing pH, TSS, and metals and only an active treatment system would meet the water quality goals. Capturing and treating the stormwater runoff appeared to be the most feasible solution for this facility. We proposed a system that captured stormwater runoff and amended the water with a coagulant to settle the fine particles and with an acid to lower the pH. With the client’s support, we designed an active treatment system. Our design was based on the 2014 IGP flow-based treatment control BMP design standards. The 2014 IGP presents three design approaches that can be selected for sizing flow-based treatment BMPs. The first approach involves calculating the runoff resulting from a storm intensity of 0.2 inches per hour; however, the rainfall in the Santa Cruz area is historically more intense than this value. Therefore, we selected the second design standard from the permit which required using the 85th percentile hourly rainfall intensity from local rainfall records multiplied by two. We developed a hydraulic model for the facility using this design standard and used the model as a tool to select the most appropriate system design flow rate.
The designed treatment system consisted of a stormwater interceptor sump with two capture pumps, an overflow “bypass” system, acid and coagulant dosing and mixing system, and a clarifier. The system was designed to capture and treat runoff from a storm intensity of 0.4 inches per hour, which is equivalent to 375 gallons per minute. The treatment system is fully automated and has been successfully treating the stormwater to below the IGP benchmarks. The facility now has coverage under the current 2014 IGP since July 2015 and has successfully maintained baseline status. The citizens suit was terminated and the facility maintains full IGP compliance with very high quality stormwater discharge.
Main audience takeaways: the importance of conducting a pollutant source assessment to characterize a facility stormwater runoff, selecting a design standard for treatment BMPs, conducting column settling tests using facility runoff to optimize treatment system design.
Laila Nasr, PE, QISP graduated from Johns Hopkins University with a BS in Environmental Engineering and continued her education at Stanford University where she obtained her Masters in Environmental Engineering. She joined Geosyntec Consultants after graduating from Stanford and has been actively supporting clients for three years. Ms. Nasr’s experience varies from developing SWPPPs for industrial sites, supporting clients under citizen lawsuits, assisting clients with IGP compliance, and supporting clients with SMARTS. For her IGP clients, she develops decision-making tools, conducts engineering evaluations and prepares designs for innovative treatment BMPs.