The Department of Defense is under increasing pressure from regulators and local communities to protect water bodies by reducing the total magnitude and concentration of industrial site pollutants being discharged within storm water runoff into harbors, bays, lakes, and streams. The objective of this project was to demonstrate a small footprint stormwater technology for industrial areas that merges structural Best Management Practice (BMP) and Low Impact Development (LID) principles to decrease the concentration of pollutants such as suspended solids, dissolved and particulate metals, and oil and grease to the National Pollution Discharge Elimination System (NPDES) permit requirements. The hybrid technology can be applied to new construction of industrial facilities, or retrofit existing industrial sites that are faced with meeting increasingly stringent NPDES discharge limits.
The stormwater technology is a full-scale 100 gallons per minute hybrid LID/BMP system designed to decrease contaminant concentrations within runoff to ultra-low NPDES permit effluent limits. The system’s innovative feature is the merging of a sustainable LID with a structural BMP along with a 1,100 gallon water storage system. The LID’s engineered soil and plant matrix mimics the contaminant removal mechanism of a natural swale within a small footprint and exceeds traditional swale percolation rates, while the structural BMP media bed polishes the LID effluent. The accompanying storage tank holds a portion of the effluent to irrigate LID plants during dry periods, or it can be used for other site-specific applications. The multi-stage passive treatment allows the system to operate without an operator for multiple rain events and seasons. The small footprint and rapid processing times of this technology are desirable at industrial sites where usable space is at a premium.
System monitoring occurred from February 2018 through May 2019 at the installation location at the Metals Finishing Complex at Naval Base Point Loma (NBPL). The system capital cost was $157,010 for the one-acre site. All effluent event mean concentration (EMC) values for copper were below the NBPL permit limit of 33.2 µg/L. For total copper, only one out of fourteen effluent results met the ultra-low NPDES permit limit of 2.9 µg/L for areas such as Hawaii. The seasonal effluent EMC was 5.2 µg/L and seasonal efficiency ratio (ER) was 97%. For dissolved copper, which is thought to be the more toxic fraction, the seasonal effluent EMC was 2.8 µg/L and the average seasonal ER was 97%. All effluent EMC values for total zinc were well below the NBPL permit limit of 260 µg/L. The average seasonal ER for both total and dissolved zinc was 98%. All effluent EMC values for total suspended solids (TSS) were well below the NBPL permit limit of 100 mg/L and the ultra-low benchmark of 50 mg/L. The average seasonal ER was 95%.
The hybrid system achieved high metals and suspended solids removal consistently over two rain seasons. The average removal percentage for total and dissolved copper was 97%, total and dissolved zinc was 98%, and TSS was 95%. Over the project life, the system received minimal maintenance and only two contractor maintenance cycles were performed to replace the top three inches of mulch on the LID biofilter. Multiple research and development projects continue to use the site as a test bed. The system is still in place at NBPL and actively treating storm water runoff.