PASSIVE SYSTEM TREATS INDUSTRIAL STORMWATER FOR CLASS V INJECTION CRITERIA

EPA has begun to more closely regulate Class V stormwater injection wells under the UIC permitting program. In cases where karst groundwater eventually becomes a source of drinking water, EPA typically sets discharge limits at drinking water standards, posing a distinct challenge to property owners. The project involves a large transportation facility in the Southeast, which was informed by EPA that the state NPDES permit would be replaced by a federal Class V UIC permit. Over 8 acres of paved areas drain into two improved sinkholes near the facility. EPA elected to regulate several constituents in the injectate, to protect drinking water sources: benzene, ethylbenzene, toluene, xylene, cadmium, copper, lead, and Total Suspended Solids (TSS). Brown and Caldwell (BC) was retained to help the owner evaluate this challenge and develop a solution.EPA suggested that the owner/operator may want to consider stormwater management/treatment options that have been considered and/or implemented elsewhere, including grassed filter strips, or sand and organic filters (surface sand filter, underground sand filter, perimeter sand filter, organic media filter, and the multi-chamber treatment train). BC evaluated these types of options as well as others, such as infiltration basins, spray irrigation, constructed wetlands, and mechanical systems, to arrive at the site solution.The selected treatment system, which has been approved by EPA, designed, and is presently being constructed, includes the following principal components: (1) a new oil/water separator for the highest-strength stormwater; (2) a flow collection and diversion structure, followed by a first-flush retention basin; and (3) a constructed wetlands. The stormwater drainage system improvements were designed to accommodate peak flows from a 25-year, 24-hour storm. However, EPA determined that it would only be necessary to capture, treat, and monitor the “first-flush” stormwater from the site. By mutual agreement, the “first-flush” was designated as the first one-inch of runoff from the total impervious area. The sedimentation or retention basin is lined with compacted clay.The constructed wetlands is also a clay-lined basin, atop which is to be placed a 9-inch thickness of planting medium suitable for the plants specified. The wetlands is expected to remove 50 to 98 percent of the influent metals and 80 percent or more of the influent suspended matter. This will be a free surface water (FSW) wetland utilizing persistent emergent vegetation.The stormwater treatment cost equates to 23,000 per acre of impervious surface, which is comparable to or less than other passive systems achieving similar effluent quality, such as the multi-chamber treatment train.