Stormwater from Maple Grove Road and Ustick Road will be routed to and treated by a stormwater treatment filter located within the wetland. The stormwater treatment filter chosen for this project is a vegetated sand filter.
A vegetated sand filter has a grass surface underlain by 6 inches of topsoil, 18 inches of sand, and 12 inches of crushed drain rock. The filter is sized to treat stormwater runoff from approximately 80% of all the precipitation events occurring within the drainage basin. Twenty percent of storms will be too large for complete treatment. What can't be treated through the filters will be discharged into the wetland. Vegetated sand filters will remove oil, grease, heavy metals, bacteria, and other pollutants that may be contained in stormwater.
Specially selected grasses will be placed on top of the filters supported by a layer of topsoil. The selected grasses not only tolerate both wet and dry conditions, but also have extensive roots systems that maintain soil porosity and maximize the grass’s ability to absorb nutrients contained in stormwater. The grass and topsoil also help remove smaller sediment particles that enter the filters.
Beneath the topsoil is a layer of sand. The sand layer removes removes nutrients, bacteria, and metals. After the stormwater passes through the sand it enters a layer of crushed rock which conveys treated stormwater to the wetland. The water quality entering the wetland after it passes through the filters is anticipated to be clear, free of sediment, oil, grease, other pollutants, and will not be detrimental to the function of the existing wetland.