The Effectiveness of Vegetated Drainage Swales in Nutrient Management

Abstract

Abstract Excessive nutrient runoff to the surface waters of the United States through stormwater has been identified by the Environmental Protection Agency as one of the largest current contributors of water pollution. Stormwater that runs off of the land surface with no identifiable origin is known as Nonpoint Source Pollution (NPS). NPS Pollution control is typically performed from either a Structural or Non-structural best management practice (BMP). Nonstructural BMPs, also known as Passive BMPs, often are less costly, quicker to design and easier to install and maintain. One of the more common passive stormwater BMPs are vegetated swales. The vegetated swale has traditionally been used solely as a primary conveyance device for NPS stormwater. Because the cost and installation of swales is reasonable, the view of swales in recent years has shifted from simply a stormwater conveyance system to a possible effective pollution removal BMP. The general pollution control from these established passive systems are thought to rely on a number of factors. Because one of the primary factors influencing pollutant removal in an open channel drainage swale may be the swale's soil medium, the relationship soil media has with dissolved nutrient pollution warrants investigation. Site tests were conducted in Gaston County to determine the effect that a soil’s medium would have on nutrient absorption within vegetated swale. A solution mixture was released in four separate channels, whose soil was composed of either clay or sand. Samples were then collected within these channels at fifty foot intervals. When tested at a private lab it was determined that nutrients in the released solution did show varying degrees of nutrient reduction. Although solution samples showed reductions at least 37% for N and 24% for P, the different soil media being examined provided no substantive difference in the removal of nutrients.