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### Abstract

Growth of developed areas and impervious surfaces in the U.S. has increased the environmental impacts of stormwater runoff and the public’s interest in regulation of those who discharge it. Growth of communities in the urban-wildland interface is an important reason why risks of wildfire have increased and government agencies have undertaken new collaborative efforts to reduce them. A bioretention cell is a space-saving method to manage stormwater runoff from highways, streets, and parking lots. Widespread use of this structural practice could improve the quality of stormwater runoff and expand the market for small-diameter woody material. As a result, widespread use might also reduce risks of wildfire because mulch could be the source of the material, which provides carbon that the cell requires. The purpose of this project was to demonstrate a bioretention cell and the use of woody material in it along a major highway in South Carolina and evaluate the environmental performance and costs of this cell. All storm water discharges from highways and other property of South Carolina’s Department of Transportation (SCDOT) will be regulated under Phase I of the National Pollution Discharge Elimination System (NPDES) permit program for municipal separate stormwater sewer systems (MS4s). After consultation with and permission from officials of the South Carolina Department of Transportation (SCDOT), we selected a site for a bioretention cell in one part of the landscape at the interchange of Interstate 85 and South Carolina Highway 81, which is near Anderson, South Carolina. We also designed and managed the installation of the cell. The cell is 20’ wide, 25’ long, and 4’ deep with a one-foot thick layer of single-ground pine mulch. In general, the highway bioretention cell substantially reduced the peak discharge and total quantity of stormwater runoff to the existing storm sewer. The cell’s efficiency of trapping zinc was five percent short of perfect and similar to the trapping efficiencies for zinc of previous ii bioretention cells. Nitrate removal of this bioretention cell was appreciably higher than the nitrate removal of previous bioretention cells that had not been anaerobically enhanced. The efficiency with which the cell removed copper was, on average, 45% and substantially less than the trapping efficiencies for copper of previous bioretention cells. The efficiency with which the bioretention cell removed phosphates was extremely variable and, on average, negligible. Regardless of trapping efficiencies, concentrations of measured pollutants in the discharge were substantially below regulatory thresholds for water quality. The highway bioretention cell near Anderson cost $9,250. This cost includes imputed expenses of project personnel who designed, engineered, and helped to install the cell and SCDOT workers who also helped to install the cell. These imputed expenses are based on typical hourly rates for similar types of work in Anderson. The largest portion of these costs,$5,489, was for the construction of the cell. Bioretention cells appear to exhibit economies of water-quality size. If the volume of water that a cell treats for pollutants increases by one percent, the total costs of the cell increase by an estimated 0.74 percent in coastal areas of mid-Atlantic states, 0.63 percent in the Piedmont region, and 0.55 percent in the Sandhill region. Hence, costs per unit of water-quality volume decrease as the volume of water that a cell treats for pollutants increases. Meaningful comparisons of costs of bioretention cells and stormwater ponds are difficult to make for a number of reasons. One reason is that stormwater ponds have been designed primarily to reduce stormwater runoff while most bioretention cells have been designed primarily to remove pollutants. Determination of the precise ranges of water-treatment and water-storage volumes over which bioretention cells are cheaper than stormwater ponds to meet regulatory standards for stormwater runoff remains an important question for research. iii The South Carolina Department of Transportation has 93 county maintenance yards and section sheds with National Pollution Discharge Elimination System (NPDES) permits to discharge storm water. These permitted sites cover approximately 826 acres. Although obviously interested, SCDOT has not decided to what extent to use bioretention cells to remove pollutants in runoff from its yards and sheds. However, if SCDOT were to eventually retrofit all 93 sites with bioretention cells to treat one inch of storm water runoff, design the cells to have nine inches of ponding depth, and create a one-foot deep layer of ground woody material in the cells, this state organization would use 148,039 yd3 of the material. If single-ground pine mulch costs $14 per yd3, then SCDOT would use$2.1 million of this material in the bioretention cells.