DEMONSTRATION AND EVALUATION OF PROCESSED FOREST BIOMASS IN BIORETENTION CELLS FOR REDUCTION OF WILDFIRE HAZARD AND TREATMENT OF STORMWATER RUNOFF

Abstract

Growth of urban 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 streets and parking lots. Widespread use of this structural best management practice could expand the market for small-diameter woody material and might reduce risks of wildfire because processed residues from logging decks could be the source of organic material that the cell requires. The purpose of this project was to demonstrate the use of processed forest biomass in a bioretention cell at an industrial park in a developing area of South Carolina and evaluate the environmental performance and costs of this cell. A bioretention cell, approximately 25’ wide, 75’ long, and 4’ deep with a 12” layer of chipped logging residue, was designed and installed at the Orangeburg County and City Industrial Park. The cell and the use of processed forest biomass in the cell were publicized in a newsletter of the state’s environmental regulatory agency, are publicized by an on-site sign, and will be publicized by a magazine article for Clemson University. Aged chips of woody forest residues remove positive amounts of nitrate nitrogen, zinc, and copper from polluted solutions in laboratory tests, even though the removal efficiencies never exceed the efficiencies of at least one of the two commercial hardwood mulches. This chipped woody material does not add more phosphorous than the two commercial mulches. Although the level of total organic carbon decreases slightly over time as pine chips age, this type of processed forest biomass can serve as an adequate source of carbon for denitrification within the bottom iii chamber of a bioretention cell. In general, the bioretention cell in Orangeburg reduces the quantity of runoff to the existing storm sewer. Ninety four percent of the first inch of runoff enters and is treated by the cell. The cell apparently removes zinc and copper. Although the cell did not always remove phosphate and nitrate, removal of these pollutants improved as time passed. Concentrations of measured pollutants in the discharge were substantially below regulatory thresholds for water quality. The bioretention cell in Orangeburg cost $28,860. The largest portion of these costs was $23,500 for the contractor, his sub-contractor, and their materials. Unusually but justifiably large excavation and grading expense for innovative design, insufficient bid competition, and contractor inexperience are reasons why the costs per unit of water-quality volume were higher in this project than the average of others. Bioretention cells 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 0.765 percent in coastal areas of mid-Atlantic states, 0.734 percent in the Piedmont region, and 0.629 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. Regardless of region, a one percent increase in the hourly wage of engineers in the area where a cell is located leads to a 6.69 percent increase in the total costs of the cell. Meaningful comparisons of costs of bioretention cells and stormwater ponds are difficult, if not impossible, to make because 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-quality volumes and drainage areas over which bioretention cells are cheaper than stormwater ponds to meet regulatory standards for iv quality and quantity of stormwater runoff remains an important question for research. In August 2004 South Carolina had 803 industrial sites and 241 industrial parks that covered 189,605 undeveloped acres. If owners or tenants of these industrial parks and sites eventually develop all of the land, manage stormwater exclusively with bioretention cells, allocate an average of 0.0525 of each developed acre for the surface area of cells, and use one foot of chipped woody material in the cells, they would use 16.06 million yds3 or 4.553 million tons of this material. If the real cost were to remain $22 per delivered ton, then developers of these parks and sites would spend $100.2 million over time to use the material in bioretention cells.