Risk Assessment Methods for Establishing Clean Closure Levels

Abstract

Establishment of clean closure concentrations for low levels of hazardous constituents in soils, ground water, and other environmental media by assessing risks to human health and the environment is a tool that is being used with increasing frequency. The four-step process sanctioned by the U.S. Environmental Protection Agency (EPA) includes: 1) hazard identification, 2) exposure assessment, 3) toxicity assessment, and 4) risk characterization. This paper describes the methodology and applies it to the closure of a hazardous waste surface impoundment in Florida. To date, the closure process has taken nearly five years and, pending the EPA's and Florida Department of Environmental Regulation's (FDER's) final approval, will result in clean closure of a small facility that has spent in excess of $450,000 U.S. during this period for removal of 516 cubic metres of sludge and other solids. If clean closure is approved, as is concluded to be protective of human health herein, the facility owner stands to save $275,000 U.S. that is estimated would be required to monitor ground water and to maintain an impermeable cap for the 30-year post closure period currently required under U.S. and Florida regulations. The risk assessment discussed in this paper began when soil samples collected after excavation of 516 cubic metres of sludges, soils, and other solids showed that the preliminary soil closure levels sanctioned by the FDER for chromium and nickel had not been achieved. Continued excavation would have cost a minimum of $50,000 to $100,000 U.S. (in 1988 dollars) and still may not have reached closure levels for these two metals. It is concluded in this paper that the maximum levels of chromium and nickel remaining onsite after remedial excavation (2.1 and 3.7 mg/kg, respectively) do not pose a risk to human health, and that the agency could raise the allowable closure level for these two metals from the current values of 1.1 and 0.70 mg/kg to these maximum values.