Natural Stabilization of Stored Industrial Sludges

Abstract

Current regulations governing the closure of a waste impoundment require either complete removal of the contained material or appropriate remediation of the material in the impoundment followed by application of a permitted cover to prevent exposure and off-site migration of residual chemicals. With the emergence of risk based corrective action decisions, there has been interest in other approaches that may be as protective as the more traditional impoundment closure approaches. This paper describes the extent to which natural ecological processes can result in the stabilization and sequestration of chemicals of concern, polynuclear aromatic hydrocarbons (PAHs). A field study in Texas offered the opportunity to evaluate the effectiveness of natural processes to reduce the mobility of PAH in impounded industrial sludges. Grasses, bushes, and trees had been growing on the surface of a sludge impoundment for years. Separate surface and subsurface samples were obtained from different locations in the impoundment. Analyses to determine PAH release and leachate data and relative toxicity data were used to assess the impact of natural processes. The results of these analyses indicated that the PAH in the surface samples were considerably less available than the PAH in the subsurface samples. The processes and factors associated with the natural conditions and growth of vegetation on the surface layers of this impoundment have resulted in the sequestration and stabilization of the PAH in the waste matrix. The reduced PAH availability in the surface layers indicates that the remaining PAH are likely to be a low risk to environmental receptors associated with this impoundment.