Kinetic constraints on the loss of organic chemicals from contaminated soils: Implications for soil-quality limits.

Beck, Angus J. and Wilson, Susan C. and Alcock, Ruth E. and Jones, Kevin C. (1995) Kinetic constraints on the loss of organic chemicals from contaminated soils: Implications for soil-quality limits. Critical Reviews in Environmental Science and Technology, 25 (1). pp. 1-43. ISSN 1064-3389

Abstract

Generic or site-specific soil-quality limits for organic contaminants have been adopted or proposed in various countries in an effort to control or assess contamination. The most comprehensive generic limits, established in the Netherlands, distinguish between background concentrations for naturally occurring substances and analytical detection limits for man-made organic compounds (target values) and threshold concentrations above which remediation may be necessary (intervention values). These are being employed to highlight contaminated sites that require treatment/remediation, ideally resulting in the reduction of the compounds of concern. These limits have been derived in a number of ways but are largely based on professional judgment, usually on information concerning pathways of human exposure, phytotoxicity, and ecotoxicological effects. Effective remediation of contaminated soils is frequently limited by the slow kinetics of contaminant loss from soils under prevailing environmental conditions. This is frequently underestimated, even though it may be of fundamental importance in determining the success of treatment/remediation with respect to the assigned quality limits. This is because loss of contaminants from soils is often biphasic, whereby a short period of rapid dissipation is followed by a longer period of contaminant release. Dissipation processes, including leaching and volatilization, exhibit similar behavior. The primary rate-limiting factors governing this behavior are postulated to be fundamental sorption/ desorption mechanisms, including intraparticle diffusion, intrasorbent diffusion and chemisorption, which control the distribution of contaminant between the solid and aqueous or gaseous phases of soils and, hence, the supply of contaminant available to the various dissipation processes. In this article, we consider the relevance of biphasic desorption kinetics to existing soil-quality limits. We contend that the concentration of a given contaminant in a given soil when the “residual” phase of dissipation is reached represents an intrinsic kinetic constraint to the remediation of contaminated soil. Because of the effects of the primary rate-limiting mechanisms discussed above, it is unlikely that (in the absence of any engineering solution to the problem) any appreciable change in the contaminant concentration will occur over practical time scales under prevailing environmental conditions for a given soil. We define this contaminant concentration as a kinetically constrained soil quality limit (KCSQL) and apply it to selected examples of contaminant dissipation from the literature, including polychlorinated biphenyls, polynuclear aromatic hydrocarbons, industrial solvents, and pesticides in a range of soils. Finally, we examine the relationship between the derived KCSQLs and the Dutch “cleanup” values and soil-quality criteria adopted in other countries.