Understanding generic soil cleanup levels: Implication on agricultural chemicals

Federal and state regulatory water quality standards have existed in the US for more than a decade while none existed for soil. More recently, the US Environmental Protection Agency (US EPA) developed a procedural model to determine minimum contaminant levels in soil that may require further investigation at the federal level. This model to determine federal soil screening levels (SSLs) by the US EPA has been slightly modified to determine state regulatory soil residual contaminant levels (RCLs) in Wisconsin. We present simplified equations for use on semivolatile compounds to evaluate these regulatory soil levels in residential settings, and in the process, we show where regulatory federal and state soil contaminant levels may differ. Establishing generic soil cleanup levels requires determining the smallest of the acceptable contaminant levels that are still considered protective of human health for all exposure pathways of concern. For the protection of direct human exposure pathways, the State of Wisconsin uses residential assumptions which are generally more conservative than federal defaults to determine acceptable levels. However, when indirect ingestion pathway via leaching through groundwater is considered, the federal generic SSLs maybe more conservative than Wisconsin's generic RCLs when an organic contaminant's sorption coefficient, K_oc falls between 4123 and 39,000 ml/g for non-carcinogens, and between 8568 and 45,525 ml/g for carcinogens. The simplified equations are used on several agricultural chemicals with current generic SSLs. Agricultural chemicals are unique because, unlike other compounds, they are designed for dispersal into the environment at legal application rates, and their generic cleanup levels may be developed based on their legal use rates. However, both the generic US EPA and Wisconsin models imply that if some agricultural chemicals are found at depth, even at use-rate levels, groundwater quality may be adversely affected.