Modeling contaminant leaching and transport to groundwater in Tier 2 risk assessment procedures of contaminated sites

The current approach to the management of contaminated sites includes a risk assessment procedure as a crucial step. In some jurisdictions (e.g., Italy), groundwater must be considered as a target of contamination. In other cases (e.g., USA), risks from exposure to contaminated groundwater (e.g., through the ingestion of contaminated water) must be taken into account. If the contaminant source is located in the vadose zone, the risk assessment requires modeling of contaminants leaching and subsequent transport to groundwater. In Tier 2 risk assessments, analytical transport models are used for this purpose. This paper compares steady state and a transient approach for modeling the leaching and migration of contaminants to groundwater. The steady-state model that was used considers soil–water partitioning, soil attenuation during transport and dilution with groundwater in a defined mixing zone. The transient model that was used considers source depletion due to leaching and volatilization, one-dimensional solute transport and a novel approach to the modeling of dilution with groundwater. Comparative simulations were performed with regard to a given site geometry and to index substances representing different contaminant classes: aromatic solvents, chlorinated solvents, and heavy metals. The results of the study indicate that, although steady-state models are simple and easy-to-use, taking into account the time variable is essential for a reasonable simulation of contaminant transport to groundwater, particularly for contaminants with high partition coefficients migrating through fine-grained soils.