Modeling arsenic sorption in the subsurface with a dual-site model.

Arsenic is a well-known groundwater contaminant that causes toxicological and carcinogenic effects in humans. Predicting the transport of arsenic in the subsurface is often problematic because of its complex sorption characteristics. Numerous researchers have reported that arsenic sorption on soil material is initially fast and then subsequently slow. A dual-site numerical sorption model was previously developed to describe arsenic desorption from arsenic-contaminated soils in batch experiments in terms of two different release mechanisms. Experiments involving synthetic acid rain leaching of four arsenic-contaminated soil columns were performed to verify the dual-site numerical sorption model in the context of one-dimensional vertical transport. The fitted models successfully simulated the signature long tailings and the two-stage arsenic leaching patterns for all four soil columns. The dual-site sorption model was incorporated within the general solute transport simulation code Modular Three-Dimensional Multispecies (MT3DMS), version 5.10. The resulting version was named MT3DDS and is available for public access. This experimental study has shown that MT3DDS is capable of simulating phase redistribution during transport, and thus provides a new numerical tool for simulating arsenic transport in the subsurface.