BIOSCREEN, AT123D and MODFLOW/MT3D a Comprehensive Review of Model Results

The Domenico equation is commonly used to evaluate long term risks associated with contaminated groundwater. Numerous groundwater models are based on it, including BIOSCREEN and BIOCHLOR. This paper compares the results from BIOSCREEN, AT123D and MODFLOW/MT3D groundwater models. Results from the AT123D and MODFLOW/MT3D models indicate that BIOSCREEN significantly under estimates contaminant mobility and thus exposure risks. This was unexpected as BIOSCREEN results are commonly assumed to be extremely conservative. In fact BIOSCREEN did produce the highest downgradient concentrations; however it took unreasonable long periods of time to achieve them. Such lengthy time periods are not typically evaluated as part of a risk evaluation. Even more surprisingly BIOSCREEN produced the same peak concentration for all contaminants and for all aquifer types tested. Both contaminant concentration and travel times from AT123D and MODFLOW/MT3D models were almost identical. Furthermore these results varied with contaminants and aquifer properties as expected. The influence of biodegradation was also evaluated. Inclusion of conservative biodegradation rates made BIOSCREEN the least conservative model by far. This is because the lengthy travel times produced by BIOSCREEN provide a longer period of time over which biodegradation works.