ESCI - Version Bioscreen - Groundwater Transport and Fate Model
BIOSCREEN is a screening level groundwater transport and fate model and is based on the Domenico analytical model. It simulates contaminant transport based on advection, dispersion, adsorption and biological decay. Contaminant transport can be simulated: Without biological decay, With biodegradation simulated as a first-order decay process, or with biodegradation based on instantaneous reactions.
When used with SEVIEW, BIOSCREEN can be calibrated to the time-dependent contaminant load to groundwater from SESOIL. Results can be used to estimate how far a contaminant plume will migrate and can be compared to groundwater standards to evaluate risk at specific locations and times.
SEVIEW can now be used to run BIOSCREEN and BUSTR-Screen simulations in AT123D. Although these models use almost identical input parameters the resulting groundwater concentrations typically differ by one to three orders of magnitude. This means that you can significantly reduce your remediation costs just by using AT123D.
This doesn't mean that BIOSCREEN and BUSTR-Screen are the more conservative models. Although BIOSCREEN and BUSTR-Screen produce higher downgradient concentrations it often takes significantly longer to reach them. For example according to BIOSCREEN it would take MTBE in a silty aquifer 245 years to reach the maximum concentration at a point 350 feet downgradient. While AT123D predicts it would take only 94 years. There are several reasons for this but, the principal reasons are associated with contaminant load and the processes used to simulate contaminant transport.
Groundwater contamination typically exists as a three dimensional plume consisting of a width, breadth, and depth. However, BIOSCREEN and BUSTR-Screen only use the depth and width perpendicular to groundwater flow. The third dimension along the x-axis is assumed to be infinite (Figure 1). This was not done because it is difficult to establish the x-axis distance in the field. Nor was it done because it is conservative. Instead it was done to improve computational speed. Contaminant load in AT123D is established in all three dimensions.
Contaminant Transport Processes
Analytical models like AT123D, BIOSCREEN and BUSTR-Screen use mathematical equations to simulate transport and fate processes. Common transport and fate processes include advection, dispersion, diffusion, adsorption and biological decay. Diffusion is the process by which contamination migrates from areas of higher concentration to areas of lower concentration even if there is no groundwater flow (Figure 2). Although not considered an important transport process in sand and gravel aquifers it can become the dominant process in silts and clays. However, BIOSCREEN and BUSTR-Screen do not simulate the diffusion process. This process was not left out because it is conservative to do so. In fact leaving it out actually reduces the rate of contaminant migration. Nor was it left out because it is difficult to find chemical diffusion values. It was left out simply to improve performance.