How Far? How Long? The Bioscreen Natural Attenuation Decision Support System

Newell, McLeod, Gonzales

BIOSCREEN is an easy-to-use screening tool for simulating the natural attenuation of dissolved hydrocarbons at petroleum fuel release sites. The software, programmed in the Microsoft Excel spreadsheet environment and based on the Domenico analytical solute transport model, has the ability to simulate solute transport and estimate source zone decay. BIOSCREEN attempts to answer two fundamental questions regarding intrinsic remediation projects; 1. How far will the plume extend if no engineered controls or further source zone reduction measures are implemented? BIOSCREEN uses the Domenico analytical solute transport model to simulate advection, dispersion, adsorption, and aerobic decay as well as anaerobic reactions, which have been shown to be the dominant biodegradation processes at many petroleum release sites. Three different biodgradation models are provided: a) solute transport without decay, b) solute transport with biodegradation modeled as a first order decay process (lumped-process approach), and c) solute transport with biodegradation modeled as an "instantaneous" biodegradation reaction (approach used by the BIOPLUME model). Each biodegradation model will predict the maximum extent of plume migration, which may then be compared to the location of potential points of exposure (e.g. drinking water wells, groundwater discharge areas, or property boundaries). 2. How long will the plume persist until natural attenuation processes cause it to dissipate? BIOSCREEN uses a simple mass balance approach to estimate the decline in source zone concentration vs. time. The mass balance is based on a) the mass of dissolvable hydrocarbons in source zone soils and NAPL, b) the rate of hydrocarbons leaving the source zone, and c) an assumed exponential decay relationship for source concentration over time. While this relationship is unverified (there are little data showing source concentrations vs. long time periods), it should provide a relative estimate of the time required for the source to completely dissipate. The Air Force Center for Environmental Excellence (AFCEE) is distributing BIOSCREEN software and manuals (Newell et al. 1996) via the EPA's Center for Subsurface Modeling Support (CSMoS) at the National Risk Management Research Laboratory in Ada, Oklahoma (Phone: 405 436-8594; Fax: (405) 436-8718; Web: http://www.epa.gov/ada/bioscreen.html. Electronic manuals will be in pdf format; users must download Adobe Acrobat Reader to read and print pdf files). The following sections are adapted from the BIOSCREEN User's Manual (Newell et al., 1996). An example application of BIOSCREEN is provided as Appendix 1.