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1 <br /> 1 <br /> ➢ solute transport without decay <br /> ➢ solute transport with biodegradation modeled as a first-order decay process (simple, <br /> lumped-parameter approach) <br /> ➢ solute transport with biodegradation modeled as an "instantaneous"biodegradation <br /> reaction(approach used by BIOPLUME models) <br /> In our case all three models types would be applicable for the site, although the solute transport <br />' without decay model will be used as a worst-case scenario Based on the actual observed r <br /> groundwater conditions at the site, the solute transport first-order decay model appears to be <br />' most representative of actual site conditions, including plume sizes and concentrations for <br /> MTBE If natural attenuation analytical results were present for the site, these values were used <br /> for input parameters in the "Instantaneous" Biodegradation Reaction If site data was not <br /> 1 available, model default parameters were used <br /> The model is designed to simulate biodegradation by both aerobic and anaerobic reactions It <br /> was developed for the Air Force Center for Environmental Excellence (AFCEE) Technology <br /> Transfer Division at Brooks Air Force Base by Groundwater Services, Inc , of Houston, Texas <br /> BIOSCREEN attempts to answer the two fundamental questions regarding RNA <br /> ➢ How far will the dissolved contaminant plume extend if no engineered controls or <br /> further source reduction measures are implemented? <br /> ➢ How long will the plume persist until natural attenuation processes cause it to <br /> dissipate? <br /> BIOSCREEN has the following limitations <br /> ➢ As an analytical model, BIOSCREEN assumes simple groundwater flow conditions <br /> ➢ As a screening tool, BIOSCREEN only approximates more complicated processes <br />' that occur in the field <br /> Site-specific data for both releases was entered into BIOSCREEN to determine the degree of <br /> RNA Site-specific data such as hydraulic conductivity and porosity were based on text book <br /> values for similar as observed at the site (Todd 1980) The site-specific groundwater gradient, � �� <br /> which was obtained from the latest quarterly sampling event, was used and the model length was <br /> set at the distance from the closest groundwater production well (which was 1,500 feet from the ��� 1 <br />' source areas) Input parameters such as the estimated plume length and the concentrations of <br /> MTBE were also based on the actual site data collected this quarter The estimated source mass <br /> of MTBE for the model run was set at the estimated mass of MTBE calculated for soil at the site, <br /> and rt was conservatively assumed that all of this mass would be an available source for <br /> groundwater contamination Since the estimated mass of MTBE in soil beneath the groundwater <br /> table was so low for the site, the source mass was increased to 0 5 kg to represent the mass of <br /> MTBE within the soil column and the groundwater plume The partitioning coefficient for <br /> MTBE (12 59 L/KG) was obtained from the American Petroleum Institute's Strategies for <br /> Characterizing Subsurface Releases of Gasoline Containing MTBE (Regulatory and Scientific <br /> 9 <br />