ARCHIVED REPORTS_XR0006323 - Laserfiche WebLink

Home Browse Search

. INTRODUCTION Upgradient Environmental Consultants was retained by Mrs Marjorie Conrady in March, 1998 to provide site remediation and groundwater monitoring services at her property located at 1002 Yosemite Street in Stockton, Califorrna (Figure 1) The property is the former location of a gasoline service station that was demolished about 1980, when the underground fuel storage tanks were removed At the end of 1998, Upgradient recommended preparing a No Further Action Request and Site Closure Report, and was given authorization to do so by the UST Cleanup Fund in early 1999 The Closure Report was submitted to PHS/EHD in May of 1999, and in June, PHS/EHD scheduled a Technical Review Committee (TRC) meeting to review the case The meeting took place in July, and the TRC requested a computer modeling study to predict the ultimate fate of the contaminants Upgradient then submitted a cost estimate for fate and transport modeling, and the estimate was approved by the UST Cleanup Fund in September This report presents the results of the study COMPUTER MODEL AND INPUT PARAMETERS The groundwater model AT123D (Version 13 1) was used for the fate and transport simulations AT123D, a public domain simulation program, was originally developed by the Oak Ridge National Laboratory and is currently distributed by the International Groundwater Modeling Center, Colorado School of Mines, Golden, Colorado The purpose of the simulation was to help the regulatory agencies determine whether residual hydrocarbons remaining in groundwater beneath the site pose a risk to the environment or people living in the vicinity Because of its known carcinogenic properties, we elected to model the future migration and attenuation of benzene, assuming no further action on the part of the Responsible Party to reduce its concentration The modeling program uses chemical, soil and groundwater parameters as input data (Table 1) in order to predict the fiiture dispersion and concentration of the contaminant In general, chemical parameters are known constants,but soil parameters are site-specific Some of the site-specific parameters were measured in previous drilling and groundwater ' sampling activities, but for those parameters that were not measured, initial values were conservatively estimated based on experience i 'I Simulated Release III A mass ("plume") of benzene-contaminated groundwater is known to exist beneath the site in the general vicinity of rriomtonng well MW-2 and air sparging well SW-1, buf ' extending outward to monitoring well MW-I and sparging well SW-2 (Figure 2) For the purpose of the simulation, this area is considered a"point source", and the model ' simulates the "release" of this plume from the source and its subsequent movement and r dissipation iln the subsurface environment The release was modeled as occurring over a I 24-hour period at some point in the future, and,the computer,calculated-the benzene II co'ncentr'ation at'various,distances from the source location at 182,-day (6-month)rintervals ' I for a period of,9 51 years � ,,dl I p[ i i I pa( litI lt rillIh1 rll� I I,I'I1 1rol,[l IEI,I l�rl$ Ila pl ,I it 1 E1n„