ARCHIVED REPORTS_XR0005008 - Laserfiche WebLink

Home Browse Search

L ATg e � Vol Q9 A S S O C I A T E S I N C / 1 The model shows the highest concentrations of hydrocarbon contaminants in well MW3 Data ,from the model predicts that groundwater TPHg concentrations in MW3 decreasing from over �, ��.� 6,906 ugll in 1998 to 270 ugll in 2004 The model shows the TPHg concentrations reaching a I ti l state of equilibrium at approximately 270 ug/l and decreasing less than 1 ug/l over the period from 1999 to 2015 This appears to be a state of equilibrium between the recharge flux rate ' and the biodegradation rate Figures 12, 13, and 14 show hydrographs of wells MW1, MW3, and MW4, respectively The ' hydrographs show TPHg concentrations over time predicted with the MT31) model These hydrographs predict that hydrocarbon contaminant concentrations will drop significantly for the ' first three years followed by constant levels as the plume stabilizes An isocontour map of the predicted concentrations after 15 years (2010) is shown in Figure 17 The 0 1 ug/l isocontour extends approximately 150 feet to the southwest of the site in the hydrologically downgradlent 45``� •� I4 direction The model shows that this plume will not expand beyond these extents It should be �,� ' noted that laboratory analytical data from historical groundwater monitoring events shows that 01.4 e i the plume is decreasing m size and TPHg concentrations are decreasing more rapidly than I Y predicted by the MT31) model This may be partially due to the operation of the groundwater .AQP eqa� extraction and treatment system that has been operating since April 2002 However, hydrographs of groundwater monitoring wells MW1, MW3, MW4, and MW5 (figures 8, 9, 10, and 11) show s that the largest decreases of TPHg and benzene concentrations in groundwater occurred prior tov#� the startup of the groundwater extraction and treatment system Therefore, it appears that natural biodegradation is a mayor factor in decreasing concentrations of hydrocarbons in groundwater at the site This MT31) model simulation represents a very conservative estimation since the extents of the soil contaminant source is based on laboratory analytical collected in 1995 prior to the startup of soil vapor extraction system The MT31) output file is included in Appendix E ' 6.0 GROUNDWATER RECEPTOR SURVEY ' A well survey for the site was performed by SEACOR in 1994 The survey revealed six wells within approximately one half mile of the site Only one of the wells was identified within 500 feet of the site Well 1N-6E/4J2, located approximately 250 feet to the north, was the closest to ' the site However, this well was located in a hydrologically upgradlent direction from the site and according the EHD, the well has been abandoned and is no longer considered a receptor The nearest well in the assumed hydrologically downgradient direction is a domestic well located approximately 1,400 feet from the site The City of Stockton operated a municipal well ' approximately 1,200 feet downgradient from the site until the late 1970s when it was shut down because of its brackish and salty content due to its proximity to the Stockton Deep Water Channel A copy of the well survey showing the well locations is included in Appendix D Y I I� w 127142Vepons%closure doe 19