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ASSOCIATES I N C . <br /> representing an average decrease in the water table elevation of approximately 0.55 feet since <br /> September 1999. The water level data were used to develop the groundwater elevation contour <br /> map (Figure 2). Assuming that horizontal isotropic conditions prevail, groundwater in the <br /> uppermost aquifer beneath the site flowed towards the east. The average hydraulic gradient on <br /> December 13, 1999 was calculated to be 0.0006 ft/ft or approximately 3 ft/mile. A summary of <br /> groundwater monitoring data is presented in Table 1. <br /> CONCLUSIONS <br /> Concentrations of petroleum hydrocarbons remained the same in wells MW1 and MW2 as the <br /> previous sampling event. The concentration of TPHg decreased in wells MW3, the <br /> concentration of TPHss increased in well MW3, and the concentration of TPHd remained the <br /> same in well MW3. The direction of groundwater flow has been consistently northeast since the <br /> groundwater wells were installed in December 1998. Groundwater elevations have decreased <br /> since September 1999. <br /> RECOMMENDATIONS <br /> Based on the results of the fourth quarter 1999 monitoring episode, we recommend the <br /> following: <br /> • Conduct the First Quarter 2000 groundwater monitoring and sampling of wells MW1 <br /> through MW3 and analyze samples for TPHg, TPHd, TPHss, and BTEX. <br /> • Groundwater concentrations and gradient have remained relatively consistent during four <br /> quarters of monitoring subsequent to the installation of the groundwater monitoring <br /> wells. ATC recommends installing one additional groundwater monitoring well <br /> southwest of well MW3 in the sidewalk of Harding Way, to define lateral extent of <br /> impacted groundwater. This well will be located directly upgradient of well MW3. <br /> • ATC also recommends installing one angle soil boring south-southwest of the former <br /> UST location under the building to attempt to define the lateral extent of impacted soil. <br /> ATC has submitted a workplan showing the proposed location of the additional monitoring well <br /> and permit application for the installation of the groundwater monitoring well. PHS/EHD <br /> comments are pending. <br /> wAj 72951quartiyl4gr-1999.doc 3 <br />