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try <br /> Contaminant Extent, Site Cleanup and Contaminant Mass <br /> • <br /> Historic groundwater monitoring showed that the dissolved plume remained centered around <br /> MW-1 The concentrations fluctuated but showed an overall slow declining trend The BTEX <br /> values suggested a relatively degraded mass, although occasional concentration "spikes" of <br /> contaminants in MW-I occurred after aquifer recharge (assumed caused by precipitation) The <br /> "spikes" are attributed to water levels using in the well causing their desorption into <br /> groundwater These spikes appeared to decline following the rainy season The dissolved plume <br /> did not show movement to and beyond Well MW-4 and is interpreted to be in a stable condition <br /> Since the tank source was removed, the plume is interpreted to be degrading at about the same <br /> rate as the groundwater motion where the more aerated edges of the plume allowed in-situ <br /> degradation processes to operate upon the plume <br /> Since the plume was relatively small, stable and the concentrations appeared slowly declining <br /> around MW-1, groundwater pumping and treatment was not considered a cost effective cleanup <br /> "best technology" approach However, there did appear to be a residual source of absorbed <br /> contaminants in the former tank pit area and excavation/water, which warranted removal Wright <br /> r submitted a work pian to PHS-EHD for additional site excavation and pit groundwater removal <br /> This excavation work was completed in October 1999 <br /> The residual soil and groundwater contaminant source was removed by excavation and pit <br /> groundwater pumping Exploratory boring sampling and groundwater monitoring data indicated <br /> that the plume was limited and was interpreted to extend dust under 10th Street The City of <br /> Tracy stated that excavation encroachment into 10th Street was unfeasible, hence, the cleanup <br /> effort was limited to the site property up to the curb of 10th Street (see Figure 4) <br /> Estimated Contaminant Mass Removed and Mass Remainin <br /> An estimate of contaminant mass removed and remaining from the Wright excavation is <br /> estimated as presented below An estimate of contaminant mass is based upon the samples <br /> collected, chemical analytical data and the quantity of soil removed Please note that these are <br /> estimates based upon the available data Wright has forwarded a letter to PHS EHD regarding <br /> the quantity of soil and groundwater removed from the former tank pit vicinity A total of 227 71 <br /> tons (or about 170 cubic yards) of soil and gallons of water were removed and properly <br /> disposed for this 1999 work Stockpiled soil was properly disposed off-site (see attached <br /> manifests and tickets) The soil stockpile, tank pit and water samples from the 1999 tank pit data <br /> are tabulated below <br /> Page 4 <br />