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Closure Documentation—7500 Meos't Eleventh Street, Tracy, CA. Page 16 The methods used to remove LNAPL from the groundwater in the area around Monitoring Well MW-7 after authorization to employ the BAT was denied are documented in: The San Joaquin Company Inc. (2004), Report on Extended Plume Definition, 2004, Former Fueling Station, 7500 West Eleventh Street, Tracy, California. (October 2004) 2.12 Reasons Why Background was/is Unattainable Using BAT Background concentrations are attainable by natural attenuation at the 7500 W. 11th Street site. If the regulatory agencies had permitted use of BAT (see Section 2.11 above) background concentrations could have been achieved at greatly accelerated rate. Calculations to show the time required before no detectable chemicals of concern will be present in groundwater are documented in: Dietz Engineering and Construction, Inc. (2011), Groundwater-quality Monitoring Report-July 2011, Former Fueling Station, 7500 West Eleventh Street, Tracy, California. October 2011. 2.13 Mass Balance Calculation of Subsurface Treated Versus that Remaining It is difficult to compute the total mass of dissolved and adsorbed contaminants that is distributed in the subsurface due to a release of fuels from an underground storage tank unless the volume of fuel that flowed from the tanks is known, which is rarely the case. The difficulty arises because the concentrations of adsorbed contaminants in the soil and dissolved contaminants in the groundwater are usually distributed in a very complex pattern throughout the area of the contaminant plume, while the number of locations at which the concentrations are known (i.e., at wells and borings) are relatively few. This difficulty is particularly pronounced in the case of petroleum hydrocarbon contaminants in an alluvial fan such as is present at the Navarra Site. Site-specific procedures for estimating the total mass of contaminants in the subsurface at the 7500 W. 1 I th Street site were presented in Section 2 of: The San Joaquin Company Inc. (2007), Corrective Action Plan Addendum for Former Fueling Station, 7500 West Eleventh Street, Tracy, California. October 2007 The estimates were made for site conditions in 2007, which was the earliest time that there were sufficient subsurface data available to perform a credible estimate of the contaminant mass in the subsurface. However, an arithmetic error occurred in calculation of the estimated mass of dissolved contaminants in groundwater. Correcting for that error, there was an estimated mass of 3,179,746 grams of contaminants in floating DEC