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CAP Addendum:Former FueM g Station, 7500 West Eleventh Street, Tracy, G l� Page 15 which prevailed in February 2007 when the concentration of dissolved fuel hydrocarbons in the groundwater was considerably greater than was the case in July 2007. Unfortunately, that is not the case. A sufficiently large reduction in the concentration of components of fuel hydrocarbons dissolved in groundwater can permit Redox Potential and the percentage of dissolved oxygen to rise and induce aerobic processes of natural attenuation to be initiated. However, although either anaerobic or aerobic processes can readily produce such a reduction in the absence of a continuing source of petroleum hydrocarbon solutes, aerobic conditions will not be initiated unless the source of the dissolved contaminants is removed. 2.3.4.1 Continuing Sources of Contaminants in Groundwater: LNAPL and Soil . When all leaking storage tanks or other causes of releases of petroleum hydrocarbons into the subsurface have been removed, there are two continuing sources that contribute dissolved contaminants to groundwater. These are: 1) adsorbed contaminants on soil that slowly desorb and dissolve in the water column; and 2) any fuel components in the form of floating LNAPL which, due to its contact with the groundwater, will contribute dissolved contaminants that, where present, will maintain the concentration of those dissolved contaminants close to their solubility limits. Given those conditions, it is clear that what must be achieved by a groundwater remediation technology designed to increase the rate at which groundwater contaminant concentrations are reduced by natural attenuation applied at sites where soil is affected by hydrocarbons and LNAPL is to sufficiently suppress the continuing sources that contribute dissolved contaminants to the groundwater. We should now consider each of the two sources of dissolved contaminants to the groundwater — adsorbed fuel components on soil and fuel components in the form of LNAPL. As was noted in Section 2.3.4, petroleum hydrocarbons in soil amount to an estimated 45.52 % of the total mass of contaminants in the subsurface beneath the area of the Navarra Site that is affected by LNAPL. That is an overwhelmingly large fraction compared to the 0.0012% of the total contaminant mass that is estimated to be dissolved in the groundwater in the same area. However, it is important to consider the mechanism by which adsorbed contaminants contribute to the quantity of dissolved contaminants in groundwater. Contaminants adsorbed to soil desorb and are leached into groundwater at a very slow rate,particularly from clastic soils that contain a high fraction of clays having a high cation exchange capacity, as is the case at the depth of the water table at the Navarra Site. As is evidenced by the aerobic conditions that prevail in groundwater in all areas of the site where LNAPL is absent but soil is affected by components of hydrocarbon fuels, the rate at which desorbtion from soil is occurring is significantly less than the rate at which aerobic processes of natural attenuation are remediating the dissolved contaminants in the groundwater. This is the typical condition at all sites except those where soils contain unusually high concentrations of petroleum hydrocarbons and contains no clay, or where the clay fraction of clastic deposits is composed entirely of minerals, such as kaolin, that have low cation exchange capacities.