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7.0 CONCLUSIONS AND RECOMMENDATIONS Based on this extensive review of data from both sites, we make the following conclusions and recommendations: o The Kwikee and Chevron sites are underlain by relatively low-permeability clay deposits from the surface to q depth of at least 40 feet, but encased within this aquitard is 4 curvilinear sandy aquifer bed that has moderate permeability and is water bearing. This sand body becomes thicker and wider in a general eastward direction, and is the principal transport path for gasoline and other soluble hydrocarbons. o Groundwater flow directions vary both spatially and temporally, but the prevailing direction has been toward a groundwater "sink"that is located a few miles to the northeast of the site. Along the southwest margin of the former Chevron site, the northeast gradient is steep and persistent, but the gradient tends to flatten and become more variable beneath both sites, except where the sandy aquifer bed is absent. o Water depths have varied over time, and a long-term shallowing trend is more dramatic than short-term seasonal variations. These changes in water depth are likely to be accompanied by changes in flow direction and hydraulic gradient. ' - e Ample evidence of an unauthorized fuel release at the Chevron site is available. Soil contamination was fairly extensive both laterally and vertically prior to site remediation. In contrast, no evidence of widespread soil contamination has been documented at the Kwikee . site, and almost all hydrocarbons detected to date have been dissolved in groundwater. o Groundwater elevation maps track the history of groundwater flow patterns and reveal an intriguing picture of the site's hydrogeology. Primarily eastward flow in the early to middle 1990's was replaced with primarily northward flow for a period in 1996-97. Groundwater subsequently reversed and began flowing southward, and a groundwater depression then developed at the Chevron site that has since been enhanced by dual-phase extraction at that site. During operation of the system, flow in the area is radial toward the Chevron site. o Gasoline that leaked from the Chevron fueling facilities impacted the groundwater prior to 1986 and began migrating eastward within the aquifer sand body under the prevailing hydraulic gradient. Westward migration was inhibited by 1) absence of the aquifer sand to the west and 2) the steep hydraulic gradient along the west and southern margins of the Chevron site. Contamination in the western portion of the Chevron site may have had a different source. o Gasoline that leaked from the Kwikee UST's did not leach through the underlying clay soil and has not been detected in subsurface borings above the water table. ® By the time the first well was drilled at the Kwikee site in 1990, dispersion of the gasoline hydrocarbons from the Chevron site had contaminated the groundwater beneath the Kwikee site, and residential properties to the north may also have been impacted. ® Hydrocarbon plume maps, in conjunction with groundwater elevation maps, can be used to track the migration of the contaminants beneath both ,sites. Due to the regional northeastward hydraulic gradient, gasoline spread a few hundred feet northward from the 25