ARCHIVED REPORTS_DRYWELL CLOSURE AND INVESTIGATION

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NOA Drywell Closure and Investigation Page 24 of 26 May 30, 2014 with detectable levels of TPH as Diesel, with a concentration of 8.4 mg/Kg, far below the �. regulatory limit. This saturated soil sample was collected beneath the water table. The minute " ' presence of TPH as Diesel in this sample is likely due to impacted groundwater present in the s ,IaI pore spaces of the soil. Soil above groundwater has not been impacted with any COPCs, 54(%)Z4' including petroleum hydrocarbons. When liquid hydrocarbons are released into the subsurface, they migrate downward under the influence of gravity and capillary forces. If the volume of release is large or continuous, the C�,J4,k� underlying soil will become saturated and remaining liquid contaminant will migrate further into no� the subsurface leaving immobile hydrocarbons behind in the soil. If the volume of release is " �a small, hydrocarbons will sorb onto soil particles and will be immobilized. Therefore, regardless s7�rs� ' of the size of release, petroleum hydrocarbons will be present in the underlying soil of the release, above the water table. Considering the above discussion, it would follow that if the Concrete Cure Room discharged petroleum hydrocarbon liquid waste into the drywells and contaminated underlying groundwater, soil above groundwater would additionally be impacted. Concentrations of TPH as Diesel in groundwater are detected at levels up to 2700 Ng/L in DW-1. This significantly elevated concentration would leave detectable residual levels of petroleum hydrocarbons if it were to migrate downward through soil. However, there is no evidence to suggest any soil contamination. Furthermore, the shallow perched lens of groundwater at 58.5 feet bgs should ' have a higher concentration of COPCs than the deeper aquifer if the source of contamination is waste water discharge from the above drywells. The perched lens and deeper aquifer are separated by what is interpreted to be an impermeable aquitard. Therefore, if petroleum hydrocarbons were able to permeate through the aquitard, it would not be at significant concentrations. Instead, the regional aquifer is impacted at significantly higher concentrations of petroleum hydrocarbons than the perched lens and is therefore interpreted to be the impacted aquifer. The perched lens may have been slightly impacted by drilling activities or capillary suction of petroleum hydrocarbons through the aquitard. ' The data indicates that 1) soil beneath the drywell has not been impacted and 2) the deeper, regional aquifer is the impacted aquifer. It therefore follows that groundwater contamination is due to lateral migration from a regional source upgradient (north) of the subject property. ' The subject property is located in an industrial area. There have been eight (8) leaking underground storage tank (LUST) cleanup sites within one-half mile of the Site. Groundwater flow direction in the region is south-southeast. Three (3) LUST cases are north (upgradient) of the subject property. All LUST cases within the area have been closed. However, some cases, such as ARCO #0760, were closed with COPCs still present in groundwater with the reasoning that chemicals will attenuate over time and the closest sensitive receptors are upgradient and ' therefore should not be impacted. However, groundwater at the subject property, which is not considered a sensitive receptor, may have been impacted. In addition, there is an 'other" cleanup case for the Lucky Store #110-310 northwest (upgradient) of the Site that has been open — inactive since 1997. A leak was reported in 1965, volatile organic compounds are listed as the potential contaminants of concern. Furthermore, there are numerous commercial/industrial businesses in the area. Facilities to the north (upgradient) within one-half mile north of the subject property include Armor Struxx, Koehler Heating & AC, Trailers Plus 902 Industrial Way• Lodi, CA 95240•209.367.3701 • Fax 209.333.8303 02014 Neil 0.Anderson Fi Associates, Inc.