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Corrective Action Plan highest concentrations were reported in monitoring well SRK-MW2, where TPHg and benzene ' were detected at 18,000 µg/1 and 670 µg/l,respectively. These are also the highest concentrations ever detected for this well. Although the screen in all of the SRK wells are presently below the water table, OST was able to obtain a representative sample from SRK-MW2 by using aggressive ' purging to depress and sustain groundwater drawdown. A summary of the most recent groundwater monitoring is presented in Table 3 and is illustrated on Figure 3. ' 4.0 REMEDIAL ACTION ALTERNATIVES The objective of the proposed remediation program is to reduce the concentrations of gasoline constituents in the impacted soil and groundwater to acceptable levels in the most cost-effective manner. ' A discussion of the specific processes which provide the basis and rationale for the selection of the proposed remediation system and its major components are presented below. 4.1 Feasible Remediation Alternatives for Soil There are two immediately apparent alternatives for remediating the soil at the site: cleaning the soil either by excavation and ex-situ treatment or by in-situ treatment utilizing soil vapor extraction. A third alternative is to take no further action and allow passive bio-degradation processes to reduce the petroleum hydrocarbons in the soil over time. 4.1.1 Excavation and Ex-Situ Treatment If the present groundwater table at the site were at its historically low level and if the groundwater ' beneath the surface were not so severely impacted,excavation and above-ground treatment would probably be the most cost-effective and preferred alternative. However, such is not the case at this time. The groundwater elevation is near its recorded history high, and the groundwater at the ' site, particularly near the location of the former UST, has been badly impacted with petroleum hydrocarbons. The most severely impacted soil in the area of the former UST is below the water table. The dilemma with using this alternative under these conditions is two-fold: ' 1 If the impacted soil were to be excavated including that below the water table the clean P � g , backfill material would be immediately impacted by the groundwater. 2) Vice versa, removing impacted groundwater by pump and treat methods, without prior ' removal or treatment of impacted soil, has proven to be generally ineffective. It has even been specifically questioned by the State Water Resources Control Board as an effective treatment method. ' OST 701-1-2-cap Page 7