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Work Plan for the
<br /> Former Two 20,000-Gallon USTs Location at Stockton AASF
<br /> California Army National Guard
<br /> 3.3.2 Potential migration pathways at the AASF former tank location include outdoor air,surface
<br /> water,soil,and groundwater. The primary release mechanism is the former UST location;thus,
<br /> leaching of contaminants downward within the subsurface to groundwater is the potential pathway.
<br /> 3.3.3 Based on the limited information available and site conditions at the two former 20,000-gallon
<br /> USTs location at the AASF, the potential for the exposure of receptors(e.g.,workers at the AASF)to
<br /> any petroleum constituents from the former two 20,000-gallon USTs location through inhalation,
<br /> ingestion,or dermal contact is possible;therefore,there is a potential health risk at the site.Potential
<br /> petroleum releases at the former UST location occurred primarily below grade.Most of the site is
<br /> paved;therefore,the transport of near-surface petroleum constituents through outdoor air or surface
<br /> water runoff is minimal.The transport of petroleum constituents through outdoor air,surface water
<br /> runoff,and infiltration to the subsurface is possible beyond the two former 20,000-gallon USTs
<br /> location. The presence of groundwater impacts originating from possible releases from the two
<br /> former 20,000-gallon USTs may have occurred.
<br /> 4.0 CONCLUSIONS
<br /> 4.1 Based on information obtained during the preliminary assessment and previous investigation
<br /> efforts by Versar,Inc.,soil and groundwater adjacent to the two former 20,000-gallon USTs area at
<br /> the AASF has been impacted by releases of petroleum and associated constituents to the environment
<br /> from either the three former 5,000-gallon or two former 20,000-gallon USTs and from surface spills.
<br /> 4.2 The results of the soil investigation conducted in February 2004 and described in Section 3.2
<br /> determined that soil south and southeast of both the three 5,000-gallon and two 20,000-gallon former
<br /> USTs had no detectable petroleum constituents(TPHG,TPHD,TPHJ,TPHO,BTEX,TAME,TBA,
<br /> DIPE,ETBE,MTBE,or lead)at concentrations greater than their respective reporting limits. The
<br /> February 2004 soil sampling event determined that the soil adjacent to the southeast and south corner
<br /> of the three 5,000-gallon and two 20,000-gallon former USTs location defined the lateral extent of
<br /> the previously discovered petroleum impacts at the three former 5,000-gallon USTs location;
<br /> however,the extent was not defined laterally from the previously discovered petroleum impacts.
<br /> 4.3 The soil samples(boring B-28 at 2 feet bgs and at 5 feet bgs[between the surface and 5 feet
<br /> bgs])that contained TPHG concentrations greater than Tri-Regional Board guidelines were
<br /> considered the result of minor fuel spills and did not impact soil from 10 feet bgs or deeper.The
<br /> limited area of TPHG impacts was considered to pose no significant threat to human health or the
<br /> environment because the vertical migration of heavy hydrocarbons is typically limited to the soluble
<br /> fraction and is not considered a concern in shallow soils at concentrations less than 1,000 mg/kg
<br /> (Versar,Inc.,2004).
<br /> 4.4 The lateral extent of groundwater(downgradient) impacts within the area of the two former
<br /> 20,000-gallon USTs has not been determined.Previous groundwater sampling by Versar, Inc.,
<br /> determined that the groundwater beneath the vicinity of the two former 20,000-gallon UST location
<br /> contains TPHG and benzene concentrations at levels that exceed taste and odor thresholds and
<br /> primary MCLS,respectively.Additional petroleum constituents have been detected in groundwater at
<br /> concentrations greater than their respective reporting limits but less then their respective action levels
<br /> as follows: ethylbenzene,xylenes,and MTBE.All three MWs exist within the western vicinity of the
<br /> two former 20,000-gallon USTs location.No MWs have been installed downgradient of the two
<br /> former 20,000-gallon USTs location;therefore, it is unknown whether petroleum impacts are present
<br /> in groundwater beyond(downgradient)this location.
<br /> KAWprocess\0025 MA ARNG\Stockton AASF WP\FINAL\WP text.doc 6 June 2007
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