Final Second Five-Year Review Report Fort Ord Superfund Site ...

and 0.098 ppbv). Both the indoor and outdoor samples collected at Lexington Court were within the
range of background concentrations 0.067 ppbv and 0.13 ppbv measured in outdoor air during the Fort
Ord outdoor air monitoring. These results then support the conclusion that groundwater contamination
appears to be an insignificant contributing source of VOCs to indoor air in the source area
(MACTEC, 2006b).
11.1.2 Groundwater
The upper three aquifers at the former Fort Ord, none of which are currently used as a drinking water
source within the OUCTP, have been found to contain concentrations above MCLs for CT and other
VOCs within the OUCTP. The aquifer cleanup levels are listed in Table 7. The presence of CT in these
three aquifers is described below.
A-Aquifer Groundwater
The length of the CT plume in the A-Aquifer is approximately 1.6 miles, and ranges from 500 to 750 feet
in width along the length of the plume (see Plate 3). The State MCL for CT in groundwater is 0.5 μg/L,
and the maximum historic detected concentration in the A-Aquifer since groundwater monitoring was
initiated in 1992 was 19 μg/L. The most recent maximum concentration of CT detected in the A-Aquifer
was 15 μg/L. Low levels of PCE and TCE have also been identified in the A-Aquifer within the OUCTP.
Hydraulic communication between this A-Aquifer and underlying aquifers is limited to those areas west
of the OUCTP where the Fort Ord-Salinas Valley Aquiclude clay unit pinches out, or where it has been
penetrated by wells without adequate sanitary seals. Two such vertical conduits have been identified and
have resulted in the migration of CT from the A-Aquifer to the underlying Upper and Lower 180-Foot
Aquifers. All identified vertical conduits have been destroyed (grouted and sealed) eliminating hydraulic
communication between the A-Aquifer and the underlying aquifers.
Upper 180-Foot Aquifer
There are two narrow, parallel plumes in this aquifer as shown on Plate 3. The western CT plume in the
Upper 180-Foot Aquifer is approximately 0.7 miles in length and 400 feet in width. The eastern CT
plume in the Upper 180-Foot Aquifer is approximately 0.9 miles in length and ranges from 200 to 600
feet in width. The maximum historic detected concentration in the Upper 180-Foot Aquifer since
groundwater monitoring was initiated was 9.8 μg/L. The most recent maximum concentration of CT
detected in the Upper 180-Foot Aquifer was 3.5 μg/L.
Lower 180-Foot Aquifer
There are two separate plumes in this aquifer. The northern CT plume in the Lower 180-Foot Aquifer is
approximately 0.75 miles in length and 1,000 feet in width. The maximum historic detected
concentration in the Lower 180-Foot Aquifer since groundwater monitoring was initiated was 6.95 μg/L.
The most recent maximum concentration of CT detected in the Lower 180-Foot Aquifer was 3.6 μg/L.
Low levels of 1,2-DCA have also been detected in the Lower 180-Foot Aquifer.
11.2 Remedial Actions
11.2.1 Remedy Selection
The following four alternatives were evaluated in the FS (MACTEC, 2006b).
Final
11-2
FORMER FT ORD 5YR REVIEW 2007_FINAL United States Department of the Army