| Comments: |
A new contract is being awarded to conduct supplemental work related to the FS for this site. See below of background and proposed actions. Contract will be awarded NLT 30 Sep 2023:
Previous investigations at CG019 identified VC in shallow (5 to 20 ft bgs) monitoring wells and
deeper (30 to 40 ft bgs) piezometers, but not in wells screened below this depth (40 to 50 ft bgs) in the southern portion of Guard Central. More current sampling indicates VC exceedances are limited to the deeper portion of the shallow zone. Although no known source has been identified, the impacted area appears localized along the east-west drainage ditch on the north side of Prime Beef Drive (Exhibit 1-6).
Groundwater monitoring for the Jet Fuel Offloading Facility (JFOF) in 2013 identified VC at
concentrations exceeding the ES (0.2 micrograms per liter [μg/L]) at six of 48 monitoring wells sampled. Concentrations ranged from 0.22 to 4.8 μg/L. VC was
not attributed to JFOF activities, and the JFOF monitoring well network was subsequently sampled
quarterly through 2014, and semiannually through October 2018, during which time VC was detected at concentrations exceeding the ES in groundwater sampled from eight wells. Highest concentrations were consistently detected at MW-100P, at concentrations that remained relatively constant across sampling events, (4.8 μg/L in August 2013 and 4.92 μg/L in October 2018). Based on detections of VC in groundwater during sampling for the JFOF site, CG019 was investigated for
a Site Inspection in 2014. Four soil borings and three groundwater samples were collected and analyzed for VOCs. VC was not detected in soil samples, and was detected at 0.69 J μg/L, exceeding the ES in one groundwater sample. During the 2016 RI, 14 existing and ten new monitoring wells were sampled, and 18 soil samples were collected from 10 borings. VOCs were not
detected at concentrations exceeding criteria in soil, and VC was the only analyte detected in groundwater at concentrations exceeding the ES. Exceedances were detected only in deep (30 to 40 ft bgs) groundwater samples. Additionally, wells installed and screened at 40
to 50 ft bgs did not contain VC at concentrations exceeding the ES, although the detection limit (0.6
μg/L) exceeded the ES for this event. The CG019 VC plume is localized along the drainage
ditch on the north side of Prime Beef Drive. Contaminant concentrations do not appear to be
migrating vertically or horizontally or changing significantly over time. This may be due to multiple
factors, including groundwater flow that is influenced by seasonal variations of high and low water levels, with shallow water tending to flow toward stormwater drainage channels, and anaerobic conditions which may result in stalled natural attenuation of VC. Because
current wells represent up-, cross, and down-gradient potential source areas within CG019, no additional monitoring wells are proposed for this site; rather the existing wells will be monitored to confirm RI findings, and monitoring wells located east of Prime Beef Drive
for investigation at CB018b will be included in the sampling program.
|
