Monitoring and data analysis for the Vadose zone monitoring system (VZMS), McClellan AFB - Quarterly Status Report - 11/15/97-2/20/98 Page: 12 of 167
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Monitoring and Data Analysis for the Vadose Zone Monitoring System -- Quarterly Status Report, 2/98
2.4 Liquid-Phase VOC Concentrations
Liquid-phase VOCs are sampled using two-chamber suction lysimeters designed for use at
depths greater than 7-8 m. One 1/4-in and one 1/8-in tube connect the lysimeter to the surface. A
miniature check valve separates the lower chamber from the upper chamber. A 0.5 gm porous
stainless steel cylinder permits the collection of the sample which is drawn by vacuum through the
check valve into the upper chamber. To withdraw a water sample from the soils into the suction
lysimeter, a vacuum is applied to the tube connected to the top of the upper chamber. In order to
bring the water sample to the surface, dry, purified gas, either N2 or Ar, is used to pressurize the
upper chamber, forcing the water sample up through the second tube that connects the bottom of
the upper chamber to ground surface. The check valve closes, preventing liquid from being forced
back into the lower chamber. During the last quarter, lysimeter samples were extracted on
12/15/97, 1/21/98, and 2/19/98. Some of these samples were removed as part of the instrument
testing process and were not analyzed. Due to the relative dryness of the formation, extracting
water continues to be difficult. Generally, small, less than 20 mL samples, are collected over a
period of a week. In many cases, samples are no greater than 5 mL. Therefore, 4- and 6-mL vials
have been used to collect the samples in order to prevent or minimize headspace. Unfortunately,
some samples are smaller than 4 mL and a headspace cannot be prevented. In the future, any
headspace will be filled in the field with a known volume of distilled water.
Although several compounds have been found to occur in the aqueous phase (LBNL, 1997a),
TCE, cis-1,2-DCE, and Freon 123a are by far the dominant contaminants and only their
distributions are presented in this report. Unlike in the gas phase, the recalculation of Freon 123a
levels in older sample sets was possible in the liquid phase. Therefore, both new results and
newly calculated results are shown. Temporal changes in TCE concentrations in Well A and Well
B are shown in Figs. 8 and 9, respectively, while cis-1,2-DCE values are shown in Figs. 10 and
11. These data exhibit trends similar to those seen in the gas phase. Specifically, there are two
peaks of TCE, one near the surface and one at the water table, but only one peak in cis-1,2-DCE,
near the surface. The first significant increase in TCE and cis-1,2-DCE was observed on 2/19/98,
the increases occurring in the top 30 ft, and being more pronounced in Well A, where downward
movement of both contaminants is apparent. This agrees qualitatively with gas-phase data, though
the pattern is not as clear. Furthermore, increases observed in the liquid phase at 30 ft do not
correlate with the rather small changes observed at that depth in the gas phase.
Freon 123a concentrations are shown in Table 1. Freon 123a was only detected in pore water
at a depth of 112 ft in Well A and 109 ft in Well B. Sample was not always available from the next
shallowest depth, 105 ft in each well, but it never contained Freon 123a above the quantification
limit of 5 ppb. Freon 123a concentrations appear to be fairly stable in both wells, with a range of
50 to 100 ppb.
Table 1. Freon 123a concentrations in pore water samples collected from specified depths.
Date Freon 123a at 112 ft, Well A Freon 123a at 109 ft, Well B
(ppb) (ppb)
5/7/97 51 80
7/22/97 76 51
10/23/97 101 65
1/21/98 75 58
2/19/98 91 65
10o' I '
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Zawislanski, P.; Mountford, H. S.; Dahlquist, R. & Rodriguez, S. Monitoring and data analysis for the Vadose zone monitoring system (VZMS), McClellan AFB - Quarterly Status Report - 11/15/97-2/20/98, report, May 5, 1998; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc719654/m1/12/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.