Quick run through of the four projects

Quick run through of the four projects 2

Vandenberg AFB Study Site. Mobile lab in center. San dune environment. Ground water contamination of TCE at ~18 to 20 feet bgs. 5

This research program addressed three key parameters that are expected to have a material impact on soil-gas sample representativeness and analytical results: sample purge rate, purge volume, and sample volume. Experiments were conducted to evaluate the effect of each principal parameter while holding all other principal parameters constant at the baseline settings. Each of the three experiments was conducted on separate days, which allowed the sample probes to re-equilibrate over night and minimized the effect of total purge volume. A subset of five of the fifteen soil vapor probes was selected for each of the experiments. 6

Tests were conducted on five probes (15-SV-B1 through 15-SV-B5) all at depths approximately 2 to 4 feet above the water table (~17 bgs to 20 bgs). 7

In general, there was a very modest increase, less than a factor of two, in measured TCE concentrations with increasing purge rate over the range of 100 ml/min to 5,000 ml/min. Furthermore, the relative percent difference (RPD) between the maximum and minimum concentrations measured at each of the probes tested ranged from 37 to 56 percent with the exception of the results from probe 15-SV-B1, which had an RPD of 82 percent. These RPDs can be compared to the RPD for the temporal control sample of 44 percent. While these data appear to show a trend toward increasing TCE concentrations with increased purge rate, changes in concentration of this magnitude would not be considered significant for site characterization or vapor intrusion applications. 8

Tests were conducted on five of the probes in the array. Two of the five array probes (15-SV-B1 and 15-SV-B4) had been used for the purge rate experiment the day before. The probe depths were approximately 2 to 4 feet above the water table (~17 bgs to 20 bgs). 9

A statistically significant positive correlation was observed between sample concentrations and purge volume; however, variations were less than a factor of three over the range of purge volumes tested (1-20 system volumes). The variation over purge volumes typically recommended for soil gas surveys (i.e., 3 to 5 volumes) was less than a factor of two. The concentrations measured after purging two system volumes were consistently higher than those measured after withdrawing only one volume. Increasing purge volumes above 6 system volumes did result in higher concentrations. While these variations would not generally be considered significant from a site characterization or vapor intrusion perspective, based upon these data, it appears that purge volumes of 2 to 6 system volumes are most appropriate under the conditions samples. Based upon the initial results, a shallow (5 feet bgs) temporary vapor probe was installed (15-SV-C4HP) to test the hypothesis that the increased TCE concentrations in the two probes were due to larger purge volumes drawing in vapors from the capillary fringe. This shallow probe showed little increase in concentration from six to twenty purge volumes suggesting that interference with the capillary fringe could have been occurring with the deeper probes. 10

Tests were conducted on five of the probes in the array. Two of the five array probes (15-SV-B1 and 15-SV-B4) had been used for the purge rate and the purge volume experiments on two previous days. The probe depths were approximately 2 to 4 feet above the water table (~17 bgs to 20 bgs). Because the purge volume necessarily increased during this experiment as consecutive samples were collected; this experiment was similar to the prior experiment except that the sample volume was integrated in this experiment (60 ml grab samples were collected at the end of the purge volume in the prior experiment). Samples were collected from five probes, with sample volumes ranging from 25 to 6,000 milliliters (ml). 11

In four of the five probes, the variation in concentration was less than a factor of two over 25 ml to 1,000 ml sample volumes. In one probe (15-SV- B3) the variation was a factor of four. These results are fairly consistent with the purge volume test results over the volumes extracted (the maximum volumes of soil gas extracted were approximately 1,400 ml and 1,700 ml for the purge volume and sample volume experiments, respectively. Measured concentrations decreased in the 6,000 ml samples relative to the 1,000 ml samples. This drop in measured TCE concentrations from the 1,000 ml to the 6,000 ml samples is noteworthy as the 6,000 ml sample volume is often used in the industry (i.e., 6-liter SUMMA canisters). This study suggests that 6-liter samples may result in lower concentrations than 500 or 1,000 ml samples. One potential cause for the observed drop in measured TCE concentrations in the 6,000 ml samples is over-purging of the system.. 12

This research program focused on the temporal variations in soil gas due to changes in meteorological conditions. A subset of twelve of the existing probes was utilized for this investigation. The probes utilized for this investigation were at depths ranging from 3 feet to 17.5 feet below ground surface (bgs). An Onset HOBO weather station integrated with a SolarStream Satellite data transceiver was deployed to continuously monitor and record atmospheric temperature, relative humidity, barometric pressure, wind speed, wind direction, and precipitation. Soil temperature and soil moisture probes were installed at depths of 3 feet, 8 feet and 17 feet bgs and integrated with the HOBO weather station data logger. Soil-gas pressure probes were also installed at the same depths and integrated with a second data logger. 13

A unique part of this program consisted of the analytical system used to measure soil gas concentrations. An automated gas chromatograph (GC) measured soil gas concentrations every hour from each of the 12 probes continuously around the clock for a period of approximately 6 weeks from March 16 Soil-gas pressure probes were installed at 17 feet, 8 feet and 3 feet bgs and integrated with a second data logger. to April 30, 2007. A low flow rate pump drew vapor through a stream selection valve integrated with the GC. The entire system was controlled remotely from the H&P office through a wireless connection and the data were reported back to the H&P office in real time. 14

This is a plot of data recently collected for an EPA funded study by an automated instrument at at Vandenberg AFB site from three probes at the same location but at different depth (3, 8, & 17 bgs). This plot consists of over 750 points per probe collected once per hour over a 6 week period from mid March to mid April 2007. The soil gas concentrations varied by less than 10% over these four days even for probes only 3 feet below the surface. 17

This is a plot of data recently collected for an EPA funded study by an automated instrument at at Vandenberg AFB site from three probes at the same location but at different depth (3, 8, & 17 bgs). This plot consists of over 500 points per probe collected once per hour over a 4 week period from mid March to mid April 2007. The soil gas concentrations varied by less than 10% over these four days even for probes only 3 feet below the surface. 18

Brief info about site setting 24

1. Groundwater at this site flows in an easterly direction and is slow-moving. 2. Plume originated over 20 years ago when releases from sewer lines and USTs occurred it is now about 1,000 feet long (large plume). 3. All sources were removed or sealed in the early part of this decade; thus, there are no continuing sources of chlorinated solvents. 25