OBTAINING HIGH-RESOLUTION SITE DATA
Inner sleeves of the DPT system were used to extract continuous cores from the boreholes. Samples of soil from the cores were collected from discrete zones of apparent impact, based primarily on readings from a photo-ionization detec- tor using ambient-temperature-head- space-analysis methods. When there was no apparent impact, samples were collected from intervals of approximately 30 centimeters (cm) (1,291 samples in all). Detailed boring logs were kept to document lithology, color, relative mois- ture content, and visual and/or olfactory evidence of volatile organic compounds (VOCs). In fine-grained deposits, frac- ture patterns and/or other pathways were noted which might affect leakage, and hence, solute fate and transport.
The soil samples were analyzed for VOCs using US Environmental Protection Agency (EPA) Method 8260B. As listed in Table 1, TCE concentrations could vary 5 orders of magnitude in 46 cm, indicating that preferential path- ways steered solute circuitously through the matrices, but with apparent preci-
Figure 5-High Resolution Data obtained from soil boring and groundwater well locations.
A primary “take away” from the detailed logs of the continuous borings was that matrix architecture was com- plex. Subsequently, selecting appropri- ate well-screen locations was critical in representing the heterogeneity that caused variability in solute distribution.
The wells were constructed using 2.54 cm diameter, flush-threaded polyvinyl chloride pipe and 10-slot screen. During the characterization phase, as many as five wells were clustered at a single location in the DNAPL area (Figure 6) to generate a vertical profile and to determine the “basement” of impact. These, and all other wells, were later used to obtain high-resolution analytical data during implementation and closure phases of the project.
A total of 5,515 groundwater sam- ples were collected from site wells using dedicated bailers and/or low-flow methods and analyzed for VOCs using EPA Method 8260B. Low-flow meth-
sion. Analytical results were used to gen- erate vertical profiles of pre-treatment solute distribution and post-treatment BOS 100® distribution.
Temporary Well Installation. DPT was used to install 1,349 temporary mon- itoring wells throughout the DNAPL and dissolved-phase plume areas (Figure 5). The purpose of the wells was to characterize impacted regions, evaluate treatment performance during remedy implementation, and provide confirma- tion that target cleanup levels had been achieved.
12 TPG •
Oct.Nov.Dec 2017
ods involved using a pump to purge groundwater at rates comparable to ambient groundwater flow (typically less than 500 milliliters/minute) to minimize drawdown and mixing of borehole stor- age, i.e., stagnant water in the well. Characteristic parameters pH, dissolved oxygen, conductivity, temperature, and turbidity were monitored until stable prior to sampling to facilitate equilib- rium with the surrounding formation to ensure samples were representative of formation water (Solinst 2014). As with soil profiles, facies changes in the native geologic units resulted in TCE concen- trations in groundwater that could vary 3 orders of magnitude between adjacent screened intervals.
Cross Sections and Transects. Detailed geologic cross sections were generated from the boring logs and anno- tated with TCE concentrations from the extensive array of soil and groundwater samples (Figure 7on the folowing page). These quantitative data were preferred over qualitative data that might other- wise have been obtained from a standard MIP survey.
In the early stages of the project, the cross sections were used to identify data gaps in understanding the complex geologic setting. Additional soil borings/ wells were installed and/or additional sampling activities were conducted, accordingly.
Figure 6- Well Cluster
Once site characterization was con- sidered complete, cross sections orthogo- nal to groundwater flow were used as transects to quantify source-zone and plume strength. Plume stability was of
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