Formatted Title
Pneumatic Characterization of Gas Flow and Contaminant Concentrations in Unsaturated, Fractured Bedrock
Background/Objectives
Contamination in fractured bedrock systems above groundwater is remediated most often with soil vapor extraction (SVE). SVE relies upon gas flow through fractures to transport contaminants to the surface for treatment. Characterization of connected pathways in these scenarios is difficult. Depth-discrete pressure changes recorded in surrounding monitoring wells in response to an applied well vacuum indicate the connectedness of fractures; however, a pressure response does not guarantee significant flow through a fracture nor can the response be correlated to a specific fracture in the extraction well. Common well logging techniques, such as video logs, can identify fractures including the aperture size and radial completeness; however, connectedness is not evaluated. Traditional techniques to estimate flow and contaminant concentrations in individual fractures along a corehole in unsaturated bedrock require the repeated placement of packers and piping to isolate and test fractures at numerous depths. This process is time-consuming and expensive.
Approach/Activities
This presentation describes an innovative well logging system that simultaneously measures the cumulative flow and contaminant concentration along the depth of a core while a vacuum is applied at the sealed wellhead. A flow sensor is connected at the end of cabling with an embedded Teflon® sampling tube that terminates just behind the flow sensor. During a log, the device is lowered and raised through the screen interval, the flow signal is recorded, and a small substream of the extracted gas is pulled from depth through the embedded tube for sampling and analyses on the surface. The vapor samples are logged by a photoionization detector and at discrete depths samples are collected in Tedlar bags for analyses by on-site gas chromatography (GC) and/or in canisters for analyses at an analytical laboratory. Methane, carbon dioxide and oxygen can also be logged to assess biological activity over the depth interval. Interpretation of the data is a simple mass balance between depth intervals to produce a vertical profile of the flow and contaminant concentrations from individual fractures or groups of fractures. These results can be combined with pressure and concentration responses at surrounding monitoring wells to better define flow paths and fracture connectedness across a site.
Results/Lessons Learned
Logging results and interpretation are presented from three contaminated sites with suspected DNAPL (TCE) in fractures. At the first site, a single complete fracture just above the water table at 168 feet below the surface (ft bgs) produced 80 standard cubic feet per minute (scfm) representing the total flow from the open core despite the identification of numerous partial and complete fractures all along the open core in a video log. The TCE concentration was constant (105 ppmV) from the fracture up to the bottom of surface casing at 30 feet below grade. An applied well vacuum of 8 inHg at the second site yielded a total extraction rate <10 scfm from the core open 69 to 245 ft bgs. Numerous fractures were identified along the core and the low flow suggests little connectivity among fractures. The total flow was below the threshold velocity for the flow sensor; however, the TCE concentration profile identified a DNAPL-contaminated fracture (17,000 ppmV) at 233 ft bgs. Extraction at a third site yielded a well flow of 20 scfm with an applied vacuum of 20 inHg. The log revealed several fractures producing low flow with TCE concentrations peaking at 900 ppmV.