BBC: Hydraulically-Connected Wells

 


During Phase III, we propose to drill four new boreholes in each of the same regions of Site 32 as the preliminary boreholes (i.e., control, TCE, DCE, and VC regions). [Note: These will be referred to as test boreholes or wells throughout the proposal.] Using the geophysical evaluations, particularly the radar data, we will locate the test boreholes so that each is hydraulically-connected by intersecting fracture planes to the preliminary boreholes. We assume that we will need to locate each test borehole within 10 meters of its paired preliminary borehole because extrapolation of the borehole radar data further than 10 meters in the saturated zone may be difficult.

Cores will be collected from the test boreholes at depths where the borehole radar predicts that fractures will occur. These cores will be processed using the drilling, extraction and microbial identification methods validated in Phase II. Because we anticipate processing as many as 6 cores per borehole for complete microbial analyses, there will be a lag time of 1 month between drilling the test boreholes. It is also proposed that these cores be 10 cm in diameter (vs. 5 cm in Phase II) to maximize the surface area of the fractures from which to obtain microorganisms. The results of these analyses will identify components of the bacterial community that are actively degrading TCE and its breakdown products and their genetic capacity for performing other metabolic reactions with chlorinated ethenes. These data will be compared to groundwater quality data (e.g., hydrogen) to verify the metabolic process(es) responsible for in situ TCE biodegradation at Site 32 (Chapelle et al., 1995; Yaeger et al., 1997). We will also determine if protists are present in the bedrock aquifer.

After microbial processing is complete, the cores recovered from the test boreholes will undergo petrographic analysis and structural description as outlined for Phase II. Omni-directional borehole geophysical radar measurements will be taken in the test boreholes, however, the complete suite of borehole geophysical tests will not occur until Year II due to budgetary constraints. All of the geological and geophysical data will be used to refine our knowledge of the subsurface bedrock at Site 32. Packer tests will be conducted in the test boreholes to determine the hydraulic conductivity and connectivity of discrete fractures. This will help verify if fractures in the preliminary and test wells are connected. The same suite of chemical analyses outlined in Phase II will be conducted in the preliminary and test wells (for discrete fractures) on a quarterly basis. This data, along with the USAF natural attenuation data from Site 32, will be used to define the plume and to help document the extent to which biodegradation is occurring. We are anticipating isolating specific fractures in the preliminary and test boreholes using in-well isolation devices located above and below the fractures. Groundwater samples will be collected from several of the discrete fractures and the results compared as a means of evaluating the interconnectivity of the fractures.





 

 
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