Fractured Bedrock Aquifer Characterization Using Borehole Geophysical Logging and FLUTe Multilevel Well Systems

The presentation will address challenges associated with designing and installing monitoring wells screened in the proper water-bearing intervals within a fractured bedrock aquifer at a Superfund site in Puerto Rico. Contamination has been detected at the site in municipal supply wells that draw from the bedrock aquifer. A site conceptual flow model was developed using published geologic data, topography, and surface water drainage features. The model was used to identify drilling locations along the likely groundwater flow path(s) between potential source areas and the supply wells. After completion of each borehole, mechanical caliper, natural gamma, acoustic televiewer, optical televiewer, fluid temperature and fluid conductivity logs were run. These data were compiled in WellCAD software and were used to identify potential water-bearing zones for heat pulse flow meter logging under ambient and pumped conditions. Logging results were used to identify groundwater sampling points in the water column above and below water entry and exit points.

The EPA Region 2 low-flow sampling method was used to collect groundwater samples from target depths in each borehole. The samples were analyzed for volatile organic compounds (VOCs). To determine transmissivity with depth, drop tests using blank borehole liners from Flexible Liner Underground Technology (FLUTe) were completed in six of the seven boreholes. Logging data, VOC groundwater sample results, and transmissivity data were evaluated collectively in WellCAD software to identify multilevel monitoring intervals for each borehole. Seven multilevel well designs, including precise monitoring intervals, and the WellCAD file were provided to the multilevel well manufacturer to ensure proper construction of the Water FLUTe^® systems. The results of groundwater sampling and water level monitoring from the multilevel bedrock wells will be used to update the conceptual flow model. The model supports ongoing site characterization and efforts to confirm the source of contamination affecting the municipal wells.