HIGH-RESOLUTION FRACTURE NETWORK DELINEATION FOR IN SITU BIOREMEDIATION USING FLUORESCENT TRACER DYES

High-resolution mapping of a source area fracture network and associated aquifer characteristics was performed for the purpose of finalizing the design of amendment injections to remediate targeted areas of a chlorinated solvent-impacted bedrock aquifer. The detailed characterization of a network of shallow-dipping bedding-plane partings and steeply-dipping tectonic fractures was accomplished using fluorescent tracer dyes, aquifer pumping response, down-hole geophysics and 3D-visualization. Previous estimates of hydraulic conductivity rates from short-term pumping tests in bedrock were not consistent with estimates made from plume migration data, underestimating the groundwater flow along the discrete bedding-plane fractures that dominate contaminant transport in this type of fractured bedrock aquifer. We performed fluorescent tracer-dye tests in bedrock at two injection wells, designed for the application of bioremediation amendments, to evaluate transport rates and migration pathways in bedrock. The bedrock in this New Jersey study area is the Passaic Formation of the Newark Basin consisting of interbedded shales, siltstones, mudstones and sandstones.

Fluorescent dyes Fluorescein and Sulforhodamine B (SRB) were selected because they have no matrix interference effects with site groundwater, are stable in slightly acidic conditions characteristic of in situ bioremediation, have acceptably low or not-detected background concentrations, and have low toxicity.

The tracer-dyes were introduced into two bedrock injection wells using straddle packers to target fracture zones identified by down-hole geophysics and to distribute the dyes more evenly. The onsite injection well received the fluorescein and the near-site injection well received the SRB. The monitoring program included periodic sampling for several months using a monitoring network consisting of 13 bedrock wells varying in depth from 50 feet to 260 feet below grade. In addition to the dye migration results, we will also present the pretest aquifer response to extracting several thousand gallons of makeup groundwater using pressure transducer data.