Formatted Title

High Resolution Site Characterization and Advanced Injection Tooling for Remediation in Fractured Rock

Background/Objectives

The characterization of fractured rock consists of identification of fractures/weathered zones within the fractured rock using high resolution tools and techniques such as two-dimensional electrical resistivity imaging survey (2-D ERI), borehole geophysical logging to delineate the vertical extent, orientation, and aperture size of the bedding planes and fractures, downhole camera, and discrete groundwater sampling from identified fractures/bedding planes using small interval straddle packer assembly. Quantification of total mass and spatial location of the contamination is crucial in the precise delivery of the selected remedial technology and in return, achieving site remediation goals. 

Selection of injection tooling and techniques, along with the proper reagent is critical in a fractured rock setting. Due to the difficulty of emplacing material in the subsurface, and the long-term source of matrix diffusion, identifying remediation products that will quickly control aqueous phase contaminants, while providing long term treatment of the matrix diffusion over the lifetime of the project will is a crucial step in site success in fractured rock. Utilization of specialized tooling and equipment is also necessary for the accurate placement of the reagent in the fracture zones identified. With the use of straddle-packer assemblies and high pressure, high flow pumps allow for more precise application of the in situ amendment into fracture zones/bedding planes identified during characterization.

Approach/Activities

A bedrock remedial design characterization (RDC) utilizing multiple geophysical investigative techniques and high-resolution field data collection was conducted to determine the transport mechanism of the dissolved BTEX plume and determine the effectiveness of the chosen in situ technology.  The RDC and pilot test included four key components: 1) a 2-D ERI survey to identify new borehole locations within the plume, 2) borehole geophysical logging to delineate the vertical extent, orientation, and aperture size of the bedding planes and fractures, 3) discrete groundwater sampling from identified fractures/bedding planes using a custom designed straddle packer assembly with an eighteen inch sample interval, and 4) high resolution water level monitoring of the surrounding well network during air rotary drilling and pilot injection test using pressure transducers to define the horizontal connectivity between existing monitoring wells and the vertical connection between formations.

Based on the data collected from the RDC and pilot test, a clear picture of the subsurface lithology and structural features was obtained.  Multiple weathered zones were identified from the 2-D ERI where the additional boreholes were installed, and the borehole geophysical logging identified the horizontal bedding planes that were targeted in the discrete groundwater sampling and pilot injection test.  The data collected from the discrete groundwater sampling, provided information to assist in injection location and remediation product loading. 

Results/Lessons Learned

Post pilot test groundwater sampling results show the targeted monitoring well with historical benzene concentrations greater than 6 mg/L, below laboratory detection limits of 0.001 mg/L for seven consecutive quarters. Using the data collected from the pilot test, a full-scale injection was implemented. The full-scale BOS 200® injection was successfully placed in the identified zones of contamination throughout the plume, intersecting all impacted monitoring wells, with three consecutive quarterly sampling results exhibiting results below the target remediation goals. Based on the post-injection groundwater results from the pilot test and full-scale injection, a No Further Action letter was issued.