Ground water assessments identified four distinct saturated units that required investigation. These units included two distinct saturated zones identified in 20 to 25 feet of unconsolidated glacial deposits and two saturated limestone bedrock formations that totaled between 80 and 100 feet underlying the glacial deposits. The two unconsolidated saturated zones included a surficial perched zone composed of fine- to medium-grained sands encountered above a laterally discontinuous clay layer and a zone of clean coarse sand and gravel encountered directly above the bedrock interface. Neither of these two upper units is used for public water supply. However, ground water in the perched unit is hydraulically connected to the bedrock interface unit, which is in direct contact with the fractured limestone bedrock. Ground water flow and contaminant migration in both of these upper units were influenced mainly by the slope of the clay layer and conductive background materials used in the utility corridors that bisected the contaminated areas.

The bedrock consists of three distinct units. The top layer is the Kokomo Limestone. The Kokomo Limestone contains thin interbedded shale layers, and its surface is extremely weathered and fractured. The Kokomo Limestone formation is underlain by the Liston Creek Limestone, a dolomitic limestone. Underlying the Liston Creek Limestone is the Mississinewa Shale, a dolomitic siltstone considered a low-yield, low-permeability unit.

• Dense Non-aqueous Phase Liquids (DNAPLs)
• Fractured Bedrock

Delineation of contamination in bedrock is complicated by differing fracture orientations within and between units. Different hydraulic gradients exist in the three bedrock interfaces.

• Borehole Geophysics
  • Natural Gamma
  • Caliper
  • Acoustic Televiewer
• Fluid Loggings
  • Conductivity/Resistivity

Comments:
The objectives of the investigation were to characterize the fracture patterns within the bedrock, define the hydrogeological characteristics of the fractured and competent portions of the bedrock, identify the vertical and lateral extent of the impacts from volatile organic compounds (VOCs) and the preferred VOC migration pathways, and assess potential receptors of ground water contaminated by VOCs.Multiple downhole geophysical investigation surveys identified the predominant bedding plane fracture intervals and orientations. These geophysical results were combined with downhole hydraulic profiling data that estimated the hydraulic properties of the fractures and identified specific bedding plane flow zones that had the greatest effect on contaminant migration. Multilevel monitoring wells constructed to screen common fractures and a network of transducers helped delineate divergent horizontal hydraulic gradients and continuously changing vertical gradients caused by off-site pumping operations.

Complex hydraulic conditions at the site complicated efforts to delineate the extent of VOC impacts. Current hydraulic data suggest that vertical fractures may be affecting ground water flow and contaminant transport. Therefore, these factors will be assessed as part of the next phase of investigation to ensure proper site characterization and subsequent remedial evaluations.

Cleanup goals are maximum contaminant levels (MCL).

Reference:
Hotchkiss, Gordon; Frederick W. Blickle; Nathan Kuhl; John P. Mather. 2004. Evaluation of Contaminant Distribution in a Complex, Three-Aquifer System. The Fourth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, California, May 24-27.