Practical Bedrock Aquifer Characterization Using Borehole Geophysics and Multilevel Wells

Abstract

Characterizing and monitoring contaminated bedrock aquifers remains an enormous challenge with important consequences. Investigations of bedrock aquifers are challenging for two principle reasons: heterogeneity and costs. This paper demonstrates that successful characterization is possible and describes a strategy using borehole geophysics and multilevel wells that greatly increases the chances of success. A cost evaluation shows that, while these methods can be expensive, proper application may reduce long-term monitoring costs. The increasing use of low-flow sampling techniques and the rising popularity of monitored natural attenuation remedies should lead to increased application of this strategy. The case study site is a hilltop landfill overlying a heterogeneous bedrock aquifer and surrounded by homes using domestic wells. Borehole geophysical and heatpulse-flowmeter data identified 1–2 water-bearing zones (WBZs) per 30 m drilled. Multilevel wells focused sampling on WBZs and provided detailed plume characterization and monitoring that will help to protect local domestic wells. Review of logs from monitoring well MW-8 shows how borehole geophysics identified seven individual WBZs, and sampling of the multilevel well revealed important variations in chemical and potentiometric signatures between the WBZs. This detailed characterization was necessary to support the site conceptual model and demonstrated that domestic wells were not threatened.