Iterative Approach to Bedrock Assessment Achieves Results, Saves Money

Iterative Approach to Bedrock Assessment Achieves Results, Saves Money

Fractured bedrock aquifers often present a daunting multitude of investigation and remediation challenges, which can commonly translate to uncertainty regarding clean-up standards and high site management costs. Through many years of experience evaluating the nature and extent of bedrock impacts, our team of practitioners has developed a well-defined, sequenced strategy for site investigation that prioritizes quality data collection to help our clients attain site closure and avoid unnecessary project costs.

A traditional approach at fractured bedrock sites is to simply drill to the deepest extent and work backwards to complete a site assessment. However, we’ve had more success using the fractured bedrock toolbox in conjunction with an iterative investigation program. This translates to an adaptive approach that prioritizes data quality using a series of integrated steps. For instance, instead of drilling to excessive depths at the onset and moving backwards to evaluate findings, we recommend drilling less on a linear foot basis and collecting and assessing data before deciding to drill deeper. Essentially these iterative steps emphasize the interpretation of high-resolution data on an elevation basis to avoid unnecessarily overdrilling to a depth where groundwater remains clean.

Relying on data before drilling

Effective bedrock site characterizations begin without drilling a borehole. Advancements in “big data” repositories have drastically increased the amount of pre-investigation resources available, and these outlets can provide valuable insights regarding local and regional bedrock type and structure. The accessibility of historic bedrock and surficial geologic maps has improved dramatically, while investments by many federal, state, and local organizations in orthophotography and LiDAR mean these even higher resolution resources are now commonly available. Consulting these types of resources, along with more traditional ones like well records, or even contracting new aerial photos or LiDAR images for sites, is a valuable first step that will help identify localized zones of groundwater flow and recharge prior to developing a proposed borehole investigation network. These resources are commonly overlooked but can provide extremely valuable insight into groundwater flow patterns that can be used to locate borehole locations more precisely relative to groundwater flow and prioritize investigation methods using a transect-based approach. This streamlines the investigation and provides optimal results at minimal cost.

Partnering with experts in geophysics

Occasionally clients can be wary of the more technical approach. The alternative approach of relying on drilling to uncover answers often seems simpler, which can sound like a cheaper and faster method. However, working with a team focused on strengthening the conceptual site model at your site can actually lead to real cost savings. Geophysics are a key component to a successful investigation program. Downhole logging tools that measure temperature, fluid resistivity, bedrock structure, and other characteristics can be instrumental in evaluating groundwater geochemistry and fluid flow, while surficial techniques can be useful in refining an investigation network.

In an iterative program, the key is to understand groundwater flow into and out of a borehole via the fracture network, isolate these zones for sample collection, and then consider the next “logical” depth interval or downgradient borehole. Our experiences have shown the value of using multiple tools during the logging process and the upside of cross-correlating data. You should feel confident in the effectiveness of geophysics in the context of these projects—fluid properties and borehole flow meters don’t lie. Plus, when considering more boreholes versus geophysical logging from a cost-benefit standpoint, there is a significant advantage to higher quality logging (and even re-logging) as opposed to the expense associated with advancing additional boreholes.

These projects are complex and can become very expensive, which makes selecting a partner that’s both an expert in the latest techniques and dedicated to running a holistic program that meets your needs a key piece of your site management strategy. For your next bedrock evaluation, consider undertaking an iterative program that relies on advanced technologies and cost-effective collection of high resolution data. The bedrock tool box is generally well-established with some new improvements developing year after year, but in our experience the sequence of implementation can make all the difference.

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