Groundwater monitoring has traditionally relied on monitoring wells to provide information regarding water level and/or groundwater chemistry changes within phreatic zones of aquifers. The accuracy of this approach is limited by the volume of the sampling which is generally less than one percent of a site’s subsurface domain. Therefore, the use of discrete wells often results in missing features of significance such as heterogeneities and preferential flowpaths which exist outside of screened interval elevation and well locations. Finally, traditional monitoring wells can provide pathways across zones leading to unplanned vertical flowpaths and resulting incorrect data or unwanted cross contamination between aquifer zones.
A modern and more robust approach to site monitoring is now available via electrical hydrogeology methods which uses an infrastructure of horizontal multicore cables trenched just below ground surface (i.e., burial depth ~ 2 feet). Data collection yields high data density continuous 2D images below the buried electrode cables. These data can be used to monitor moisture conditions in the vadose zone, fluctuations in groundwater table elevations, changes in bioactivity levels, and other subsurface reactions and changes over time.
After installing a dedicated system, the subsurface can be periodically monitored on a regular basis similar to the conventional approach of sampling monitoring wells on a specified (e.g., quarterly) basis. This approach converts a higher labor geophysical field effort to a low labor monitoring approach that significantly lowers the per sample price. The ability to monitor a continuous 2D plane worth of data allows project stakeholders to develop a more holistic understanding of the subsurface for better decisions related to technical, financial, and social issues facing project stakeholders.
In an arid region of western Oklahoma, a municipality faced the critical challenge of developing a new groundwater well field capable of meeting long-term demand, and without the benefit of viable surface water supply options. Another challenging aspect of the project was the limited amount of site-specific hydrogeologic data available to guide placement of individual wells in locations which would provide optimal yield and water quality. To minimize the risk of drilling costly low-yield wells, a targeted, high-resolution approach was needed to identify optimal well locations across a large undeveloped track of land.
Specialized electrical resistivity imaging (ERI) technology (commercially available as GeoTrax Survey™) was deployed across approximately one square mile to evaluate subsurface electrical properties in 3D and determine if zones of enhanced porosity and permeability were present which would serve as optimal drilling targets. These high-resolution data were compared with existing hydrogeologic information and borehole lithology to correlate electrical anomalies to known aquifer structures. The results were integrated with a geologic surface model and visualized in 3D, to provide a robust and detailed interpretation of subsurface conditions critical for successful groundwater production well targeting.
Seven test wells were located based on the 3D visualization of integrated data sets to target electrically anomalous zones interpreted as higher-permeability targets. All seven wells delivered strong results, with each demonstrating satisfactory quality and capacity for at least 1 million gallons per day (MGD) of production upon full development. By leveraging high-resolution electrical imaging, the project eliminated the need for multiple rounds of trial-and-error drilling, significantly reducing both capital costs and timelines for optimized development of the groundwater wellfield. Aestus’ work is estimated to have saved our client millions of dollars as well as facilitated project schedule compression.
Background/Objectives: Remedial substrate injection (light remediation) and thermal (heavy remediation) are commonly used for in situ treatment of contaminated sites. An important component of designing and monitoring these remediation programs is optimizing effectiveness and cost efficiency across varied geologic settings. Traditional monitoring techniques based on wells and tiltmeters often fail to provide sufficient data density to develop a workable conceptual site model.
Temporal electrical resistivity imaging (TERI) technology has been demonstrated to more robustly characterize NAPL and dissolved plume distribution, state of weathering, and the distribution of bioactivity. When these electrical data sets are collected over time (i.e., 4D/temporal/time-lapse imaging), valuable insights about preferential flowpaths, plume migration/behavior, radius of influence, and overall success of remedial actions can be better understood by analyzing changes in electrical properties of the subsurface.
Approach/Activities: Successful monitoring has been performed at multiple injection and thermal remediation sites using both single-event post-injection imaging events or via temporal imaging conducted over multiple time points (e.g. pre- and post-remediation). This presentation will highlight the advantages of the method through a discussion of several project examples which provided information about the benefits and challenges of subsurface electrical imaging and data integration.
Results/Lessons Learned: To date, results indicate conceptual site models (CSMs) developed from electrical imaging with targeted drilling and remediation lead to more successful remediation via injection or thermal. The further use of electrical methods via TERI to better understand the actual injectate distribution and effectiveness of thermal remediation over time has produced a number of helpful lessons. First, electrodes must remain in place throughout the monitoring period to maintain low temporal noise and ensure data quality. Second, channeling and vertical migration of injectate are common occurrences. Finally, electrical signatures can change over time in response to subsurface reactions. Incorporating these lessons into future designs will improve the planning, implementation, and monitoring of both light and heavy in situ remedies for contaminated sites.
The majority of characterization and monitoring of LNAPL impacted UST sites is conducted using a vertically emplaced slotted pipe (i.e., a traditional monitoring well) to measure site parameters. These measurements can be fluid levels, LNAPL thickness, bioactivity or chemistry collected once or over time. Although monitoring wells provide useful data, the data density across a typical site is very low and resulting understanding of plume movement and/or remediation effectiveness can be easily misunderstood.
Temporal electrical resistivity imaging (TERI) technology has been demonstrated to be able to more robustly characterize LNAPL distribution, state of weathering, and the distribution of bioactivity. When these electrical data sets are collected over time (i.e., 4D/temporal/time-lapse data sets), more information about preferential flowpaths, plume migration/behavior, and success/radius of influence of remedial actions can be imaged and better understood. To collect these data sets a cable is laid out across the ground (short-term monitoring) or buried in a shallow trench (longer-term monitoring) to protect the cable against damage from traffic or animal chews. Resulting data sets include continuous 2D static and time-lapse imagery which can inform baseline conditions and changing subsurface conditions over time.
This presentation examines the most efficient way of characterizing and monitoring an LNAPL site based on modern tools available in the present day and the current state of the science. Case studies will be presented showing real world application of TERI at leaking AST/UST and other sites to demonstrate the efficacy of these methods as well as highlight potential limitations. The goal of this talk is to allow attendees to understand the value of leveraging TERI at their project sites to increase certainty and decrease time and cost to site closure.
