Vertical borings and monitoring wells are the most commonly used methods to characterize environmentally impacted sites. These methods are useful to help characterize horizontal distribution of contaminants and geologic layers. However, the industry has found that vertical tools are largely inadequate to locate and evaluate discrete vertical features and flow paths, which can significantly contribute to contaminant migration and distribution in the subsurface and make sites appear recalcitrant.
This workshop will educate environmental professionals about a proven 5-step conceptual site model (CSM) development process which overcomes these challenges and has been applied on over 200 projects worldwide, including many leaking UST sites. 3D integration/visualization of data from conventional monitoring wells, HRSC direct push, and electrical geophysical imaging will be discussed relative to developing robust CSMs and facilitating remediation in less time and lower cost.
The workshop format will be participatory and have opportunities for roundtable discussions on current challenges with initial site characterization and RDC. Attendees will receive an interactive “behind the curtain” tutorial on integrating electrical resistivity images with other multiple lines of evidence to achieve RDC level CSMs,and guide next steps regarding drilling and/or remediation for environmental sites.
Workshop participants will be able to use their own laptops and the free trial version of RockWare’s® 3D visualization software (RockWorks® Version 20) to view and navigate through a fully constructed CSM which incorporates electrical resistivity data and traditional monitoring well data (including PIDs, analytical data, and groundwater parameter data).
By the end of the workshop, the goal is for participants to have a basic understanding of how to:
A former truck stop overlying a fractured sandstone aquifer experienced multiple fuel releases during the 1990s (including one release of ~2,100 gallons) which resulted in a significant LNAPL plume and up to 10 feet of product thickness recorded in multiple monitoring wells. Seven USTs were removed, and remediation work was performed from 2008 through 2017 which included free product recovery, enhanced fluid recovery, and surfactant injections. In 2020, free product was still measurable in a significant amount of the wells on site suggesting more robust characterization of the plume was required.
Specialty electrical resistivity imaging (GeoTrax SurveyTM) was deployed across the site to collect high resolution site characterization (HRSC) continuous 2D images of the subsurface and target confirmation borings/wells to understand subsurface conditions in anomalous zones. These data were integrated with pre-existing site and monitoring well data for the purpose of developing a data-rich 3D conceptual site model (CSM) and allowing site managers and other project stakeholders to understand how to focus appropriate next steps in the remediation process.
The HRSC electrical resistivity data provided key project insights regarding geologic structure, preferential flowpaths, and distribution of remaining LNAPL as well as evidence for the presence of bioactivity. Specifically, analysis of electrical imagery and integrated regional geology information indicated the presence of intersecting fracture zones in the sandstone which appeared to be the primary control on distribution of LNAPL, as indicated by alignment of the plume contours with the geometry of the fracture zones. The 3D CSM resulting from this process allowed the site management team to determine which of the pre-existing and targeted confirmation wells would likely produce the most NAPL extraction using high vacuum extraction.
As a result of the guided high vacuum extraction performed in the fracture zones, a product recovery radius of influence for the wells was recorded up to 40 feet laterally and significant volumes of LNAPL were recovered from the subsurface relative to previous unguided remedial efforts. To build on this success, the electrical imaging data are currently being used to select locations for future surfactant injections followed by high vacuum extraction to remove the remaining free product. This approach will leverage the delineated fracture zones where enhanced permeability allows for more efficient extraction.
The primary goal for management of environmentally impacted sites is to protect human health and the environment with a common secondary objective of restoring the property to beneficial use. This is a challenging goal which often involves costly remediation, so it is critical to take steps to minimize these costs and maximize effectiveness.
The primary reason remediation costs often exceed initial budgets is when initial remediation fails, and subsequent phases/remedies need to be deployed due to incomplete and/or incorrect conceptual site models (CSMs). Insufficient CSMs occur primarily because remedial design characterization (RDC) has historically been performed by installing wells or borings separated by distances on the scale of five to ten meters or more which industry data suggests is often inadequate data density for complex sites. Additionally, the conventional “drilling blind” or “poking and hoping” approach also results in data gaps which limit full understanding of subsurface conditions. One common significant data gap involves understanding the presence of preferential flowpaths (i.e., from faults, fractures and/or channels) which control contaminant transport and distribution.
Advances in direct push tooling have led to high-resolution site characterization (HRSC) work at many sites in recent years to improve data density. These data are useful but are limited to the domain of the boring and do not work in harder soil types/bedrock due to refusal. During the last 24 years, specialty electrical resistivity imaging (GeoTrax Survey™) has been used at hundreds of sites in varying alluvial and bedrock geology to generate ultra-high resolution 2D continuous subsurface images/scans and 3D CSMs which has further increased data density leveraging 10,000+ field data points on a typical site. The continuous imagery are able to detect and map preferential flowpaths to further strengthen the resulting CSM in the context of this RDC approach.
Industry experience shows that remediation projects are most successful when “beginning with the end in mind” (i.e., performing adequate RDC to understand the scope of site issues sufficiently to inform responsible remedial design and allow effective remediation the first time). This talk explores the benefits of a 5-Step RDC approach which has been used successfully at multiple LUST and other sites across the world in all different geologies. Utilization of this proven 5-step process involves significant data integration work to honor the benefits of leveraging multiple lines of evidence to develop a robust CSM appropriate for remedial design. Case studies will be reviewed demonstrating the efficacy of this approach in terms of accuracy and cost savings. These data demonstrate that this robust and collaborative 5-Step RDC process is more comprehensive than traditional characterization methods and helps site managers achieve faster and cheaper remediation while minimizing trailing liabilities.
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.
