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Five Steps to Developing Weighted Lines of Evidence During Site Characterization

Posted by John Sohl on May 30, 2012 12:39:43 PM
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One of the most critical points to remember when developing a site closure work plan is that it simply costs too much to assess, remediate, and make decisions on information that is incomplete, inaccurate, and too late. Traditional approaches to characterizing subsurface conditions by drilling, soil sampling, and monitoring wells screened over large intervals are too incomplete to accurately deal with site heteregoneity.

Therefore, a high resolution site characterization approach is a more attractive option as it enables projects to move forward with remediation focused activities such as corrective action, free product recovery efforts, and site closure.

Let’s face it, the heterogeneity geology of a site is going to impact the migration of subsurface contamination opposed to the age old myth that it always follows the direction of groundwater. Introduction of the Hydraulic Profiling Tool (HPT) for site investigation and remediation efforts supports contaminant mapping activities to provide a greater understanding of subsurface conditions. By measuring direct pressure response, HPT develops weighted lines of evidence indicating the tight (or loose) grains and low (or high) flow zones in order to determine the migration pathways, remediation injection regions, and placements for monitoring wells. With an integrated Soil Electrical Conductivity (EC) array this tool also defines zones of lower conductivity which allows for the movement of contaminants into the subsurface.

On a recently completed former retail station site, the main objective was to identify sources and define the extent of LNAPL petroleum contamination. Based on the results of the LIF and HPT investigation, the client was able to find additional pockets of residual products that previous borings hadn’t identified. As noted in the logs below, you can see the classic “shark fin” distribution of LNAPL accumulation and saturation near the water table. There’s also evidence of dissolved phase on the lower edge of the plume. By comparing the LIF and HPT logs side-by-side with one another, you can see how this location is supported by high pressure with a low flow zone beneath.


At another LIF and HPT point on the same property, you can easily notice the quick < 6” transition from the high pressure-no flow to the low-high flow zone and the correlation to the “shark fin.” The key here is that in comparing the LIF log with the LNAPL contaminant mass distribution to the HPT log with the EC, Corrected Pressure, HPT Flow Maximum, and Estimated K Value data, there’s a great deal of weighted lines of evidence whereas traditional approaches would not have supplied this amount of decision making information.


So, what are the five steps to developing weighted lines of evidence during site characterization?

  1. Seek to understand the site and your goals, as well as the existing site conceptual model
  2. Configure direct sensing (or other data collection tools) to detect and map contaminants of concern to meet project objectives
  3. Use high resolution tools to identify and close data gaps including determining soil stratigraphy and permeable pathways with HPT
  4. Confirmation sampling and analyses of soil, water, and vapor with quantitative results
  5. Use real-time information processing to support decisions on adjusting the survey and 3D site models while changing technologies to close data gaps, which keeps the project on track and budget.

By successfully supporting nearly 1,000 direct sensing projects throughout Mexico, Canada and 45 out of the 50 United States, including Hawaii, we’ve developed best practices for site characterization by using a combination of technologies including Membrane Interface Probe (MIP), Laser Induced Fluorescence (LIF), and the Hydraulic Profiling Tool (HPT). Since every site differs in soil types, contaminants, subsurface conditions, among other challenges,  it becomes necessary to follow these five steps in order to successfully develop weighted lines of evidence in moving forward to achieve remediation focused objectives.

What has your experience been with traditional approaches and how would you have benefited by using a holistic approach to site characterization?

Let us know in the comments section below, we’re always willing and open to discussing your site specifics in greater detail or call us at +1-888-344-2704.

Tags: Hydraulic Profiling Tool (HPT), LNAPL, Membrane Interface Probe (MIP), News, Site Characterization