- Eliot Cooper
On a typical contaminated site, low permeability zones tend to be heavily under-sampled, which means critical data about contaminant mass is missed. These zones can be “charged” or “loaded” by diffusion processes over decades and significant mass can accumulate during that time—which is information you need to have when assessing the extent of contamination. This mass is like dark matter and affects the outcome of traditional site remediation approaches. In this blog post, we’ll talk about how to take dark matter (or “back matrix diffusion”) into account to develop a comprehensive remedial strategy and expectations.
If you want a deeper dive into the topic, join me next week for our webinar, “Dark Matter: How to Use Advanced Site Characterization to Identify & Design for Contaminants in Low Permeability Zones.” I’ll be joined by Jason Flattery, our Director of Site Characterization, and Kerry Stonestreet, our Manager of Data and Visualization.
Cascade Environmental’s optimization methodology for chlorinated solvent sites
The first step is to identify and address data gaps. One of the key areas that needs to be addressed for dark matter is ensuring there is a solid understanding of contaminant mass versus lithology, both in storage and transmissive zones. High resolution site characterization (HRSC) tools are appropriate for this task, and the Membrane Interface Hydraulic Profiling Tool (MIHPT) and the Waterloo Advanced Profiling System (WaterlooAPS) are considered tried and true technology.
Following HRSC investigations that collect screening level data, confirm the results with a limited set of traditional soil and groundwater samples (WaterlooAPS programs collect fully defensible and representative groundwater samples and do not require this additional characterization step). All the data, along with that from previous characterization and remediation activities, can then be used to update the conceptual site model (CSM), often done through 3D visualization.
Once the CSM has been updated, a remediation difficulty matrix can be developed. This tool sets remedial expectations and designates the delivery and chemistry parameters that can achieve the contact and residence time required for the complex treatment of the dark matter.
Cascade Environmental’s Remediation Difficulty Matrix
Although it defies traditional remedial design methodology, selection of delivery approach should be made prior to the selection of chemistry. Traditionally, chemistries have been chosen based on bench tests conducted under ideal conditions, or on reported contaminant treatment efficacy rather than first evaluating whether they can even be effectively injected to achieve the necessary contact and residence time.
For dark matter diffused into storage zones composed of silts and clays, a solid amendment will typically achieve the best distribution, and they tend to have high persistence. For dissolved phase contaminants in coarse grained transmissive zones, injection of liquid amendments provides the best approach to distribution and contact. The complexity of heterogeneous sites becomes evident when deciding whether to target the storage and/or transmissive zones.
These first step decisions about delivery cannot be made without a solid understanding of the injection characteristics of the target intervals defined in the CSM, even when the design team has hydraulic conductivity data in hand. That’s why design optimization testing (DOT) should be conducted for both injections of liquid amendments and the emplacement of solid amendments prior to full-scale implementation. Any distribution surprises during full-scale will result in significant additional costs and/or can compromise remediation results.
By injecting water across target intervals, pressure and flow data can be collected that indicate whether the intervals need to be fractured to obtain flow or to establish maximum flow rates, to optimize liquid injection cost performance without fracturing the formation.
Dark matter poses unique challenges to meeting aggressive remediation goals. As an industry, we need to take advantage of proven characterization and injection innovations to address issues such as dark matter, and incorporate these technologies into methodologies that help us set realistic expectations and optimize our remedial approaches. In doing so, we can also achieve cost savings by reducing remediation footprints, and cost avoidances by not applying the wrong technologies or missing contamination that contributes to compliance wells.
If you’d like to learn more about addressing dark matter at your sites, register for next week’s webinar, “Dark Matter: How to Use Advanced Site Characterization to Identify & Design for Contaminants in Low Permeability Zones.” If you can’t make it, register anyway and a link to the recording will be sent to you.