- Program Areas
Direct Push Optical Screening Tool for High-Resolution, Real-Time Mapping of Chlorinated Solvent DNAPL Architecture
Objectives of the Demonstration
Chlorinated solvents are among the most common organic contaminants detected in groundwater at Department of Defense (DoD) sites. The sources of these contaminants are often historical releases of dense nonaqueous phase liquids (DNAPL). Unfortunately, chlorinated solvent DNAPL source zones are difficult to locate using conventional subsurface characterization technologies. Laser-induced fluorescence (LIF) tools are currently available for real-time, high-resolution mapping of petroleum hydrocarbon and coal tar-based NAPL source zones. These LIF tools do not work with chlorinated solvent DNAPLs because chlorinated solvents lack the aromatic structure responsible for the laser-induced fluorescence in coal tars and petroleum hydrocarbons. The objective of the field demonstration was to provide a field-scale demonstration of the new DyeLIF tool for high-resolution subsurface mapping of chlorinated DNAPLs. The real-time, high-resolution profiles generated from the DyeLIF were then compared to profiles from high resolution vertical soil sampling with subsequent dye shake tests and quantitative laboratory VOC analysis.
The DyeLIF tool is a new site characterization technology that, for the first time, facilitates rapid, cost-effective 3-dimensional (3-D) delineation of residual chlorinated solvent DNAPL in the subsurface. This type of high-resolution source characterization can identify previously unknown residual DNAPL, thereby optimizing source zone excavation or in situ treatment programs. In particular, high-resolution characterization using DyeLIF can dramatically reduce cumulative remediation costs and improve remediation performance by targeting excavation or treatment on the most impacted areas that convey mass to potential receptors. Similarly, a site investigations program using DyeLIF can also quickly determine that residual DNAPL is not present in the subsurface at a particular site. That knowledge can also be very valuable for risk evaluations and scoping of remediation systems.
In addition to yielding information on the subsurface DNAPL distribution, the recording of dye solution flow rate and injection back-pressure provides high-resolution information on the lithology and hydraulic conductivity of the soil, similar to other profiling tools such as the Geoprobe HPT™ (Hydraulic Profiling Tool) and Waterloo APS™ (Advanced Profiling System).
The DyeLIF system was field tested at a Formerly Used Defense (FUD) facility in Massachusetts in Fall 2013 (Geoprobe® delivery) and again in March 2014 (CPT delivery). The primary field demonstration completed in 2013 included two components: one week of DyeLIF probing and a second week of follow-on soil coring using research-quality direct push (DP) soil coring methods in order to compare DyeLIF results to colorimetric dye shake tests and laboratory analysis.
Several performance objectives were established in the project demonstration work plan and all were met or exceeded. The performance objective for chemical analysis was 70% consistency between positive DyeLIF responses and samples when DNAPL saturations were greater than 5%. The demonstration results showed 100% consistency between chemical analysis and DyeLIF for saturations greater than 1.9% (35 of 35 samples), and 95% consistency for estimated saturations greater than 0.5% (40 of 42 samples).
The performance objective for the dye shake tests was 70% consistency between a positive DyeLIF response and a positive colorimetric response with the dye shake test when the DNAPL saturation was estimated to be above 5%. For the dye shake tests, the demonstration results showed 100% consistency between DyeLIF and the shake tests at saturations as low as 1.3% percent (37 of 37 samples). There was 98% consistency between DyeLIF and dye shake tests above 0.5% saturation (41 of 42 samples). Therefore, the performance objective for dye shake tests was also exceeded.
The hammering and stress of percussive drilling over the one week drilling program allowed the project team to evaluate the durability of the DyeLIF tool. A performance objective of 90% uptime was specified in the work plan for the field demonstration. 100% uptime was achieved during the field demonstration.
A performance objective was also established for the average linear feet of drilling production achieved per day. A performance goal of 150 feet per day was proposed in the work plan. The production rate for the week of DyeLIF probing averaged over 400 feet of probing per day, greatly exceeding the 150 feet per day goal. The production rate, coupled with the extremely fine vertical resolution of DyeLIF (~ one data point per 0.5 centimeter probed), results in an extremely high data acquisition rate for the DyeLIF tool. Using a typical LIF production rate average of 334 feet probed per day, the number of data points generated per day would be greater than 20,000. Considering the excellent correlation between DyeLIF and colorimetric dye shake tests, one day of DyeLIF probing is essentially equivalent to conducting 20,000 colorimetric dye shake tests, something that would take several months of expensive soil coring and detailed sub-coring to complete.
No major implementation issues were identified during the field demonstration. However, a key limitation for any direct-push technology is suitability of the geological conditions for direct-push probing, i.e. absence of cobbles or other conditions that would cause tool damage and preclude advancing the probes. A frequently asked question about the dye is the potential for regulatory resistance to the technology because of the injection of dye as the probe is advanced into the subsurface. The project team does not anticipate regulatory resistance to the technology based on the following:
- The dye is relatively non-toxic (Rat LD50 [intraperitoneal] 4170 mg/kg) and is not a known or suspected carcinogen. From a toxicity standpoint, the dye is therefore similar to fluorescent groundwater tracing dyes that are released in much greater quantities during tracer studies.
- A de minimis quantity of dye is injected (only 0.11 grams per meter of probe penetration).
- Analytical testing on water left in contact with the dye for several days yielded no detectable levels of any listed VOCs or semi-VOCs.
- The dye is extremely hydrophobic and therefore it is expected that there is very little transport of the dye in groundwater away from the probed location.
- Because the DyeLIF tool is used to assess DNAPL in the subsurface, the dye is being injected at a de minimis quantity into a portion of the subsurface that is already heavily impacted by chemicals that are toxic and carcinogenic.