Direct Push Chemical Sensors for DNAPL and Other VOCs

ER-200109

Background

ER-200109 Project Graphic 1

Dual reservoir flow cell system. The system consists of: (A) water reservoir, (B) calibration reservoir, (C) switching valve, (D) peristaltic pump, and (E) flow cell

The Department of Defense (DoD) has a critical need for faster, less expensive, and more accurate methods to characterize and monitor volatile organic compounds (VOC) in the subsurface. Chlorinated solvents in the form of dense non-aqueous phase liquids (DNAPL) pose the most serious challenge. Failure to adequately define DNAPL source terms often plagues many remediation efforts.

Defining the three-dimensional subsurface distribution of VOCs traditionally relies on drilling, discrete sampling, and laboratory analysis. This strategy yields an incomplete picture, and the data are often suspect. The trial-and-error placement of soil borings and monitoring wells to locate the DNAPL source terms is inefficient; even at intensely investigated DNAPL sites with high dissolved phase concentrations, direct detection of residual or free-phase DNAPL in groundwater can be rare. Methods that attempt to delineate DNAPL distributions by extrapolating the results from indirect methods (e.g., soil gas survey results) are generally unsuccessful. Non-invasive geophysical techniques that aim to image DNAPL from the ground surface have not been successfully demonstrated to date.

A direct push method (or combination of methods) is needed to definitively identify, verify, and quantify VOCs in situations ranging from dissolved phase near the maximum contaminant levels (MCL) to high-resolution characterization/delineation of DNAPL source zones. This ESTCP project, which retains the original SCAPS vision of continuous contaminant logging as a function of depth below ground surface, extends the original SCAPS approach to DNAPL.

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Objectives of the Demonstration

The objective of this effort was to demonstrate and validate an innovative suite of VOC characterization and verification tools, with emphasis on direct push deployment and DNAPL source term delineation. By bringing together several related and complementary techniques, the project allows site managers to select the best arsenal of tools for any site with subsurface VOC contamination.

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Demonstration Results

Two technologies were evaluated: the halogen specific detector (XSD) and the high resolution fluorescence (HRF) sensing system. The XSD can be operated downhole behind a membrane interface probe (MIP) that samples the soil formation for VOCs. MIP operation in which vapors are returned to an uphole detector was previously demonstrated with ESTCP support, but moving the detector downhole and measuring while the direct push probe is continuously advanced will increase the spatial resolution of DNAPL detection by an order of magnitude (from feet to inches). Even higher spatial resolution (tenths of inches) will be obtained with a complementary HRF sensing system that can be applied whenever the DNAPL is fluorescent owing to dissolved petroleum products or humic substances. The ability of the characterization techniques to find DNAPL was verified via GeoVIS, an in situ video imaging system developed by the Navy. Demonstrations were conducted at Naval Air Station North Island, IR Site 9, Coronado, California, and at Marine Corps Base Camp Lejeune, Site 89, located in Onslow County, North Carolina.

At North Island, the XSD clearly showed several areas that were heavily impacted by halogenated compounds, indicating heavy dissolved phase halogenated compounds in areas not determined in extensive previous studies. The XSD showed very high halogen concentrations in shallow regions along with several sharp features in deeper regions. The XSD indicated saturated halogen conditions below 30 feet that were confirmed with water validation samples. Correlation between uphole XSD-MIP measurements of water samples and laboratory measurements was quite good. Two water samples, both from areas below heavily impacted areas, did not show good agreement with either uphole or downhole measurements. Further, both samples were from the same hole, which raised the concern about contamination being dragged down. In general, averaging the XSD signal, especially when contaminant layers are thin, effectively dilutes the XSD signal levels. Averaging does not give a representative signal for the XSD when narrow bands are encountered.

At Camp Lejeune, the XSD clearly delineated zones of chlorinated VOC contamination and water sampling confirmed the relative accuracy of the XSD concentration vs. depth profiles. Maintaining an acceptable MIP temperature was difficult. While this negatively affected mass transport since a compound (1,1,2,2-tetrachloroethane) that has a high boiling point (146º C) and relatively low vapor pressure (6.36 torr) was involved, successful profiling was still accomplished.

XSD signal levels were not saturated at Camp Lejeune, even in areas later found to have DNAPL. It is unclear at this time if this was because of the deviation from the normal operating conditions, inability to maintain higher MIP temperature, or length of time required for 1,1,2,2-tetrachloroethane to pass through the membrane. The calibration system worked well, with some minor improvements needed, most notably, temperature stabilization of saturated response calibration solution. Correlation between uphole XSD-MIP measurements of water samples and laboratory measurements was very good. Averaging the XSD signal when contaminant layers are relatively homogenous over a large depth range produces a good correlation with water sample data.

With the LIF at Camp Lejeune, false positives were encountered. Elevated LIF responses did not always prove to be a positive indicator for DNAPL. Fairly high heterogeneity could explain the inability to confirm DNAPL at some locations. Elevated LIF responses were sometimes seen when the GeoVIS passed through at high speed; LIF and GeoVIS window positions should be swapped for optimum performance as a DNAPL validation system.

ER-200109 Project Graphic 2

Schematic of the flow cell used to calibrate the XSD

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Implementation Issues

The average rate used for cost comparisons of $4,675/10-hr day appears to represent a fairly conservative estimate that would cover not only operations but also account for basic data analysis and reporting. This rate is based on a CPT truck equipment rate of $2,750/day (includes the CPT equipped 20-ton penetrometer system plus two support vehicles, push room operator, push room helper), XSD-MIP data logging service with data acquisition specialist ($1,500/day), and labor for a project geologist or project manager charged at $67.50/hr for a 10-hr day. The XSD-MIP logs generated during this demonstration have the same innate ability to convey relative concentrations of contaminant vs. depth as SCAPS and ROST LIF.

The XSD-MIP logs provide an intuitive graphical representation of halogenated VOC (HVOC) concentrations vs. depth. Even though the logs are not analytically accurate in a quantitative sense, they are invaluable in their ability to instantly paint a picture of the subsurface HVOC distribution. The CPT-delivered XSD-MIP system provides an unparalleled ability to “hunt and seek” DNAPL and its associated high dissolved phase HVOCs. It is difficult to fully convey the benefits of the immediate feedback the logs give the geologist in the hunt for DNAPL source terms when one is necessarily constrained to follow up the logging with validation and confirmation sampling. Deciding where to place a transect, conducting tests along that transect, and then sampling and comparing soils and waters retrieved from that transect are obviously necessary and desirable for the purposes of a demonstration. But the true value of tools such as the XSD-MIP are their ability to be used in a real-time sense to adaptively move about the site, follow gradients toward locations with higher signal levels both laterally and horizontally, and finally pinpoint the true hotspots and likely DNAPL source term areas. (Project Completed - 2007)

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Points of Contact

Principal Investigator

Dr. Stephen Lieberman

Space and Naval Warfare Systems Command (SPAWAR)

Phone: 619-553-2778

Fax: 603-909-8049

Program Manager

Environmental Restoration

SERDP and ESTCP

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