U.S. EPA Contaminated Site Cleanup Information (CLU-IN)

U.S. Environmental Protection Agency
U.S. EPA Technology Innovation and Field Services Division

Dense Nonaqueous Phase Liquids (DNAPLs)

Detection and Site Characterization

Hydrophobic Flexible Membranes and Dyes

The following discussions are taken from Site Characterization Technologies for DNAPL Investigations, EPA 542-R-04-017Adobe PDF Logo.

Two variants of hydrophobic flexible membranes have been employed in DNAPL characterization. The basic device is built around an inflatable tubular membrane. The membrane can be fitted with either discretely spaced hydrophobic sorbent packs, or with a dye-impregnated hydrophobic ribbon, or with a cover that changes color in the presence of DNAPL chemicals. It is effective in both the vadose zone and beneath the water table.

The membrane can be deployed using one of two techniques. The deployment method chosen depends on site geology and whether there is a risk of mobilizing the DNAPL. The first technique deploys the membrane directly into an open borehole. The tubular membrane is everted into the open hole using air pressure. As the membrane everts and expands, the sorbent pads or ribbon, which is attached to the membrane, are pressed against the sides of the hole and held there by air pressure. After a period of time ranging from minutes to hours the membrane is removed by pulling up on a removal tether that is attached to it. The retrieval process rolls the outside of the membrane back inside so that the reactive material does not touch the wall as it ascends. Once the membrane is on the surface, it is again turned inside out to expose the reactive ribbon/membrane pads. The presence of colored spots indicates the ribbon has come into contact with a DNAPL. If positive identification of the chemical is required, the stained area of the ribbon/membrane cover or sorbent pad can be preserved and transported to a laboratory facility for analysis. Field GC or GC/MS equipment is usually used to perform this analysis.

If it is unlikely that the borehole will stay open, then the membrane can be deployed through the rods of a cone penetrometer test rig or dual-tube direct push rig. In this configuration, the membrane is pushed directly into the rods with the hydrophobic ribbon/membrane cover facing out. Water is used to carry the membrane down to the bottom of the cased hole. As the rods are pulled up, more water is added to ensure that the membrane and reactive ribbon are flush against the surrounding soil. The water should also supply sufficient strength to keep the hole open. Retrieval is the same as with the open borehole method.

When continuous coring is not done, this method can provide a relatively cheap ($15/ft sampler and ribbon) way to determine if there is residual or mobile NAPL chemicals in the subsurface both above and below the water table.

Hydrophobic dye testing uses a hydrophobic dye to determine the presence of DNAPLs in ex situ soil samples suspected of DNAPL contamination based either on OVA screening results or on MIP results that show high concentrations of total VOCs in subsurface soils. The technique is designed to work with other analytical technologies. The sample is placed in a sample jar and a suitable dye is introduced (e.g., Sudan IV, Oil Red O). The jar is capped and vigorously shaken. A bright red coloration appears in the presence of DNAPL. The lack of a color change does not necessarily mean there is no DNAPL in the soil sample or the soil horizon from which it was taken. Sample handling during extraction of the core sample from the formation may affect results (e.g., draining while being brought to the surface), and poor dye mixing with clayey soil may mask DNAPL presence. Note that Sudan IV is highly toxic and should be carefully handled and disposed of.

For Further Information

Adobe PDF LogoDNAPL Characterization Using the Ribbon NAPL Sampler: Methods and Results
B. Riha, J. Rossabi, C. Eddy-Dilek, D. Jackson, and C. Keller.
WSRC-MS-2000-00182, 10 pp, 2000

This paper provides a description of how the ribbon sampler works and offers several case studies on its deployment.

Adobe PDF LogoInnovative Technology Summary Report: Ribbon NAPL Sampler
U.S. DOE, Office of Environmental Management.
DOE/EM-0522, 23 pp, 2000

This document describes a demonstration of a ribbon sampler at DOE's Savannah River facility at Aiken, SC. It includes a description of the technology, performance, costs, and lessons learned.

Abstracts of Journal Articles

A Comparison of Field Techniques for Confirming Dense Nonaqeuous Phase Liquids

Subsurface Characterization in Karstic, Fractured Bedrock, Redstone Arsenal, AL

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