The Low Level Membrane Interface Probe (LLMIP) is a direct-push screening tool with semi-quantitative analytical capabilities that is used to detect and log the relative concentration of volatile organic compounds (VOCs) with depth in soil. The LLMIP can also log electrical conductivity (EC) that can be used to infer soil texture (i.e., clay content typically exhibits a higher conductivity). The LLMIP was developed by Geoprobe Systems® in the United States (U.S.) and has been used extensively in the U.S., Europe and Canada for mapping the extent of VOC contamination in the subsurface. As a screening tool, the LLMIP offers many some extra benefits to site investigators for the fringe/edges of the suspected plume:
- Detecting and logging both chlorinated (PCE, TCE, Carbon Tet, etc.) and non-chlorinated VOC contaminants (BTEX, PHCs, etc.) down to 20 ppb.
- Able to detect contaminants in both coarse and fine-grained soils.
- Works in both saturated and unsaturated soils of varying types (ie. clay/silt to sand/gravel).
- Real time contaminant screening information is generated, allowing field adjustment of the site investigation program.
- Cost-effective in terms of the amount and level of detail of in-situ site characterization data as compared to traditional Phase II Environmental Site Assessment techniques for fringe/edge of plume situations.
In standard MIP operation, the carrier gas continually sweeps behind the membrane transporting VOCs to the on board Gas Chromatograph (GC) and the detectors at the surface. In the LLMIP operation, the trunkline flow is temporarily halted when the probe is stopped at a discrete depth in the soil. Stopping the gas flow allows the contaminant concentration to build behind the membrane allowing for smaller amounts of contaminants to travel past the membrane. This results in a larger and narrower contaminant response peak at the detectors for the same concentration versus traditional MIP operations.
The standard detectors within the GC include a photoionization detector (PID), a flame ionization detector (FID), and a halogen-specific detector (XSD). These detectors are employed in series with each detector providing sensitivity to a particular group or type of VOC contaminant. The XSD is highly specific to halogenated compounds and is the best detector when the MIP is used for logging chlorinated solvent plumes or source areas (e.g. TCE, PCE, Carbon Tet, etc.). The PID provides sensitivity to aromatic compounds (e.g. BTEX compounds) as well as confirmation of chlorinated ethylene compounds detected by the XSD. The FID is a general detector useful for petroleum hydrocarbon detection as well as confirmation of high concentrations of all compounds recorded by the other two detectors.