LL MIP (Low Level Membrane Interface Probe)
Low Level MIP (LL MIP) is a technology developed by Geoprobe Systems® that greatly increases the sensitivity (and therefore utility) of the MIP logging tool. The primary feature of LL MIP technology is that the carrier gas stream that sweeps the internal surface of the MIP membrane is pulsed. This results in an increase in the concentration of VOC contaminant delivered to the MIP detectors.
Low Level MIP can be performed with standard MIP probes, MiHpt probes or with MIP-CPT probes. To perform this method an operator will need to add the MP9000 Pulse Flow Controller (Fig. 1) along with an updated version of the FI6000 acquisition software to the current MIP system. The addition of the MP9000 to the system is simple and requires only the rearrangement of gas line connections. This controller can then be easily removed from the system to return to standard MIP logging. Switching between methods requires only a few minutes of time.
Figure1: MP9000 Pulse flow controller (patent pending)
In standard MIP operation, the carrier gas continually sweeps across the membrane transporting contaminates to the detectors at the surface. In the LL MIP method, the trunkline sweep flow is temporarily stopped when the MIP probe is brought to rest at a discrete depth in the soil. Stopping the sweep gas flow allows the contaminant concentration to build behind the membrane. This results in a larger and narrower contaminant response peak at the detectors (Fig. 2 and 3) for a given chemical concentration. Switching valves located inside the MP9000 create separate flow paths for the MIP trunkline and detectors; trunkline flow can be stopped and restarted without impacting detector baseline or stability. When the trunkline flow is restarted the contaminant mass (peak) is quickly swept to the surface with a trunkline flow rate of approximately 60ml/min. and is routed to the detectors via a sample loop located in the MP9000.
Figure 2: Comparison of 0.5ppm TCE response between standard (50-100s) and Low level (300s) MIP methods
Figure 3: Comparison of standard and LL MIP method 0, 0.20, 1.0 & 5.0ppm TCE
The Low Level MIP operation requires the use of a specific purpose edition of DI Acquisition software to control the timed cycling of the MP9000 controller. This software also includes a subroutine for automatic triggering of LL MIP cycling at preset depth intervals during the MIP logging process.
Low level MIP will not replace standard MIP logging, but will expand MIP capabilities. LL MIP is most useful when low contaminant concentrations are present. Figure 4 shows a comparison of closely spaced standard MIP and LL MIP logs at a location of low concentration chlorinated VOC’s. While it is difficult or even impossible to discern the presence of VOC contamination based on the standard MIP log at this location, the LL MIP log exhibits robust signal to noise and the presence and location of contamination is easily defined. Projects with contaminant concentrations too low to proceed with traditional MIP methods may be good candidates for investigation using the LL MIP method. This technology will provide operators with the ability to track and map contaminant plumes down to concentrations at or below the 100ppb range for some contaminants.
The LL MIP method will greatly increase the sensitivity of a MIP system but the resulting detection limits are dependent on the sensitivity of the detectors. To achieve the lowest possible detection limits the probe and trunkline need to be new or verified clean with a system blank. The detectors also need to be fully current within their maintenance program and sensitivity should be tested prior to mobilization. Equipment that has been used to map high level contaminants will result in false positive results due to contaminant desorption from the membrane and return carrier gas line.
Figure 4: XSD responses in side by side logs comparing standard and low level MIP methods
The MIP-XSD log in Figure 4 offers another view of LL MIP technology. On the left is a standard MIP log at a site contaminated with chlorinated VOCs. As you can see, with standard MIP logging we are at the limit of what can be detected with the signal to noise response offered by standard MIP. The log on the right is a 3 ft. (1m) offset location done with LL MIP technology. The presence and location of contaminants is obvious in the LL MIP log.