Steven Debruyne (left), Field Technician, and David Desmet, Field Engineer, both with Geosan, use the PRT system and the portable vacuum/volume system to collect soil gas samples. The collected sample (in Steven’s hand and at atmospheric pressure) was then analyzed through a GC a few minutes later. A water curtain was frequently required near above ground installations to decrease the risk of explosion hazard due to occasional release of light hydrocarbons in the atmosphere.

Portugal Cavern Tested for Hydrocarbon Leaks

What began as the only parts in the Geoprobe® toolbox 20 years ago, soil gas sampling tools continue to be a powerful way of locating and delineating contamination today. Geosan n.v., a consulting company in Belgium and northern France, knows first hand the benefits of this tooling. Coupled with the MIP (Membrane Interface Probe) system, Geosan used soil gas sampling through PRT (Post-Run Tubing) to provide quick, onsite results for a rather atypical site. Geosan was approached by an international engineering company specializing in the design, construction, and operation of underground storage facilities for liquid, liquefied, or gaseous hydrocarbons. According to Perrine Marchant, Project Manager for Geosan, the request was specific: detect the origin, characterize the nature, and delineate the extension of light hydrocarbons leaking to the top soil from an underground storage cavern. “Little did we know this experience would extend our know-how through the strategy we proposed,” she said, “and could be successfully completed thanks to the Geoprobe® team and tools.”

Previous investigations at the Portugal storage cavern had reported the presence of very light hydrocarbons (C1 to C4) in some of the in-place groundwater monitoring wells. “We knew the MIP would greatly contribute to the answers our customer was looking for,” Perrine recalled, “but we needed to qualify and quantify the hydrocarbons detected by our FID and PID detectors.”

Geosan opted for the PRT soil gas sampling technique because of its reliability. However, sample collection was only part of the uniqueness of the project. Geosan also needed to develop an analytical method to identify and quantify the light hydrocarbons eventually showing up, not to mention just getting familiar with the procedure. “We wanted to be sure to accurately measure the relative abundances of C1-C4 compounds collected from the top soil,” Perrine added, “in order to check whether the reported leaks were coming from gas pipes or from the storage cavern itself, as each had mixes of known proportions of C1-C4 gases.”

David Desmet (left) and Steven Debruyne, field team members for Geosan, know they can perform MIP investigations anywhere their 6610DT machine can go. The small trailer hosts all of the MIP-related equipment needed in the field.

The 21 X MIP logs (to 8.5 m maximum) allowed a quick exploration of the site and provided a map showing the most impacted areas. The logs also allowed the field team to identify the locations where the most representative soil gas samples could be subsequently sampled. A portable Geoprobe® vacuum/volume pump and the PRT system were then used to collect gaseous samples into a 1000 mL glass bulb for onsite analysis through GC within minutes after collection. Some samples were contained in Tedlar bags for off-site analysis as part of QA/QC procedures.

Perrine explained, “Although some silty/clayey-depth intervals were totally reluctant to yield soil gas, the correlation between MIPFID/ PID signals and the quantitative results obtained through GC analysis of the samples was largely established.” Within a few days, the field team was able to detect some locations suggesting minor leaks, but they also delineated a 50 ft. by 50 ft. area with a major flow of light hydrocarbons making their way to the surface. Both MIP and PRT systems were used with a Geoprobe ® 6610DT probing machine owned by Geosan who also owns a model 6620DT.

“We were asked to investigate some areas of the site with low accessibility,” Perrine concluded, “and again, the high maneuverability of the 6610DT was greatly appreciated, as it has been during many other projects.

ECO364. This MIP log reveals sharp, intense peaks between 1.3 m down and 5.5 m bgs on both FID and PID detectors (Detector 1 is PID, Detector 2 is FID, Detector 3 is DELCD). The observed signals are typical from light hydrocarbons. An attempt was made to collect a sample at 2.0 m bgs but the soil was too impervious and would not yield gas at that depth. Lower soil electrical conductivity values (as seen in the upper figure) suggest that a more permeable layer would not be seen before 0.6 m. The tool string and PRT system were then slowly retracted and it’s only at precisely 0.6 m that a sample could finally be collected. On-site analysis through GC yielded significant amounts of propane (C3) and lesser concentrations of C1- C4 compounds as seen on the chromatogram (lower).