Tracking underground fuel pollutants

Investigating hazardous waste sites before cleanup can be a complicated affair when there is the suspicion of subsurface contamination. Many measurements have to be taken across an area to detect and plot the distribution of a pollutant so that full remediation can take place. Various chemical sensors have been developed to perform this task and they are capable of producing accurate 3D models of the subsurface.

Despite this, a team of scientists at Tufts University, Melford, MA, recognised a deficiency in these methods because they failed to detect the full range of organic compounds likely to be present underground. In particular, the broad group known as semi-volatile organic compounds (SVOCs) remained difficult to analyse.

Over several years, the Albert Robbat, Jr. group from the Center for Field Analytical Studies and Technology at Tufts has progressively tackled the SVOC problem. It led to the design of a heated sampling probe which can extract the target compounds below ground and transfer them to the surface. Here, a photoionisation detector provides continuous, real-time detection of the pollutants and triggers a single-stage Peltier freeze trap to collect them for off-line GC/MS analysis. A specially developed ion fingerprint mass spectral deconvolution algorithm quantifies the compounds.

This system signalled a marked improvement in the 3D mapping of underground hydrocarbon pollutants but Robbat was not satisfied because some of the more volatile compounds such as the alkylbenzenes and alkylnaphthalenes were not held by the trap. So, Robbat, Thomas Considine and Patrick Antle modified the system to incorporate a three-stage Peltier trap while introducing online GC/MS analysis of the trapped pollutants.

Their improved system can bore up to 30 m deep, sampling the soil as it travels. The probe can be heated up to 400°C, which raises the soil temperature to 300°C within 5 minutes. A stream of nitrogen is pumped through to collect the volatilised compounds from the soil and transfer them to the photoionisation detector for continuous monitoring. When organic compounds are detected, the flow is switched to the Peltier trap.

In Peltier cooling, also called thermoelectric cooling, an electric current at the junction of two different metals transfers heat from one side of the device to the other, to introduce a cool surface. The novel three-stage trap contained stacked Peltier chips placed within a steel tube with the addition of a fan and a heat sink to produce temperatures of -30°C, compared with -8°C for the single-stage trap design.

The freeze trap was subsequently heated to 28°C within 10 s to desorb the trapped compounds which were led to the GC/MS inlet by a flow of helium. The gas chromatograph was fitted with an accelerated column heater to reduce the analysis time, bearing in mind that the system was intended for real-time, on-site analysis. After electron ionisation, the data were analysed by the aforementioned deconvolution software, with quantitation by comparison of the signal intensities of each analyte to that of an internal standard.

The sensitivity of the system was illustrated by data for coal tar sampled from soil at a former manufactured gas plant. As the probe travelled down, negative readings from the photoionisation detector indicated no detectable organic compounds. But the readings switched to positive within 1 cm and passed through a maximum with distance, before becoming negative at 14.5 m.

Over the positive section, the organics were analysed by GC/MS to identify benzene and alkylbenzenes, naphthalene and naphthalenes, acenaphthylene, acenaphthene, fluorene and phenanthrene. Comparison of the field data with GC/MS analyses in the lab revealed that measurement accuracy was acceptable except for fluorene and phenanthrene which were underestimated and naphthalene which was overestimated. In addition, no false positives or false negatives were encountered.

A probe was also fitted with a membrane to sample organic compounds in groundwater. It could be heated to 140°C but no higher, because water began to infiltrate the membrane. This temperature restriction limited the system to the analysis of the more volatile compounds.

In a second test case, soil contaminated with fuel oil was collected from the vicinity of an underground storage tank containing industrial heating oil and sampled by pushing the probe through by hand. Series of C₁-C₆-substituted benzenes and PAHs were detected, demonstrating the improved trapping ability of the modified Peltier device for VOCs.

The real-time system can perform analyses within 5 minutes. In contrast, lab analyses took about 55 minutes, which must be added to the time taken to extract drill samples and transport them to the lab. The speed of the on-site analysis ensures that personnel time is used efficiently on location.

The improved system will permit the detailed analysis of hazardous waste sites, delivering a 3D picture of the extent of contamination by an extended range of compounds, including VOCs and SVOCs. The research group are now focusing on the development of a single high-temperature probe suitable for soil and groundwater testing of VOCs and SVOCs.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Albert Robbat