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Double probe picks up oil site contamination

SITE INVESTIGATION contractor Fugro recently used its membrane interface cone penetration test (MIP CPT) to map contamination on the site of a former oil depot in Essen, Germany.

The single probe MIP CPT combines two of Fugro's inhouse developed tools for contamination investigations the electrical cone penetrometer (ECP) and the membrane interface probe (MIP).

The ECP measures compressibility, permeability data and detailed soil lithology at 20mm intervals.The MIP uses a heated Teflon membrane that allows organic hydrocarbons in vapour phase to cross into a sampling chamber which is continuously swept by a controlled flow of an inert gas (nitrogen).

Contaminants are driven, via the gas flow, to a set of standard gas chromatograph detectors comprising a photo ionising detector and a flame ionising detector.These pick up a wide range of organic compounds, specifically aromatic and halogenated hydrocarbons (chlorinated solvents).

The probe is pushed into the ground using a standard truck-mounted hydraulic CPT rig.Data is collected at 10mm to 20mm intervals, with 500 data points typically being collected for a 10m push.The tool goes into the ground at about 20mm/s, with a 20m push taking 90 minutes.

Between 80m and 100m of probing can be achieved in a working day.

The Essen oil supply depot used large volumes of Trichloroethylene (TCE) as a degreasing agent, which leaked into the groundwater over a number of years. The degraded compounds were found in water wells in local gardens and discharging from land drainage into the water course over 700m from the site.

A number of investigations using drilling, sampling and monitoring wells had given no clear and consistent information on the extent of the contamination. Ground conditions comprised sand and gravel to 11.4m depth, underlain by a clay with sandy layers.

For this further investigation, 29 MIP CPT probes were put in to a maximum depth of 16m. In all, some 450m of probing was carried out in just four days.Using the real time data,14 ambient water samples were taken to depths of up to 11.2m with a Swedish BAT sampler.

Data was displayed in the CPT truck in real time and a hardcopy printed out at the end of each test.This allowed control over the actual end depth of the test, based on the geotechnical and environmental data so that tests could be extended or shortened as necessary. And because data from each test was immediately available to the site engineer, the test programme could be continuously modified based upon findings.

The plume was found to stretch over 700m from the depot site and is up to 100m wide in places. Highest concentrations were found at the source of the spillage.

Results showed that contamination was predominantly in the sand and gravel layers, with strongest concentrations between 9.5m and 11.4m depth. DNAPL contamination was also found in the top 3m of the clay.

The other advantages of the MIP CPT were that it produced no hazardous drilling spoil and because the trucks were only in position for a short time, there was minimum disruption to residents.

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