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Chapter 4
Part 2: Special cones: geo-environmental cones
Hydrocarbon cone
| HYDROCARBON CONE 4.11 | |
 | Hydrocarbons The hydrocarbon cone (Figure 30) is a testing instrument for the in-situ detection of light, non-aqueous phase liquids (LNAPLs) of hydrocarbons within soils. Beneath chemical plants, refineries and petrol stations, hydrocarbons are frequently encountered within the soil in a pure form, either as droplets or as a distinct floating layer. This pure product requires close attention, particularly where mobility is high (ie within the zone of rising and falling groundwater, and within permeable soils). |
| Mapping and modelling Where cost effective assessment and remediation is required, the importance of accurate mapping and modelling of the spatial distribution and volumes of the contaminants within the soil is high. The location and presence of hydrocarbons within the soil is particularly difficult to establish. It is even more difficult to establish the size and mobility of these layers and the proportions that can be removed by pumping. Traditional methods of hydrocarbon identification and assessment include chemical soil analysis and measuring the thickness of floating layers within observation wells. Both methods require boreholes, and are time consuming and costly. This is because aquifer protection systems are required and the equilibrium time for monitoring wells can be significant, with durations of one week and longer not uncommon. Furthermore, the floating layer in a well represents the mobile part of the LNAPLs in the soil and not its in-situ location. Detection The hydrocarbon probe detects, continuously with depth, in-situ, the presence of total pure product contained within the soil. The system is pushed into the soil using standard cone penetration testing plant and equipment. The detection of hydrocarbons as a pure product is enabled because the hydrocarbon mixtures produce fluorescence when they are irradiated with UV light. |
| The cone The hydrocarbon probe has the appearance of a normal CPT cone. However, it contains a light source as well as the detection system. The total diameter of the system is 55 mm. During the penetration process, measurements are carried out by illuminating the soil from a UV light source placed behind a window. The fluorescent light emitted by the hydrocarbons is detected by a photomultiplier tube in the cone. Detection limit The detection limit of the system is set at 50 mg/kg dry weight for a LNAPL. The intensity of the radiation emitted by the hydrocarbon is an indication of the concentration of pure product contained within the soil. The system can also detect other wavelengths for analysis of other products contained by soils. Quality The instrument is calibrated. During the test programme, the occurrence of ‘smearing’ and ‘displacement’ is examined. In practice however, it has been demonstrated that soil effectively cleans the probe from hydrocarbons as it passes through the soil. Displacement of the LNAPL in front of the penetrating probe tip does occur to a limited extent. This means that the depth at which the LNAPL occurs in the soil is slightly less than the depth observed by the hydrocarbon probe. However, this slight variation is considerably less than drilling-induced disturbance experienced using traditional cable tool-boring methods of exploration. Speed and reliability The speed and reliability with which the method can establish the presence and extent of LNAPL contaminants means that cost savings can be obtained. These savings can be made in the reduction of chemical sampling required and laboratory testing required. Where necessary, Mostap samples can be obtained to correlate the results of the hydrocarbon probe and provide samples for subsequent laboratory testing. Significantly more exploratory locations can be achieved in less time, in comparison with traditional sampling methods. Therefore, improved and more accurate contamination models through contouring and volume calculation can be achieved. Remediation The system can also be used for the monitoring of in-situ clean-up operations. The probe can be used to detect petrol, diesel and motor oils following calibration. It is also possible to establish whether or not the LNAPLs can be pumped-off, based on the results of the hydrocarbon probe. Typical test results are shown in Figure 31 |
