Direct Push Technology : an efficient screening technique for an optimized site characterization
Résumé
Chlorinated solvents are amongst the most common soil and groundwater contaminants due to their widespread use as cleaning solvents or degreasing agents. Due to their physico-chemical properties, chlorinated solvents produce large scale plumes of pollution in the groundwater. In the densely populated north western Europe, these pollution plumes are often located under residential and urban development areas and therefore not easily accessible. Vapours can migrate through building slabs and affect the quality of indoor air. That is why the CityChlor project was created. It aims at improving the quality and minimizing pollution of soil and groundwater by developing an integrated approach to tackle the threats caused by contamination with chlorinated solvents in urban areas. One of the goals of the project is to define the most reliable, fast and cost effective techniques to detect, characterize and delineate chlorinated solvent pollution in groundwater. In this context, Direct Push Technology (DPT) is an innovative technique in Europe to characterize soil, soil gas and groundwater pollution. The goal of this work was to assess this technique in order to evaluate its efficiency to locate polluted zones. This could be an important step of the characterization process of a polluted site which could help consultants to decide where to install groundwater and soil gas wells for an efficient site monitoring. This work was carried out on an in-service facility located 60 km from Paris, in an urban area. In this factory, door locks and metal fittings are produced and chlorinated solvents are used to remove grease marks from metal fittings. On site, soils are composed of 50 cm to 1 m of embankments, then about 3 m of clay and finally fine sand to 10 m deep. The alluvial aquifer was studied during the project: the mean depth of the water table was 1.5 to 2 m below the ground surface and was about 10 m deep. Amongst the DPT, 3 specific techniques were used: the CPT (cone penetration test) which gives indications on the lithology, the MIP (Membrane Interface Probe) giving semi-quantitative concentrations of pollutants in soil and groundwater along a vertical profile and the BAT sampling allowing us to take depth discrete samples of groundwater. The DPT work was carried out in 2 steps. First of all, it was used as a screening technique: the tool was equipped with a CPT cone and a MIP probe and 19 locations were investigated outside and inside buildings near supposed pollution sources and downstream these pollution sources. Then, in the most polluted zones, 10 BAT sampling were carried out at 2 different depths. Thanks to the Robertson CPT soil classification, the CPT allowed us to determine the soil characteristics which were in accordance with the lithology observed during soil sampling. The MIP was very efficient as well to obtain a good picture of polluted areas. In addition, the results obtained after analyzing groundwater from the BAT sampling confirmed the ones given by the MIP. This work allowed us to highlight general advantages of the DPT. This technique is mobile and as we can get information on the contaminant concentration almost in real time thanks to the MIP system, this implies a better reactivity on site. As DPT systems are hold by mobile structures of different sizes, investigations can be carried out almost everywhere (in small buildings or outside). In addition the BAT sampling is a very efficient technique to carry out depth discrete sampling without cross contamination between the samples. Nevertheless other limitations have to be taken into account when planning to use such a technique. For example, DPT is not suitable when soils are too hard (the CPT cone could not get deeper than 6-7 m on this pilot site). Finally results are not obtained in real time with the MIP system because post treatment of raw data is needed before having final results on polluted areas.