Land Contamination: Technical Guidance on Special Sites: Acid Tar ...

The choice of investigation technique will depend on site-specific considerations. A numberof techniques are suitable for acid tar lagoons, as follows:• whilst increasingly sophisticated drilling techniques are being utilised in contaminatedland investigation, the cable percussive (“shell and auger”) technique is usuallyadequate for acid tar lagoons. This technique enables reasonable depths to beachieved, both chemical and physical samples to be collected and facilitates theinstallation of gas and groundwater monitoring wells. Whilst the weight of the drillingrig may be less than some other types, it may still require specialist pontoon structuresif the lagoon is unstable and, as with some other techniques, it cannot penetrate largelumps of concrete, steel or thick wood;• dynamic probing can also be used, which uses either a continuous pneumatic systemor a drop hammer to drive the probe. Depending upon the rate of penetration, anindication of the thickness and type of materials present can be determined. Samplesfor analysis can be collected using a hollow stem or split spoon attachment;• trial pits and trial trench excavations allow greater inspection of the waste types, largersamples to be taken and can be extremely valuable in helping to determine the lateralextent of the wastes. However, it will usually be necessary to import clean covermaterial to cover over the larger “scar” that is left at the ground surface from trial pitscompared to boreholes;• window sampling can be carried out to obtain in-fill information between trenchexcavations although it should not be the sole method of investigation, as individualpockets of tar can be easily missed;• rotary drilling techniques are unlikely to be effective in acid tar lagoons due to thephysical characteristics of the acid tars themselves and the greater weight of themajority of rotary rigs;• gas and groundwater monitoring wells can be installed with response zones bothbelow and in the acid tar lagoon. However, great care must be taken with boreholedesign and construction techniques when penetrating through the base of a lagoon intonatural ground to avoid driving down contamination and short-circuitingcontamination migration routes;• drilling and well construction materials that are resistant to aggressive groundconditions will be needed e.g. stainless steel equipment is preferable to mild steel asthis latter type can be corroded by the acidity of the tars. Similarly, slotted pipe andcasing should be of high density polyethylene (HDPE) for maximum chemicalresistance.Adequate time and appropriate facilities will be needed for the decontamination of equipmentcoated in tars. For example, steam cleaning of augers will be needed between locations andall equipment, including vehicle tyres, will need to be thoroughly cleaned before leaving thesite at the end of each day. Depending on the cleaning equipment used, it may be necessaryto have access to mains power and water close to the wash down point (the location of whichalso needs to be carefully chosen).R&D <strong>Technical</strong> Report P5-042/TR/04 24