Radon, helium and CO2 measurements in soils overlying a former exploited oilfield, Pechelbronn district, Bas-Rhin, France.

The Pechelbronn oilfield (Rhine Graben, France), where mining activity ended in the 1960s, has been used for waste disposal for twenty years. Since the wastes are varied, work is underway to identify the discharged materials and their derivatives, as well as to locate and quantify potential discharge sites. Two major goals were assigned to the present work. The first was to identify or refine the location of hidden structures that could facilitate gas emanation up to the surface, by studying soil gas concentrations (mainly (222)Rn, CO(2), CH(4) and helium) and carbon isotope ratios in the CO(2) phase. The second was devoted to examining, from a health and safety viewpoint, if the use of the oilfield as a waste disposal site might have led to enhanced or modified gas emanation throughout the area. It appeared that CO(2) and (222)Rn evolution in the whole area were similar, except near some of the faults and fractures that are known through surface mapping and underground observations. These (222)Rn and CO(2) anomalies made it possible to highlight more emissive zones that are either related to main faults or to secondary fractures acting as migration pathways. In that sense, the CO(2) phase can be used to evaluate (222)Rn activities distant from tectonic structures but can lead to erroneous evaluations near to gas migration pathways. Dumping of wastes, as well as oil residues, did not appear to have a strong influence on soil gaseous species and emanation. Similarly, enhanced gas migration due to underground galleries and exploitation wells has not been established. Carbon isotope ratios suggested a balance of biological phenomena, despite the high CO(2) contents reached. Other monitored gaseous species (N(2), Ar, H(2) and alkanes), when detected, always showed amounts close to those found subsurface and/or in atmospheric gases.