Ionic contribution to the self-potential signals associated with a redox front.

In contaminant plumes or in the case of ore bodies, a source current density is produced at depth in response to the presence of a gradient of the redox potential. Two charge carriers can exist in such a medium: electrons and ions. Two contributions to the source current density are associated with these charge carriers (i) the gradient of the chemical potential of the ionic species and (ii) the gradient of the chemical potential of the electrons (i.e., the gradient of the redox potential). We ran a set of experiments in which a geobattery is generated using electrolysis reactions of a pore water solution containing iron. A DC power supply is used to impose a difference of electrical potential of 3 V between a working platinum electrode (anode) and an auxiliary platinum electrode (cathode). Both electrodes inserted into a tank filled with a well-calibrated sand infiltrated by a (0.01 mol L(-1) KCl+0.0035 mol L(-)(1) FeSO(4)) solution. After the direct current is turned off, we follow the pH, the redox potential, and the self-potential at several time intervals. The self-potential anomalies amount to a few tens of millivolts after the current is turned off and decreases over time. After several days, all the redox-active compounds produced initially by the electrolysis reactions are consumed through chemical reactions and the self-potential anomalies fall to zero. The resulting self-potential anomalies are shown to be much weaker than the self-potential anomalies observed in the presence of an electronic conductor in the laboratory or in the field. In the presence of a biotic or an abiotic electronic conductor, the self-potential anomalies can amount to a few hundred millivolts. These observations point out indirectly the potential role of bacteria forming biofilms in the transfer of electrons through sharp redox potential gradient in contaminant plumes that are rich in organic matter.

Evaluation of streamwater composition changes in the Vosges Mountains (NE France): 1955-2005.

In 1995, in the southwestern Vosges Mountains (NE France), 158 of 395 streams (40%) had a pH lower than 5.5 at baseflow. As elsewhere in Europe, acid deposition has decreased since the seventies, as has base cation deposition. In order to assess the response of streamwater to decreasing deposition, we compared their present chemical composition to their former composition. All comparisons showed a decrease in sulphate concentration, which was greater on granite than on sandstone. In addition calcium, magnesium and aluminium concentrations generally decreased. Acidity in streams draining granite decreased in spring, especially during the eighties; decreases were not observed on sandstone. Continuous monitoring of 5 streams since 1998 confirmed that Al concentrations decreased while changes in pH were small. Chemical trends in streams from the Vosges massif fell between those measured in Northern Europe and Central Europe. This study provides the first broad-scale overview of surface water acidification and recovery in France and emphasizes the need for continuous monitoring to assess long-term changes in aquatic ecosystems.