Hydrogen as indicator for redox conditions and dechlorination

The use of intrinsic degradation processes (natural attenuation) is becoming the basis for the restoration of many contaminated sites. Within this context, measuring concentrations of hydrogen in groundwater is important in two ways. Firstly, the hydrogen concentration can be used to characterize redox conditions. Secondly, hydrogen may be an indicator for the occurrence of dechlorination processes. These two potential applications of hydrogen measurements were tested by field research at three sites contaminated with chlorinated solvents and at one landfill site. Besides hydrogen concentrations, other geochemical parameters were also determined here. Based on these parameters, different methods for data interpretation were applied and compared. Redox characterisation using hydrogen measurements proved to be a (cost) efficient method to identify the redox condition at the investigated sites, including chlorinated solvent sites. Hydrogen measurements can be used in the following ways: - quick on-site scan of the redox situation can be performed by comparing hydrogen concentrations to an empirically determined range; the so-called empirical hydrogen range. - detailed redox characterisation can be obtained by the partial equilibrium approach, which is based on thermodynamic calculations of hydrogen concentrations in combination with concentrations of redox species. At all investigated sites a consistent characterisation of the redox conditions could be deduced from hydrogen measurements. In contrast, the use of redox potentials and/or macro chemistry alone, would have led to an inconsistent redox characterisation at the investigated sites. Hydrogen has also been shown to be an indicator for the occurrence of dechlorination processes. Hydrogen concentrations above 2 nM correlated with a high degree of in-situ dechlorination. In wells where relatively high amounts of DOC, BTEX, mineral oil, or 1,2-dichloroethane were detected, elevated hydrogen concentrations together with higher degrees of dechlorination were also observed.