Experimental determination of isotope enrichment factors – bias from mass removal by repetitive sampling

Application of compound-specific stable isotope approaches often involves comparisons of isotope enrichment factors (ε). Experimental determination of ε-values is based on the Rayleigh equation, which relates the change in measured isotope ratios to the decreasing substrate fractions and is valid for closed systems. Even in well-controlled batch experiments, however, this requirement is not necessarily fulfilled, since repetitive sampling can remove a significant fraction of the analyte. For volatile compounds the need for appropriate corrections is most evident, and various methods have been proposed to account for mass removal and for volatilization into the headspace. In this study we use both synthetic and experimental data to demonstrate that the determination of ε-values according to current correction methods is prone to considerable systematic errors even in well-designed experimental setups. Application of inappropriate methods may lead to incorrect and inconsistent ε-values entailing misinterpretations regarding the processes underlying isotope fractionation. In fact, our results suggest that artifacts arising from inappropriate data evaluation might contribute to the variability of published ε-values. In response, we present novel, adequate methods to eliminate systematic errors in data evaluation. A model-based sensitivity analysis serves to reveal the most crucial experimental parameters and can be used for future experimental design to obtain correct ε-values allowing mechanistic interpretations

Tracers as Essential Tools for the Investigation of Physical and Chemical Processes in Groundwater Systems

In complex environmental systems, tracers are indispensable tools for the investigation of various physical, chemical, and biological processes. From the large variety of tracers employed by EAWAG in the aquatic environment, we present some examples relevant to groundwater research. Some tracers (e.g., 222Rn, 3H/3He, chlorofluorocarbons) allow the time since groundwater infiltration to be determined; this information can be used to quantify process rates such as flow velocities, recharge and reaction rates. Other tracers (e.g., conductivity, isotopes of oxygen or boron) can be used to quantify mixing ratios between waters of different origin, for instance, to study the admixture of leachate from a landfill to an aquifer. In contrast to these inert tracers, chemically reactive tracers (e.g., nitroaromatic compounds) can be used to study subsurface biogeochemical transformation processes, e.g. the reduction of contaminants under anaerobic conditions. The successful use of tracers in the study of environmental systems requires highly developed analytical facilities for a broad palette of tracer measurements and a careful choice of appropriate tracers for each specific problem

Natural Organic Matter as Reductant for Chlorinated Aliphatic Pollutants

Humic acids (HA) are ubiquitous redox-active compounds of natural aquatic and soil systems. Here we studied the potential of HA as reductants for chlorinated aliphatic pollutants. To avoid artifacts potentially involved when studying chemically reduced HA, we prepared electrochemi cally reduced soil, aquatic and synthetic HA, and anthrahydroquinone-2,6-disulfonic acid (AHQDS), a model compound for hydroquinone moieties in HA. Both reduced HA and AHQDS reduced hexachloroethane (HCE) at appreciable rates. Some reduction of HCE by HA, however, occurred even before electrochemical reduction of the humic acids. This indicates that a small fraction of reduced moieties in HA persists at oxic conditions for some time. The initial reaction followed pseudo-first-order reaction kinetics, and tetrachloroethylene was the only halogenated product. The relatively small variations in carbon-normalized rate constants, kDOC, found indicate that despite inherent variations in concentration, accessibility, and reactivity of redox-active groups in HA of various origins their overall dechlorination activity is fairly constant. However, HCE transformation rate constants and reducing capacities of different HA did not correlate. Rate constants normalized to both carbon content and reducing capacity of HA clearly indicate that reduced functional groups in different HA exhibit different reactivities. Our results together with the fact that reduced HA can be formed by a variety of microbiological and chemical processes suggest that HA could play a significant role as reductants in the reductive transformation of subsurface contaminants and that such a process could potentially be enhanced at contaminated sites by addition of reducible natural organic matter