Transport and degradation of chlorofluorocarbons (CFCs) in the pyritic Rabis Creek aquifer, Denmark

Abstract

[1] Vertical profiles of the chlorofluorocarbons CFC-11, CFC-12, and CFC-113 penetrating aerobic and anaerobic parts of a shallow sandy aquifer show that the CFC gases are degraded in the <1 m thick transition zone from aerobic to anaerobic groundwater in a pyritic sand aquifer at Rabis Creek, Denmark. Two-dimensional solute transport simulations with either zero-order or first-order degradation in the anaerobic zone corroborate this interpretation. The transport model was previously calibrated against detailed tritium profiles in the same wells. First-order degradation is found to best match the observed CFC profiles yielding an approximate half-life of a few months for CFC-11. Degradation is not as clearly recognized for CFC-12 and CFC-113, but it may occur with rates corresponding to a half-life of a few years or more. Data indicate a geochemical control of the CFC concentration gradient at the redox front and that denitrification and denitrifiers are not of major importance for the observed CFC degradation. The responsible mechanism behind the observed degradation is not known but we suggest that reductive dehalogenation by surface-bound Fe(II) on pyrite possibly enhanced by the presence of Fe(III)-bearing weathering products (green rust) may be a plausible mechanism. The observed data and the performed simulations confirm the potential application of the CFC gases as age-dating tools in the aerobic part of the investigated aquifer, but also that CFC data must be analyzed carefully before it is used as a dating tool in reducing aquifers because degradation may have occurred. The use of multiple or alternative tracers should be considered in anaerobic environments.

Identifying the contribution of regional groundwater to the baseflow of a tropical river (Daly River, Australia)

“…For suspected anaerobic conditions, Hinsby et al (2007) advocated use of multiple tracers in groundwater age studies. On a plot with hypothetical mixing models (Fig.…”

Coupled Reactive Transport Modeling of Redox Processes in a Nitrate-Polluted Sandy Aquifer

“…1). The Rabis Creek area has been intensively studied during the last 19 years (Postma et al 1991;Olsen et al 1993;Engesgaard et al 1996;Engesgaard et al 2004;Hinsby et al 2007) with a view to obtaining a more in-depth insight into the groundwater geochemistry, groundwater dating, transport and degradation of chlorofluorocarbons, and dispersive processes. The geology, geochemistry, and hydrogeology (including water tritium dating and dispersivities) of the region have been described in detail in the aforementioned papers.…”

Evidence of CFC degradation in groundwater under pyrite-oxidizing conditions

“…Thus, further assessment of CFC persistence under varying redox environments is warranted at this time. Furthermore, only a few field studies (Cook et al, 1995;Oster et al, 1996;Hinsby et al, Accepted for Publication) have been of sufficient detail to allow estimation of in situ reaction rates when CFC degradation has been suspected.…”