Linking chloride mass balance infiltration rates with chlorofluorocarbon and SF₆ groundwater dating in semi-arid settings

Potential and limitations

authored by

Susanne Stadler, Karsten Osenbrück, Wilhelmus H.M. Duijnisveld, Martin Schwiede, Jürgen Böttcher

Abstract

In the framework of the investigation of enrichment processes of nitrate in groundwater of the Kalahari of Botswana near Serowe, recharge processes were investigated. The thick unsaturated zone extending to up to 100 m of mostly unconsolidated sediments and very low recharge rates pose a serious challenge to study solute transport related to infiltration and recharge processes, as this extends past the conventional depths of soil scientific investigations and is difficult to describe using evidence from the groundwater due to the limitations imposed by available tracers. To determine the link between nitrate in the vadose zone and in the uppermost groundwater, sediment from the vadose zone was sampled up to a depth of 15-20 m (in one case also to 65 m) on several sites with natural vegetation in the research area. Among other parameters, sediment and water were analysed to determine chloride and nitrate concentration depth profiles. Using the chloride mass balance method, an estimation of groundwater infiltration rates produced values of 0.2-4 mma^-1. The uncertainty of these values is, however, high. Because of the extreme thickness of the vadose zone, the travel time in the unsaturated zone might reach extreme values of up to 500 years and more. For investigations using groundwater, we applied the chlorofluorocarbons CFC-113, CFC-12, sulphur hexafluoride (SF₆) and tritium to identify potential recharge, and found indications for some advective transport of the CFCs and SF₆, which we accounted for as constituting potential active localised recharge. In our contribution, we show the potential and limitations of the applied methods to determine groundwater recharge and coupled solute transport in semi-arid settings, and compare travel time ranges derived from soil science and groundwater investigations.

Organisation(s)

Institute of Soil Science
Section Soil Biophysics

External Organisation(s)

Leibniz Institute for Applied Geophysics (LIAG)
Federal Institute for Geosciences and Natural Resources (BGR)
Helmholtz Zentrum München - German Research Center for Environmental Health

Type

Article

Journal

Isotopes in Environmental and Health Studies

Volume

46

Pages

312-324

No. of pages

13

ISSN

1025-6016

Publication date

2010

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Environmental Chemistry, General Environmental Science, Inorganic Chemistry

Electronic version(s)

https://doi.org/10.1080/10256016.2010.508124 (Access: Unknown)