The objective of this study is the assessment of
interactions between the sulfur and carbon cycles in a Tertiary aquifer, both in the water as well as in the host sediment. This includes the quantification of all available species and their subsequent isotopic characterization. Results
will contribute to our general understanding of processes
like bacterial sulfate reduction, which are fairly well
characterized in the marine realm but less well studied in the terrestrial environment.
The general lithostratigraphic column in the study
area (Niederrheinische Bucht, Germany) comprises a succession of fine- to medium-grained, well sorted, marine sands of Tertiary age, with up to three interbedded lignite seams, resulting in a total of up to four groundwater
horizons. The uppermost horizon is composed of Quaternary sand and gravel deposits of fluvial origin. Geochemically, the uppermost aquifer is aerobic while all lower levels can be characterized as anerobic. Locally, a clear anthropogenic input through leakage inbetween groundwater horizons can be observed through high nitrate, sulfate and chloride concentrations.
For sediments from the main horizon under study, the following abundances have been determined: total sulfur: 0.41 wt.%; pyrite sulfur: 0.36 wt.%; organic sulfur: 0.03 wt.%; sulfate sulfur: 0.008 wt.%; acid-volatile and elemental sulfur: 6 x 10-5 wt.%. All values represent average compositions, not reflecting stratigraphic variations due to slight changes in lithology (silt content). Average isotopic compositions for these species are -16.4 ” for total sulfur, -22.8 ” for pyrite sulfur, +1.2 ” for organic sulfur and +2.8 ” for sulfate sulfur. Abundances as well as the isotopic compositions indicate that pyrite sulfur represents the
main sulfur constituent of these sediments, likely a result of bacterial sulfate reduction as reflected by its 34S-depleted isotope signature.
Organic carbon abundances in these sediments lie around 0.5 wt%, with significantly higher values close to the lignite seams. Carbonate carbon is present only in minor amounts. The organic carbon isotopic composition displays a mean value of -25.8 ” with little variation up section. This is in good agreement with expected values for terrestrial organic matter.
Dissolved sulfur species in the water include both, sulfate and hydrogen sulfide. Sulfate concentrations range from
< 1 mg/l to > 100 mg/l. Hydrogen sulfide concentrations of up to 120 µg/l have been determined, with higher concentrations accompanying low sulfate values. Isotope values for sulfate are quite variable, ranging from +2.2 to +22.5 ”. Strongly 34S-enriched values have been observed for low sulfate concentrations. These observations are consistent with the process of bacterial sulfate reduction occuring in the aquifer today.
Dissolved inorganic carbon (DIC) represents the main carbon component in the water. Its concentration ranges from 100 - 400 mg/l and its isotopic composition between -20 to -11 ”. These values indicate that biogenic carbon (oxidized organic matter) can be regarded as the major source for DIC. However, increasing abundances and progressively more positive carbon isotope values reflect an influence of carbonate dissolution. Dissolved organic carbon (DOC) is present in the aquifer with concentrations of up to 4 mg/l, averaging 2 mg/l.
Results discussed above indicate that bacterial sulfate reduction appears to be one of the major processes affecting water chemistry in the anaerobic parts of the studied aquifer. Future work will include a detailed compositional assessment of the dissolved organic carbon as likely substrate for this process.