Short Communication METHANE IN GROUNDWATER AND ITS ...

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Nov 24, 1980 - phase to that of the liquid phase (Henry's law) for an initial water volume of 103 ml with ... Barker, J.F., Fritz, P. and Brown, R.M., 1978.
Journal of Hydrology, 5 2 ( 1 9 8 1 ) 3 5 5 - 3 5 8 Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands

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Short Communication [2]

METHANE IN GROUNDWATER AND ITS EFFECT ON WATER DATING

14 C

GROUND-

M E B U S A . G E Y H and R U D I K Ü N Z L Niedersächsisches of Germany)

Landesamt

für Bodenforschung,

3000 Hannover (Federal

Republic

(Received November 2 4 , 1 9 8 0 ; accepted for publication January 1 5 , 1 9 8 1 )

ABSTRACT Geyh, M . A . and Künzl, R., 1 9 8 1 . Methane in groundwater and its effect on 1 4 C groundwater dating. J. Hydrol., 5 2 : 3 5 5 — 3 5 8 . The methane content of 1 2 3 representative groundwater samples with 1 4 C ages exceeding 2 0 , 0 0 0 yr. B.P. collected in the Federal Republic of Germany was determined. It has been confirmed that methane only affects the C groundwater dating in the exceptional case. A correction which is necessary when only traces of dissolved gas are present is described.

Discussions on the reliability of 14 C dating of groundwater have been stimulated by studies on the effect of secondary hydrochemical reactions on its 14C content (Wigley, 1975, 1976; Wigley et al., 1979). One of the aspects considered is the presence of methane (Fritz et al., 1976; Barker et al., 1978). 14C ages are increased if fossil organic matter in the form of liquid hydrocarbons has been decomposed to methane and carbon dioxide, e.g., during the process of sulfate reduction or fermentation (Davis, 1967). In these cases 513C-values might also be changed. A methane content of 50/xmol/l would correspond to an increase in 14C age by up to 160 yr. if no gas was lost. In order to study the importance of the methane effect, 123 samples were collected of dated groundwater from various aquifers throughout the Federal Republic of Germany. The sampled areas included the coal mining area Ibbenbühren and Schleswig—Holstein, the latter of which has occurrences of natural gas. Preferentially, groundwater with an age exceeding 20,000 yr. B.P. was examined. The samples were stored upsidedown in 103-ml glass bottles to avoid gas losses. The methane concentrations were measured with a Packard 421® gas Chromatograph, calibrated with Linde® methane standard gas of 4.14 /imol/1,

0022-1694/81/0000—0000/S02.50

© 1 9 8 1 Elsevier Scientific Publishing Company

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Fig. 1. Calculated and measured relationship of the methane concentrations of the gas phase to that of the liquid phase (Henry's law) for an initial water volume of 1 0 3 ml with various bubble sizes. The analyses were run after 5 0 % of the water was displaced by helium and equilibrium was established.

connected to a LDG® 304 computing integrator. Half of the water in each bottle was withdrawn by helium displacement. The analysis was done when the methane in the gas phase had established equilibrium with that of the liquid phase. The reproducibility was ±10%. The calibration line deviates from the theoretical one by 40% (Fig. 1). This is explained by the presence of a gas bubble in the sample bottle into which the dissolved methane migrated during the storage. As the methane content in the gas phase is 19 times greater than that of the liquid phase under equilibrium conditions, bubbles decrease the methane content in the liquid phase considerably. However, a bubble correction is possible (Fig. 1). The few samples of young groundwater which contained oxygen showed only initially high methane contents due to the activity of methane-oxidizing bacteria. Hence, bactericides were generally added to avoid methane loss. The methane content in water in boreholes varied with time when the wells were idle, therefore samples were taken after the water in the casing had been pumped out. Obviously, methane-producing bacteria can occur near the well filter. Seasonal changes observed in the methane content of shallow groundwater seem

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Fig. 2 . Histogram of the methane concentrations of 1 2 2 groundwater samples collected in the F . R . G .

to be due to an increase in the oxygen content during the period of groundwater recharge rather than to temperature variations, which normally are small in the aquifers. During the summer, when oxygen is consumed by methane-oxidizing bacteria and little groundwater recharge takes place, the life conditions for anaerobic methane-producing bacteria may be improved, which is why the methane content increases. Our results are compiled in the histogram of Fig. 2. Eighty percent of the analyzed groundwater samples contained less than 0.5 /umol/1 methane and only 5% had concentrations between 5 and 178 pimol/1. However, the samples with a high methane concentration may be non-representative of groundwaters in general due to their preferred collection. Thus, our results are in agreement with those by Dyck et al. (1976), who found only 14 groundwater samples out of 1677 exceeding 8.2jumol/l methane content in Canada. Even degassed methane-bearing groundwater seems to be an exception, as an anomalous 513C-value was found in only one case among our samples.

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Hence, we conclude that a methane effect on rare exception.

14 C

groundwater dating is a

ACKNOWLEDGEMENTS

The financial support for the study by the Deutsche Forschungsgemeinschaft was highly appreciated. The gas chromatographic analyses were done with the assistance of Mr. M. Schmitt. Special thanks are due also to all the geologists of the various State Geological Surveys who carried out the groundwater sampling and to Dr. R.C. Newcomb as well as Dr. G.H. Davis, who assisted in the formulation of the English text.

REFERENCES Barker, J.F., Fritz, P. and Brown, R . M . , 1 9 7 8 . 1 4 C measurements in aquifers with methane. Proc. Int. Symp. on Isotope Hydrology 1 9 7 8 , I A E A , Vienna, pp. 6 6 1 — 6 7 8 . Davis, J.B., 1 9 6 7 . Petroleum Microbiology. Elsevier, Amsterdam, 1 8 5 pp. Dyck, W., Chatterjee, A . K . , Gemmell, D.E. and Hurricane, K . , 1 9 7 6 . Well water trace element reconnaissance, eastern maritime Canada. J. Geochem. Explor., 6: 1 3 9 — 1 6 2 . Fritz, P., Barker, J.F. and Reardon, J.A., 1 9 7 6 . Methane in groundwater, remarks on its chemistry and isotope composition. 4th Eur. Coll. of Geochronology, Cosmochronology and Isotope Geology, April 5—10, 1 9 7 6 , Amsterdam. Wigley, T . M . L . , 1 9 7 5 . Carbon-14 dating of groundwater from closed and open systems. Water Resour. Res., 1 1 : 3 2 4 — 3 2 8 . Wigley, T . M . L . , 1 9 7 6 . Effect of mineral precipitation on isotope composition and 1 4 C d ating of groundwater. N ature ( L o n d o n ) , 2 6 3 : 2 1 9 — 2 2 1 . Wigley, T . M . L . , Plummer, L.N. and Pearson, Jr., F.J., 1 9 7 9 . Mass transfer and carbon isotope evolution in natural water systems. Geochim. Cosmochim. Acta, 4 2 : 1 1 1 7 — 1138.

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