The Ribe Formation in western Denmark — Holocene and Pleistocene groundwaters in a coastal Miocene sand aquifer

Abstract

The Ribe Formation is a regionally extensive Miocene sand aquifer that is present in western Denmark at depths ranging from 100 to 300 m below ground surface. Groundwater chemistry and isotope data collected from more than 40 wells show that the Ribe Formation mainly contains high quality Ca-bicarbonate type groundwater of Holocene age (100-10 000 a BP). Pleistocene age groundwaters, identified on the basis of stable isotopes, noble gases and corrected 14C values, are present below the island of R0m¢, in discharge areas near the coast, and in hydraulically isolated inland areas. The groundwater age distribution in the Ribe Formation was successfully simulated with a numerical groundwater flow model and particle tracking only when the 14C content in groundwater was corrected for both geochemical reactions and diffusion. The results indicate that geochemical and physical processes significantly influence the 14C content of groundwater and that the correction factors required for the two processes are of the same magnitude. Flow modelling results indicate that Pleistocene groundwaters were emplaced at depth within the Ribe Fromation under low base-level conditions that prevailed throughout the late Pleistocene -near the coast these waters are essentially isolated from the present flow system, and Pleistocene freshwater may be present offshore. Seismic surveys show that conditions offshore are favourable for the presence of Pleistocene freshwater within the Ribe Formation and other aquifers.

The Miocene Sand Aquifers, Jutland, Denmark

“…This is corroborated by the relatively low 18 O content in well 148.52 and by noble gas measurements in well 149.377, which indicate a recharge temperature 5-6°C lower than the present annual average temperature (Hinsby et al 2001a). However, the high DOC and/or sodium concentrations in well 148.52 infl uences the inorganic carbon pool either directly through production of CO 2 from fermentation processes and/or indirectly through the triggering of carbonate dissolution/precipitation processes (Appelo 1994).…”

“…A thick succession of Eocene and Oligocene clays, of which some are swelling, forms the lower boundary of the Ribe Formation (Hinsby et al 2001a), grey circles were sampled in a PhD project (Jakobsen 1995 aquifer and an efficient seal to deeper saline waters (Dinesen 1961;Hinsby et al 2001a et al 2001a). Hence the topography of the pre-Quaternary surface and the depths to the Miocene sediments vary considerably in the investigated area.…”

“…During this period the Ribe Formation sand aquifer must have been salinised, when the sea transgressed and covered the area with marine sediments. However, the Ribe Formation was generally completely freshened again during the Pleistocene possibly partly through fl ushing with glacial meltwaters at high hydrostatic pressure at lower sea levels (up to 130 m at the glacial maximum) and later during the Holocene (Hinsby et al 2001a). …”

“…This is demonstrated by the high DIC and CH 4 concentrations, and the relatively enriched ('heavy') δ 13 C values. These processes will lower the measured percentage of modern carbon ( 14 C) and increase the uncertainty on the groundwater age estimate, although the effect is partly taken into account by the δ 13 C correction (Hinsby et al 2001a).…”

Organic Quality of Groundwaters

“…Such occurrences are however extremely rare, the 95 percentile is below 10 mg C/L TOC for all Danish aquifer types. Organic carbon rich waters may be found both in deep aquifer systems (> 100 m below surface) like in the Danish cases (Grøn et al 1996, Jørgensen et al, 1999, Hinsby et al 2001) and in shallow soil, bog and aquifer sediments (e.g. Weis et al, 1989, Ernstsen et al 2005.…”