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Age structure and recharge conditions of a coastal aquifer (northern Germany) investigated with 39Ar, 14C, 3H, He isotopes and Ne |
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| Authors: | J��rgen S��ltenfu�� Roland Purtschert Jens F F��hrb?ter |
| Institution: | 1. Institute of Environmental Physics, University of Bremen, Otto-Hahn-Allee, 28359, Bremen, Germany 2. Climate and Environmental Physics, University of Bern, Sidlerstrasse 5, 3012, Bern, Switzerland 3. Leichtwei?? Institute for Hydraulic Engineering and Water Resources, Technical University of Braunschweig, Beethovenstr. 51a, 38106, Braunschweig, Germany |
| Abstract: | A tide-influenced two-layer aquifer system in northern Germany was investigated using environmental dating tracers (3H, 39Ar, 14C), the noble gas isotopes 3He, 4He and Ne. The study area is a marshland at the River Ems estuary, exposed to regular flooding until?AD 1000. The construction of dykes, artificial land drainage and groundwater abstraction define the hydraulic gradient. The aquifer depicts a pronounced age stratification with depth. Tritium concentrations above 0.03 TU are found only in the top 30?m. Two tritium-free samples between 20 and 30?m depth show 39Ar ages of 130 and 250?years. Below a clay layer?Cabout 50?m below surface level (mbsl)?Call analysed samples are 39Ar free and, thus, older than 900?years. The initial 14C activities were about 70 pmC. Resulting 14C ages increase with depth and increase up to 9,000?years, in agreement with minimal 39Ar ages. Concentrations of radiogenic 4He correlate with 14C ages. Samples in a mid-depth range (20?C70 mbsl) show significant gas loss. The gas loss is assigned to recharge in a methane producing environment. Deduced 4He ages were used to assign this water to a infiltration period of about AD 1000. |
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