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Age structure and recharge conditions of a coastal aquifer (northern Germany) investigated with 39 Ar, 14 C, 3 H, He isotopes and Ne

Age structure and recharge conditions of a coastal aquifer (northern Germany) investigated with 39 Ar, 14 C, 3 H, He isotopes and Ne,10.1007/s10040-01

Age structure and recharge conditions of a coastal aquifer (northern Germany) investigated with 39 Ar, 14 C, 3 H, He isotopes and Ne  
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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–about 50 m below surface level (mbsl)–all 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 (20–70 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.
Journal: Hydrogeology Journal - HYDROGEOL J , vol. 19, no. 1, pp. 221-236, 2011
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