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

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.