Distribution of 36Cl/Cl in a river-recharged aquifer: Implications for the fallout rate of bomb-produced 36Cl

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Distribution of 36Cl/Cl ratios in a river-recharged aquifer was investigated in the Oderbruch area, northeastern Germany. The aquifer is confined up to 3.5–4 km inland, where it changes to an unconfined condition. The 36Cl/Cl ratios in the confined area were in the range between 4.6 × 10−14 and 23.1 × 10−14, showing a peak at 2–3 km away from the river. A plot of 36Cl/Cl vs. reciprocal Cl− concentrations indicated possible effect of the Cl− concentration variation on the observed 36Cl/Cl ratios. After accounting for this effect, the estimated 36Cl fallout rates for the last 30 yrs show reasonable agreement with the Dye-3 data and the mid-latitude background value. The results suggest that a local 36Cl fallout curve can be constructed from groundwater when dispersive mixing is of minor importance.