A pilot study to use the ³⁶Cl bomb peak as a tracer for groundwater flow velocities in the Western Dead Sea catchment

The aquifer system of the western Dead Sea catchment is stressed by semi-arid to arid climate conditions, limited groundwater recharge rates and increasing water abstractions for human water needs. The groundwater flow system is dominated by two main Cretaceous limestone aquifers with karst characteristics and discharging in springs in the Lower Jordan Valley and Dead Sea region. The karst properties give reason to assume parts of the flow system having high transmissivities and groundwater flow velocities, respectively. For estimating recharge rates and rain water infiltration time periods, ³⁶Cl and ³H were used, with the anthropogenic bomb peaks as input functions.
The chloride content in groundwater of the limestone aquifers enriches after contact with the saline Quaternary sediments and groundwater in the Lower Jordan Valley. The ³⁶Cl/Cl ratios in groundwater were found to be up to 1E-12 in the recharge area and decrease to 1E-14 in the discharge area. Groundwaters in the recharge area show partly ³⁶Cl/Cl ratios comparable to those in recent precipitation. The wide range of ³⁶Cl/Cl in the recharge area indicates different stages of chlorine isotope and elemental mixing within the recharge area or aquifer system. This may be due to varying Cl input (dependent on altitude and coastal proximity), varying ³⁶Cl input (regional variation in fallout) or both. Together with ³H analyses it is possible to evaluate the recent rain water component in the springs emerging from the uppermost part of the Cretaceous aquifers.
Our results show that a combination of the ³⁶Cl/Cl and ³H measurements in groundwater and a correlation to the atmospheric input curves of ³⁶Cl/Cl and ³H allow estimating the admixture of post-bomb recharge in groundwater.