Strontium isotopes as a tool for estimation of groundwater recharge and aquifer connectivity

Aflaj waters and groundwaters hosted in limestones, ophiolite and alluvium of northern Oman were investigated for their strontium content and ⁸⁷Sr/⁸⁶Sr ratio. The main objective of this investigation is to identify different sources of recharge and to establish hydrological inter-aquifer connectivity. Most of the water seems to acquire its isotope signature from interaction with carbonate and evaporite rocks. Based on the correlation between ⁸⁷Sr/⁸⁶Sr and 1/Sr, the contribution of groundwater from both the Hajar Super Group (HSG) and ophiolite aquifers in addition to direct infiltration of running waters can be identified for the Aflaj samples. The relationship between ⁸⁷Sr/⁸⁶Sr and 1/Sr indicates aquifers' inter-connectivity and suggest that the ophiolite aquifer is recharged from the alluvium and HSG aquifers. Groundwater of the Tertiary aquifer is from: 1) mixture of groundwaters from the ophiolite, alluvium and HSG aquifers and 2) direct infiltration of rainwater and its interaction with evaporite rocks. Mixed waters between those from HSG and ophiolite aquifers is one source of recharge to alluvium aquifer. A mass balance for the ophiolite aquifer was calculated in a binary mixing using the Sr content and the ⁸⁷Sr/⁸⁶Sr ratio. The results indicates that about 85–98% of the groundwater of the ophiolite aquifer is recharged from HSG aquifer.