Isotopic and geochemical evolution of rainwater percolating through the rocky outcrops: Judaea mountain case study.

Groundwater recharge of mountain aquifer requires detailed knowledge of the hydrologic system and adequate monitoring and modeling methods to determine water amount and water quality evolution. Mountain aquifers are well known of their highly complex lithologic structure and surface morphology. These become more significant in dry climate regions (<300 mm rainfall/year) which are characterized by erratic rain pattern and extreme deep thickness unsaturated zone.

In this study we monitor the isotopic and geo-chemical evolution affecting the composition of the unsaturated porewater during deep infiltration, from surface to depth that is not affected from evaporation. The geo-chemical processes were characterized related to land surface morphology and climate conditions.

The research setup includes instrumentation of first-order stream which is characterized by two main typical geomorphologic setting: rocky terrain and deep soil along the stream channel. Each plot was instrumented with a monitoring setup that include a meteorological station and Vadose Zone Monitoring System (VMS) that enables continuous water content measurement and collection of unsaturated porewater from the vadose zone.

Fast increases in water content and arrival of depleted δ¹⁸O (VSMOW) reveal quick and deep infiltration of rainwater during storm events, while enriched δ¹⁸O arrival indicates slower infiltration of water that is exposed to evaporation. In addition, the geo-chemical processes exhibited depletion in δ¹³C (PDB) of rainwater during the infiltration (-19 to -11 ‰) which indicates on dominant of biogenic activities and relatively low rock-water interactions. Major elements correlation network expresses the contribution of dust and rain to the rock evolution across the water flow path.

The study results clearly exhibited different infiltration rates in each site. Fast infiltration at the rocky terrain due to rock outcrops on the surface create funnels for collecting the local runoff and delivering it into high permeability fractured zones where the water penetrates directly to the deep sections. In contrast, the bare soil areas such as hilltops or man-made terraces in streams with highly developed soil cross-section, reveal limited infiltration. Also, the annual rainfall pattern impacts the geochemical process and finally impacts the groundwater quantity and quality.