Recharge and regional circulation of thermal groundwater in northern Jordan using isotope geochemistry.

Abstract

The scarcity of water resources in Jordan poses difficulties for the development of the country and its relationship with its neighbours. Thermal groundwaters recently identified in the northern part of the country represent a 40 MCM/y resources for exploitation to meet the increasing demand for water. These thermal groundwaters are found in three well fields (Mukhebeh, JRV and Ramtha) and were investigated to determine their recharge origin, mean subsurface residence times, and the source of heat. They discharge in the northern part of Jordan Rift Valley and the rifted Yarmouk Valley, which are low elevation (50 to 150 m below sea level) zones of recent tectonism and volcanic activity. The range of temperatures is 30 and 56$\sp\circ$C and salinities vary between 500 and 2500 mg/l. Non-thermal groundwaters within the study area have also been studied. In particular shallow groundwater in adjacent highlands region (Ajloun Mountains) are examined to determine their role in recharge to those regional flow systems. The principal aquifer is the Upper Cretaceous B2/A7 group, a package of carbonate formations with high kerogen content at depth. This aquifer outcrops in the Ajloun Mountains and flanking regions and is confined by overlying marls in the down gradient regions. A deeper sandstone aquifer underlies the study area and hosts thermal groundwater which was sampled in the Ramtha area. The major geochemical processes in the subsurface have generated various geochemical facies in the thermal waters. These include carbonate dissolution to calcite saturation in the recharge areas. The thermal groundwater in Mukhebeh and JRV well fields are found to be chemically similar to the carbonate groundwater from Ajloun mountains recharge area. All thermal waters are characterized by sulphate reduction, driven by oxidation of kerogen. Sulphate is of marine evaporite origin dissolved from within the aquifer with a component of volcanogenic sulphur. Some thermal waters have also Na-Cl salinity component related to evaporite dissolution. The thermal waters are of meteoric provenance, originating as rain falling over the carbonate highlands in Jordan and Syria. The $\delta\sp $O and $\delta$D isotopic data show that all thermal groundwaters are largely associated with Eastern Mediterranean Meteoric Water Line, signifying recharge under the climate regime which dominates today in Jordan. The exception is groundwater from the deep sandstone aquifer which is associated with Global Meteoric Water Line, signifying recharge during Pleistocene time. The isotopic composition of groundwater suggests two distinct recharge areas for the Mukhebeh well field: Ajloun Mountains (Jordan) and Mount Hermon (Syria). (Abstract shortened by UMI.)