Using environmental isotopes in the study of the recharge-discharge mechanisms of the Yarmouk catchment area in Jordan

The recharge sources, the flow mechanisms and discharge areas of the different groundwater bodies underlying the Yarmouk River catchment area in Jordan, have, until now, not been adequately explained, although a wide range of hydrological, hydrogeological, and hydrochemical studies have been done. Along the Jordanian part of the catchment area of the Yarmouk River, groundwater issues from different aquifers with a variety of chemistries and types within the same aquifer and in between the different aquifers. Conventional recharge/discharge mechanisms, water balances and chemical analyses did not adequately explain the chemical variations and the different water types found in the area. Applying environmental isotopic tools combined with their altitude effects due to topographic variations (250–1,300 m a.s.l. within a distance of 20 km), and taking into consideration re-evaporation effects on the isotopic depletion and enrichment of rainwater, has greatly helped in understanding the recharge discharge mechanisms of the different aquifers. Precipitation along the highlands of an average of 600 mm/year is found to be depleted in its isotopic content of δO¹⁸ = −7.0 to −7.26 and δD = −32.2 to −33.28, whereas that of the Jordan Valley of 350 mm/year is highly enriched in isotopes with δO¹⁸ = −4.06 and δD = −14.5. The groundwater recharged along the highlands is depleted in isotopes (δO¹⁸ = ~ −6, δD = ~ −30), groundwater at the intermediate elevations is enriched (δO¹⁸ = ~ −5, δD = ~ −23) and that of the Jordan Valley aquifers containing meteoric water is highly enriched (δO¹⁸ ~ −3.8, δD = ~ −18). The deep aquifers in the Jordan Valley foothills are depleted in isotopes (δO¹⁸ −18 = −6, δD = −30) and resemble those of the highland aquifers. Only through using isotopes as a tool, were the sources of the different groundwater bodies and recharge and discharge mechanisms unambiguously explained. It was found that recharge takes place all over the study area and produces groundwater, which, from the highlands towards the Jordan Valley, shows increasing enrichment in isotopes. The highlands aquifer, with its groundwater depleted in isotopes, becomes confined towards the Jordan Valley; and, due to its confining pressure, leaks water upwards into the overlying aquifers causing their water to become less enriched in isotopes. Water depleted in its isotopic composition also seeps upward to the ground surface at the mountain foothills through faults and fissures.