Isotope response to hydrological systems for long term exploitation, case of Azraq Basin, Jordan

Azraq basin supplies the capital Amman with high quality drinking water, but due to over-pumping from the Amman Water and Sewage Authority well-field (AWSA), tapping the upper unconfined Basalt aquifer; The water level dropped dramatically and signs of salinization and depletion are starting to occur. This study includes hydrological, hydrogeological, hydrochemical and isotope hydrology investigation for the upper unconfined aquifer from the early 80's until present to come up with the causes of groundwater salinization of AWSA wells. The possible salinization scenarios of the upper aquifer could be attributed to: (a) Threat of reverse flow from Sabkha to AWSA well field resulting from low hydraulic gradient of the aquifer water table and increased draw down in the AWSA well field, (b) The probable upward leak of middle aquifer brackish water to the upper aquifer fresh water and the presence of faults that might be conductive, (c) The dissolution processes between water and rock matrix due to lowering the dynamic water levels during pumping which reached the mineralized formations underlying the Basalt. The results indicate that the first scenario is not yet occurring as the hydrodynamic system shows that groundwater flow is still in the direction of Azraq depression. There is no evidence that upward leak from the brackish middle aquifer is taking place; The salinization trend in transient conditions was accompanied by consistent increase in nitrate levels in some wells and high 14C contents in others while they are both not detected in the middle aquifer. The third scenario is the most likely to occur; The salinization trend of some AWSA wells represented by the gradual consistent increase of EC, Na, Cl, SO4 and Mg associated with rather constant stable isotopic contents indicate that these constituents originate from the main salts existing in the geochemistry of the lower rock matrix. (author)