Water chemistry of the Dhuleil aquifer (Jordan) as influenced by long-term pumpage

The Dhuleil aquifer is located in the northwest of Jordan and represents a major groundwater resource for domestic and agricultural purposes. At present, there are more than 80 wells extracting water from basaltic or limestone formations. Long-term observations have indicated an increase in the groundwater salinity. Chemical analyses of water samples collected from seven wells over a minimum of 12 years have been employed to characterize and determine the cause(s) of increasing water salinity. The results show that the electrical conductivity of each well's water is linearly associated with Cl concentration. Slopes of such relations are almost identical for the basaltic well waters and, thus, reflect the influence of the geological formations on water chemistry. Increasing water salinity was mainly due to increasing Cl salts of Na, Ca, and Mg as indicated by the linear relation between the concentration factor of each cation (a given cation concentration divided by the reported minimum cation concentration) and that of the C) anion. The S04 concentration factors for all well waters did not increase in harmony with these of Cl. The Cl fractional negative charge tended to increase to an average steady-state value of approximately O.8 for all basaltic wells, irrespective of the variation in irrigation practices. These observations suggest that return flow of irrigation leachate was not the cause of the increasing water salinity. Alternatively, deterioration in water quality was hypothesized to be due to excessive pumpage which rendered the exhausted aquifer more susceptible to the limited rainwater infiltrating the natively saline parent materials. This conclusion is further supported by the results of chemical speciation which indicate that all well waters were always undersaturated with respect to gypsum (ion activity products of (Ca2+) (SO2-4) were smaller than Ksp of gypsum by more than one order of magnitude). In addition, all well waters were undersaturated with respect to caclite with only a few cases where the waters were in equilibrium with that solid phase. After 25 years of irrigation, soil solution chemistry became similar to that of the DH-16 well water employed in irrigation.