Hydrogeologic effects on the quality of water in the Oued Issen watershed,western Upper Atlas Mountains, Morocco

The study of spatial and temporal variations of some hydrochemical properti es in the Oued Issen watershed, Morocco, has revealed their close relation to the area's lithologic and geologic characteristics and to variations in hydroclimatological cycles. High concentrations of sodium chloride are meas ured during flood periods, when the outcrops of the basin are leached by st reams draining the rather dense hydrographic network. These variations depend also on the relative abundance of tributaries on bo th sides of the Oued, their respective contributions being hydrochemically very different: (1) northern-side tributaries, which mainly leach evaporiti c terranes dating from the Late Triassic and Late Liassic periods and which are rich in gypsum and halite, increase the mineralization of the Oued Iss en waters flowing toward the Abdelmoumen Dam, (2) the low salinity of water observed in the upstream portion of the watershed is due to dilution by wa ters from the southern-side tributaries. which are derived from snowmelt an d the leaching of the high Paleozoic massif; (3) in the middle part of the basin, the increase in mineralization of the Oued Issen becomes substantial between the Abdelmoumen Dam and the Dkhila Dam, the next dam downstream, d ue to the absence of dilution by waters from the southern-side tributaries. which are sparse in this part of the basin. In addition. during flood peri ods, the outflow from the first dam is stopped. Thus, the main contribution of NaCl to the Oued is from the very salty Tirkou spring, which is situate d downstream from the first dam. The geological characteristics of the basin have induced the development of high-salinity zones, which are particularly evident in the southwestern Da rt of the basin in the vicinity of the confluence of the Oued Boulebaz with the Oued Issen. The discharge of very salty springs is controlled by fault s that offset the formations that underlie the hydrographic network and deg rade the quality of the Oued Issen. This water later recharges the unconfin ed aquifer along the Oued where it: flows on the Oued Souss Plain.