HYDROGEOLOGICAL BEHAVIOR OF A LONG AND NARROW KARST AQUIFER, GILAN KARST SPRING, IRAN

The Gilan aquifer is located southwest of Kermanshah in west Iran. This aquifer discharges at Gilan spring, with an average discharge rate of 938 l/s. The catchment area of Gilan spring was determined using hydrogeological approaches. Specific conductance (SP), water temperature, pH, major ions and discharge were measured at a two week interval within the period of Sept. 2000 to Sept. 2002. Water temperature, SP, and stage were measured once every two days in the wet season. Calcite, dolomite and gypsum saturation indices and partial pressures of CO₂ were calculated using the WATEQF hydrochemical model. The hydrochemical data, percentages of base flow and recession coefficients imply that the type of flow is mainly diffuse in the Gilan aquifer. The SP time series of the Gilan spring implies that a high percentage of the spring water is supplied by conduit flow. The peak discharge occurs when conduit water near the Gilan spring is the main source of discharge, while conduit water from the remote area reaches the spring during the recession period. The contribution of conduit flow during the dry period reduces the variations of physio-chemical parameters and produces a low recession coefficient. This kind of behavior occurs in an aquifer with a long and narrow catchment area, where the rain duration is significantly less than the time lag thus resulting in a decrease in SP during the dry period.