The existence of industrial centers and agricultural activities has a main role on surface and ground water resources pollution in Isfahan province. Agricultural drainage water contains dissolved salts, pesticide residues and chemical fertilizers especially nitrate. The aim of this study was the possibility of existence of diazinon, a pesticide with the highest use in Zayandehroud river banks (upstream of Baba-Sheykh-Ali refinery of Isfahan ). Samples of water were taken from 10 sites (Zayandehroud regulator dam, Zamankhan Bridge, Cham-Tagh Bridge, Cham-Aseman dam, Kalleh Bridge and Nekouabad dam, three selected drainages and one drinking well) from July 2006 to March 2007. Chemical properties of water (temperature, pH, Ca 2+ , Mg 2+ , total hardness, Na + , SAR, EC, Cl - , HCO 3 - , SO 4 2- , TDS, BOD, COD and N-NO 3 ) were measured by the standard methods. Clean-up, detection and quantification of diazinon residue, was measured by Thin Layer Chromatography (TLC) and High Performance Liquid Chromatography (HPLC) respectively. Simulation of some of chemical and physical properties in river's water was carried out using QUAL2K model. The results showed that the quantity of most measured parameters in surface water were increased from upstream to downstream similarly from summer to winter. Maximum concentration of most parameters was observed in February. The river discharge was at the minimum level in February. The chemical parameters were also increased in drainage and groundwater from summer to winter. In Cham-Aseman Dam (upstream of Baba-Sheykh-Ali refinery plant), total hardness, Na + and EC were higher than the national and international drinking water standards (100 mg/l CaCO 3 , 20 mg/l, and 0.4 dS/m respectively) especially in winter. Generally, Zayandehroud regulator dam and Nekouabad dam had the best and lowest water quality, respectively. The maximum values of most parameters in Zayandehroud water were higher than some of national and international drinking water standards. The entrance of Steel Melting Factory wastewater to Zayandehroud water, from Kalleh Bridge to Nekouabad dam, caused the increasing of Na + , EC, Cl - and SO 4 2- in river's water. The both quality of surface water and groundwater were permissible level for irrigation and drinking. The characteristics and the discharge of drainages and the river discharge had an important effect on the quality of water. The N-NO 3 concentration of groundwater decreased from fall to winter
