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During recent years, worldwide concern has been focused on the potential for contamination of surface waters and ground waters by agrochemicals in runoff and soil water from irrigated agricultural fields. Given this perspective it is very important to correctly evaluate the levels of different agrochemicals in water. The main objective of this study was to assess the changes in the quality of tailwater due to use of agrochemicals in order to determine contamination loads. Four farms A, B, C and D was chosen in the Fouman region in the F2 unit in 1976. The areas of these fields ranged from 0.22 to 0.60 ha having a number of unequal successive basins with variable inflow and outflow rates. A quantitative analysis showed that the tailwater ratio in the farms of A, B, C, and D ranged from 2% to 64% during irrigation season. The outflow water classification was similar (C3S1) the change of the qualitative factors was not significant and were not following any certain pattern during the irrigation season. The concentration changes of SAR, ESP, EC, Cu, Zn, B, P, and K, DO in the inflow and outflow waters was also insignificant. After fertilizer application, the concentration of the nitrogen in the outflow was considerably significant. The average pollution loads in the inflow were about 1618 and 1476 kg/24hr/ha, respectively. Remaining loads in rice fields was about 142 kg/24hr/ha, which was significant at 1% level. The fifth farm (E) located in Lahidjan was sprayed with diasinon pesticide then water samples were analyzed each day for diazinon residual for 10 days after application. Sample analysis showed that the concentration of the insecticide was 93.08 mg/l immediately after application, which was gradually reduced and reached 0.98 mg/l ten days after spraying.