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The effects of the Khirbet Es-Samra Waste Water Treatment Plant (KS) on the deterioration of ground-water quality at Seil Zerqa basin were investigated in this study. The plant is located in an arid area in the eastern part of Jordan. It was built in 1985 to treat the waste water of about 60% of the Jordan population. It is composed of two anaerobic and four facultative ponds which empty into four maturation ponds. The reservoir of these ponds is impermeable but, due to geologic and hydrogeologic features, leakage of waste water into the ground-water began to occur and the Upper Aquifer Complex (Amman-Wadi Sir) started to deteriorate. In addition, the water quality of Sell Zerqa and Wadi Dhuleil has deteriorated during the flow to the King Tallal dam, due to the increase of waste water and limited fresh water resources. Topography differences plays a large role in changing climatic elements, especially rainfall, due to orographic effects and temperature. The results of periodical chemical analysis show that the electrical conductivity increased from 700 mu s cm(-1) at Hanna well in the KS upstream which is not affected, to more than 7000 mu s cm(-1) at Muawad well in the KS downstream. The other cations and anions have also increased by many times. The electrical conductivity of the soil water increased from 5800 mu s cm(-1) at 30 cm depth to more than 35,000 mu s cm(-1) at 140 cm depth. The heavy metals and bacterial count of soil irrigated with waste water have also increased more than the normal dry soils of the arid and semi-arid soils. This continual seepage of waste water may change the ground-water flow direction, in addition to the increased recharge which is many times greater than it was before the plant was built. However, the recharge of fresh water is very limited in this area, which is characteristic of an arid environment. Moreover, Sell Zerqa and Wadi Dhuleil have become breeding sites for insects and developed a foul odour, due to the low levels of rainfall that may otherwise have diluted the treated waste water through the flow to the King Tallal dam. (C) 1999 Academic Press.