The amount of waste generated each year is increasing. Sultanate of Oman produces about 1 million tons of waste per year. The un-engineered landfills used for waste disposal in the Sultanate lack gas and leachate collection systems. Moreover, most of these landfill sites lack the main criteria required for landfill site selection such as vicinity of sites to residential areas. The main complaints about these landfills are due to population movement towards the site, continuous flaring and leachate flow in wadies. For example, the closest house to Al Amirat landfill is only 300 m away. These problems can be tackled from the beginning by proper site selection. Therefore, the main objectives of this study are to determine the environmental suitability of three un-engineered landfills in Muscat (Al Amirat, Al Mabella and Bousher) using multi criteria decision analysis, to assess leachate of Al Amiratunengineered landfill and to identify its potential impact on groundwater and soil in vicinity to the landfill site. Leachate, groundwater and soil samples were collected and analyzed using the standard methods. Ninety parameters were used for leachate and groundwater analysis. Although all the studied landfills are vulnerable due to the lack of proper design, the landfill site suitability analysis has showed that Al Amirat un-engineered landfill is the most suitable site for waste disposal practices (Score: 0.375) in comparison to Al Mabella and Bousher landfill sites due factors like the geological setup of the area. Moreover, the assessment of leachate of the same landfill has showed a complex nature. The leachate was polluted with elevated concentrations of chlorides (Average: 6710 mg/l), Na (Average: 5380 mg/l), heavy metals (AI, V, Cr, Mn, Co, Ni, Cd, Ba, Pb and Fe), volatile fatty acids (Maximum: 173.8 mg/l), ammonia (Maximum: 990 mg/l), Poly chlorinated biphenyles (PCBs) (Maximum: 1.4 mg/l), organochlorine pesticides (Maximum: 0.083 mg/l), phthalates (1.615 mg/l) and coliforms (Maximum: 9300 MPN/100ml) which were exceeding the Omani waste water re-use and discharge standards. The presence of heavy metals in leachate along with hazardous organic contaminants is a direct indicator of disposal of hazardous wastes in the landfill which might be of household origin. Furthermore, the leachate pollution index (LPI: 26.33 to 27.67) of Al Amirat landfill leachate indicates contamination and thus proper treatment is required before discharge. The high organic content of waste in Oman (60%) has leadto the production of high organic strength leachate. Al Amirat landfill was found to be highly biodegradable (Biodegradability: 3 – 4.6). Moreover, its leachate composition reflects characteristics of different stages of landfill decomposition and heterogeneity of waste. Boreholes construction, rock porosity analysis (1.38% porosity) and soil textural class analysis (Sandy clay loam) have showed very low permeability and porosity indicating either no groundwater or very limited groundwater resources at Al Amirat landfill site. In addition, the groundwater evaluation in the area has showed that the upstream groundwater wells were contaminated. The concentrations of parameters like total dissolved solids (TDS), electrical conductivity (EC), chlorides, nitrates, sulfates, bicarbonates, Mg, Na, Fe, coliforms were considerably high, exceeding the Omani drinking water standards. Xenobiotic contaminants were detected in concentrations less than 0.1 mg/l in the groundwater samples. Theseresults indicate that leachate may not be the sole source of contamination of groundwater in the area surrounding Al Amirat landfill. Natural and anthropogenic sources of contamination may exist like rock weathering, evaporation, domestic sewage and agricultural practices in the area. Furthermore, soil assessment has indicated that soil samples in vicinity to Al Amirat landfill do not pose any toxicity as it is not considered hazardous, but since most of the metals show mobility within the soil profile, an effect on groundwater might occur on the long run if no action is taken to prevent it. Knowledge of the fate of these pollutants in these multicopartment heterogeneous landfills is poor and cannot be predicted, making it difficult to assess its environmental risks. Therefore, any adverse impacts on the environment should be minimized by sucking out the leachate by means of newly constructed boreholes and transferring it from pools and boreholes to specialized treatment facilities with continuous monitoring of leachate quality and quantity. Thus, the findings of this study highlight the importance of multicriteria decision analysis in landfill site selection to prevent any vulnerability to the environment from landfills. The criteria used in this study should be considered and buffer distances should be established and maintained by decision makers in order to carry out assessment studies on different existing landfill sites and to select the most suitable sites for future landfills in order to reduce any potential impacts on the environment. Rehabilitation of such old un-engineered landfills should take place with continuous monitoring programs of groundwater resources in the area. Further investigations of pollution sources of groundwater should be conducted and criteria for use of such quality of groundwater should be set. Standards including the parameters studied should be established and implemented and regulations should be set by regulatory bodies in the country regarding treatment and final discharge of leachate and closure of landfills. Also, legislations regarding protection of environment and water resources should be implemented in order to prevent pollution and protect the environment.