Abstarct: In this study, the impacts of urbanization on the quantity and quality of groundwater resources in Mashahd Mega-city is evaluated. Available data which have been collated from a variety of water organizations include quality and chemical composition data and the water level. For more accurate examination of water quality, 46 water samples from various depths of about 100 m were collected and were subjected to chemicals and biological analysis. Results show that increased water consumption (with the visits of million pilgrims in a year) has led to water level rise and contamination of aquifer. Elevated water level induces contamination as well as a decrease in ground strength, demolition of qanats and liquefaction in the case of earthquake. Contamination is higher in southern and western part of the aquifer. In north eastern part, with clay texture soils, observed contamination is a result of dense population. The old graveyard in east and centre of the city is also associated with groundwater contamination. Concentration of nitrate, sulphate, chloride and phosphate is high and especially nitrate concentration reaches 202 mg/l. High concentration of ammonium is a reason to show modernity and continuity of contamination. Combined assessments of both contaminated samples and city development map show that contamination started since 1986; peak of urban development is during 1980’s. The current contamination may treat drinking water bores. Iso concentration maps show that contamination is in southern, western and northeastern and old parts of the city. Aquifers in the older parts of the city, although sited on the fine texture soils, have been contaminated because of the role of qanats in facilitating contaminant transports. Diagrams prepared using Aquachem show various water types in the city aquifer which may be a reflection of contamination. Construction of sewer network , prevention of pipe leakage, restriction on residential developments and surface water collection and injection into aquifers all can help improving the quality of groundwater.