Groundwater (deep and shallow) in Islamabad particularly in vicinity of Lei Nala were characterized by environmental stable (?13C) and Radioactive (tritium) isotopes as well as hydrogeochemical parameters such as physiochemical (pH, EC, TDS, DO and temperature), major cation (Ca+2, Mg+2, Na+1, K+1), anion (NO3-1, SO4-2, CO3-2, HCO3-1, Cl-1) and toxic metals (Zn, Cd, Cu, Pb, Ni, Fe, As, Cr and Co). Piper diagram reveals that Ca-Mg- type of water is dominant in the studied area. The percentage of samples falling under Ca-Mg-type was 53.86 %. For anion concentration, HCO3-type of water predominated with 96.15 % samples. Stable isotope techniques in combination of conventional classical techniques have been applied for this investigation Stable carbon isotope techniques have helped to understand the level and source of contamination in groundwater. The more depleted ?13C (TDIC) values around –8 to –11 ‰ PDB are mainly indicative of inventory of domestic sewage pollution. As such, this study indicates the potential of ?13C (TDIC) technique in relation to physiochemical techniques for assessment of the sources of pollution.