More than 7 million people of Lahore city, Pakistan, use groundwater for drinking and other household purposes. The quality of drinking water from source and distribution system was investigated, and the hydrochemical characteristics and formation mechanisms of groundwater were analyzed. Statistical summary showed higher mean values of major descriptors in distributed groundwater samples compared with source groundwater samples. A total of 50 drinking water samples (16 from source, and 34 from distribution system) were examined for physical, chemical and bacteriological parameters, including pH, turbidity, electrical conductivity, total dissolved solids, total hardness, total alkalinity, Ca2+, Mg2+, Na+, K+, SO4 2−, Cl−, NO3 −, F−, arsenic (As) chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), cadmium (Cd) and lead (Pb). Varying concentrations of trace amounts of Cr (in 8 samples), Fe (18 samples), Cu (18 samples), Zn (13 samples) and Pb (14 samples) were detected. However, the detected trace elements were within the WHO permissible limits for drinking water except Pb where 5 out of 14 samples showed higher than WHO limit value (Pb > 0.01 mg/L). 19 out of 50 samples (6 from source and 13 from distributed samples) showed high values of alkalinity (>250 mg/L). All of 16 groundwater samples from source exceeded 0.01 mg/L WHO limit value for arsenic (As). On the basis of bacteriological analysis, 42 % samples (12 % from source, 55 % from distribution) did not meet WHO guidelines and were unsafe for drinking, especially distributed groundwater samples. Mg–HCO3 contents predominated the groundwater type followed by Ca–HCO3. Carbonate weathering was revealed to be the dominant process controlling dissolution/precipitation processes. PHREEQC modeling showed that the aqueous phase was undersaturated with respect to significant gypsum, halite and mirabilite while equilibrium to saturated with respect to aragonite, calcite and dolomite was observed. Principal component analysis (PCA) revealed that the concentration of As was strongly associated with that of SO4 2−. Reductive dissolution and pH-dependent desorption are the plausible processes responsible for the observed high As concentrations in Lahore.