In this study, a reservoir in Shenzhen was taken as the object. In September, 2021, the water in the reservoir area, the surrounding surface water (pits, ditches, catchments, etc.) and the shallow groundwater in the muck landfill area where the groundwater flows upstream were sampled and analyzed. Combined with the hydrochemical characteristics analysis of surface-groundwater, isotope radon (222Rn) tracer, Pearson correlation analysis and principal component analysis, the pollution status and causes of Tl, Fe, Mn and Iodide in the reservoir were deeply analyzed.The results show that Iodide, Mn and Tl are the main pollution indexes of the reservoir. The hydrochemical composition of reservoir water, surrounding surface water and groundwater in residue landfill area is similar and has certain connectivity. Both surface runoff and groundwater recharge can carry pollutants into the reservoir. Iodide comes from the same source as Mn, mainly from the recharge of groundwater to the reservoir; Tl mainly comes from the leakage of 1# pumping station and water collection tank.