The Ni solubility and Ni-colloid association were studied in natural groundwater, cement-conditioned groundwater, and synthetic saline groundwater. The experiments were performed under N2 at ∼28°C. The duration of the experiments was up to 177 days. The Ni solubility was studied by adding 63Ni-spiked NiCl2 to the samples and by measuring the activities using liquid scintillation counting. The initial Ni concentrations were 1 × 10-6 and 1 × 10-3 M. The nickel solubilities in the water samples differed significantly. The association of Ni with colloids in the natural groundwater was detected.