Nickel can cause allergic reactions so it is often considered to be a toxic metal. In some areas of Denmark and northern Europe, where drinking water comes predominantly from chalk aquifers, Ni concentrations in groundwater are higher than safety limits. The contamination is natural in the sense that it comes from oxidation of small grains of pyrite in the chalk that contain Ni as a trace element (Larsen and Postma, 1997). Because of Danish water-treatment practice, the most politically acceptable water-supply protocol is not to use secondary water treatment, so in order to minimize allergy risks for consumers, high-Ni water is either mixed with low-Ni water to bring concentrations below the Danish limit, or new, uncontaminated wells are drilled. Some recent efforts focused on finding a method to minimize nickel concentration while the water is still in the ground, before it is pumped to the primary treatment facility and distributed.

Unfortunately, only a few experimental studies of the interactions between Ni and calcite have been published (Lakshtanov and Stipp, 2007; Zachara et al., 1991; Hoffmann and Stipp, 2001) and we found no previous work on Ni uptake by chalk. Experiments with calcite show that Ni forms surface complexes and it is incorporated into the solid as solid solution. Chalk aquifers often contain as much as 99% calcite, so the same processes ought to control Ni uptake, but some very preliminary experiments suggested that chalk behaviour might not be what one would expect by analogy with calcite (Stipp et al., 2005). The aim of this study was to examine the Ni-uptake capacity of chalk and compare it with that of synthetic, pure calcite.