Reactions controlling Aspo groundwater chemistry

Samples of groundwater are often found to be close to saturation with respect to some minerals. A common approach to modelling of groundwater chemistry is to assume that such minerals control some chemical property of the groundwater. As an example, calcite is often supposed to control the pH value (in addition to concentrations of calcium and carbonate). Here, three models have been used to investigate the control of chemical properties of a groundwater from the Aspo site at the Swedish East Coast. In the first model, a set of minerals has been selected, in which all minerals are close to saturation with respect to the groundwater. Then an attempt has been made to model formation of the groundwater from rain by letting PHREEQE equilibrate the rain with the minerals. The second model is similar, but the mineral set used is similar to the one observed at Aspo, and includes also minerals that are far from saturation. To handle the complex situation with numerous minerals and spatial variability, the CRACKER program was used to simulate formation of groundwater from rain. In the third model, the CRACKER program has been used with the observed groundwater. In the simulation, the water has been disturbed locally, and the relaxation of water chemistry has been studied. Results from all three kinds of models show that no conclusions on regulation of chemical properties in groundwater can be drawn from saturation properties of minerals. Rather, most properties are controlled by complicated interactions, including spatial variability, irreversible reactions and mixing.