Calcium carbonate scaling under alkaline conditions – Case studies and hydrochemical modelling

Calcium carbonate scaling poses highly challenging tasks for its prediction and preventative action. Here an elemental, isotopic and modelling approach was used to decipher the evolution of alkaline tunnel drainage solutions and sinter formation mechanisms for 3 sites in Austria. Drainage solutions originate from local groundwater and form their characteristic chemical composition by interaction with shotcrete/concrete. This interaction is indicated by a positive correlation of dissolved K+ and pH (up to 12.3), and a decrease of aqueous Mg2+ by the formation of brucite (pH > 10.5). Variability in Ca2+ and DIC is strongly attributed to portlandite dissolution, calcite precipitation and CO2 exchange with the atmosphere, where the 13C/12C and 18O/16O signatures of calcite can be traced back to the source of carbonate. The internal PCO2 value is a reliable proxy to evaluate whether uptake of CO2 results in an increase or decrease of the degree of calcite saturation with a threshold value of 10−6.15 atm at 25 °C (pH ≈ 11). Precipitation rates of calcite are highest at pH ≈ 10. Mixing of groundwater-like solutions with strong alkaline drainage solutions has to be considered as a crucial factor for evaluating apparent composition of drainage solutions and calcite precipitation capacities.