Regional-scale reactive transport modelling of hydrogeochemical evolution in a karstic carbonate aquifer

A regional-scale reactive transport model is used to conduct a quantitative assessment of the chemical and isotopic processes that form a conceptual model of geochemical evolution. The primary geochemical reactions described in the conceptual model are incongruent dolomite and gypsum dissolution followed by a series of redox reactions and sulphur isotope fractionation with closed-to-atmosphere groundwater evolution. The investigated aquifer comprises karstic carbonate bedrock with a hydraulic conductivity (K) range spanning several orders of magnitude. Hydrochemical evolution was simulated with a fully saturated one-dimensional model using the multicomponent reactive transport code MIN3P. Five steady -state model scenarios representing the known range of K and porosity simulate geochemical and isotopic evolution along a hypothetical 50-km flowpath. Simulation results are compared with sparse field observations along the flowpath. Although field observations show similar trend directions for all parameters, the magnitude of these trends varies due to differences in residence times. The model results bracket the field observations well for all parameters, except for Mg, and thus these results confirm that variability in field trends can be attributed to physical heterogeneity. The good agreement between models and field observations demonstrates that the geochemical and isotopic processes forming the conceptual model can be quantitatively reproduced. This supports water management activities by establishing hydrochemical end members that may be used to constrain recharge area mapping, assess flow zone continuity and identify areas of older evolved waters. These results also support the use of reactive transport models for quantifying chemical processes in regional-scale groundwater flow systems.