Abstract
Continental scale aquifers can store significant amounts of carbon as a result of immense water volumes, substantial concentrations of dissolved inorganic carbon (DIC) and its reactions with a matrix, thus contributing the global carbon storage and cycle. However, concentration of dissolved solutes may vary significantly over distances, which causes interpretative challenges and difficulties in process quantification. This occurs in the Guarani Aquifer System in South America, which is a subject of extensive research due to a significant strategic role in water supply. Dissolved CO2is expected to dissociate and undergo reactions with aluminosilicate minerals, but it is unknown how much DIC may get immobilised in the aquifer. To quantify the processes, we performed reactive transport modelling which combines hydrological and geochemical information followed by global sensitivity analysis. We show that more than a half of the infiltrated CO2may be consistently precipitated as CaCO3. The DIC concentrations across the aquifer depend primarily on the input carbon concentrations and the plagioclase hydrolysis rate, while other parameters including hydraulic conductivity, recharge rate and mineral stability are of the minor importance. We present how advanced modelling techniques may be used to interpret and quantify processes controlling water quality in continental scale groundwater systems.
Highlights
Groundwater systems are important inorganic carbon reservoirs, and they are gaining attention as to which extent they contribute to the global carbon cycle
Reactive transport models (RTMs) are useful in addressing these uncertainties because they (1) combine complexity of water flow with a set of biogeochemical reactions (Maher and NavarreSitchler, 2019), (2) enable to test a variety of conceptual scenarios when they are run in a predictive mode (Jessen et al, 2017; Jakobsen et al, 2018), (3) may be evaluated using global sensitivity analysis (Dai et al, 2014)
Plagioclase hydrolysis drives the stoichiometric increases in Ca2+ and Na+ and the intensity of the process depends on the chemical rate (R in Equation 1)
Summary
Groundwater systems are important inorganic carbon reservoirs, and they are gaining attention as to which extent they contribute to the global carbon cycle. The reaction pathways are complex and heterogeneous and may depend on a variety of factors including: (1) groundwater flow velocity (Maher and Chamberlain, 2014), (2) thermodynamic equilibria and dissolved ion availability (Zhang and Planavsky, 2020) or (3) rates of dissolution and precipitation (Pogge von Strandmann et al, 2019). These pathways can be explored with RTMs that rely on both hydrological and biogeochemical principles (Steefel et al, 2005). We performed global sensitivity analysis to identify the importance of individual model parameters
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