Improved parameterization of the weathering kinetics module in the PROFILE and ForSAFE models

Abstract

<p><strong>Abstract.</strong> Although the PROFILE and ForSAFE model can accurately reproduce the chemical and mineralogical evolution of the soil unsaturated zone, it overestimates weathering rates in deeper soil layers and in groundwater systems. This overestimation has been corrected by improving the kinetic expression describing mineral dissolution by adding or upgrading <q>braking functions</q>. The base cation and aluminium brakes have been strengthened, and an additional silicate brake has been developed, improving the ability to describe mineral- water reactions in deeper soils. These brakes are developed from a molecular-level model of the dissolution mechanisms. Equations, parameters and constants describing mineral dissolution kinetics have now been obtained for 113 minerals from 12 major structural groups, comprising all types of minerals encountered in most soils. The PROFILE and ForSAFE weathering sub-model was extended to cover two-dimensional catchments, both in the vertical and the horizontal direction, including the hydrology. Comparisons between this improved model and field observations are available in Erlandsson Lampa et al. (2019, This special issue). The results showed that the incorporation of a braking effect of silica concentrations was necessary and helps obtain more accurate descriptions of soil evolution rates at greater depths and within the saturated zone.</p>

Catchment export of base cations: improved mineral dissolution kinetics influence the role of water transit time

“…For a more complete description of base cation release from catchments, especially on a longer timescale, a kinetic description of the gradual transformation from primary to secondary minerals would be useful. While the chemical se-quence for such mineral alterations, for example feldspars to illite or biotite to vermiculite, are known (Sverdrup et al, 2020), the kinetics are not, and this would probably require a calibration approach as well as data from catchments with substantially older water fractions than those studied here.…”

“…BC and Al brakes were applied to the terms for which they were not defined in the original PROFILE equations, i.e., the CO 2 term, the R − term and the OH − term. The parameters of the new terms can be found in Sverdrup et al (2020). The new PROFILE equations require aqueous Si to be modeled in order to make the Si brakes work properly.…”

“…While this is less important in the unsaturated zone where aqueous silica concentrations are low (typically <0.3 mM), it has been suggested that the overprediction of weathering in the saturated zone is caused by neglecting the silica effect. In Sverdrup et al (2020), a revised set of PRO-FILE equations is presented which contain a number of expanded functions, including retardation due to silica concen-trations. In the present study, we use both the original and the revised PROFILE equations to evaluate how the mineral dissolution rates interact with the WTT.…”