Abstract. Accurate estimates of base cation weathering rates in forest
soils are crucial for policy decisions on sustainable biomass harvest levels
and for calculations of critical loads of acidity. The PROFILE model is one
of the most frequently used methods to quantify weathering rates, where the
quantitative mineralogical input has often been calculated by the A2M
(“Analysis to Mineralogy”) program based solely on geochemical data. The
aim of this study was to investigate how uncertainties in quantitative
mineralogy, originating from modeled mineral abundance and assumed
stoichiometry, influence PROFILE weathering estimate, by using measured
quantitative mineralogy by X-ray powder diffraction (XRPD) as a reference.
Weathering rates were determined for two sites, one in northern (Flakaliden)
and one in southern (Asa) Sweden. At each site, 3–4 soil profiles were
analyzed at 10 cm depth intervals. Normative quantitative mineralogy was
calculated from geochemical data and qualitative mineral data with the A2M
program using two sets of qualitative mineralogical data inputs to A2M: (1) a
site-specific mineralogy based on information about mineral identification
and mineral chemical composition as determined directly by XRPD and electron
microprobe analysis (EMPA), and (2) regional mineralogy, representing the
assumed minerals present and assumed mineral chemical compositions for large
geographical areas in Sweden, as per previous published studies. Arithmetic
means of the weathering rates determined from A2M inputs (WA2M)
were generally in relatively close agreement with those (WXRPD)
determined by inputs based on direct XRPD and EMPA measurements. The
hypothesis that using site-specific instead of regional mineralogy will
improve the confidence in mineral data input to PROFILE was supported for
Flakaliden. However, at Asa, site-specific mineralogies reduced the
discrepancy for Na between WA2M and WXRPD but produced
larger and significant discrepancies for K, Ca and Mg. For Ca and Mg the
differences between weathering rates based on different mineralogies could be
explained by differences in the content of some specific Ca- and Mg-bearing
minerals, in particular amphibole, apatite, pyroxene and illite. Improving
the accuracy in the determination of these minerals would reduce weathering
uncertainties. High uncertainties in mineralogy, due for example to different
A2M assumptions, had surprisingly little effect on the predicted weathering
of Na- and K-bearing minerals. This can be explained by the fact that the
weathering rate constants for the minerals involved, e.g. K feldspar and
micas, are similar in PROFILE. Improving the description of the dissolution
rate kinetics of the plagioclase mineral group as well as major K-bearing
minerals (K feldspars and micas) should be a priority to help improve future
weathering estimates with the PROFILE model.
In Situ Quantification of Degradation Is Needed for Reliable Risk Assessments and Site-Specific Monitored Natural Attenuation
