Da. Stonestrom et al., DETERMINING RATES OF CHEMICAL-WEATHERING IN SOILS - SOLUTE TRANSPORT VERSUS PROFILE EVOLUTION, Journal of hydrology, 209(1-4), 1998, pp. 331-345
SiO2 fluxes associated with contemporary solute transport in three dee
ply weathered granitoid profiles are compared to bulk SiO2 losses that
have occurred during regolith development. Climates at the three prof
iles range from Mediterranean to humid to tropical. Due to shallow imp
eding alluvial layers at two of the profiles, and seasonally uniform r
ainfall at the third, temporal variations in hydraulic and chemical st
ate variables are largely attenuated below depths of 1-2 m. This allow
s current SiO2 fluxes below the zone of seasonal variations to be esti
mated from pore-water concentrations and average hydraulic flux densit
ies. Mean-annual SiO2 concentrations were 0.1-1.5 mM. Hydraulic conduc
tivities for the investigated range of soil-moisture saturations range
d from <10(-9) to >10(-6) m s(-1). Estimated hydraulic flux densities
for quasi-steady portions of the profiles varied from 6 x 10(-9) to 14
x 10(-9) m s(-1) based on Darcy's law and field measurements of moist
ure saturations and pressure heads. Corresponding fluid-residence time
s in the profiles ranged from 10 to 44 years. Total SiO2 losses, based
on chemical and volumetric changes in the respective profiles, ranged
from 19 to 110 kmoles SiO2 m(-2) of land surface as a result of 0.2-0
.4 Ma of chemical weathering. Extrapolation of contemporary solute flu
xes to comparable time periods reproduced these SiO2 losses to about a
n order of magnitude. Despite the large range and non-linearity of mea
sured hydraulic conductivities, solute transport rates in weathering r
egoliths can be estimated from characterization of hydrologic conditio
ns at sufficiently large depths. The agreement suggests that current w
eathering rates are representative of long-term average weathering rat
es in the regoliths. Published by Elsevier Science B.V.