『Abstract
A numerical model of chemical weathering in soil horizons and
underlying bedrock (WITCH) has been coupled to a numerical model
of water and carbon cycles in forest ecosystems (ASPECTS) to simulate
the concentration of major species within the soil horizons and
the stream of the Strengbach granitic watershed, located in the
Vosges Mountains (France). For the first time, simulations of
solute concentrations in soil layers and in the catchment river
have been performed on a seasonal basis. The model is able to
reproduce the concentrations of most major species within the
soil horizons, as well as catching the first-order seasonal fluctuations
of aqueous calcium, magnesium and silica concentrations. However,
the WITCH model underestimates concentrations of Mg2+
and silica at the spring of the catchment stream, and significantly
underestimates Ca2+ concentration. The deficit in calculated
calcium can be compensated for by dissolution of trace apatite
disseminated in the bedrock. However, the resulting increased
Ca2+ release yields important smectite precipitation
in the deepest model layer (in contact with the bedrock) and subsequent
removal of large amount of silica and magnesium from solution.
In contrast, the model accurately accounts for the concentrations
of major species (Ca, Mg and silica) measured in the catchment
stream when precipitation of clay minerals is not allowed. The
model underestimation of Mg2+ and H4SiO4 concentrations when precipitation of well crystallized
smectites is allowed strongly suggests that precipitation of well
crystallized clay minerals is overestimated and that more soluble
poorly crystallized and amorphous materials may be forming. In
agreement with observations on other watersheds draining granitic
rocks, this study indicates that highly soluble trace calcic phases
control the aqueous calcium budget in the Strengbach watershed.
1. Introduction
2. Model description
2.1. The WITCH model
2.2. Coupling with a model of water and carbon cycles in forest
ecosystems
2.3. Sites, run design, forcing functions and parameters
3. Results of the reference simulation
3.1. Reference run, PP location
3.2. Reference run, HP location
3.3. Calculating the chemical composition of the main stream
at the spring collector
4. AP and APL10 sensitivity tests: Looking for calcium at the
spring collector
5. NOPREC and KSP simulation: Looking for silica at the spring
collector
6. Trace mineral and base cation budget of a granitic catchment
7. Some uncertainties and limitations
8. Conclusions
Acknowledgments
References
Main stream BC flux (weathering contribution only) (meq/m2/year) |
Percentage of the Ca flux originating from trace mineral dissolution relative to the total BC flux | |
Hubbard Br. | 101 (Drever and Clow, 1995) | 23.5% (trace apatite) (Blum et al., 2002) |
Estibere(最初のeの頭に`) | 167 (Oliva et al., 2004) | 80% (trace silicates) (Oliva et al., 2004) |
Loch Vale | 42 (Mast et al., 1990) | 41% (trace calcite) (Clow et al., 1997; White et al., 1999) |
Strengbach | Measured: 281 | 15% (trace apatite) (Aubert et al., 2001) |
KSP simulation (spring collector): 285 | 26% (HP) to 69% (PP) (trace apatite) (This study) |