『Summary
Weathering rate models designed for watersheds combine chemical
data of discharging waters with morphologic and hydrologic parameters
of the catchments. At the spring watershed scale, evaluation of
morphologic parameters is subjective due to difficulties in conceiving
the catchment geometry. Besides, when springs emerge from crystalline
massifs, rock structure must be accounted in formulas describing
the area of minerals exposed to the percolating fluids, for a
realistic evaluation of the rates. These particular features are
not included in the available approaches and for that reasons
a new model was developed, coined THROW model. This is a lumped
approach that integrates (T)opography, (H)ydrology, (RO)ck structure
and (W)eathering in a single algorithm. The study area comprises
several stream watersheds and spring sites of the Vouga River
basin (northern Portugal), shaped on granites. Firstly, the THROW
model couples a terrain modeling analysis with hydrologic models
based on discharge rates, to determine hydraulic conductivities
(K), effective porosities (ne) and annual
recharges (Vr) at the stream watershed scale.
Subsequently, these parameters are used in a water balance model
to estimate concomitant groundwater travel times (t). The mean
K [(4.7±3.2)×10-7 m s-1] and ne[(2.0±1.3)×10-2]
values are adopted as proxies for the spring watersheds and a
firm regression equation is defined between time and stream watershed
area (A). Secondary, two more runs of terrain modeling analysis
are executed to extrapolate morphologic parameters for the spring
watersheds. The first run hinges on scaling properties of the
drainage networks, known as Horton laws, and is used to scale
watershed areas across stream orders (i). The scaling function
is described by another regression equation. The second run evaluates
the order of a spring watershed, defined as equivalent order (ieq) and equated to the mean order of the surrounding
stream watersheds. Having calculated the ieq,
spring watershed areas and travel times were downscaled using
the regression equations (A＜10 km2 and t = 1.4-2.8
year). Standing on the physical and hydrologic parameters of the
spring watersheds, the THROW model finally calculates plagioclase
weathering rates in the vicinity of the spring sites. The SiB
model (Pacheco and Van der Weijden, 1996) was used before to estimate
the contribution of plagioclase dissolution to the chemical composition
of these springs (Van der Weijden and Pacheco, 2006). The chemical
data were now coupled with K, ne and t in
a rate equation to estimate chemical weathering rates of plagioclase
in the basin. In the THROW model, the rate equation describes
the exposed surface area as a function of fracture spacings, openings
and porosities (Pacheco and Alencoao（後のaの頭に〜）,
2006). The calculated rates (WPl = (2.5±1.2)×10-14
mol m-2 s-1) are consistent with previous
reports and with results of experimental kinetic models. The SiB
results predict formation of halloysite and gibbsite along the
flow path, which were indeed close to equilibrium with the dissolved
Al and Si activities.
Keywords: Spring watersheds; Hydrology; Travel time; Surface area;
Weathering rates; Modeling』
1. Introduction
2. Study area
3. Spring water sampling and analytical techniques
4. The throw model
4.1. The topographic module
4.1.1. The Horton laws of drainage network composition
4.1.2. Carrying out the topographic module on a GIS platform
4.2. The hydrologic module
4.2.1. Spring outflows and aquifer hydraulic parameters
4.2.2. Spring base flows and aquifer recharge
4.2.3. Groundwater travel times
4.3. Weathering module
4.3.1. Geochemical mass balance calculations
4.3.2. Rate equation
5. Results
5.1. Results of the topographic module
5.2. Results of the hydrologic module
5.2.1. Aquifer hydraulic parameters and travel times of stream
watersheds
5.2.2. Aquifer hydraulic parameters and travel times of spring
watersheds
5.3. Results of the weathering module
6. Discussion
6.1. Thermodynamic validation of the SiB results
6.2. Residual undersaturation of spring waters with respect to
plagioclase
6.3. Rates of plagioclase weathering
6.3.1. Comparison with reported weathering rates
6.3.2. Influence of the exposed surface area
7. Conclusions
References