『Abstract
Water-rock reactions are driven by the influx of water, which
are out of equilibrium with the mineral assemblage in the rock.
Here a mass balance approach is adopted to quantify these reactions.
Based on field experiments carried out in a granito-gneissic small
experimental watershed (SEW), Mule Hole SEW (〜4.5 km2),
quartz, oligoclase, sericite, epidote and chlorite are identified
as the basic primary minerals while kaolinite, goethite and smectite
are identified as the secondary minerals. Observed groundwater
chemistry is used to determine the weathering rates, in terms
of ‘Mass Transfer Coefficients’ (MTCs), of both primary and secondary
minerals.
Weathering rates for primary and secondary minerals are quantified
in two steps. In the first step, top red soil is analyzed considering
precipitation chemistry as initial phase and water chemistry of
seepage flow as final phase. In the second step, minerals present
in the saprolite layer are analyzed considering groundwater chemistry
as the output phase. Weathering rates thus obtained are converted
into weathering fluxes (Qweathering) using
the recharge quantity.
Spatial variability in the mineralogy observed among the thirteen
wells of Mule Hole SEW is observed to be reflected in the MTC
results and thus in the weathering fluxes. Weathering rates of
the minerals in this silicate system varied from few 10μmol/L
(in case of biotite) to 1000 s of micromoles per liter (calcite).
Similarly, fluxes of biotite are observed to be least (7±5 mol/ha/yr)
while those of calcite are highest (1265±791 mol/ha/yr). Further,
the fluxes determined annually for all the minerals are observed
to be within the bandwidth of the standard deviation of these
fluxes. Variations in these annual fluxes are indicating the variations
in the precipitation. Here, the standard deviation indicated the
temporal variations in the fluxes, which might be due to the variations
in the annual rainfall. Thus, the methodology adopted defines
an inverse way of determining weathering fluxes, which mainly
contribute to the groundwater concentration.
Keywords: Mass transfer coefficient (MTC); Groundwater chemistry;
Mineralogy; Silicate chemical weathering; Regolith; Weathering
fluxes』
1. Introduction
2. Field settings
2.1. Protolith
2.2.Regolith
2.3. Hydrology and hydrogeology
3. Methodology
3.1. Water sampling and analyses
3.2. Observation well network
3.3. Theoretical mass balance approach
4. Conceptual inverse models for determining weathering rates
and fluxes
4.1. Models for soil layer (Step 1)
4.2. Model for saprolite layer (Step 2)
5. Results
6. Discussion - application of inverse models
6.1. Weathering rates (MTC) of minerals in the topsoil zone
and saprolite
6.2. Weathering fluxes (Qweathering) in
the soil and saprolite
6.2.1. In the soil
6.2.2. In the saprolite
6.3. Variability in the weathering fluxes
6.3.1. Spatial variability
6.3.2. Temporal variability
7. Conclusions
Acknowledgement
References