『Summary
The study area is situated in a fault zone with fractured granites
and metasediments. In a conceptual model, infiltrating water first
passes the bedrock cover of soil and saprolite and then partly
enters the fractures. Weathering reactions of minerals occur in
small pores and fissures in the bedrock cover zone to continue
in the larger fractures. Pumping tests were carried out in a number
of boreholes to measure the drawdown as a function of pumping
time. From the results, values of transmissivity (T) could be
derived. In combination with the storage coefficient (S) for similar
fault zones, the hydraulic diffusivity (D = T/S) could be computed.
Water samples, collected from the boreholes, represent fluid
packets with a history of weathering reactions in the bedrock
cover and in the larger fractures. The major element composition
of these samples was used by means of the SiB mass balance algorithm
(Pacheco and Van der Weijden, 1996) to calculate the moles L-1
of dissolved plagioclase (oligoclase with An≒0.20) and the moles
L-1 of secondary phases (gibbsite, halloysite, smectite)
precipitated along the flow paths of the samples. These results
were then used to calculate the net dissolved silica concentrations
([H4SiO40])
related to dissolution of plagioclase followed by precipitation
of each of the secondary phases. An interpretation of a plot of
each of these [H4SiO40]'s
versus D is that at D＜0.7 m2 s-1, dissolution
of plagioclase is followed by precipitation of halloysite in the
large fractures of the fault zone (open system), whereas at D≧0.7
m2 s-1 precipitation of both halloysite
and smectite occurs in the rock matrix with small fissures and
pores (semi-open system ). Before being pumped, the percolating
fluids traveled 0.01-13.7 years. During these periods, plagioclase
weathered at rates (WPl) of 10-(12.9±1.1)
moles m-2 s-1, which are approximately 2.2
orders of magnitude higher than solid-state weathering rates reported
in various field studies. In this study, it suggested that part
of the apparent discrepancy between the results is due to changes
in hydraulic diffusivity of the weathering environments occurring
over the geologic times.
Keywords: Gravity flow fracture; Capillary flow microfracture;
Advective transport; Diffusive transport; Open system; Semi-open
system』
1. Introduction
2. Study area
3. Hydrogeologic setting
3.1. Conceptual flow and weathering models
3.2. Boreholes
3.3. Pumping tests and hydraulic parameters
4. Sampling and analysis
5. Weathering reactions and rates
5.1. Mass balances
5.2. Moles of dissolved plagioclase
5.3. Plagioclase weathering rates
6. Groundwater travel times
7. Results and discussion
7.1. Hydraulic tests
7.2. Weathering reactions
7.3. Weathering reactions and hydraulic diffusivity
7.4. Groundwater travel times, plagioclase weathering rates and
hydraulic diffusivity
8. Conclusions
References