『Abstract
We analysed the water chemistry of a 6-yr time series of the
Liwu River, Taiwan, as well as spot samples of tributaries and
groundwater. In this steep and well-drained catchment impacted
by typhoon event, the river carries a high dissolved load (average
of 320 mg/L) dominated by Ca2+ and Mg2+
(73 Eq% and 21 Eq% of the cationic charge), and SO42-
and HCO3- (48 Eq% and 51 Eq% of
the anionic charge). Hysteresis in Ca/Na-discharge data and absence
of hysteresis in matching Ca/Sr-discharge data are interpreted
as reflecting a mixing of three components. While river chemistry
at low-to medium-flow reveals a binary mixing between a rapid
surface runoff characterised by the chemistry of first-order tributaries
and a component likely to have a deep groundwater chemistry, samples
collected at high-flow show the contribution of a third water
source not immediately mobile and therefore called slow surface
runoff. The relative contribution of these end-members has been
estimated through hydrograph separations, and then validated by
modelling of the observed riverine 87Sr/86Sr
variations (0.0013) during a typhoon. The 37-yr average estimates
that 16±3% of the river discharge comes from the deep groundwater
reservoir while slow and rapid surface runoff provide 21±5% and
63±5%, respectively. The slow surface runoff is the least well
characterised of the end-members but the modelled evolution of
water chemistry along its flow-path suggests that cations (except
Na) and sulphate are removed from the water in the slow surface
runoff source region by the combination of biological uptake,
bacterial reduction and pedogenesis (clays and carbonate precipitation)
- processes that might be partly driven by the evapo-transpiration
that removes 75% of the water within this reservoir. The silicate
cation denudation rate for the Liwu is 18 t km-2 yr-1,
which is one of highest measured so far in the world for felsic
lithologies. This high value has to be modulated by the facts
that 1) deep chemical weathering of silicate accounts for more
than 1/3 of this flux, and 2) sulphuric-acid weathering is significant
in this catchment. In Taiwan, the input of chemicals in river
flux from deep reservoirs is likely to be promoted by rapid uplift
of fractured rocks, but the underlying processes (chemical reaction
in a porous media) responsible for such significant groundwater
contribution are also present in alluvial and sedimentary basins.
Keywords: chemical weathering; hydrology; Taiwan; groundwater;
time series; river water; weathering rate』
1. Introduction
2. Materials and methods
2.1. Setting
2.2. Sample collection
2.3. Analytical techniques
3. Results
3.1. Riverine chemistry and cyclic inputs
3.2. Seasonal to multi-annual variations in river chemistry:
the long-term record
3.3. Hourly to daily variations in river chemistry: the typhoon
events
3.4. Tributary and groundwater chemistry
4. Discussion
4.1. Causes of temporal variations in river chemistry
4.2. Hydrograph separations
4.3. Sr-isotropic validation of the hydrograph separation
4.4. Location of weathering reactions and associated weathering
fluxes
4.5. Silicate weathering rate
5. Conclusion
Acknowledgements
Appendix A. Supplementary data
References