Contribution of deep groundwater to the weathering budget in a rapidly eroding mountain belt, Taiwan

Calmels, Damien and Galy, Albert and Hovius, Niels and Bickle, Mike J. and West, A. Joshua and Chen, Meng-Chiang and Chapman, Hazel (2011) Contribution of deep groundwater to the weathering budget in a rapidly eroding mountain belt, Taiwan. Earth and Planetary Science Letters, 303 (1-2). pp. 48-58. ISSN 0012-821X DOI

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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 events, 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.

Item Type: Article
Uncontrolled Keywords: 2010AREP; IA62;
Subjects: 02 - Geodynamics, Geophysics and Tectonics
Divisions: 02 - Geodynamics, Geophysics and Tectonics
Journal or Publication Title: Earth and Planetary Science Letters
Volume: 303
Page Range: pp. 48-58
Identification Number:
Depositing User: Sarah Humbert
Date Deposited: 25 Mar 2011 16:14
Last Modified: 23 Jul 2013 10:02

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