The sources and fluxes of dissolved chemistry in a semi-confined, sandy coastal aquifer: The Pingtung Plain, Taiwan Caroline E.A. Martin a,⇑ , Albert Galy a , Niels Hovius b,a , Mike Bickle a , In-Tian Lin c , Ming-Jame Horng d , Damien Calmels e , Hongey Chen c a Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, United Kingdom b Earth Surface Dynamics Section, GeoForschungsZentrum, Potsdam, Germany c Department of Geosciences, National Taiwan University, Taiwan d Water Resources Agency, Ministry of Economic Affairs, Hsin-Yi Road, Taipei, Taiwan e Equipe de Geochimie des Isotopes Stables, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ Paris Diderot, UMR 7154 CNRS, F-75005 Paris, France article info Article history: Received 31 August 2012 Accepted 21 February 2013 Available online 4 March 2013 Editorial handling by I. Cartwright abstract Groundwater chemical fluxes from the Pingtung Plain in SW Taiwan to the ocean were determined by analysing waters from 43 wells at varying depths through a 237 m deep window across the Pingtung Plain, for major dissolved cations, anions, dissolved SiO 2 , and stable isotopic composition of O and H, and computing their subsurface water fluxes from measurements of hydraulic heads and formation per- meabilities. The results show that between 1.5% ðSO 2 4 Þ and 12.3% (Ba 2+ ) of the total chemical weathering flux discharged to the ocean (Kaoping River combined with groundwater) can be attributed to the groundwater. Estimated propagated errors at 1r on subsurface fluxes are ±20%. Multi-year daily hydrau- lic head data give the direction of groundwater flow through the plain, and indicate that pumping has led to episodic reversals of flow, facilitating seawater intrusion in the near-coast aquifer. Tracing end-mem- ber proportions using mixing relationships shows that, in addition to seawater and meteoric water, hot- spring activity contributes to the dissolved chemistry of the groundwater. In addition to these three end- members, the weathering of carbonate and silicate minerals in the plain accounts for the remainder of the chemical budget. Hydrological connectivity exists throughout the drilled depth of the basin, but chemical gradients show that flow is stratified, with up to a twofold increase in silicate-derived Na + seen in deeper horizons as compared to the near surface. For all ions except SO 2 4 , the average concentrations of dis- solved species in the coastal groundwaters exceed those of the river, ranging from a factor of 1.33 in the case of Li + to 27.29 in the case of Cl  . The results suggest that submarine groundwater chemical fluxes through the drilled depth of the Pingtung Plain into the Taiwan Strait are modest in comparison to those related to surface runoff. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Chemical weathering of the Earth’s surface is a primary control on ocean geochemical budgets. Silicate weathering in particular, through consumption of atmospheric CO 2 , is thought to regulate the global C cycle and thus mediate climate over geological time- scales (Walker et al., 1981; Berner et al., 1983; Aumont et al., 2001; Gislason et al., 2009). It is, therefore, important to have good estimates of chemical weathering rates, the first step to which is calculating dissolved fluxes from varied surface and subsurface environments across different climatic zones and spatial scales. River catchments have input primarily in the form of rainwater, and outputs, runoff and groundwater discharge, with their dis- solved chemistries reflecting internal processes causing net chem- ical denudation of the landscape. Much of this chemical interchange is thought to occur in the soil zone (Birkeland, 1999) and most delivery of dissolved chemistry to the oceans is attrib- uted to rivers, which are easily sampled (Garrels and MacKenzie, 1971; Meybeck, 1987; Gaillardet et al., 1999). Catchments, how- ever, are hydrologically complex, with groundwater flow pathways penetrating below the well-studied soil and saprolitic zones. For example, Calmels et al. (2011) estimated that 40% of the silicate weathering flux carried by the Liwu River, Taiwan, is derived from meteoric water that had circulated at depth within surrounding bedrock. In coastal regions groundwater may discharge directly to the oceans. This deeper groundwater, which can be several or- ders of magnitude enriched in certain elements relative to surface waters (Zektzer et al., 1973; Basu et al., 2001), is a largely unknown contributor to global dissolved weathering fluxes. 0883-2927/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2013.02.016 ⇑ Corresponding author. Tel.: +44 (0)1223 768327; fax: +44 (0)1223 333450. E-mail address: ceam4@cam.ac.uk (C.E.A. Martin). Applied Geochemistry 33 (2013) 222–236 Contents lists available at SciVerse ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem