Geochemical processes, evidence and thermodynamic behavior of dissolved and precipitated carbonate minerals in a modern seawater/freshwater mixing zone of a small tropical island Chin Yik Lin a,⇑ , Baba Musta a , Mohd Harun Abdullah b a School of Science and Technology, Universiti Malaysia Sabah, Jalan UMS, 88400 Kota Kinabalu, Sabah, Malaysia b Water Research Unit, School of Science and Technology, Universiti Malaysia Sabah, Jalan UMS, 88400 Kota Kinabalu, Sabah, Malaysia article info Article history: Received 24 July 2012 Accepted 16 October 2012 Available online 10 November 2012 Editorial handling by D. Gooddy abstract The geochemical processes and thermodynamic behavior of dissolved and precipitated carbonate miner- als controlling the hydrochemistry of an aquifer in the seawater/freshwater mixing zone of a small island are identified. Field and laboratory analyses, geochemical modeling (PHREEQC) and multivariate statisti- cal analysis (MSA) provide a quantitative interpretation for the geochemistry of the carbonate-dominated aquifer. Geochemical analyses and modeling results show that dissolution and re-precipitation of CaCO 3 are the prevalent processes governing geochemical reactions in the mixing zone. Furthermore, this was confirmed by coherent statistical output that incorporates Principle Component Analysis (PCA) and k- means Cluster Analysis (k-CA). Generally, the composition of the lowland sandy soil was rather homoge- neous and was primarily composed of quartz, aragonite, calcite and Mg-calcite. Thermodynamic model calculations indicate that the carbonate minerals calcite, aragonite and dolomite are supersaturated in the mixing zone. Nevertheless, Powder X-ray Diffraction (PXRD) and Scanning Electron Microscope (SEM) examination verified the occurrence of low-Mg-calcite (LMC) and the absence of dolomite, attrib- uted to thermodynamic/kinetic hindrance, cation disorder and the presence of dolomite crystal growth rate inhibitors (such as SO 4 ). The results suggest that dissolution of aragonite and precipitation of LMC drives the solid phase geochemistry in the small tropical island aquifer. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Many shallow water carbonates, such as carbonate soils, coral fragments, remnant shell fragments, echinoids, mollusks, forami- nifera and algae are very common and can be ubiquitous in most small tropical islands (Kim et al., 1999; McGregor and Gagan, 2003). Calcite, aragonite and Mg-calcite are thus by far the most abundant carbonate minerals present in this shallow marine envi- ronment (Morse and Mackenzie, 1990; Morse and Arvidson, 2002; Nehrke et al., 2007). In the seawater/freshwater mixing zone, car- bonate materials are periodically bathed with either seawater or freshwater. As a result, chemical reactions (dissolution, re-precip- itation, re-suspension and sorption) in such an environment are strongly promoted. Owing to their geochemical reactivity, as a function of ionic strength, it is suggested that carbonate minerals can potentially release trace elements from the metastable carbon- ate lattice that will percolate into shallow groundwater (Charette et al., 2005). Furthermore, these trace elements are susceptible to accumulation in response to the relatively sluggish movement and circulation of groundwater, which may eventually lead to haz- ardous concentrations which could have adverse health effects on humans. The situation is particularly evident in most small tropical islands, where drinking water resources are extremely scarce, a fact exacerbated by large numbers of visitors. In view of this, the present study was undertaken to investigate the occurrence of carbonate minerals that are commonly distrib- uted over a small island, in order to understand their thermody- namic behavior and infer the geochemical processes that largely control the dissolution/precipitation of elements in the shallow aquifer. An understanding of the factors that control the hydro- chemical behavior and thermodynamic state of carbonate minerals is essential to advance the modeling of biogeochemical cycles. It is also important to investigate the impact of dissolution/precipita- tion on small island groundwater chemistry and the diagenetic patterns in the shallow phreatic environment. Further, the present study also contributed some insights into how a small tropical is- land ‘‘evolves’’ in terms of its sediment structure, degree of cemen- tation and microfabric alteration. To date, there has been limited holistic research, which integrates multiple interpretation techniques, on this topic. Past 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2012.10.029 ⇑ Corresponding author. Address: 14, Lorong Nipah Lima, Taman Lip Sin, 11900 Bayan Lepas, Pulau Pinang, Malaysia. Tel.: +60 4 6430769, +60 17 4839158. E-mail addresses: chinyik85@gmail.com, cy_lin_ars@hotmail.com (C.Y. Lin). Applied Geochemistry 29 (2013) 13–31 Contents lists available at SciVerse ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem