A combined modelling and geochemical study of the fate of terrigenous inputs from mixed natural and mining sources in a coral reef lagoon (New Caledonia) Jean-Michel Fernandez a,b, * , Sylvain Ouillon a , Christophe Chevillon a , Pascal Douillet a , Renaud Fichez a , Romain Le Gendre a a UR Camelia, IRD, BP A5, 98848 Noumea Cedex, New Caledonia b LMTE/SMTM CEA-Cadarache BP1, 13108 Saint Paul Lez Durance, France Abstract Open-cast mining for Ni, Cr and Co was conducted in the south-west part of New Caledonia during the 20th century. Abandoned mining and prospecting sites were severely affected by erosion, resulting in an increase in the load of terrigenous particles transported to the coral reef lagoon. This article assesses the impact of a typical small catchment area (La Coule ´e River, 85 km 2 watershed) on two bays (Boulari and Sainte Marie) located near Noumea, New CaledoniaÕs main city. This multi-disciplinary study combines geochemical, sed- imentological, and hydrodynamic approaches. Ni and Cr concentrations contained in the geochemical matrix phase of the pelitic fraction were determined. The study of the geochemical signatures together with sedimentological data and 3D numerical simulations of the transport of non-settling particles throughout the lagoon demonstrated that terrigenous inputs from the Coule ´e River were mainly trans- ported and deposited along the shoreline, reaching areas as distant as Sainte Marie Bay. Although quantitatively low (about 3% of the pelite mass of the bay sediments), the terrigenous inputs in Sainte Marie Bay originating from the Coule ´e River could be traced. The metal content in suspended matter was over 7000 mg kg 1 for Ni and 4200 mg kg 1 for Cr. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Open-cast mining; Ni; Cr; Geochemistry; Sedimentology; Hydrodynamics; New Caledonia 1. Introduction Despite current environmental awareness, biologically degraded conditions in coral reef lagoons bordering land masses and subject to significant terrigenous and anthropo- genic inputs are still poorly documented (Mc Laughlin et al., 2003; Muslim and Jones, 2003). Questions about input and transport of land derived material and associated metals in lagoon waters have rarely been addressed specif- ically, even in target study sites such as the coral reef lagoons of Florida or Australia (e.g. Liston et al., 1992; Caccia et al., 2003; Muslim and Jones, 2003) and most of the existing data have been relegated to grey literature reports. New Caledonia currently ranks as the third nickel pro- ducer in the world. A certain number of saprolitic layers and enriched laterites have been exploited by open-cast mining techniques for more than a century. Open-cast min- ing requires removal of vegetation and of the overlaying ferricrete and laterite layers (low in nickel content) before the exploitable saprolitic layer can be reached. This method produces huge volumes of overburden (2.5 times the vol- ume of extracted ore), generally left on the spot and at best confined by filtering dams of uncertain efficiency. It is esti- mated that a total of 300 Mm 3 of soil has been displaced by direct extraction since the beginning of mining activities (Labrosse et al., 2000). Prospecting and open-cast mining 0025-326X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2005.09.010 * Corresponding author. Address: UR Camelia, IRD, BP A5, 98848 Noumea Cedex, New Caledonia. Tel.: +687 26 07 19; fax: +687 26 43 26. E-mail address: jean-michel.fernandez@noumea.ird.nc (J.-M. Fernan- dez). www.elsevier.com/locate/marpolbul Marine Pollution Bulletin 52 (2006) 320–331