Estimating the spatial distribution of dredged material disposed of at sea using particle-size distributions and metal concentrations Tomonari Okada a , Piers Larcombe b, * , Claire Mason b a National Institute for Land and Infrastructure Management, 3-1-1 Nagase, Yokosuka, Kanagawa 239-0826, Japan b Centre for Environment, Fisheries and Aquaculture Science, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK article info Keywords: Dredged material Disposal site Particle-size distribution Entropy analysis Metal concentration abstract We present a method to estimate the spatial distribution of dredged material disposed of at sea. Using both dredged sediments and samples of sea-bed sediment from near the Rame Head disposal site, Plym- outh, UK, we applied entropy analysis to the <63 lm sediment fraction and combined the results with the trace metal data in the same fraction, to form a series of groups. We interpret the distribution of sedi- ments in one group (F1) to approximate the distribution of material affected by the disposal site. This dis- tribution includes locations close to the disposal site, and also locations <4 km to the SE and SW, <6 km to the NW and <2 km to the N. This approach demonstrates the feasibility of using trace metal analysis of particular grain size fractions to reduce uncertainty in interpreting the spatial distribution of impacts of dredge disposal. Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved. 1. Introduction 1.1. The issue of marine disposal of dredge material Disposal of dredged material at sea is accepted throughout the world as necessary, and is explicitly permitted under the OSPAR Convention (www.ospar.org), which covers the North-West Europe and under the London Convention of 1972 (www.londonconven- tion.org), which has global coverage. Today, however, there is in- creased awareness of the environmental consequences of marine disposal of dredged material. As the port channels in several major UK estuaries are in or adjacent to Natura 2000 sites (www.natu- ra.org) designated under the EU Habitats and Species Directive and the Wild Birds Directive, it has become more important that strategies for monitoring activities such as dredging are formalized. Disposal of dredged material at sea has the potential to affect the water column, sea-bed conditions, and the biota of both. How- ever, impacts are generally most significant at the seabed, where the material comes to rest. Effects in the water column are gener- ally very limited in space and time and are dependent on the phys- ical characteristics of the location and the hydrodynamics of the water column. Most UK disposal sites are dispersive in nature (some highly dispersive) due to the prevailing tidal regime. Tem- porary reductions in water clarity can occur because of increased turbidity, but are unlikely to affect primary production by phyto- plankton in most circumstances due to the very limited spatial and temporal reductions in water clarity. While the release into the sea of materials contained within the dredged sediment, either in the water-soluble fraction, or as particulate matter, has the po- tential to change the chemical environment including creating a water-column oxygen demand, these effects are generally of minor concern due to the highly dispersive nature of most UK disposal sites. Based on 35 years of experience with disposal of dredged sediment at sea in New England (USA), Fredette and French (2004) concluded that impacts of organisms via the water column are generally minimal. The deposited material can change the character of the sea-bed sediment, for example through smother- ing, and consequently may affect the structure of the sea-bed com- munity and, in turn, the demersal and benthic fish that feed on that community (Stronkhorst et al., 2003; Bolam et al., 2006). There are thus a variety of potential effects of the disposal of dredged mate- rial in the sea but the effects on the seabed are usually of most sig- nificance (Fredette and French, 2004; Bolam et al., 2006). 1.2. Techniques of analysis and assessment The essential aspect of monitoring such activities is to identify that distribution, because the potential effects are controlled pri- marily by the spatial distribution of the disposed material. Previous studies have used numerical simulations to investigate the spatial distribution of material disposed of in the sea (e.g., Clarke et al., 1982 (sewage waste); Gallacher and Hogan, 1998; Van den Eynde, 2004), and metal concentrations in sediments (e.g., Matthai and Birch, 2001; Rowlatt et al., 1991 (sewage sludge); Kress et al., 2004 (sewage sludge)). Recent progress in the development of numerical simulation techniques means that it is theoretically pos- sible to achieve accurate estimates of the spatial distribution of 0025-326X/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2009.03.023 * Corresponding author. E-mail address: piers.larcombe@cefas.co.uk (P. Larcombe). Marine Pollution Bulletin 58 (2009) 1164–1177 Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul