Geochemical Changes in Metal and Nutrient Loading at Orplands Farm Managed Retreat Site, Essex, UK (April 1995±1997) C. L. MACLEODà, M. D. SCRIMSHAW , R. H. C. EMMERSON , Y.-H. CHANG and J. N. LESTER *  T.M. Huxley School of the Environment, Earth Sciences and Engineering, RSM Building, Imperial College of Science, Technology and Medicine, London SW7 2BP, UK àSchool of Earth and Environmental Sciences, The University of Greenwich, Medway Campus, Chatham, Kent, UK Salt marshes have recently been considered to be a major part of the coastal system and have played a key role in the development of the UK coastal management strategy. Managed Retreat (MR) is a process aimed to restore salt marshes by realignment of the seawalls allowing tidal in- undation of low value agricultural land. The resultant marshes are expected to function both as an integral part of the ¯ood defence system and as an ecological conser- vation area. We report on the eects of salt marsh res- toration on metal and nutrient loading of the sediment at the Orplands Farm MR site, Essex, UK. Sur®cial grab and sediment cores were collected from the two ®elds that comprise the site. The heavy metals, Cd, Cr, Cu, Hg, Pb, Ni and Zn were analysed to determine changes in an- thropogenic inputs to sediments. The major ions, Al, Ca, Fe, K, Mg, Mn and Na were also monitored to identify changes in sediment geochemistry. Analysis of the cored sediments after inundation for Na and Sr demonstrated that penetration of estuarine water had, within 2 yr of exposure, reached an average depth of 20 cm. The study observed that input of heavy metals had occurred to the sediments with the most signi®cant being that of Pb, however increases were also observed for Cr and Cu. However, concentrations of Cd in the MR sediments de- creased from 1995 to 1997. For the major metals within both ®elds it was found that the dominant changes were those of enrichment of marine associated metals, Ca, K, Mg and Na via inputs from tidal inundation. The con- centration of Ca in the sediments was further enriched by the deposition of carbonates to the sediments. One ®eld demonstrated a signi®cant loss of Fe from sediments which corresponded to changes in redox potential of the sediments. Dierences observed in geochemical pro®les between the two ®elds of the site were attributed to dif- ferences in land use prior to ¯ooding. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: saltmarsh; pollutant; Blackwater; estuary; ¯ood defence. Introduction Salt marsh chemistry is highly variable and dependent upon a number of factors, most importantly the nature of the mineralogical composition of the marsh sediments and their interactions with estuarine water. In North America salt marshes are increasingly being developed in inner estuaries as a means of improving water quality due to the high anity of some heavy metal contami- nants for marsh sediments. In the Blackwater estuary, Essex, UK, the need to re-establish salt marshes due to the high rate of erosion caused by the inability of ex- isting marshes to retreat landward beyond existing sea- walls is of primary importance, however, anthropogenic contaminants may also have an impact on salt marsh development (OÕReilly-Wiese et al., 1994). Thus under- standing the implications of the changing geochemistry of the sediments on the establishment of salt marsh ¯ora is of importance. Geochemically, salt marsh restoration by the Managed Retreat (MR) technique involves two distinct but synergistic processes: accretion of sediments and the eects of periodic inundation on the existing soil pro®le. An inherent requirement of the successful de- velopment of salt marsh at the MR sites on the Essex coast, UK, is the accretion of new sediment on the pre- inundation surface, as this surface is below the level, due to the extent of tidal inundation, at which establishment of salt marsh plants is normally favoured. Below the newly accreting layer the old soil pro®le will be modi®ed by the in®ltration of seawater, some minerals will ¯occulate, exchanges of adsorbed cations with seawater Marine Pollution Bulletin Vol. 38, No. 12, pp. 1115±1125, 1999 Ó 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0025-326X/99 $ - see front matter PII: S0025-326X(99)00141-1 *Corresponding author. 1115