Bioassay responses and effects on benthos after pilot remediations in the delta of the rivers Rhine and Meuse Pieter J. den Besten a, * , Paul J. van den Brink b a Institute for Inland Water Management and Waste Water Treatment (RIZA), Ministry of Transport, Public Works and Water Management, PO Box 17, 8200 AA Lelystad, The Netherlands b Alterra Green World Research, Wageningen University and Research Centre, PO Box 47, 6700 AA Wageningen, The Netherlands Received 28 May 2004; accepted 6 January 2005 Capping contaminated sediments can be an effective remediation measure in two large river deltas. Abstract Chemical and biological monitoring was carried out for 5 years following pilot remediations at two locations in the Rhinee Meuse delta. The remediations consisted of partial excavation of the contaminated sediments, followed by applying a clean layer of sandy material on top. After the remediation, a new silty sediment top layer was formed exhibiting a lower toxicity in five sediment/ sediment pore water bioassays. Compared to the unremediated sites, lower metal and PAH concentrations were found at the remediated sites, but in one location at the same time elevated HCH, PCB and HCB levels were recorded. One year after the remediation, the differences became smaller, although effects-based classification showed that the remediated site showed a higher quality up to the last year. In both remediated sites a rapid recolonization of nematodes, oligochaetes and chironomids was observed, while the recolonization by bivalves was slower. A few years after the remediation the differences decrease. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Sediment contamination; Sediment bioassays; Meiofauna; Macrofauna; Principal response curves 1. Introduction From autumn 1995 until spring 1996, two pilot remediation projects were carried out simultaneously to investigate whether dredging polluted sediment can reduce environmental risks. One project was situated in a large creek in the Brabantsche Biesbosch. This creek, named Spijkerboor, receives water from the river Meuse. The second one was situated in a groyne section of the Nieuwe Merwede, a watercourse in the Rhine delta (Fig. 1). In these locations a high risk of sediment contamination for the ecosystem was concluded on the basis of effects in bioassays, of surveys of the benthic community and of studies on bioaccumulation (Den Besten et al., 2000). The remediations consisted of partial excavation of the contaminated sediments, followed by applying a clean layer of sandy material on top of the remaining contaminated sediment (for a detailed description, see Van Meel et al., 1997). An extensive monitoring program accompanied both projects. The aim of the investigations was to evaluate the recovery of the meio- and macrozoobenthos communities, and to compare pollution-related effects before and after the remedia- tion. The latter was done on the basis of physical- chemical analyses of sediment, sediment bioassays, * Corresponding author. Tel.: C31 320 298411; fax: C31 320 249218. E-mail address: p.dbesten@riza.rws.minvenw.nl (P.J. den Besten). URL: http://www.akwa.info. 0269-7491/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2005.01.016 Environmental Pollution 136 (2005) 197e208 www.elsevier.com/locate/envpol