1. Soil Quality Department, Wageningen University, P.O. Box 8005, 6700 EC, Wageningen, The Netherlands, Laboratory for Physical Chemistry & Colloid Science, Wageningen University, Dreijenplein 6, 6703 HB, Wageningen, The Netherlands, British Geological Survey, Wallingford, Oxon OX10 8BB, UK, and Laboratoire de Géochimie des Eaux, Université Denis Diderot-Paris 7, UMR CNRS 7154, IPGP, Case 7052, 2 Place Jussieu, 75251 Paris Cedex 05, France