Gross fluxes and estuarine behaviour of pesticides in the Scheldt Estuary (1995-1997)

As part of the Fluxes of Agrochemicals into the Marine Environment (FAME) p roject, the gross fluxes of selected pesticides (i.e. the herbicides atrazi ne, simazine, alachlor and metolachlor, the atrazine degradation product de sethylatrazine, the insecticide dichlorvos and the antifouling agent Irgaro l 1051) transported by the river Scheldt and the Canal Ghent-Terneuzen were determined from March 1995 through February 1997. In general, the observed temporal trends were related to the application period of the pesticides, except for metolachlor for which elevated concentrations were observed in t he winter of 1995-1996. Relatively large gross fluxes were found for deseth ylatrazine compared with its parent compound. A study on the estuarine beha viour of pesticides showed distinct differences between the compound classe s. The mixing plots of the organophosphorus insecticides dichlorvos and dia zinon revealed clear evidence of estuarine loss processes which agrees with their low DT50 values reported for water/sediment systems, their relativel y high Henry's law constants and, for diazinon, its relatively high K-oc va lue. The mixing plots of the acetanilides alachlor and metolachlor were str ongly influenced by an additional direct emission into the estuary, which w as evident from a maximum in dissolved concentration near a salinity of 10 parts per thousand. An apparent conservative behaviour was observed for the triazine compounds atrazine and Irgarol 1051. This was in contrast to sima zine, which showed an apparent non-conservative behaviour. However, the tim e profiles of the riverine concentrations of simazine did not exclude that the observed curvature was solely caused by estuarine losses; therefore, ad ditional modelling is required. In a follow-up study a suitable hydrologica l model of the Scheldt estuary was constructed; the results will be present ed in a forthcoming paper (Steen, R.J.C.A., Evers, E.H.G., Van Hattum, B., Cofino, W.P. and Brinkman, U.A.Th. Net fluxes of pesticides from the Scheld t estuary into the North Sea: a model approach. Enviromental Pollution, sub mitted). (C) 2001 Elsevier Science Ltd. All rights reserved.