130 Earth and Planetary Science Letters, 40 (1978) 130-136 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands I61 THE REMOVAL OF DISSOLVED HUMIC ACIDS AND IRON DURING ESTUARINE MIXING E.R. SHOLKOVITZ l, E.A. BOYLE : and N.B. PRICE 1 i Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW (Scotland) 2 Department o f Earth and Planetary Sciences, Massachusetts Institute o f Technology, Cambridge, MA 02139 (U.S.A.) Received December 24, 1977 Revised version received March 15, 1978 The estuarine chemistry of dissolved humic acids was determined by carrying out both field and laboratory studies. These approaches were combined in an investigation of the Amazon estuary while laboratory mixing experi- ments were performed using filtered (0.45-0.001 #m) river water fractions of the Water of Luce (Scotland). The results demonstrate that a small fraction of river dissolved organic matter is preferentially and rapidly floccu- lated during estuarine mixing. This fraction is the high molecular weight component of dissolved humic acids (0.45- 0.1/zm filtered). Approximately 60-80% of the dissolved humic acid in these rivers flocculates during estuarine mix- ing. This represents a removal of only 3-6% of river dissolved organic matter and is responsible for the non-conser- vative behaviour of dissolved humic acid in the Amazon estuary even though total dissolved organic carbon appears conservative. The salinity dependence with which humic acid flocculates in estuaries is similar to that of iron. This implies that both constituents may be removed from river water by a common mechanism of colloid flocculation. 1. Introduction During estuarine mixing the removal of dissolved organic matter (DOM), iron and other inorganic trace constituents appear to be related to a common pro- cess [1-3]. The salinity dependence of the removal of DOM and iron are similar [ 1,2]. However, dissolved iron is almost totally removed while only 3-11% of river DOM is flocculated [1,3]. This small reactive fraction of DOM appears to be colloidal humic mate- rial which forms part of DOM in river water [ 1,2]. A better understanding of the processes which control DOM behaviour in estuaries will also help to establish the relationship between DOM and trace metal reac- tivities. Apart from the references cited above, little work has been undertaken to determine either the concen- tration of DOM in estuaries or its estuarine chemistry [4]. For example, Duursma [5] has presented a salin- ity-dissolved organic carbon (DOC) diagram for an estuary (Wadden Sea) indicating that DOC behaves conservatively;he also concludes from laboratory experiments that there is no measurable precipitation of DOC from river water when it is mixed with sea- water. Sieburth and Jensen [6] and Swanson and Palacas [7] have demonstrated that land-derived dis- solved humic substances precipitate in seawater. Hair and Bassett [8] and Brown [9,10] have suggested that the high molecular weight fractions of dissolved humic acid (DHA) in river water flocculate during estuarine mixing. This conclusion is based on monthly averages of the concentrations of dissolved and par- ticulate humic acids [8] and spectrophotometric mea- surements of estuarine waters [9,10]. Although the above studies reach what appears to be an intuitively correct conclusion, in neither case do their results actually demonstrate the preferentive precipitation of high molecular weight humic acids during estuarine mixing. In this paper we have attempted to extend and refine our understanding of removal mechanisms and reactivity of DOM in estuaries by combining both field and laboratory studies of dissolved organic mat- ter and humic acid in the Amazon and Water of Luce