Name /entc/27-09-15 06/02/2008 02:26PM Plate # 0-Composite pg 104 # 1 Environmental Toxicology and Chemistry, Vol. 27, No. 9, pp. 000–000, 2008  2008 SETAC Printed in the USA 0730-7268/08 $12.00 + .00 CREATING METAL-SPIKED BED SEDIMENTS: A CASE STUDY FROM OREWA ESTUARY, NEW ZEALAND XUEQIANG LU,*†‡§ REBECCA L. BIBBY,† RICHARD B. FORD,‡ and JENNY G. WEBSTER-BROWN*† †School of Geography, Geology and Environmental Science, University of Auckland, Private Bag 92019, Auckland, New Zealand ‡Leigh Marine Laboratory, University of Auckland, Private Bag 92019, Auckland, New Zealand §Tianjin Academy of Environmental Sciences, Tianjin 300191, China ( Received 27 August 2007; Accepted 18 March 2008) Abstract—Spiking sediments to achieve target concentrations of heavy metal pollutants is a key step in sediment toxicity tests. It is difficult, however, to ensure that metals in an artificially spiked sediment will behave naturally. A method has been developed in the present study to create Cu-, Pb-, and Zn-spiked sediments in which naturally occurring adsorption onto sediment surfaces is the dominant process binding the metals and in which precipitation of readily redissolved minerals and other metal-bearing phases (artifacts of the spiking procedure) are avoided. Uncontaminated bed sediment from an intertidal mudflat in the Orewa estuary, New Zealand, was characterized in terms of existing metal content, optimal adsorption pH, and adsorption capacity. Competitive adsorption between Cu and Pb as well as complexation by seawater anions only slightly affected metal adsorption from seawater. Surface complexation modeling indicated that iron oxide surfaces in the sediment likely were dominating metal adsorption processes. Spiking experiments were designed using these established adsorption characteristics but with significantly higher (100-fold) concentrations of sediments and dissolved metals and a liquid to solid (L:S) ratio of approximately 5.5. An equilibration time of at least 36 h was required to achieve a reproducible target metal concentration, which could be reliably predicted from the L:S ratio and the initial metal concentration in the spiking solution. Adsorption equilibrium remained the process governing metal binding to the sediment, and no indication was observed that the adsorption capacity of the sediment had been exceeded or that additional metal-bearing phases had been formed. Keywords—Estuarine bed sediment Metal adsorption Spiking INTRODUCTION Increasing urbanization and industrialization of catchments have led to higher concentrations of heavy metals, such as Cu, Pb, and Zn, in wastewaters and in storm-water runoff. These urban metals subsequently accumulate over time in estuaries and coastal marine environments [1–5]. Estuarine bed sedi- ments play an important role as both sinks and sources of contaminants in the water column and biota. Ecotoxicological studies on bed sediment–associated contaminants have been used to evaluate environmental risks and to derive sediment- quality guidelines [5–7]. The evaluation of sediment toxicity through laboratory or field bioassays is critical to the effects- based approach to developing sediment-quality guidelines [6] (http://www.mincos.gov.au/publications). For in situ or laboratory-based sediment toxicity testing, sediments can be spiked with the contaminant of interest, or with multiple contaminants, to create the conditions of interest [8,9]. It is important, however, to recreate a natural partitioning of contaminants between sediment and water, because this is a crucial factor determining trace metal bioavailability, trans- port, and fate in coastal estuarine environments. An important consideration is the adsorption capacity of the sediment. If this is exceeded during the spiking process, metals will be bound to the surface by processes other than adsorption and, there- fore, may be released by a different mechanism and at different rates relative to the adsorbed metals. Thus, the adsorption char- acteristics of the sediment must be known before spiking can * To whom correspondence may be addressed (xueqianglu@gmail.com). Published on the Web 4/17/2008. be undertaken. In addition, inert gas typically is used to form an anoxic condition for preventing oxidation of the sediment during the spiking procedure [8,9]. For oxic sediment, how- ever, the anoxic condition is not suitable to adsorb metals onto the sediment naturally because of the changed redox potential. Many studies have addressed the adsorption of heavy met- als onto pure mineral phases present in bed sediment or, less commonly, onto specific size fractions of natural sediment [10–14]. Few studies have spiked heavy metals onto whole sediment [8,9]. In those few studies, however, relatively high concentrations of metals (650–3250 mg/kg for Zn [8] as well as 2,000 mg/kg for Cu and 4,000 mg/kg for Zn [9]) were targeted, which possibly exceeded the sediment adsorption ca- pacity. In the Auckland region of New Zealand, the effect of ur- banization on estuarine and coastal sediment quality is in- creasing in intensity (see, e.g., [1,15,16] and http://www.arc. govt.nz/plans/technical-publications). Copper, Pb, and Zn are contaminants of interest, because these are the principal heavy metals in the suspended sediments of Auckland urban streams [17]. Benthic fauna have been impacted by the increasing heavy metals in estuarine bed sediments of the Auckland area [18]. A research program therefore is being undertaken on an intertidal mudflat of the Orewa estuary, north of Auckland city, that is designed to test the effects from multiple stressors (Cu, Pb, and Zn) on benthic ecology. Intertidal, soft, silty, midestuary mudflats are considered to be representative of a whole estuary system [19]. Because it is removed from the urban center and is relatively uncontaminated with respect to urban heavy metals, the intertidal mudflat at the Orewa estuary ?1 ?2 ?3