Is the 1:4 elutriation ratio reliable? Ecotoxicological comparison of four different sediment:water proportions
Introduction
Most contaminants originating from human activities and discharged into surface water are eventually deposited and concentrated in sediment. Due to its propensity to sequester both organic and inorganic contaminants, sediment can be defined as the main sink and source of pollutants. The sediments in estuaries and coastal areas thus constitute important reserves of contaminants and represents potential sources of pollution. In particular, marine disposal of dredged material, dumping of wastes, and shipping and fishing activities can play major roles in sediment particulate resuspension, a possible source of pollution (USEPA, 1977; Forstner and Wittmann, 1983). In sediment, in addition to independent actions of contaminants, the presence of complex mixtures could produce additive, synergic, and antagonistic interactions. Their concentrations in sediment may be several orders of magnitude higher than that in overlying water; however, bulk sediment concentrations are not highly correlated to bioavailability (Burton, 1992).
Elutriate is an environmental matrix that enables the replication of sediment mobilization phenomena (Shuba et al., 1978) and the prediction of the release of contaminants from the sediment to the water column (ASTM, 1990). It was first developed for evaluating the potential effects of disposing of dredged material in open water and is nowadays also applied to the quality evaluation of in situ sediment (Beiras et al., 2001; Lazorchak et al., 2003).
Briefly, the elutriation procedure consists of the vigorous shaking of a predetermined part of sediment with parts of water to release sorbed pollutants. This mixture is allowed to settle and the liquid phase is centrifuged (ASTM, 1991; Burton and MacPherson, 1995). Analyses of elutriate samples provide information on the water-soluble constituents potentially released from the sediment to the water column. The method has been proved suitable for detecting altered and toxic sediment, supplying information on water-bioavailable components (Williams et al., 1986).
Several methods of elutriate preparation have been proposed in the literature. The main differences consist of sediment:water ratios, sediment shaking techniques, times, and temperatures, and supernatant centrifuging techniques. The 1:4 sediment:water ratio, suggested by USEPA (1991), is the most commonly employed sediment:water proportion (USACE, 1978; Burton et al., 1989; Daniels et al., 1989; Munawar et al., 1989; Long et al., 1990; Ankley et al., 1991; USEPA, 1991; Vashchenko and Zhada, 1993; Andreatta et al., 1994; Hurkey et al., 1994; Bridges et al., 1996; Schuytema et al., 1996; Da Ros et al., 1997; Sibley et al., 1997). Nonetheless, a wide series of alternative ratios have flourished: the 1:2 ratio (Meador et al., 1990; Vashchenko and Zhada, 1993), 1:3 ratio (Matthiessen et al., 1998), 1:5 ratio (Lee et al., 1978; Da Ros et al., 1997), 1:8 ratio (McFadzen, 2000), 1:10 ratio (Daniels et al., 1989; Da Ros et al., 1997), 1:20 ratio (Lee et al., 1978; Daniels et al., 1989), 1:25 (Lee et al., 1978), 1:50 ratio (Long et al., 1990; Van den Hurk, 1994; Van den Hurk et al., 1997; Da Ros et al., 1997), 1:200 ratio (Da Ros et al., 1997), 2:1 ratio (Williams et al., 1986), and 2:5 ratio (Thain et al., 1996). Moreover, Daniels et al. (1989) reported that the choice of the dry or wet weight of sediment to achieve the sediment:water proportions is questionable.
Different sediment shaking devices are also employed: rotary shaker tables (Williams et al., 1986; Ankley et al., 1991; Matthiessen et al., 1998), rotary tumblers (Daniels et al., 1989; Munawar et al., 1989), air systems (USACE, 1978; Daniels et al., 1989; McFadzen, 2000), shakers (Burton et al., 1989; Daniels et al., 1989; Long et al., 1990; Van den Hurk, 1994; Matthiessen et al., 1998; His et al., 1999), magnetic stirrers (Bridges et al., 1996), and ultrasonic baths (Andreatta et al., 1994) have all been proposed. Shaking times varied from about 30 min to 24 h (Ankley et al., 1991; ASTM, 1991; Burton and MacPherson, 1995; Liß and Ahlf, 1997).
Despite all these methodological differences, only limited studies have been conducted to highlight the procedure best able to simulate natural sediment resuspension phenomena or to render pollutants more bioavailable (Daniels et al., 1989; Da Ros et al., 1997).
The aim of this work was to assess the discriminatory capability of three toxicity bioassays toward four different elutriation ratios (1:4, 1:20, 1:50, and 1:200) to evaluate which ratio can produce greater effects for water-column bioindicators. Moreover, this research aimed to determine whether the widely used 1:4 sediment:water ratio represents the best method for a quality control sediment diagnosis.
Static nonrenewal bioassays using sea urchin sperm cells, embryos, and larvae, and oyster embryos and larvae, widely used in biomonitoring programs (ASTM, 1998a, ASTM, 1998b), have been employed to evaluate elutriates toxicity. To evaluate the reliability of the results, the toxicity data are discussed in relation to possible confounding factors (ammonia and sulfide).
Section snippets
Sampling sites
The sediments were sampled in late summer (August–September 1998) at six sites in the Lagoon of Venice, northeast Italy, as part of the “Orizzonte 2023 Project.” The stations were located along a pollution gradient: two presumably unpolluted sites (CE(a) and CE(b)), two oil-refinery and industrial sites (BR and SA), and two estuarine sites (DE and OS) close to agricultural areas.
At Centrega Marsh (CE), in the northern Lagoon, CE(a) and CE(b) were chosen as two possible reference sites,
Quality assurance/quality control for toxicity tests
In the tests performed, controls showed 83±5% of fertilized eggs, 70±3% of normally developed plutei of P. lividus, and 83±4% of normally developed embryos of C. gigas, according to each methodology's limits. Experiments using copper as reference toxicant confirmed the good repeatability of all assays. The sperm cell test had a mean EC50±standard deviation (SD) of 49±3 μg/L (CV=6%, ). These data are well within the EC50 acceptability range (39–71 μg/L) (Volpi Ghirardini and Arizzi Novelli, 2001
Discussion
Toxicity results showed that contaminant bioavailability seemed to be sampling station based and highlighted that the commonly employed 1:4 sediment:water ratio (USEPA, 1991) has not always been useful in detecting sediment toxicity, generally demonstrating a lower discriminatory capability than the intermediate ratios (1:20 and 1:50) and, sometimes, than the 1:200 ratio. A particularly interesting result is that the two possible reference sites, CE(a) and CE(b), which present low
Conclusions
A preliminary understanding of the discriminatory capabilities of the four different elutriation ratios chosen for this study (1:4, 1:20, 1:50, and 1:200) has been reached. It has been highlighted that there is no best sediment:water proportion, but rather that the different ratios can contribute in specific manners to the sediment quality assessment procedure. In general, the samples generated from the widely used 1:4 ratio have shown to be less toxic than those from intermediate ratios, 1:20
Acknowledgments
The authors are very grateful to Michele Cornello and Giuseppe Pessa for sampling activities and to Eugenia Delaney for laboratory assistance. This work was partly funded by the Magistrato alle Acque di Venezia through the Consorzio Venezia Nuova within the Project “Orizzonte 2023” and by the Consorzio Ricerche Laguna (Co.Ri.La.) of Venice (Italy). Alison Garside revised the English text.
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Current address: CNR-Istituto di Scienze Marine, Riva Sette Martiri, 1364/a, I-30122 Venice, Italy.