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POREWATER TOXICITY TESTING: AN OVERVIEW
Citation:
Doe, K., J. Burton GA, AND K Ho. POREWATER TOXICITY TESTING: AN OVERVIEW. Chapter 6, RS Carr, M Nipper (ed.), Porewater Toxcity Testing: Biological, Chemical, and Ecological Considerations. SETAC Press, Pensacola, FL, , 125-142, (2003).
Description:
Sediments act as sinks for contaminants, where they may build up to toxic levels. Sediments containing toxic levels of contaminants pose a risk to aquatic life, human health, and wildlife. There is an overwhelming amount of evidence that demonstrates chemicals in sediments are responsible for toxicological (Williams et al. 1986; Chapman 1988; Ankley et al, 1989; Giesy and Hoke 1989, 1990; Swartz et al. 1989) and ecological effects (Swartz et al. 1982, 1994; Anderson et al. 1987; Bailey, Day et al. 1995; Hartwell et al. 1997; Hatakeyama and Yokoyama 1997).
Pore water (interstitial water) is a major route of exposure to contaminants for many benthic organisms. Contaminants in pore water can be transported to ground water or to overlying water by a variety of processes, thus exposing hyporheic and water column orgnaisms. Porewater assessments are chanllenging because sediment gradients and microenvironments that control the physicochemical characteristics are disrupted during the porewater collection process, thus bioavailability may be significantly altered. However, if proper in situ and ex situ collection methods are used, then reliable and accurate conclusions can be obtained. The optimal methods used depend on the questions being asked and the site-specific conditions.
An accurate assessment of the importance of porewater contamination in an ecosystem requires an understanding of
1) the in situ bioavailability of contaminants,
2) the exposure of indigenous organisms,
3) the predictive capability of surrogate species effects to the ecosystem, and
4) the predictive capability of laboratory results to field conditions.
Historically, assessment of sediment quality has been carried out by using 3 types of characterization techniques: analysis for chemical contaminants, benthic community structure, and toxicity. A weight-of-evidence approach, using a combination of these techniques (the Sediment Quality Triad [SQT] is an example; see Chapters 8 and 9) is now generally accepted as the superior approach to sediment assessments Burton 1998). Toxicity testing may be carried out on whole sediment using benthic organisms, and this has been the approach for assessing the suitability of dredged material for different disposal options in the regulatory arena in the U.S. (U.S. Environmental Protection Agency/U.S. Army Corps of Engineers [USEPA/USACE] 1991). However, measurement of total levels of contaminants in whole sediments or in the pore water does not necessarily relate the observed bioavailability of the contaminants. In the mid 1980s, the USEPA proposed an approach for developing sediment quality guidelines (SQGs) based on equilibrium partitioning (EqP) of contaminants in pore water. EqP theory predicts that pore water is the controlling exposure medium in the toxicity of sediments to organisms (USEPA 1993). The USEPA Equilibrium Partitioning Approach suggests that a major route of exposure is pore water, and it accounts for acute toxicity effects when sediment concentrations of nonpolar organics and metals are normalized to the primary sorption sites (total organic carbon) [DOC] and acid volatile sulfide [AVS], respectively). For this reason, TOC and AVS measurements are useful in sediment and porewater toxicity assessments. This relationship has been shown to work in many lentic and estuarine environments with few chemicals. However, its applicability to flowing water systems and more dynamic sediment environments has not been well tested. In addition, the influence of sediment sampling, manipulation, and other confounding factors (discussed later in this chapter) is likely, yet ill defined. These uncertainties provide compelling reasons to assess porewater toxicity directly.
Both lethal and sublethal endpoints have been used in porewater tests with a variety of test organisms (Car 1998). Well-developed toxicity identification evaluation (TIE) procedures exist for liquid samples, and these can be applied to porewater samples, thus allowing the ability to identify which class or specific chemical is responsible for toxicity in contaminated sediments (Ho et al. 1997). Whole sediment TIEs are under development but are not as advanced as those for pore waters. Knowing the contaminants responsible for toxicity can help improve regulatory control, source identification, and/or remediation techniques. For these reasons, porewater tests are a useful complement to whole sediment toxicity tests; they have been included in batteries of tests to assess the quality of contaminated sediments (e.g., Long et al. 1990; Burton 1998; Porebski et al. 1999; Car et al. 2000) and are used in Canada as part of a suite of regulatory tests to assess the suitability of dredged material for differtent disposal options (Porebski and Osborne 1998; see Chapters 10 and 12.