Deltares - R&D Highlights 2009

Contact: jos.vink@deltares.nl +31 (0)88 335 7154 The origin of speciation Chemical availability of dissolved copper over time (above) measured for water-sediment interfaces as performed with the EU-patented SOFIE® cell (below) Although it is well known that environmental contamination with heavy metals has adverse eff ects on organisms, surprisingly little has been achieved in recent years in terms of actually linking chemical speciation to bioavailability (such as accumulation in organisms) in natural systems. This is the result of a lack of techniques for quantifying or actually measuring free ion activities (FIA) in pore water in sediments/aquatic sediments. The FIA concept assumes that metal ions in the Mn+ form, rather than total content or total dissolved levels, largely determine the biological eff ect observed in organisms exposed to water or sediment containing heavy metals. The methodological criteria for a suitable technique require it to be: 1) non-destructive for the sample; 2) capable of measuring small layer increments following redox gradients across watersediment interfaces; 3) capable of time-dynamic measurements; 4) suitable for implementation in bioassay settings. A novel experimental “cell” was developed that meets these criteria. For the fi rst time, it is possible to quantify free metal ion activities in undisturbed pore water both repeatedly and in a non-destructive way for the sample over a natural redox gradient. Bioassays are conducted simultaneously in the same setting. The design of the cell is based on using undisturbed samples in such a way that redox status and geochemical integrity is guaranteed. For the purpose of conducting bioassays, this setting is preferable to conventional methods which, by defi nition, produce unrealistic results for redox-sensitive compounds such as heavy metals. For a variety of surface water types, chemical speciation was measured for the water-sediment interface. Simultaneously, aquatic organisms were exposed to these sediments to relate chemical availability to biological uptake. It was shown that the uptake of Cd, Cu, Ni, and Zn by the aquatic biota can be directly linked to free metal ion activity concentrations present in the overlying surface water. These 54

fi ndings make a signifi cant contribution to our understanding of chemical speciation in water-sediment systems and the subsequent impact on organisms. We have demonstrated that the presence of benthic organisms in a water-sediment system has a signifi cant eff ect on chemical speciation, and that it must therefore be considered during bioassays and chemical tests. Body concentrations are regulated by external concentrations that vary in time or, more specifi cally, by free ion activities. The conclusion is that an integrated cell of the type presented here provides the necessary experimental tool to support, in a mechanistic way, environmental risk assessments of contaminants and sounder quality criteria for aquatic sediments. Bioassay performed in the cell. Uptake of heavy metals by aquatic organisms (Limnodrilus) can now be estimated accurately with the free ion activity in overlying water as a driver force Further reading J.P.M. Vink, The origin of speciation: trace metal kinetics and bioaccumulation by Oligochaetes and Chironomids in undisturbed water-sediment interfaces. Environmental Pollution 157 (2009) 519-527. This paper received the 2009 TNO publication award for Environmental Sciences 55

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