A system dynamic model for the assessment of different exposure routes in aquatic ecosystems

With the exception of a limited number of highly persistent lipophilic poll utants, waterborne exposure is considered the most relevant exposure route for aquatic organisms, and even the only relevant exposure route in lower t ier assessment, while the potential for bioaccumulation is only evaluated a s secondary poisoning for birds and mammals. In addition, some evaluations consider that only the dissolved fraction is bioavailable for pelagic organ isms. The lack of easy and cost-effective assessment methods is probably th e main reason to disregard the contribution of other routes. This paper pre sents a system dynamics model that estimates the time-dependent accumulatio n of toxic chemicals through the food chain, and has also the potential to include the exposure from sediment. The generic ecosystem includes a primar y producer and three levels of consumers, represented by unicellular algae, cladoceran invertebrates, cladoceran-eating fish and fish-eating fish, res pectively. Nevertheless, the model, built using Microsoft Excel software, a llows any number of levels and ecological-exposure relationships. The resul ts obtained for four illustrative pollutants demonstrate that factors such as the binding to algae and other edible particles or food chain exposure f or lipophilic non-persistent chemicals may constitute significant exposure routes. The current low tier European ecological risk assessment guidelines do not cover these aspects, and therefore may under estimate the real risk for both aquatic organisms and their predators. The model includes a simpl ified worst-case alternative that normally can be calculated using the exis ting information. Under a tiered approach, the simplified alternative is pr oposed to trigger the need of the toxicokinetics assays required to perform the complete time-dependent calculation. (C) 2000 Elsevier Science B.V. Al l rights reserved.