Pellston Workshops

Figure 1 presents an overview of the model testing/valida-tion process as developed at the Pellston workshop. A distinction is drawn between validation of empirical versus theoretical models as discussed by Lassiter (4 ). In reality, many models are combinations of empiricism and theory, with empirical formulations providing process descriptions or interactions lacking a sound, well-developed theoretical basis. The importance of field data is shown in Figure 1 for each step in the model validation process considerations in comparing field data with model predictions will be discussed in a later section. 

Although the existence or absence of a particular process can often be determined from observed data, an assessment of how well an algorithm represents the process is often difficult to make due to observation errors, natural variations in field data, and lack of sufficient data on individual component processes. In such circumstances, model validity must be inferred or possibly based on comparisons with laboratory data obtained under controlled conditions. Often laboratory data provide the basis for developing an algorithm since field data are so much more difficult and expensive to collect and interpret. Examples of system representation errors and their analysis were presented at the Pellston workshop (6 ). 

A dramatic example of this type of error was discussed by Donigian, (6j at the Pellston workshop based on the Iowa study described earlier (8 ) Figure 3 shows the calibration (top figure, 1978 data) and verification (bottom figure, 1978) results. A simulated alachlor concentration value of greater than 0.1 mg/1 occurred on May 27, 1978, (top figure) whereas the observed... 

Pavlou, S.P (1987) The use of equilibrium partition approach in determining safe levels of contaminants in marine sediments, p. 388 -12. In Fate and Effects of Sediments-Bound Chemicals in Aquatic Systems. Dickson, K.L., Maki, A.W., Brungs, W.A., Editors. Proceedings of the Sixth Pellston Workshop, Florissant, Colorado, August 12-17,1984. SETAC Special Publ. Series, Ward, C.H., Walton, B.T., Eds., Pergamon Press, N.Y. 

Grothe DR, Dickson KL, Reed-Judkins DK (1995) SETAC Pellston workshop on whole effluent toxicity, 16-25 Sep 1995. SETAC Press, p 340... 

Uncertainty analysis is increasingly used in ecological risk assessment and was the subject of an earlier Pellston workshop (Warren-Hicks and Moore 1998). The US Environmental Protection Agency (USEPA) has developed general principles for the use of Monte Carlo methods (USEPA 1997), which provide one of several approaches to incorporating variability and uncertainty in risk assessment. 

This chapter considers the role of variability and uncertainty in ecological risk assessment and discusses whether it is necessary to quantify them. It concludes by setting out the objectives and key issues that were considered at the Pellston workshop in February 2002, which are addressed in the following chapters of this book. 

A previous Pellston workshop listed the benefits of uncertainty analysis in regulatory programs as follows (Warren-Hicks and Moore 1998) ... 

In higher tier assessments, the question is not so much whether uncertainty analysis is required, but rather whether it should be quantitative and what methods should be used for it. The previous Pellston workshop made the following recommendations as a general guide (Warren-Hicks and Moore 1998) ... 

The Pellston workshop in February 2002, which produced this book, aimed to develop guidance and increased consensus on the use of uncertainty analysis methods in ecological risk assessment. The workshop focused on pesticides, and used case studies on pesticides, because of the urgent need created by the rapid move to using probabilistic methods in pesticide risk assessment. However, it was anticipated that the conclusions would also be highly relevant to other stressors, especially other contaminants. 

Norton SB. 1998. A ecological risk assessor s perspective of uncertainty. In Warren-Hicks WJ, Moore DRJ, editors. Uncertainty analysis in ecological risk assessment. Proceedings of the Pellston Workshop on Uncertainty Analysis in Ecological Risk Assessment, 23 to 28 August 1995. Pensacola (FL) SETAC. 

An important question when planning a probabilistic assessment is whether to separate variability and uncertainty in the analysis and results. This is one of the key issues that were given special consideration at the Pellston workshop that developed this book. While there was not a consensus, the majority view was that there are potential advantages to separating variability and uncertainty, but further case studies are needed to evalnate the benehts and practicality of this for routine pesticide assessment. 

Another important qnestion when planning a probabilistic assessment is how to deal with dependencies. This also is one of the key issues that were identified for the Pellston workshop. Fnrther work is needed to evaluate these options. Some additional points are made here. 

Grothe, D.R., Dickson, K.L. and Reed-Judkins, D.K. (eds.) (1996) Whole effluent toxicity testing an evaluation of methods and prediction of receiving system impacts, Proceedings from a SETAC -sponsored Pellston Workshop, Society of Environmental Toxicology and Chemistry, Pensacola, FL, 346 pp. 

Wenning, R.J. and Ingersoll, C.G. (eds.) (2002) Use of sediment quality guidelines and related tools for the assessment of contaminated sediments, Executive Summary Booklet of a SETAC Pellston Workshop, Society of Environmental Toxiclogy and Chemistry, Pensacola, FL, 48 pp. 

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