Uncertainty field studies

Uncertainty may also affect the credibility of ecological risk assessment procedures. In the late 1970s, the USEPA presented a risk assessment for the use of granular carbofuran on com, including a detailed list of field studies and incidents. The Eederal Insecticide, Fungicide, and Rodenticide Act Science Advisory Panel concluded there was insufficient information to justify restricted use labeling and recommended further testing. Nearly 20 years later, the accumulation of additional field studies and incidents provided sufficient evidence such that approvals for use of carbofuran were withdrawn. 

Extrapolation from field studies with other pesticides, and/or sampling uncertainty due to limited numbers of field sites... 

Methods for evaluating the performance and utility of uncertainty analysis in the context of probabilistic pesticide assessments are needed. This should include comparisons between assessment outputs and existing field data (e.g., avian field studies) to evaluate whether decision makers can rely on the assessment methods. Consideration should also be given to existing field data to refine generic assessment models, using Bayesian updating methods. 

The third need is standardization. The receptor model applications need to be written as standard computer routines, common data structures that can accomodate uncertainties of the observables need to be created, and sampling and analysis equipment and procedures must produce equivalent results. Field study "scenarios" for typical situations should be proposed. 

There is also evidence from field studies for the generation of HONO, first detected unequivocally in the atmosphere by Perner and Platt (1979), at surfaces. Interpretation of such studies is complex due to the contributions of meteorology and the uncertainty in the nature of the available surfaces for reaction (14) (e.g., Lammel, 1996 Hjorth et al., 1996). However, in semirural areas, the surface appears to be a net source of HONO at concentrations of N02 above 10 ppb (Harrison and Kitto, 1994 Harrison et al., 1996). 

Although random labeling with tritium usually involves some element of uncertainty as to the identity of the labeled molecules with the original sample, this labeling method frequently offers the only practical means of preparing the necessary amounts of tagged materials for field studies. Carbon- 14-labeled gibberellic acid has been prepared, but the yields have been too low to make its use practicable (8). 

Table 3-6 distinguishes between correlational evidence from field observations and experimental evidence. Correlational evidence from field studies inherently has multiple sources of uncertainty, many of which are controlled in experimental studies. Observations that indicate a positive relation between environmental PCB exposure (sometimes represented by PCB concentration in tissue) and an adverse health effect in free-ranging wildlife are represented in Table 3-6 as correlational field observations. Effects that were observed in experimental studies under controlled or closely monitored exposure conditions were included in the table as experimental observations. However, no entry was made in Table 3-6 for responses that were reported in an experimental study to be equivocal, ambiguous, or not statistically significant. 

Overall it is concluded that population modeling is of potential use, particularly to refine the risk to terrestrial vertebrates where the risk is very often not sufficiently addressed by refinements focusing only on exposure. Experiments with vertebrates should be reduced to a minimum because of animal welfare considerations. Hence, further testing to reduce uncertainty related to differences in species sensitivity should be avoided as a standard risk refinement option. Field studies investigating effects on population level are cost-intensive, and population modeling may provide a more cost-effective alternative. 

Values for K sa observed in this study were consistently lower than those predicted although higher values have been reported in field studies of PCBs. ° Predicted values are based on empirical relations between Aiow and and K, which would result in some uncertainty. For example, using different Aiow/ < and Ks relations a K sa of 1 -9, has been reported, which is independent of However, these projections do provide a basis for assessing the transfer of compounds from sediment to the biota but tend to overestimate the accumulation of compounds with log Kov, > 6. This would parallel the relations observed with and Kovi (Fig. 4.20) that illustrates the limited abihty of these compounds with high Kovi to move across membranes. Variations among species and environmental conditions will also influence the distribution between sediments and biota and further studies are needed to develop a more comprehensive analysis of this process. 

Another set of uncertainties involve how background tissue concentrations for essential metals influence the results of field studies, and whether it is possible to distinguish true increases in metal residues due to trophic transfer from inherent differences between species. Unfortunately, background metal concentrations in tissues are rarely measured. This hinders interpretation of data from lab and field studies, but not biokinetic studies that use radio-isotopes, where TTFs are based on accumulation and efflux of new metal, and therefore are not subject to interference from background metal concentrations. 

Patches are essentially spot or grab samples (typically 25 cm ) of the skin, whereby dermal exposure is assessed by extrapolating the patch loading level to the surface of the entire body. Given that only a small area of exposed skin is covered by the patches used, the uncertainties in calculating true exposure are likely to be high. Furthermore, the exposure pattern may vary with time, with activities, and both within and between individuals, days and shifts. However, several field studies have shown that this technique is a useful tool in the identification and evaluation of dermal exposures to a range of workplace contaminants and in the assessment of the effectiveness of personal protective equipment (Fenske et al. 1990). 

Although the movements have been recorded in many field studies, there is still uncertainty as to when they occur. Probably this is early or late winter when the frozen ground is relatively warm. Temperatare change (which occins constantly in the near surface layers) is important. 

In the feasibility phase the project is tested as a concept. Is it technically feasible and is it economically viable There may be a number of ways to perform a particular task (such as develop an oil field) and these have to be judged against economic criteria, availability of resources, and risk. At this stage estimates of cost and income (production) profiles will carry a considerable uncertainty range, but are used to filter out unrealistic options. Several options may remain under consideration at the end of a feasibility study. 

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