Dames & Moore, 1999 - USDA Forest Service

7.2.5 SOURCES OF UNCERTAINTY
Limitations associated with any risk assessment have a number of components. including degree of
success in meeting objectives, the range of conditions over which conclusions can be applied, and the
certainty with which conclusions can be drawn (EPA 1989a). The conclusions of a risk assessment are
useful only once they have been placed in perspective relative to the uncertainties associated with the
evaluation.
7.2.5.1 General Components of Uncertainty
Toxicology and risk assessment are relatively new sciences, and, as with all risk assessments. there are a
variety of uncertainties in this baseline ERA. The ERA process relies on assumptions that have varying
degrees of accuracy and validity. Uncertainty in risk estimation has both qualitative and quantitative
components. In general, the uncertainty surrounding a risk estimate consists of:
real variation, reflecting actual ranges in biological responses
lack of certainty, regarding basic physical, chemical, and biological properties and
processes (e.g., bioavailability, bioaccumulation)
assumptions in the models used to approximate key input values (e.g., dose-response an
exposure "models" used for criteria derivation)
measurement error
It is important to understand that uncertainty includes both real variation (reflecting actual, mechanistic
biological response ranges and variability in ecosystem conditions) and error. Thus, because biological
systems are inherently uncertain and variable, some component of variability in risk estimation is due to a
realistic reflection of ecological conditions, while another component is due to "error" or uncertainty
introduced by the overall analytical process. However, it is critically important to understand that natural
ecosystem variability represents an important source of uncertainty to the evaluation of the ecosystem.
While this parameter is paramount to the making of risk management decisions, it cannot be controlled.
Therefore, "error" is the component to be minimized, because this encompasses undesirable uncertainty
that has been introduced by the assessment process and can. to some degree, be controlled.
Database
The analytical database has inherent uncertainties. For example, sampling was generally concentrated in
areas of known elevated concentration so that estimates of the actual underlying distribution of PCOCs
made from the dataset may be biased conservatively. In addition, when analytes were not detected in
environmental media, these data points were not included in the statistical analysis. Although this
conclusion is based upon the available evidence, it is possible that metals could have been present at
concentrations below the method detection limit. This was not felt to impact the conclusions reached
because the detection limits were always lower than the concentrations known to cause adverse effects.
Nevertheless, this method introduces uncertainty which cannot be quantified. An attempt to limit this
source of uncertainty was made by using the median concentrations rather than the means. Medians are an
expression of central tendency which is less biased by either high or low values and is more appropriate for
animals which may or may not contact a specific area where data were collected.
G:\wpdurW~~\boIdm-2\ni74.doc 7-7 1
17693405419Uuly 27.1999;5:16 PMDRAFT FINAL RI REPORT