Establishing Background Values in Geochemical Data

An important component of risk assessments is the comparison of guidelines derived through toxicological studies for elements or compounds against measures, or ranges of measures, that are considered to be "background" for the media for which the guideline has been developed. Over any given region of interest, there can be great variability in the geochemistry of the medium (e.g., soil, stream/lake sediment, water, etc.) for which a guideline was estimated. The element of interest will exhibit a range of values that reflect the underlying geology and more recent surficial and hydrological processes. When evaluating geochemical data, there are typically two issues of concern. Firstly, does the measurement represent natural abundances that occur for the specific medium, or, secondly, is the value the result of anthropogenic activity? Furthermore, does the value exceed guideline levels, e.g., those of the Canadian Council of Ministers of the Environment (CCME), considered harmful to human health or the surrounding natural ecosystem?

Natural geochemical variability cannot be expressed as a single number, but rather as a range of values. As a consequence, values that generally exceed (but also fall below) the expected range of values can be considered atypical and thus warrant further scrutiny in terms of risk management.

Tukey boxplots and other graphical descriptions provide a summary of background variability of trace elements upon which CCME guideline values can also be plotted. Data from the 2007 field studies in the Canadian Maritimes will be presented using these methods.