Statistical issues in assessing anthropogenic background for arsenic

Conceptual and statistical issues surrounding the estimation of a backgroun d concentration distribution for arsenic are reviewed. How background area is defined and samples collected are shown to impact the shape and location of the probability density function that in turn affects the estimation an d precision of associated distributional parameters. The overall background concentration distribution is conceptualized as a mixture of a natural bac kground distribution, an anthropogenic background distribution and a distri bution designed to accommodate the potential for contamination site samples being included into the background sample set. This concept is extended to a discussion of issues surrounding estimation of natural and anthropogenic background distributions for larger geographic areas. Finally, the mixture model is formally defined and statistical approaches to estimating its par ameters discussed. (C) 2001 AEHS.