Dennis R. Helsel

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A great many papers and one textbook have been published on the topic of how to incorporate 'nondetects', low-level values reported only as below a detection limit, into statistical analyses. This is of interest not only in occupational hygiene but also in environmental sciences and astronomy, among other fields. Here, the literature is reviewed from the(More)
Throughout the United States increasing demands for safe drinking water and requirements to maintain healthy ecosystems are leading policy makers to ask complex social and scientific questions about how to assess and manage our water resources. This challenge becomes particularly difficult as policy and management objectives require scientific assessments(More)
Low-level contaminants often are present below the detection or reporting limits of a laboratory, resulting in values reported as a nondetect or less-than. How can these values be summed along with detected concentrations to obtain a total, particularly when weighting factors such as toxic equivalence factors (TEFs) are used? The most common method employed(More)
Perchlorate ion (ClO4-) is an environmental contaminant of growing concern due to its potential human health effects, impact on aquatic and land animals, and widespread occurrence throughout the United States. The determination of perchlorate cannot normally be carried out in the field. As such, water samples for perchlorate analysis are often shipped to a(More)
Previously reported dendrochemical data showed temporal variability in concentration of tungsten (W) and cobalt (Co) in tree rings of Fallon, Nevada, US. Criticism of this work questioned the use of the Mann-Whitney test for determining change in element concentrations. Here, we demonstrate that Mann-Whitney is appropriate for comparing background element(More)
AIM Analysis of wastewater samples can be used to assess population drug use, but reporting and statistical issues have limited the utility of the approach for epidemiology due to analytical results that are below the limit of quantification or detection. Unobserved or non-quantifiable-censored-data are common and likely to persist as the methodology is(More)
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