Dennis R Helsel

Dennis R Helsel
Practical Stats · Main Office

Ph.D Environ. Sci & Engineerin

About

71
Publications
19,113
Reads
How we measure 'reads'
A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more
9,388
Citations
Additional affiliations
May 2008 - present
Practical Stats
Position
  • Applied Statistician
May 2008 - December 2015
Independent Researcher
Position
  • Applied Statistician
June 1978 - May 2008
United States Geological Survey
Position
  • Geologist/Hydrologist/Statistician
Education
August 1975 - May 1978
Virginia Polytechnic Institute and State University
Field of study
  • Environmental Science and Engineering
August 1973 - June 1975
University of Delaware
Field of study
  • Geochemistry
September 1969 - May 1973
University of Rochester
Field of study
  • Geology

Publications

Publications (71)
Article
Full-text available
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 fr...
Article
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...
Book
Praise for the First Edition . . . an excellent addition to an upper-level undergraduate course on environmental statistics, and . . . a 'must-have' desk reference for environmental practitioners dealing with censored datasets. -Vadose Zone Journal. Statistical Methods for Censored Environmental Data Using Minitab® and R, Second Edition introduces...
Book
Statistical Methods in Water Resources 2020 is available as a free pdf. Download at https://doi.org/10.3133/tm4a3 . It is also available as a hardback book for under $20 USD at https://store.usgs.gov/product/533012 (extra for shipping outside of the US). It is intended to be a text in applied statistics for hydrology, environmental science, envir...
Presentation
Full-text available
Seven common errors in statistical analysis by environmental scientists all stem from an outdated understanding of statistics. I’ll define the seven ’perilous errors’ and how each can be avoided. They revolve around old ideas about hypothesis tests, p-values, using logarithms of data, evaluating what is a good regression equation, evaluating outlie...
Experiment Findings
Past webinars are available for viewing at https://practicalstats.teachable.com . Click on "View More Courses". New webinars are announced on our webpage, practicalstats.com, and in our free newsletter. Next webinar is Sept 29, 2020.
Experiment Findings
Statistical Methods in Water Resources 2020 is now available for download (free pdf) at https://doi.org/10.3133/tm4A3 A hardcover textbook version is available for purchase at https://store.usgs.gov/product/533012 . It costs $18.49 USD. There is a $5 shipping and handling fee for standard delivery in the US. There are extra fees for rush shipping...
Presentation
R is one of the most widely used statistics software packages in the world. Its versatility as a programming language and its interconnectivity with email, web page generation and other computer processes make it a bit daunting for people just starting to use it for data analysis. It need not be that way. This webinar introduces you to R software a...
Article
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 methodo...
Presentation
Full-text available
Shortly after World War II, conventional military munitions were disposed of in relatively shallow (<100 m) waters off the leeward side of O‘ahu, Hawai‘i. The Department of Defense refers to this area as Hawaii Sea Disposal Site 6 (HI-06). Local residents, who refer to the area as Ordnance Reef, expressed concerns regarding the effects of these dis...
Conference Paper
Optical models have traditionally been computed using Model I least squares linear regression. However, least squares linear regression is appropriate only when the causal (x) variables are measured with little error in comparison to the response (y) variable. We instead use Model II geometric mean functional regression (GMFR), as all our variables...
Article
Regional-scale variations in soil geochemistry were investigated in a 20,000-km2 study area in northern California that includes the western slope of the Sierra Nevada, the southern Sacramento Valley and the northern Coast Ranges. Over 1300 archival soil samples collected from the late 1970s to 1980 in El Dorado, Placer, Sutter, Sacramento, Yolo an...
Chapter
Detection limits are established within a chemical laboratory to designate low-level data that cannot be distinguished from a zero concentration. Higher limits also are used to denote data with low, nonzero concentrations that are too imprecise to report as distinct numbers. Data below both types of limits are reported to the customer as a “nondete...
Article
Full-text available
Methyl tert-butyl ether (MTBE) concentrations > or = 0.2 /microg/L were found in samples of untreated water in 18% of public-supply wells (n = 284) and 9.1% of private domestic wells (n = 264) sampled in 2005 and 2006 in New Hampshire. In counties that used reformulated gasoline (RFG), MTBE occurred at or above 0.2 microg/L in 30% of public- and 17...
Article
Analysis of low concentrations of trace contaminants in environmental media often results in left-censored data that are below some limit of analytical precision. Interpretation of values becomes complicated when there are multiple detection limits in the data—perhaps as a result of changing analytical precision over time.Parametric and semi-parame...
Article
The most commonly used method in environmental chemistry to deal with values below detection limits is to substitute a fraction of the detection limit for each nondetect. Two decades of research has shown that this fabrication of values produces poor estimates of statistics, and commonly obscures patterns and trends in the data. Papers using substi...
Article
Trends in environmental variables are often investigated within a study region at more than one site. At each site, a trend analysis determines whether a trend has occurred. Yet often also of interest is whether a consistent trend is evident throughout the entire region. This paper adapts the Seasonal Kendall trend test to determine whether a consi...
Article
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 ship...
Article
Trace contaminants in water, including metals and organics, often are measured at sufficiently low concentrations to be reported only as values below the instrument detection limit. Interpretation of these “less thans” is complicated when multiple detection limits occur. Statistical methods for multiply censored, or multiple-detection limit, datase...
Article
Environmental data often include low-level concentrations below reporting limits. These data may be reported as “< RL,” where RL is one of several types of reporting limits. Some values also may be reported as a single number, but flagged with a qualifier (J-values) to indicate a difference in precision as compared to values above the RL. A current...
Article
Fifteen years after writing a feature article about methods for the statistical analysis of non-detect data-those concentrations that are only known to be somewhere between zero and a laboratory's reporting level-Dennis R. Helsel of the U.S. Geological Survey returns to survey the current state of the art. Although the statistical methods have impr...
Article
Trace-element concentrations in baseline samples from a survey of aquifers used as potable-water supplies in the United States are summarized using methods appropriate for data with multiple detection limits. The resulting statistical distribution models are used to develop summary statistics and estimate probabilities of exceeding water-quality st...
Book
Out of print. Was replaced by the 2nd edition, titled "Statistics for Censored Environmental Data Using Minitab and R". See that entry on my list of publications.
Article
Complications arise when ratios are used to present environmental data because ratios are an unbounded, multiplicative scale that can lead to asymmetrical (skewed) data distributions. Enantiomeric ratios (ERs), historically used in discussions of chiral signatures, often are published as mean ER+/-single-value standard deviation. Application of sta...
Data
Data from the 2002 electronic version of Statistical Methods in Water Resources by D.R. Helsel and R.M. Hirsch. Published by the U.S. Geological Survey in 2002 as an update to the 1992 print version published by Elsevier Publishers.
Article
Skewness of environmental data is often caused by more than simply a handful of outliers in an otherwise normal distribution. Statistical procedures for such datasets must be sufficiently robust to deal with distributions that are strongly non-normal, containing both a large proportion of outliers and a skewed main body of data. In the field of wat...
Book
Has been replaced by the 2020 edition, with all new R code and updated methods. Download the 2020 version at https://doi.org/10.3133/tm4a3 . The ebook sold by Elsevier is the outdated first edition. The full text of the newer 2020 version at https://doi.org/10.3133/tm4a3 is much more current, and free.
Article
Full-text available
Data assembled by the U.S. Geological Survey (USGS) from nearly 30,000 sources suggest that ground water in the United States typically contains low arsenic concentrations: less than 1 part per billion (ppb), which is equal to about one drop in an Olympic-size swimming pool. However, moderate to high concentrations do occur in some areas throughout...
Article
Concentrations of naturally occurring arsenic in ground water vary regionally due to a combination of climate and geology. Although slightly less than half of 30,000 arsenic analyses of ground water in the United States were 1 μg/L, about 10% exceeded 10 μg/L. At a broad regional scale, arsenic concentrations exceeding 10 μg/L appear to be more fre...
Article
A total of 136 stream water and 143 groundwater samples collected in five important hydrologic systems of the US were analyzed for microbiological indicators to test monitoring concepts in a nationally consistent program. Total coliforms were found in 99%, Escherichia coli in 97%, and Clostridium perfringens in 73% of stream water samples analyzed...
Chapter
Publisher Summary High arsenic (As) concentrations in ground water have been documented in many areas of the United States. Within the last decade, parts of Maine, Michigan, Minnesota, South Dakota, Oklahoma, and Wisconsin have been found to have widespread As concentrations exceeding 10 μg/L. These high concentrations most commonly result from the...
Article
Nitrate contamination of groundwater occurs in predictable patterns, based on findings of the U.S. Geological Survey's (USGS) National Water Quality Assessment (NAWQA) Program. The NAWQA Program was begun in 1991 to describe the quality of the Nation's water resources, using nationally consistent methods. Variables affecting nitrate concentration i...
Technical Report
Full-text available
Online at: http://pubs.usgs.gov/fs/fs218-96/ Nutrient pollution is not a new problem, but it is among the most persistent. Consequently, the status of nutrients is one of the first water– quality issues evaluated by the U.S. Geological Survey (USGS) National Water–Quality Assessment (NAWQA) program. Public concern about water pollution from nutri...
Article
Nutrients are essential for plant and animal growth and nourishment, but the overabundance of certain nutrients in water can cause a number of adverse health and ecological effects. To determine the extent of nutrient and other types of contamination in the Nation's streams and ground water, Congress has appropriated funds for a National Water-Qual...
Article
Full-text available
Broad patterns in the chemistry of surface or ground waters are the result of regional patterns of human activities and natural features. The relations between these patterns and water chemistry provide insight into cost-effective management methods for improving water quality. They allow prioritization of areas that have the greatest risk of conta...
Article
Water-quality conditions in surficial unconsolidated aquifers were assessed in five agricultural regions in the United States. The assessment covers the Delmarva Peninsula, and parts of Long Island, Connecticut, Kansas, and Nebraska, and is based on water-quality and ancillary data collected during the 1980s. Concentrations of nitrate in ground wat...
Article
The usefulness and sensitivity, of a portable immunoassay test for the semiquantitative field screening of water samples was evaluated by means of laboratory and field studies. Laboratory results indicated that the tests were useful for the determination of atrazine concentrations of 0.1 to 1.5 μg/L. At a concentration of 1 μg/L, the relative stand...
Book
This is the original first edition published as a physical book by Elsevier. It is woefully out of date. An updated electronic version was published in 2002 by the U.S. Geological Survey, and a completely revised 2020 version with updated methods and supporting materials is listed in my publication list, and is available for download at https://doi...
Article
The U.S. Geological Survey through its Toxic-Waste— Ground-Water Contamination Program began studies during 1984 to evaluate the degradation of regional ground-water quality as a result of human activities. The studies are designed to quantitatively relate human activities, expressed as land use, to the quality of ground water at a regional scale....
Article
This paper extends the work of Gilliom and Helsel (1986) on procedures for estimating descriptive statistics of water quality data that contain “less than” observations. Previously, procedures were evaluated when only one detection limit was present. Here we investigate the performance of estimators for data that have multiple detection limits. Pro...
Conference Paper
Water-quality data from 90 shallow wells screened in the upper glacial (water-table) aquifer beneath five different land-use areas in Nassau and Suffolk Counties, Long Island, were compared to assess the effects of human activities and land-use practices on ground-water quality. The areas, which range from 22 to 44 square miles, represent suburban...
Article
Montgomery and Loftis (1987) have listed several situations for which the t-test does not accurately reproduce Type I errors, and should therefore be avoided. Characteristics common to water quality data (skewness or other non-normality, presence of outliers and less-thans) also reduce the power of the t-test, in relation to nonparametric alternati...
Article
Water quality data are usually analysed with parametric statistical procedures requiring the normality assumption for accuracy of their attained significance levels. However, these data are typically non-normally distributed. When applied to non-normal data, the power of parametric procedures is low, and their results may be in error. Three typical...
Article
Bottom-material samples were collected at two sites on the Cuyahoga River in Cuyahoga and Portage Counties, Ohio. Multiple samples were collected from one cross section at each site and separated into three fractions based on particle size. Significant differences in metal concentrations were observed within cross sections as well as between sites....
Article
A recurring difficulty encountered in investigations of many metals and organic contaminants in ambient waters is that a substantial portion of water sample concentrations are below limits of detection established by analytical laboratories. Several methods were evaluated for estimating distributional parameters for such censored data sets using on...
Article
Estimates of distributional parameters (mean, standard deviation, median, interquartile range) are often desired for data sets containing censored observations. Eight methods for estimating these parameters have been evaluated by R. J. Gilliom and D. R. Helsel (this issue) using Monte Carlo simulations. To verify those findings, the same methods ar...
Article
Publisher Summary This chapter describes the aspects of estimating distributional parameters from censored data. The key aspects of estimating distributional parameters from censored data include (1) the performance of several estimation methods when estimating distributional parameters from small samples drawn from a wide range of underlying distr...
Article
Coal mining and reclamation activities have influenced low-flow water quality of streams in Ohio. In order to document how water quality has changed, and by how much, data collected from 779 sites during June 1975 through September 1982 were compiled and analyzed. Sites were classified into five categories: unmined (100% unmined land), abandoned (5...
Article
Stream water during fair weather (base flow) is largely ground water discharge, which has been in contact with minerals of the underlying aquifer. Base flow water quality should therefore reflect aquifer mineralogy as well as upstream land use. Three upstream mining categories (unmined lands, abandoned coal mines, and reclaimed coal mines) differed...
Article
The Occoquan and Four Mile Run watersheds drain lands ranging from farms and undisturbed forests to commercial and high-density residential areas. Stormwater drainage from 19 subbasins of differing land use characteristics were sampled during a 1-year period to determine the impact of nonpoint sources to the watershed. Seven metals were monitored....
Article
Composite precipitation samples were collected by recording raingages at nine stations in the Virginia portion of the greater Washington, D.C. metropolitan area. The samples were analyzed for COD, TOC, all nitrogen and phosphorus forms, and seven heavy metals; iron, zinc, lead, copper, cadmium, chromium and manganese. The results showed that wash o...

Network

Cited By

Projects

Projects (3)
Project
To demonstrate what is new in statistics as applied to environmental and natural resources science.
Project
Two of our long-time courses are now available at our Online Training Center, https://practicalstats.teachable.com. Your registration fee allows you one year access to the materials for one person, with emaill support from the Instructor, Dennis Helsel. Courses: 1. Applied Environmental Statistics, 2. Nondetects and Data Analysis. More information on each course is available on our training page, http://practicalstats.com/training/