D.3 ChemStat

Approximate Cost: $990; ChemPoint, which is data management software that supports ChemStat is an additional $300.

Source: http://www.pointstar.com/

Current Version: v6.3

Operating System Needs: Windows

Input Structure: tab delimited text file; Sanitas, DUMPstat, GRIT, ChemPoint files; converter program to import Microsoft Excel, Microsoft Access, and DBF files.

Overview

ChemStat can be used as an assessment tool for analyzing groundwater contamination. The software can calculate upper confidence limits, upper prediction limitsIntervals constructed to contain the next few sample values or statistics within a known probability (Unified Guidance)., and upper tolerance limitsThe upper or lower limit of a tolerance interval (Unified Guidance). for comparison to backgroundNatural or baseline groundwater quality at a site that can be characterized by upgradient, historical, or sometimes cross-gradient water quality (Unified Guidance). concentrations in groundwater or a designated value (such as MCLsmaximum contaminant levels). Other available tests that may apply to groundwater monitoring include one-way analysis of variance (ANOVA)A statistical method for identifying differences among several population means or medians., trend evaluation including seasonal analysis, outlier, and goodness-of-fit tests.

ChemPoint 7.0 is a data management system that organizes environmental data collected during water, soil and air sampling. Although not required, ChemPoint is commonly purchased along with ChemStat and serves as the data repository. ChemStat also interfaces directly with files from ChemPoint, Sanitas, DUMPstat, and GRITS programs.

In conjunction with the data repository, ChemStat is used to plot time-series graphs of data, identify outliersValues unusually discrepant from the rest of a series of observations (Unified Guidance)., test for normal data, and calculate concentration limits of both parametricA statistical test that depends upon or assumes observations from a particular probability distribution or distributions (Unified Guidance). and nonparametricStatistical test that does not depend on knowledge of the distribution of the sampled population (Unified Guidance). monitoring well data to be used for both intrawellComparison of measurements over time at one monitoring well (Unified Guidance). and interwellComparisons between two monitoring wells separated spatially (Unified Guidance). comparisons in order to evaluate potential groundwater contamination. ChemStat generates reports and various plots including box plots, concentration versus time plots, and probability plotsGraphical presentation of quantiles or z-scores plotted on the y-axis and, for example, concentration measurement in increasing magnitude plotted on the x-axis. A typical exploratory data analysis tool to identify departures from normality, outliers and skewness (Unified Guidance)..

Add-Ins Available

None

Ease of Use and Data Import

To input data, format the data into columns and generate an ASCII text file. A conversion utility is included to convert tab delineated text, Microsoft Excel files, Microsoft Access files and DBF files. You can provide additional variables for regression analysisA statistical tool for evaluating the relationship of one of more independent variables to a single continuous dependent variable (Kleinbaum et al. 2007). as part of concentration versus time plots; however, all other statistical methods are performed one parameter at a time. Select desired statistical tests from drop-down menus and relevant options from subsequent menus. ChemStat does support batch processing of statistical methods and graphs.

ChemStat is relatively straightforward to use; however, make sure that you understand the assumptions and input requirements for any statistical tests used to ensure validity and applicability in making decisions.

Types of Distribution

Data can be evaluated for fit to normal, lognormalA dataset that is not normally distributed (symmetric bell-shaped curve) but that can be transformed using a natural logarithm so that the data set can be evaluated using a normal-theory test (Unified Guidance)., or gammaA gamma distribution or data set. A parametric unimodal distribution model commonly applied to groundwater data where the data set is left skewed and tied to zero. Very similar to Weibull and lognormal distributions; differences are in their tail behavior, and the gamma density has the second longest tail where its coefficient of variation is less than 1 (Unified Guidance; Gilbert 1987; Silva and Lisboa 2007). distributions. Statistical interval test results are available for all of these distributions in addition to several nonparametric options.

Visualization

Plots available in ChemStat include box plots, concentration versus time plots, probability plots, and Shewhart-CUSUMcumulative sum control chart control charts for both intrawell and interwell comparisons. Limited options are offered for editing the output plots, and you can only export output to Microsoft Word or Acrobat PDF files. 

Primary Uses for Groundwater Data Analyses

ChemStat 6.3 is a Windows-based program used for analysis of groundwater, surface water, soil, or air quality monitoring data at RCRAResource Conservation and Recovery Act facilities. Analysis methods are consistent with the USEPA Unified Guidance document.

Benefits

• Relative simplicity of use
• Developed specifically for environmental applications
• Documentation is well-written and generally easy to understand

Limitations and Data Requirements

• Cost
• Primary use is calculation of upper statistical limits
• Cannot set percent false positive rates or false negatives to other values than those preset for statistical tests
• Limited options provided to modify plots and reports

 

Publication Date: December 2013