C.2 Study Question 2: Are concentrations greater than background concentrations?

Determining whether a site's groundwater has been impacted usually requires either comparison of site data to a single criterionGeneral term used in this document to identify a groundwater concentration that is relevant to a project; used instead of designations such as Groundwater Protection Standard, clean-up standard, or clean-up level. derived from concentrations measured in backgroundNatural or baseline groundwater quality at a site that can be characterized by upgradient, historical, or sometimes cross-gradient water quality (Unified Guidance). samples, or a direct comparison of the site data to the background data set. To determine the statistical tools which will meet your specific needs, identify the type of comparison to be made (that is, comparison to a single criterion, or a two data set comparison). The distribution assumption for the data set, and whether interwellComparisons between two monitoring wells separated spatially (Unified Guidance). or intrawellComparison of measurements over time at one monitoring well (Unified Guidance). tests will be used, also determine the selection of the proper statistical methods. Note that the site data and the background data to which they are compared must share the same hydrogeologic and geochemical parameters (see Section 4.3.1: Physical Site Conditions and Section 4.2.1: Background Conditions).

This question is usually relevant in the release detection, site characterization, monitoring, and closure stages of the project life cycle.

Selecting and Characterizing the Data Set

Determine that the data sets to be compared meet the assumptions of the test to be used. Verify that the background data set is representative (see Study Question 1). Refer to Section 3.4 for further discussion of how the following requirements may impact statistical analysis results.

Statistical Methods and Tools

After checking that the data meet prerequisites common to most statistical tests, determine which tests will provide the information needed using the data you have or data that you will collect. Depending on the source of the background data, comparisons will either be interwell or intrawell. Background data set development is discussed in Study Question 1.

There are two general approaches for analyzing the site data and determining whether site chemical concentrations are above those measured in the background; individual compliance samples can be compared to pooledGroundwater samples from more than one sampling point. background results, or pooled compliance samples can be compared to pooled background. Site-specific considerations and regulatory considerations usually determine which approach is used. In either case, 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). methods are available, so determining the distribution of the data is a typical initial data examination step (see Section 3.4.3). Prediction limits, tolerance limits, and control charts allow individual samples (an individual well sampled in time for control chartsGraphical plots of compliance measurements over time; alternative to prediction limits (Unified Guidance).) to be compared to pooled background samples. T-tests and ANOVAone-way analysis of variance-type tests only allow for comparing of pooled compliance samples to pooled background samples.

Tests That Support Examination of Individual Sample Points

Prediction Limits

Prediction limits (PLs) estimate an interval in which future observations will fall, with a defined probability, given the data which had been collected. The calculation of PLs takes into consideration the number of future results to be compared as well as the number of retests required to confirm a release. Once a background data set is established, prediction limitsIntervals constructed to contain the next few sample values or statistics within a known probability (Unified Guidance). based on the data are used as the criteria for comparison of compliance samples.

Control Charts (individual wells over time)

Tolerance Limits

Tests that Support Examination of Pooled Data

In many situations, such as site characterization, it is desirable or advantageous to compare pooled data sets. A key assumption when pooling data from multiple sampling points, is that variability between wells is minimal; however, in many natural systems this spatial variabilitySpatial variability exists when the distribution or pattern of concentration measurements changes from well location to well location (most typically in the form of differing mean concentrations). Such variation may be natural or synthetic, depending on whether it is caused by natural or artificial factors (Unified Guidance). is too great to be ignored and therefore, it should be tested before pooling data. This is particularly true when pooling data to represent background concentrations.

Sometimes, pooling background data is appropriate. For example, when building a background data set, it may be possible to combine data sets that are thought to be background, but which were collected at different times or were spatially separated from one another. Even though it would be exceptional, given the typical variability in groundwater chemical concentrations, monitoring networks that have low natural spatial variability like sand aquifers or artificial systems, may be examined by pooling data.

Parametric Two-Sample T-Tests

Parametric ANOVA F-test

Nonparametric Two-Sample Tests

Wilcoxon rank sum test (Mann-Whitney U-test)

Tarone-Ware Two-Sample Test for Censored Data

Nonparametric Kruskal-Wallis test

The Kruskal-Wallis test is a nonparametric counterpart to ANOVAone-way analysis of variance that does not require normality of the ANOVA residuals (see Chapter 17.1.2, Unified Guidance and Section 5.8.2). In using this test, the interpretation is similar to the parametric F-test.

Related Study Question

Study Question 1: What are the background concentrations?

Key Words: Background, Compliance Monitoring, Interwell, Intrawell, Concentration Comparisons, Release Detection, Site Characterization, Monitoring, Closure


Publication Date: December 2013

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