C.1 Study Question 1: What are the background concentrations?

Background is defined as groundwater which is not influenced by the releases from a site (Section 4.2.1). Specifically, a backgroundNatural or baseline groundwater quality at a site that can be characterized by upgradient, historical, or sometimes cross-gradient water quality (Unified Guidance). groundwater data set may represent either a location or a time period that has not been influenced by a release from the site.

While it may be convenient to think of the background concentration as a single value, the concentration of a chemical will naturally vary both spatially and temporally. The analytical process introduces additional variability. Because of both natural and introduced variability, background can best be understood as a distribution, prompting the question: “Is the distribution of concentrations consistent with the distribution of concentrations in background?”

Background data may be collected by either of two methods. Data may be collected from a number of different wells (interwellComparisons between two monitoring wells separated spatially (Unified Guidance). data collection). Data may also be collected from the same well over time (intrawellComparison of measurements over time at one monitoring well (Unified Guidance). data collection; see Section 3.6.4). If interwell comparisons are desired, a hydrogeologic assessment must be performed to evaluate whether the upgradient and downgradient wells are appropriately grouped. For example, are the wells screened in the same geologic formation (Section 4.2.2)? If representative upgradient wells are not available for use as background, or if 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). exists among background wells, intrawell comparisons may be better for evaluating background conditions. Intrawell evaluations assume that the background time period is uninfluenced by chemicals from the site.

Chemicals present at background concentrations in the groundwater may be either naturally occurring or anthropogenic. Naturally occurring substances present in the environment are those that are not a result of human activity. "Anthropogenic substances are natural and human-made substances present in the environment as a result of human activities not specifically related to the site in question" (USEPA 2002c).

Tests used to determine if a particular data set is consistent with background assume appropriate data collection and focus on data characterization.

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

Selecting and Characterizing the Data Set

Verify that the collected data set represents background. Using graphical methods (Section 3.3 and Section 5.1) and distributions, establish that a collected data set is consistent with background (natural or anthropogenic).

Statistical Methods and Tools

Probability Plots

Time Series Plots

Outlier Identification

Interpretation of Results and Associated Uncertainty

The natural variation, the anthropogenic variation, or both variations in concentrations must be understood before developing background values. The expected distribution, character of the probability plot, the potential concentration variation across seasons as well as over time, and the occurrence of apparent outliers are all a function of the chemical and its environmental setting. Examine published studies regarding the occurrence of the chemical to determine which analyses should be emphasized or more heavily weighted in decision making.

Based on the qualitative examination of the background data set, you may choose to analyze the data set and present its basic statistical characteristics. See discussions on characterizing the data set presented in Section 3.3.3, Section 5.1, and Section 5.6.

The background value is not determined only once. The individual wells or groups of wells which were used to support background determination or comparisons may develop trends. These trends could result from new contaminant sources influencing previously unimpacted wells. These trends could also result from changes in groundwater flow or chemistry. Trends that are not sustained could also result by chance.

Regardless of the reason for changes, background data must be updated. How often the data must be reconsidered for update depends on site-specific parameters such as groundwater flow velocity, nearness of other potential sources of contamination, and geochemistry. Frequency of updating the background data is also dependent on having sufficient new data to statistically identify a change; the Unified Guidance suggests four to eight new data points. The new data may be compared to the historical data by either the parametric t-test or the nonparametricStatistical test that does not depend on knowledge of the distribution of the sampled population (Unified Guidance). Wilcoxon rank sum test depending on the distribution of the pooledGroundwater samples from more than one sampling point. intrawell data. If there is no significant difference between the new data and the historical data, then the new data can be considered background. Additionally, the absence of a trend in the data when historical and new data are combined, is indicative of background; see Section 5.5, Section 5.5.1, and Section 5.5.2.

Related Study Questions

Study Question 2: Are concentrations greater than background concentrations?

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


Publication Date: December 2013

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