C.7 Study Question 7: What are the contaminant attenuation rates in wells?

When contaminant concentrations are identified as having a trend over time (see Study Question 5), this question follows to estimate the rate of change over time (attenuation rate). You can then use the attenuation rate to evaluate whether the rate of decrease in concentration is adequate to achieve the site goals. In addition, the attenuation rate can be used to estimate future concentrations including when concentrations will reach a cleanup 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. (see Study Question 4).

When using a data set from a single well or a set of wells in the source area, the attenuation rate determined from the concentration versus time data (sometimes called a "source attenuation rate") represents source depletion (Newell et al. 2002). In contrast, the decrease in concentration over distance downgradient of the source is called a "plume attenuation rate" (see Study Question 8).

This question is usually relevant in the remediation, monitoring, and closure stages of the project life cycle.

Selecting and Characterizing the Data Set

Verify that the data set can support trend analyses and modeling. Refer to Section 3.4: Common Statistical Assumptions for further discussion of how the following requirements may impact statistical analysis results.

When you evaluate multiple time intervals from a single monitoring record in order to identify changes in attenuation rates, be sure to evaluate the uncertainty in the attenuation rate estimates (the confidence bands) in order to determine whether an apparent difference in attenuation rates is most likely to be associated with true change in the source attenuation rate or an artifact of shorter-term random variability.

Determining whether two attenuation rates are different requires an evaluation of the uncertainty associated with each attenuation rate. The greater the difference in the attenuation rates relative to the uncertainty associated with each rate, the greater the confidence that the observed difference in the attenuation rates is real. See the case example in Appendix A.7.

Statistical Methods and Tools

Attenuation rates can be estimated using parametricA statistical test that depends upon or assumes observations from a particular probability distribution or distributions (Unified Guidance). or nonparametricStatistical test that does not depend on knowledge of the distribution of the sampled population (Unified Guidance). methods. Both methods require assumptions about the concentration trend, for example zero order (linear trend over time) or first order (exponential decay). For groundwater monitoring data, exponential decay is a commonly observed long-term trend (Newell et al. 2002). Therefore, the attenuation rate is usually best represented by a first-order decay rate.


Theil-Sen Trend Line

Interpretation of Results and Associated Uncertainty

When evaluating temporal trends in groundwater monitoring results, differences in results between wells are often present. Even at sites with overall decreasing contaminant concentrations, the trend analysis can identify some wells with statistically significant decreasing concentrations, some wells with apparently decreasing concentrations that are not statistically significant, and some wells with apparently increasing concentrations. In many cases, the apparent differences in concentration trends between wells can be attributed to random variability in the monitoring data rather than real differences in attenuation rates between wells.

A key challenge in the evaluation of concentration trends for multiple wells is determining whether these differences are due to random variability in monitoring results or due to true differences in attenuation between wells. This determination should be based on lines of evidence such as:

Unless a majority of the lines of evidence suggest true differences in attenuation rates between wells, then it is likely that the observed differences are due to random variation. For a group of wells without clear differences in attenuation rates, the best estimate of the overall plume attenuation rate can be obtained by either using a midpoint attenuation factor such as the average or medianThe 50th percentile of an ordered set of samples (Unified Guidance). attenuation factor or evaluating the attenuation rate for groundwater concentrations that are representative of the group of wells (such as the average, median, or maximum concentration for each monitoring period).

When comparing attenuation rates, the evaluation of whether the attenuation rates are different is not an absolute yes or no. The results should influence the level of confidence in the conclusion. If the confidence bands for two attenuation rates almost overlap, then confidence in the conclusion that the attenuation rates are different should be lower than if the confidence bands are separated by a greater distance.

The 95% confidence band for the attenuation rate reflects the uncertainty associated with the estimate of the attenuation rate based on the variability in monitoring record that is not explained by the long-term trend. However, other sources of uncertainty may not be captured by the statistical analysis. For example, if a monitoring record was collected during an extended drought when the water table was dropping over time, then you may have less confidence that the observed rate of attenuation would continue during a subsequent period of normal precipitation. For an evaluation of whether two attenuation rates are different, consider the statistical analyses discussed, the complete conceptual site model (CSM)A living collection of information about a site which considers factors such as environmental and land use plans, site-specific chemical and geologic conditions, and the regulatory environment (ITRC 2007b)., and any other available information that may be relevant to the determination.

See also Study Question 7, Section 4.5.1: Monitoring for Concentration Changes, and Section 4.6.2: Trends Toward Compliance Criteria.

Related Study Questions.

Study Question 8: How do contaminant concentrations change with distance from the source area?

Key Words: Attenuation, Contaminant attenuation rate, changing contaminant concentrations, Remediation, Monitoring, Closure


Publication Date: December 2013

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