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Adaptive Monitoring Programs

Discussion with examples of how long-term monitoring programs can benefit from dynamic work strategies.

Long-term monitoring programs can also benefit from dynamic work strategies. Long-term monitoring programs are usually associated with groundwater contamination and remedial actions that have life spans measured in years or even decades. They include caps, subsurface barriers, natural attenuation, active bioremediation, reactive subsurface barriers, sequestration techniques, and confined disposal facilities. The purpose of long-term monitoring programs is to evaluate the efficacy of the on-going remedial effort, and to identify deviations from expected performance that would be of concern. Examples deviations include elevated contaminant levels in wells down-gradient from a disposal facility, evidence from potentiometric surfaces that contaminated groundwater might be circumventing a subsurface barrier, contamination above cleanup levels in compliance wells, asymptotic behavior in groundwater contamination concentrations that leaves residual contamination levels above cleanup guidelines, etc.

Standard subsurface monitoring programs typically collect samples on a quarterly basis, or perhaps even less frequently. A justification for this monitoring frequency is that subsurface processes evolve slowly. In this context, the benefit of "real-time" measurement results is that they can be used to screen for contaminants of concern at significantly reduced costs, with their results used to determine whether additional monitoring points need to be sampled (i.e., adaptive location selection) or whether samples need to be sent for more definitive analysis (i.e., adaptive analytics selection) or whether the frequency of sampling should be revisited.

In situ depth-to-water table sensors are an example of where inexpensive real-time monitoring results can be used to significantly improve site understanding and decision-making, particularly for near surface contamination events where subsurface water movement is driven by rainfall events, there are significant seasonal changes in flow patterns, and/or the remedial activity has a significant impact on groundwater movement. These types of sensors can provide temporally dense (e.g., a measurement every hour or less) synoptic data sets that allow a monitoring intensity that is simply not possible using standard manual depth-to-water table measurements. The results can be used to better understand subsurface responses to external stresses (e.g., rainfall events, variations in pumping rates, etc.), to determine when a system is at risk of failing (or conditions under which it fails) such as loss of hydraulic control by a reactive barrier or pump and treat system, and also to identify sampling windows when a monitoring well would be most likely to yield useful information about the presence or absence of contamination.

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