In a previous blog post (Getting a Head Start on Groundwater Compliance for Coal Ash Units), we discussed common early warning signs associated with groundwater data. In this post, we talk about some of the possible actions to take once you become aware of one or more of these warning signs. By addressing potential data issues early on, it will be easier to make timely adjustments to your Detection Monitoring Program (DMP); confirm and demonstrate the cause of a statistically significant exceedance (SSE); and/or provide adequate justification of an alternate source. This could mean the difference between continuing with your DMP or initiating assessment monitoring, corrective action, or even closure of your coal ash unit sooner than planned.
At the outset, you should validate analytical data via an independent (non-laboratory) process and cross check field sampling procedures/notes. This will identify potential laboratory errors or perhaps help detect field errors (e.g. mislabeling issues).
Significant isolated data inconsistencies within the same well from one event to another, or between wells in the same event, are often attributed to poor well construction or improper sample collection. The first step in evaluating the data is to confirm that field parameters (e.g., turbidity, pH, etc.) stabilized during well purging prior to sample collection. A rapid purge rate can result in turbid samples, which may lead to higher total concentrations of constituents adsorbed to sediments suspended in the groundwater.
When sampling, consider using low-flow methods to minimize turbidity and collect more representative groundwater samples. The simplest way to verify that you have a representative sample is to resample the well(s) in question, which would also address potential mislabeling issues that could have caused inconsistent data. .
Additionally, you can analyze filtered samples to confirm if suspended sediment in the samples is influencing results. Improper well development can often result in sediment not being removed from the well or sandpack following installation. If inconsistencies exist between filtered and unfiltered data, redeveloping your well(s) may help. Higher than “normal” pH may suggest that the well seal materials have failed. If the issue is not resolved after redeveloping, the well may not be properly constructed and it may be worth reinstalling.
Seasonal variances may also contribute to inconsistent results between sampling events, but any changes would likely be consistent among multiple wells from one event to another, and not likely identified as a cause until multiple years of data are collected.
Consistently High or Upward Trending Concentrations in Isolated Downgradient Wells
Wells where specific concentrations remain elevated or trend upwards may be associated with well development or construction issues. In this case, take similar actions as you would to address data inconsistencies, as described above. The problem may be a poorly designed well due to an improper filter pack and/or screen size. Elevated data in a single downgradient well may also indicate the well is located in a different groundwater unit; additional wells will likely need to be installed to confirm.
Elevated Concentrations in Multiple Downgradient Wells
High groundwater concentrations in downgradient wells, where there is only one background well for comparison, could indicate that groundwater in the background well does not accurately represent actual conditions. This can be confirmed and remedied by installing additional background wells.
Higher than Anticipated Groundwater Levels
If water levels are higher than expected in one or more wells, it may indicate that a higher water-bearing zone is present. This condition should be carefully examined, as an outside party could argue the separation distance between the base of your impoundment and the uppermost aquifer is inadequate (i.e., less distance than required by the CCR Rule).
It may be possible to argue that some higher water-bearing units encountered beneath an impoundment are localized and/or do not produce enough water to be considered the uppermost aquifer. Consider researching what criteria Federal agencies such as the United States Environmental Protection Agency (US EPA) or the United States Geological Survey (USGS), or your State regulatory agency uses to define an aquifer.
If None of the Above
Elevated or upward trending data in your background well(s) could indicate the presence of another source of contamination. Strategically placed upgradient wells will help to confirm or refute the presence of an alternate source.
Downgradient well concentrations that appreciably exceed background concentrations may indicate a leaking CCR unit. Data should be evaluated early on to identify and eliminate potential outlying causes. Additionally, if a release is subsequently confirmed, identifying this potential situation early on, and planning ahead, can save money and future headaches.
After an SSE is confirmed, you have 90 days to take action prior to implementing assessment monitoring. If you are able to identify that an error occurred during one or more of the sampling events or a natural variance led to the SSE, a report will need to be prepared documenting how the error occurred and justifying the impact on the statistical evaluation. This report must be certified by a qualified professional engineer. Alternatively, if you can provide adequate justification that an alternative source caused the SSE, a similar demonstration report will be needed. If after 90 days you are unable to successfully demonstrate an error, natural variance, or alternate source resulted in the SSE, you must begin assessment monitoring.
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