Published in the June 2020 Focus on the Environment Newsletter
Let’s face it, many of the monitoring wells we routinely sample are getting old, just like us. Those wells born back in the glory days of the environmental industry have stood silent witness over the years as we tirelessly experimented with different purging methods and sampling protocols in search of the perfect representative sample. If there is one truth we can glean from past experience, it’s that you can change the sample method, the analyte list, the laboratory methods, and you can even change who and what collects the sample, but the monitor well is the one constant that doesn’t change. Its depth, diameter and screened interval were fixed in time on the day it was installed and short of complete replacement, there is little that can be done to alter that reality. Therefore, if the integrity of the monitor well is compromised, it does not matter what fancy sampling protocol you use, the chances of collecting a representative groundwater sample are thin. With that in mind, it makes sense that we should do our best to protect and maintain these silent, faithful sentinels. Based on our experience, a cost-effective approach to ensure the long-term integrity of a monitor well network is through routine inspection of both the outside and inside of the monitor wells.
Routine monitor well inspections and maintenance are important to ensure the integrity of a monitor well or monitor well network. Properly maintained wells are crucial to obtaining quality samples representative of natural groundwater conditions. Visual surface inspections are typically performed during event-based sampling; but often, problems affecting groundwater sample collection and quality lie hidden below the ground surface. Potential problems include grout intrusion from well installation, broken or separated casing, foreign objects preventing access to the entire screened interval and/or cross contaminating your sample, biofouling, or a silt-plugged well screen. Each of these conditions compromise the integrity of the well and the ability to collect representative groundwater samples. Furthermore, none of these conditions would be apparent from a visual surface inspection and, depending on the severity, may not be noticed during the course of sampling. Left unnoticed, any of these conditions would result in the collection of nonrepresentative samples, which over the long term leads to frustration and wasted resources attempting to validate and/or interpret the data.
Continuing to collect samples from compromised or deficient wells can lead to data sets showing trends unrelated to actual groundwater quality. When an issue as seemingly small as a silted well screen goes without proper maintenance, high turbidity and lengthy stabilization time adds irritation and wasted time during low-flow sampling. Additionally, increased turbidity due to poorly maintained wells can cause an overestimation in metals concentrations that can require costly confirmation sampling to disprove.
Most monitor well integrity evaluation is done through a combination of visual inspection of the surface components and verification of total depths. Measuring a well’s total depth is a good way to monitor for obstructions and sediment accumulation over time and to keep track of how much screen is open to the formation. Tracking depths over time is helpful for determining when a well needs redevelopment to remove accumulated sediment. Extreme changes in total depth between monitoring events can be signs of vandalism or a catastrophic failure of the well casing or screen. Physical measurement of the total depth of a well comes with its own set of potential complications, all of which can be addressed with some planning and proper equipment (look for a future article on this topic). Although total depth measurements can tell you that the well has filled with sediment or is obstructed, it does not tell you what is causing the problem. For that reason, we often recommend periodic down-hole video inspection of monitor wells. If we know the why or what is causing the problem, we can evaluate options for removing the obstruction, repairing the well and returning the well to service.
Video inspections provide real-time evidence of obstructions, foreign material, broken casings, or sediment-plugged well screens, all of which have the potential to alter apparent groundwater quality. The following is one example of how Cox-Colvin has used downhole video inspections to successfully troubleshoot monitor well issues.
A potentiometric map constructed from water levels measured during a recent sampling event indicated the unusual presence of a “groundwater mound.” Closer inspection of the data revealed that the “groundwater mound” was the result of an abnormally high groundwater level measured in a single monitor well. Based on the site conceptual model, hydrogeologic conditions at the site were such that it was unlikely that the “groundwater mound” reflected actual groundwater conditions, thus it was concluded that the monitor well was somehow damaged and consequently no longer a viable monitoring point. However, prior to abandoning and replacing the well, a downhole video inspection was performed to characterize the damage.
A damaged PVC threaded joint was noted just below the ground surface allowing for the infiltration of surface water. Instead of spending time and resources to replace the damaged well, maintenance activities were performed to repair the issue. The protective casing and well pad were removed, the damaged PVC joint was cut, and a PVC coupling put in place to complete the remaining stickup structure. A new well pad and protective casing was installed, and the well was resurveyed for accuracy. Subsequent groundwater levels are consistent with historical data. Shifting of the original concrete well pad from years of freeze-thaw activity was determined to be the underlying cause.
While a down-hole camera can easily detect a compromised PVC joint, they have also shown that the cause of elevated pH in groundwater was due to grout contamination within the screened interval of a newly installed well, and not due to a contaminated aquifer. Instead of assuming a historic release in the area and the subsequent expansion of the monitor well network, the well was over-developed to remove the grout contamination, resulting in typical pH measurements.
At another site, minor total depth discrepancies were noted for several monitor wells over time. After redevelopment efforts failed to resolve the total depth discrepancies, downhole video inspections were performed to identify what created the discrepancy. In each case, a short section of sample tubing was found at the bottom of the wells. The tubing was easily retrieved, and future sampling was completed without worries as to the representativeness of samples.
Down-hole cameras are quick to set up and inspections can be performed from start to finish in less than thirty-minutes. Most cameras are capable of recording, so imagery can be shared or viewed again on a later date.
There are many benefits to using real-time imagery for well inspections. Routine monitoring and video inspections are recommended to stay on top of your well’s health. Using these evaluation tools may help to eliminate or ease the costs of future maintenance, ineffectual well replacements, or worse yet, trying to remedy a groundwater problem that doesn’t truly exist. When was the last time you gave your wells some extra TLC?
Colton Creal is a Staff Scientist with Cox-Colvin & Associates, Inc. He holds a Bachelor of Science degree in Environmental Geology from Ohio University. He has been with Cox-Colvin since earning his degree in 2015. Mr. Creal is highly experienced in the collection and analysis of environmental data of all types, and has personally experienced the benefits of proper well maintenance.