The final chapter of a well’s life should not be discounted.
By Marvin F. Glotfelty, RG
Water well professionals give a lot of attention to the design details of newly constructed water wells, and we also pay ample attention to the structure, performance, and subsurface environments of water wells that are being cleaned or rehabilitated during their years of use.
These considerations are no less important when a well has come to the end of its useful life and has reached that point in time when well destruction is warranted.
The process for destroying a well by backfilling it to seal it up is assigned different terms from state to state. The terminology used to describe destruction of a water well is also unique for various regulatory entities and different states. Well destruction may be described in terms of the well having been “abandoned,” “decommissioned,” or “plugged.” Regardless of what term is used to describe the act of permanently destroying a well, it should be done in such a way that the conduit into the aquifer has been removed.
One of the most obvious reasons to backfill inactive wells—especially wells that have had their pump equipment removed—is the need for securing the wellhead to prevent the possibility of animals or people from falling into it.
Such an incident famously occurred in 1987 when an 18-month-old girl named Jessica Morales (known as “Baby Jessica” as the event became national news) fell down a 22-foot-deep well in Midland, Texas. Although the well was only 8 inches in diameter, it was still large enough to fit the child. She was rescued after 58 hours and reportedly had a full recovery.
The near-tragic event underscores the importance of securing inactive wells at the land surface, but appropriate destruction of a well also involves sealing off the subsurface flow paths to prevent them from allowing naturally occurring or man-caused contaminants to migrate throughout the well and between aquifers.
Destruction of Filter Packed Wells
The potential flow pathways in a well that could potentially allow migration of contaminants include:
- The interior of the well casing and screen
- The annulus outside the well casing and screen
- Annular tubing strings, such as sounding tubes or gravel feed tubes (Figure 1).
We generally consider the annulus to be completely sealed with cement or bentonite material in the upper intervals of filter packed wells, and we also assume that the annulus adjacent to the well screen is completely filled with filter pack sand, as shown in Figure 1.
However, we should be mindful that in fractured rock aquifers, and even in unconsolidated alluvial aquifers, there may actually be voids in the annular seal or fill materials, so our assumption that the annulus is completely filled may be wrong.
We draw nice as-built diagrams of wells showing near-perfect plumbness and alignment, perfectly centered well casing and screen, centralized annular tubing strings, and complete filling of the annulus (Figure 1). However, we have no practical way to actually go down in the well and confirm what we’re reporting, so well records can be incorrect, and our as-built diagrams can be misleading.
To assure a truly impervious seal in the annulus of a filter packed well, it may be necessary to cut holes in the well casing or screen or mill out the bottom cap of the well. This will allow the filter pack sand to flow into the well where it can be removed by swabbing and bailing or airlifting, which will create a void in the annulus.
Once an annular void has been created, we can confidently seal off the empty annulus we just created (and can confidently document), to prevent future vertical migration of poor-quality water through that flow pathway.
Destruction of Wells with an Open Annulus
Many old wells and some newer wells have been installed with a well casing that was perforated in place, and no filter pack media filling the annulus. These wells are commonly perforated with Mills knife cuts or similar large-sized perforations that will not prevent native sand from being produced by the well as it is pumped.
There are also cases in some wells where bar holes, corrosion holes, or tears in the casing may result in a direct hydraulic connection between the interior of the well and the annulus outside the casing. In such wells, loose sand from coarse-grained formations will tend to be pulled into the well by the flow of groundwater as it is being pumped into the well, so the size of the annulus can become extreme at certain depth intervals.
Over many years of operation, such wells may form extremely large annular voids adjacent to sand formations but maintain a relatively minor annulus adjacent to silt or clay formations.
Variability in the size of the annulus at different depths generally results from the greater sediment cohesion and lower water production in fine-grained formations. The higher flow velocities and looser sediment characteristics of coarser sand and gravel formations make them much more susceptible to sediment production while the well is being pumped.
When a loose flowing sand is located immediately beneath a fine-grained cap formation, as shown by the lower sand formation in Figure 2, the fine-grained cap strata may have a roofing effect that can result in an annular void that extends horizontally to significant distances.
Stories are told by experienced drillers about annular voids they’ve encountered that are “large enough to park a truck in.” Given the right set of hydrogeological and well design circumstances, those stories can hold true.
Lessons Learned from Well Destruction Projects
Annulus Volumes in Mills Knife-Perforated Wells
I co-authored a Water Well Journal article with Meladie Nelson in 1992 (Whew, that’s 30 years ago!) that was titled “Well Annulus Volumes . . . How Large Are They?” In the article published in the July 1992 issue, we recounted a project we did that involved destruction of 44 old agricultural wells as part of the land sale of a large farm (about 42 square miles).
Each well was cleaned out to its total cased depth, and then properly destroyed by filling and sealing up the well interior and annulus. The buyer wanted confirmation that there would be no environmental impacts or liability from remaining debris or contaminants in any of the wells. Old wells in a remote area, such as was the case with this farm, are often uncapped for periods of time, so those wells are susceptible to potentially having had debris go in them over the years.
In some cases, the actual annular volume in a well can exceed its calculated volume (based on the drill bit size, casing diameter, and well depth) due to a combination of washed-out (enlarged) intervals of the borehole or invasion of annular fill materials into the formation.
To account for this, we generally assume 30% over calculated volume (calculated volume × 1.3) when preparing technical specifications for a new well. For many older wells, much larger annular voids would be expected due to sand production over many years.
In our article, Nelson and I evaluated 30 of the 44 wells, all of which had a single non-telescoping casing diameter and Mills knife perforations. Details of those wells are presented in Table 1, which shows the wells range in diameter from 8 to 26 inches, and in depth from 48 to 848 feet.
The calculated volume of annular material was calculated for each well and compared with the actual volume of material required to fill each well. Variations in the local geology and specific designs of each well resulted in a range of annular void sizes that ranged from about 11% over calculated volume to more than 300% over calculated volume.
The average of all 30 wells provides a reasonable representation of the average annulus size one might expect from a Mills knife-perforated well in unconsolidated alluvial sediment, which is about 70% over calculated volume (calculated volume × 1.7).
Impact of Improperly Destroyed Wells
Well destruction is sometimes done incompletely or improperly when the people involved with the work are uninformed or unconcerned about the future impacts of their actions or inactions regarding the well destruction.
I once experienced such an impact when I was involved with installation of a new public supply water well to serve a residential community. We conducted depth-specific groundwater sampling of the pilot hole at the new well site, which did not indicate any water quality concerns within our designated screened interval.
However, there turned out to be an old well located about one-half mile away, which had not been properly destroyed (only capped with a farm disc with some soil pushed over it). The old well was not readily detectible, and it did not show up in state records, so unfortunately, we were unaware of its existence during the new well installation.
Although the depth-specific groundwater sampling of the pilot hole did not indicate any water quality problems, longer duration pumping of the new well pulled water from a broader radius, which resulted in breakthrough of high-nitrate groundwater that had migrated from the shallow aquifer to the deeper aquifer through that old well.
The old well was subsequently properly destroyed, but the new drinking water well had to be pumped to waste for an extended period of time to purge the contaminated water that had been introduced via flow paths in that old well.
Direct Hydraulic Communication Between Wells
Many lessons were learned from that large well destruction project that Nelson and I discussed in our 1992 article, and a couple are worth retelling. The first lesson we learned was how extensive the hydrologic connection between neighboring wells can be.
I have heard the term direct hydraulic communication applied to offset wells that are so closely associated that fluids will flow directly from one well to the other in an almost unimpeded path. Although I had previously understood this concept, I did not appreciate the magnitude to which it can occur—even in unconsolidated sediments.
During the well destruction program in the early 1990s, Nelson and I were talking on the phone at the start of a cement pour that would initiate the destruction of an old agricultural well. There was an active well about 30 feet away (about the length of a school bus), which was being pumped at the time. We wanted to confirm there would be no detrimental impact to the pumping well from our well destruction activities, so we directed the contractor to install only a small volume of cement to check the results.
Gray water was pumped from the active well, and when I asked Nelson what the time delay was between installation of the cement and production of gray water from the pumping well, she told me there was no time delay, and production of gray water was instantaneous. We informed our client that they could either destroy both wells or neither well, so the well we were working on was maintained as an observation well.
Importance of Cleaning Up the “Doctor’s Well”
Another lesson I learned during the well destruction program in the early 1990s was the tendency of some people to inappropriately use wells as disposal vessels. Dumping trash in an old water well can certainly make the rubbish “out of sight and out of mind,” but the highly inconsiderate and uncaring act of a person who would take such an action demonstrates an absolute disregard for the grossly inappropriate and highly illegal nature of that disposal practice.
During the well destruction program we did all those years ago, we began to hear folklore from local people who told of the “Doctor’s Well,” in which an unscrupulous medical doctor had reportedly dumped his waste into the well, potentially including dead bodies. (Spoiler alert: There were no dead bodies. Whew!)
The well was about 24 inches in diameter and had a hinged lid with a padlock (like you might see on a trash receptacle). The initial video survey of the well indicated debris fill that was encountered several hundred feet above the bottom of the casing.
The folklore and initial video of the “Doctor’s Well” substantially concerned us, so we subcontracted a medical doctor and an industrial hygienist to provide advice as needed. We also used a shale shaker and other appropriate measures to safely separate the solids of debris bailed from the well.
This well turned out to have been used by a retired medical doctor who—for reasons I cannot imagine—disposed of such things as paperwork (including envelopes with his name and address), pill capsules, used syringes, used bandages, a file cabinet drawer full of paperwork (file drawer and all), and an expired Arizona Liquor License. All the debris was logged and removed from the well for disposal as biohazardous
As if that were not enough, the debris was found to contain minor concentrations of PCBs (polychlorinated biphenyls), which are chlorinated hydrocarbons used in powerline transformers and other equipment until about 1979 when it was banned by the Toxic Substances Control Act (TSCA) due to adverse health effects.
After much logistical, technical, and regulatory effort, the well was finally cleaned out to its bottom with all the debris disposed of properly, and confirmation re-sampling to document the well and groundwater was clean, followed by appropriate destruction of the well.
After I delivered our report of the well destruction (which was now a three-ring binder) to the appropriate state agencies (Arizona Department of Environmental Quality and Arizona Department of Water Resources), I was also contacted by the Office of the Arizona Attorney General who was considering prosecuting the doctor who had created this mess. It turned out that doctor’s license to practice medicine had long since expired, and I never heard anything further about his fate.
General Recommendations for Well Destruction
Most of the best practices of well destruction involve common sense, but here is a list of general guidelines that may be helpful when planning to destroy a water well.
- Develop a prudent and effective well destruction design that will effectively seal off the well interior, the well annulus, and any annular tubing strings. Prepare an “As-Abandoned” diagram showing the specific depths and materials of the well destruction.
- Be sure to comply with state and local laws and ordinances associated with well destruction, which vary from place to place.
- Even if complete sealing is not a regulatory requirement, consider including a seal every 50 feet to 100 feet to prevent the potential for fluid migration through the wellbore.
- Document the location of the well to be destroyed with a GPS device so its specific location can be known for future planning. Remember that any markings or monuments showing the location of the well may be obliterated by future activities, so a GPS measurement is a good idea.
- If a replacement well is to be installed in the immediate area, a gyroscopic survey of the old well is a good idea prior to its destruction. The gyroscopic survey will provide a measurement of the extent and direction of any horizontal drift in the old well. Interference with the replacement well and original well can be avoided this way.
Thron, who served as NGWA president in 2015, will cover what decommissioning is, what factors should be considered in the sealing process, and how it can be achieved responsibly as local and state codes may require. Click here to learn more. Click here to register for Groundwater Week 2022, December 6-8 in Las Vegas, Nevada. ______________ Click here to watch Glotfelty’s video on decommissioning a well.
Thron, who served as NGWA president in 2015, will cover what decommissioning is, what factors should be considered in the sealing process, and how it can be achieved responsibly as local and state codes may require.
Click here to learn more. Click here to register for Groundwater Week 2022, December 6-8 in Las Vegas, Nevada.
Click here to watch Glotfelty’s video on decommissioning a well.
Marvin F. Glotfelty, RG, is the principal hydrogeologist for Clear Creek Associates, a Geo-Logic Associates Co. He is a licensed well driller and registered professional geologist in Arizona, where he has practiced water resources consulting for more than 35 years. He is author of The Art of Water Wells (NGWA Press, 2019) and was The Groundwater Foundation’s 2012 McEllhiney Lecturer. Glotfelty can be reached at email@example.com.