Wells get old too..
Originally written for the NGWA toolkit
Many well owners incorrectly believe that wells begin their operational lives at 100 percent efficiency. In actuality, most wells begin life at less than 15 percent efficiency. It is through the process of well development, more correctly termed “well efficiency development” that the well efficiency is increased prior to the well beginning its operational life. However, wells get old too, and as they age they start having performance issues.
With no moving parts intrinsic to the “well”, it is difficult for many well owners to grasp the idea that with time, wells lose efficiency. In other words, as wells mature, they become less efficient for several reasons, and in addition to that increasing inefficiency, they can experience structural failure. As a consequence of their maturing, wells often express their old age by requiring more energy to pump water, pumping less water, pumping more sand, and experiencing changes in water quality just to name a few. These are behaviors that can generally be measured or observed.
More insidious is the change in the interaction between the well, the filter pack and the surrounding formations. As the well matures, portions of the well screen can become partially or fully blocked, the result of chemical precipitation, biological fouling or excessive fine sediment becoming trapped in the perforations, and any combination thereof. Additionally, this very same blockage can occur in the filter pack, and not necessarily in the same place as the blocked perforations. These blockages can change the distribution of water flow into a well, along with variations in pressure distribution and water quality.
Unfortunately, these blockages can mislead us as we collect data, whether it’s pump performance data, static and pumping water levels for well performance or aquifer tests, well performance data that may be included as part of a groundwater modeling effort, or water quality data suggesting “something has happened”. One other factor to keep in mind, is that no two wells are alike, and each responds to variations is subsurface geology and geochemistry, construction practices, and usage differently. The cautionary point is to not assume that because one well is in good shape, its “neighbor” should be also.
Examples range from misinterpreting overall pumping plant efficiency (OPE) such that it seems as if the pump or motor is failing, when in fact it’s a less efficient well for one of the reasons mentioned. Furthermore, cascading water to completely blocked sections of either well perforations or filter pack may result in misleading water level data as different portions of the supply aquifers are withdrawn from during pumping, again due to block well perforations or filter pack, or both. Mature well performance data, when taken at face value, could mislead modeling efforts by producing falsely negative specific capacities, causing modeling efforts to assume lower aquifer transmissivity, when its actually just blocked well perforations.
There are but a few examples of how a mature well, with blocked perforations, might mislead us during observation, measurement and sampling. Ideally, we need to avoid or at least understand how a maturing well can mislead us as we collect data. To that end, having baseline data (“home” data as I sometimes call it) that can be used for subsequent temporal comparisons is invaluable. As I have said, video logging a new well is critical, as is collecting and analyzing water samples for both the chemical composition, but also for the bacteriological presence. Additionally, conducting spinner surveys in a new well, both static and dynamic, leads to a flow profile for water entering the well. Mature well data can be compared back to this initial data, to assess and evaluate the effects of how a particular well is maturing.
Structural failure is also sometimes experienced as a well matures. Design, materials selection, well construction and installation, along with development and maintenance will all contribute to the structural condition of a mature well. Structural failures will occur as a result of insufficient structural strength in well casing or weld seams, as a result of over-pumping and poor collapse strength design, and as a result of corrosion of various kinds as three examples of structural failures. These failures can create water inflow where it was not initially intended, through breaches in the blank well casing. These breached casings, while allowing water to flow in from unintended depths, can also alter the chemistry of the water in a well. Lastly, these breaches can allow fine sediment into a well, which can damage pumps. Structural breaches can also facilitate variations in the vertical flow in a well, which can potentially mislead efforts to characterize everything from well hydraulics to water quality.
Routinely collecting video log, water quality and spinner data, if at all possible, will aid in understanding how a well is maturing. This will aid in identifying when a well should be redeveloped or possibly be in need of rehabilitation. In general, it would be best if this type of testing occurred every two to three years, or when pumps are removed to assess the mechanical condition, or both. It is neither easy nor inexpensive to conduct these tests on a routine basis, but experience has taught us that these routine tests are far less expensive than a new pump, or long-postponed well maintenance or rehabilitation.
Lastly, it is important, almost critical, that these tests be conducted on a mature well that is either going to undergo well or aquifer hydraulic testing; or a well that is being considered for some kind of important modeling or basin groundwater study. Too often, in an effort to save the cost of such testing, assumptions are made and accepted, that lead to misleading data and an expensive re-assessment of that data, only to return to the need to evaluate the operational condition of the well.