Often idle for periods of a time, they can provide challenges in their use and operation.
By Michael Schnieders, PG, PH-GW
A groundwater well in the traditional sense is used to supply water to a home or municipal entity on an as-needed basis.
In this same sense, the well is used or cycled daily to meet the needs of the owner. In larger systems where multiple wells fill this need and storage can be utilized, some wells may experience an adjusted operating schedule, but for the most part are operated on a regular basis.
A well may sit idle or out of service for an extended time period in many circumstances. Reasons for this include conservation efforts, seasonal agricultural demand, or the well serving as an emergency backup water supply.
Additional wells that fall into this category include pressure relief wells, dewatering wells, and monitoring wells.
Aquifer storage and recovery (ASR) wells are operated variably depending on the supply and demand, and in some areas may see long periods of dormancy. As such, ASR wells would fall under this non-traditional operation pattern.
A well, when designed and constructed properly, will require routine maintenance based on the supporting aquifer system and nature of the background water quality. Periodic maintenance typically includes replacement of the pump every five to ten years on average—based on use and downhole conditions, disinfection, and a rehabilitation or cleaning effort to target the accumulation of natural fouling mechanisms such as scale or biomass.
Wells that spend a large amount of time out of service generally have a higher maintenance demand than those that are regularly operated. This is counter to conventional wisdom for many well owners who feel that the number of gallons pumped or the hours of active operation should dictate the need for maintenance, much like miles driving may necessitate a need for an oil change on your vehicle.
Identities of a Well
Each well is the sum of its parts and as such, while it may show some similarities to others, is its own entity. A well’s identity is beyond the aquifer but incorporates the materials of construction, the construction method, development, operation, maintenance (or lack thereof) and a number of driving factors that include recharge rates, flooding, external damage, and more.
Well fouling, or in a major sense, failure, stems from any change or changes that impact the use and usability of the well over its operational life. These changes can include such things as water quality impairment, scale accumulations, biomass buildup, sediment infiltration, corrosion, pathogen occurrence, as well as aesthetic impacts including taste, odor, and discoloration.
One of the main issues to address is many of these tendencies are not recognized or addressed until after the fact. That is, for expense, time constraints, or reasons unknown, we do not fully incorporate these vulnerabilities into the design and construction of the well.
As many of the non-traditional wells are not viewed as a primary water source or that they fill a secondary role of importance, they are not given the full consideration during design as needed.
Many of the noted fouling issues come from the concentrating effect—as flow into and out of the well decreases, the water begins to concentrate. As concentration occurs, be it chemical, microbial, or physical, there is a greater potential for accumulation to occur.
The accumulations—including scale, biofilm, and fine sediment—can further impact conditions downhole. The concentrating of these conditions leads to fouling, initially temporarily, but over time, more permanently.
In monitoring wells, which often only see limited pumping during a sample cycle, the concentrating effect can play out through an impaction of the geochemical profile. This is regularly seen in a significant decline in total dissolved solids (TDS) when an idle well is operated for an extended time period.
TDS is a measure of the combined total of inorganic and organic substances present in a liquid in molecular, suspended, or ionized form. Generally, TDS is a relatively stable value with minor fluctuation. However, as materials concentrate within a well, the TDS will reflect that concentration and increase. A steady decline in TDS when operated indicates that when idle, those materials that comprise TDS have been concentrating downhole.
Corrosion is a major concern in many wells. A 2016 U.S. Geological Survey study of more than 20,000 wells nationwide indicated that groundwater in 50% of the states had a high potential for being naturally corrosive.
Many non-traditional wells are not designed or constructed to account for corrosion. Due to the chemical and biological changes that occur when a well sits idle, corrosion rates generally increase, impacting the produced water quality and the lifespan of the well.
It is common for many pressure relief wells to be constructed of low-carbon or galvanized steel. These less noble metals are commonly susceptible to the chemically reducing state that develops as well as microbiologically influenced corrosion (MIC)—two conditions that often develop in idle wells.
As pressure relief wells only operate when poor water pressure reaches a threshold, they spend a large portion of their time idle. As a reflection, it is not uncommon to see high occurrences of iron fouling in those wells.
Research has shown during periods of flow in and around a well system, bacterial communities and the biofilm they have produced contracts as a near constant supply of nutrients are delivered to the well. As the flow decreases, the biofilm expands as the need for nutrient capture grows.
It is during this time period when we often see microbial population sizes within a given system expand greatly. Bacteria can be active initially in stagnant water situations as they seek to capture nutrients necessary for their growth and propagation.
Similarly, as the flow of a well system decreases, the entrance and influence of oxygen on the system decreases. This can lead to more anoxic or anaerobic environments to develop. As anaerobic conditions develop, the growth and development of anaerobic bacterial populations increase.
Increased anaerobic activity is often used as an indication of increased microbial maturity. Research has shown that with increased maturity among the microbial community downhole, there is an increase in the potential for coliforms and opportunistic pathogens to occur as well as other nuisance organisms.
This played out during the recession as many homes which sat on the market for an extended time period exhibited difficulty passing a total coliform test when returned to use. Irrigation wells and wells limited in operation due to regional conservation efforts typically see a significant increase in the levels of anaerobic growth when placed back into regular operation.
One of the large driving factors of impairment in these wells is proper development. Development has two primary objectives:
- The first is to repair aquifer damage near the borehole that occurs during drilling.
- The second is to alter the basic physical characteristics near the borehole in order to enhance flow and thereby efficiency.
If development is not achieved, the well can be plagued for its operational life.
Development remains a vital aspect of all well construction but more so with wells that sit idle. The constriction of flow from improper development can further aggravate conditions that are developing. By impairing flow, the natural evacuation (flushing) that occurs during pumping is hampered.
In wells that remain idle for a major period, this can limit the removal of material during a vital time, further aggravating the concentration effect.
Several things can be done to identify the potential for issues to plague an inactive, non-traditional well. First, the history of wells in the area and the target production zone can offer clues to likely challenges.
Evaluate the corrosion potential of the background water chemistry. Examine the likelihood of mineral scale, biofouling, and sediment infiltration. Use this information to select less reactive materials for construction and incorporate conveniences for periodic maintenance. In the construction process, allow additional time for complete development, including target benchmarks in efficiency and produced quality.
Depending on the well’s intended use, include valving to allow the well to be pumped to waste periodically in order to flush the well column and borehole. In addition to valving, this may require impoundments or structures to contain or divert the produced water.
If produced water quality will be a concern, efforts to regularly monitor the well are advised. This should include developing targets and active responses when concentrating is occurring, including both water chemistry and microbiological aspects.
For example, in hard water areas where calcium and magnesium are elevated, periodically tracking hardness levels can help alert the owner when the development of carbonate scale may be occurring. Flushing the well can aid in reducing the rapidity of scale development. Similarly, setting targets for the bacterial population can help ensure that the microbial community does not run unencumbered, potentially impacting the viability of the production and water quality.
As with many aspects of the groundwater industry, time spent understanding the potential problems before they arise is invaluable. Non-traditional wells, while often not seen as important, require just as much if not more foresight and efforts to ensure construction and use of a good well system.
Michael Schnieders, PG, PH-GW, is a professional geologist currently serving as the principle hydrogeologist and president of Water Systems Engineering Inc. in Ottawa, Kansas. Schnieders’ primary work involves water resource investigation and management, specializing in the diagnosis and treatment of fouled well systems. Schnieders was the 2017 McEllhiney Distinguished Lecturer in Water Well Technology. He can be reached at firstname.lastname@example.org.