Part 3: Maintenance and testing parameters
By Ed Butts, PE, CPI
In Parts 1 and 2, we outlined such diverse topics as well water chemistry, corrosion and encrustation processes, and well and pump investigating techniques. This month we will examine the various methods used in well and pump rehabilitation, some regular well and pump maintenance considerations, and testing procedures used to evaluate the efficiency of each element.
Does the well need rehabilitation?
In an “ideal” well (Figure 1), groundwater enters the well with a minimal amount of head (energy) loss due to the efficient combination of well construction techniques and proper selection and placement of a well screen and filter pack. Drawdown is proportional to the well’s flow rate and increases linearly as the well’s flow increases.
Unfortunately, an ideal well is not common in the real world. The occurrence of an actual water well problem (or success) is generally due to a complex set of criteria specific to a region and site. These include the type, thickness, and transmissibilty of the aquifer; well type, methods, and materials used in the original well construction; well yield vs. screened area (entrance velocity) and pumping rate; usage patterns; background water quality, including the chemistry and physical conditions of the natural groundwater; and the well’s screening (inlet) arrangements—is the aquifer to the well fully or partially screened or perforated?
Each of these parameters must be given proper consideration during determination of a well rehabilitation evaluation and procedure. However, the aquifer and background water quality and the impact on the well, well type, the original well construction methods and materials used, and the screening/inlet arrangement are usually already well established and set before the time to consider any actual well rehabilitation.
Typically, the only elements we have the ability to modify or control are well usage patterns, abrasives, and part of the water chemistry. Before undertaking any actual well rehabilitation, you should address a simple question: “Is the well inlet area actually plugged or blocked, or is the decline simply due to a normal (or abnormal) reduction in aquifer capacity/water level?”
Believe it or not, thousands of dollars have been spent to rehabilitate a well, only to discover the real problem was the aquifer was declining in yield or available head, or the original well screen or filter pack were wrongly sized or misapplied.
Tests to verify or refute either of these situations are relatively simple procedures, and assuming no nearby observation wells are available to assist in data collection, can still be performed with the well in question and the well pump in place by means of a carefully conducted step-drawdown test to determine specific capacity followed by a timed water level recovery observation.
An efficient well (Figure 2) is one delivering water with a minimal loss of energy across the well screen and through the surrounding filter pack, whether natural or artificial. Conversely, a plugged well (Figure 3) exhibits excessive head losses and usually more drawdown at a much faster rate than it would have during the well’s original pumping test.
Drawdown will usually increase rapidly as the yield increases. This means the specific capacity will decline substantially with a proportional increase in well production due to increasing head losses associated with the increase in flow rates against the currently available aquifer head and well and filter pack entrance area and volume, respectfully.
Recovery will also generally be rapid, sometimes exhibiting a full recovery up to the static water level condition in just a few minutes.
It is also wise to collect all available historical pumping and water level data—past maintenance and rehabilitation data and logs associated with the subject well and other local wells—and investigate information relative to the aquifer to help determine or verify any trends, issues, or potential long-term problems. These are the first acid tests you should perform before undertaking any specific well rehabilitation procedures—where a wrong decision could jeopardize your customer’s money and your reputation.
Let’s assume the well’s inlet area is fully or partially plugged. Proceeding with well rehabilitation along with the appropriate method should be evaluated by factoring the well usage patterns with the following questions to the customer:
- Has or is the well pump now drawing air? If so, is it during startup only or while in operation? If during operation, at what capacity does air occur and does it dissipate?
- Is the drawdown in the well at pump startup or during normal operation excessive for the design conditions of the well, or does it dewater the screen/perforations enough to expose to atmosphere?
- Is the problem with the well associated with any seasonal variables or the concurrent use of any neighboring wells?
- Can the well be used at a lower flow rate for a longer period of hours per day, or during different time periods?
- Is sand in the discharge water evident? If so, can it be controlled or lessened at lower flow rates?
- How much restoration of well yield or reduction in drawdown or flow rate is needed for a satisfactory outcome?
- Was the well ever capable of producing the desired flow rate?
These questions are the type needed to fully evaluate the amount of effort—and money—the customer should plan to invest in rehabbing an older well before seriously considering a new replacement well.
Assuming the answers to these questions are favorable and the well justifies rehabilitation, the next and most important issue is the chemistry of the well water—the electrochemistry, encrustation, and biofilm potentials of the water along with the possible impact to the physical elements of the well.
Downhole video inspections
Rehabilitation of the well will not do any good, and may in fact be counterproductive, if the screen or perforations are excessively deteriorated from corrosion or sand erosion. The result is a possible loss of well screen/casing strength or excessively open slot size or openings—leading to sand pumping, or in extreme cases, collapse of an entire wellbore.
Conversely, if the openings are severely blocked and plugged with scale or biofilms, well production will usually decline.
A video inspection of the well is often adequate to determine the answers to many of these questions. However, each well must be examined on a case-by-case basis. In some cases, geologging of the well, observing water levels in nearby wells, or isolated test pumping of selected zones as well as a comprehensive video inspection are needed to fully figure out the exact condition of the aquifer and the structural and screened components of the well.
When evaluating a well rehabilitation method and procedure, I try always to address or at least consider all these parameters as well as evaluate the water chemistry completely.
Water well testing procedures for determining performance
Performance and efficiency testing of a water well can consist of several different tests and procedures, although I have found for most cases just a few specific tests to be of the highest value.
For our purposes, we will assume the well in question has no easily accessible nearby wells to use for observing aquifer water level. This greatly limits the type and degree of testing that can be used to gauge the performance and efficiency of an individual well.
However, there are still two functional well tests that can be used to help determine if the problem is more likely an aquifer issue, caused by localized plugging of the slots or openings in the well, or a combined problem.
The first is a specific capacity test, which provides a benchmark evaluation as to the well’s relative performance on a unit basis, and where historical data is available, a timeline indication of any changes to the well’s efficiency. The specific capacity test can be performed at any flow rate throughout the normal capacity range of the well and pump and is fairly easy to conduct. In fact, conducting a specific capacity test at various flow rates of well production often provides guidance to determine the efficiency of the well. Simply:
Specific capacity = Observed flow rate in GPM/feet of drawdown
For example: a 160-foot-deep well with the aquifer between 60 to 150 feet below ground surface (BGS) and a screened interval between 80 to 150 feet BGS–static water level (SWL) = 50 feet BGS.
Upon stabilization after flow testing the well for eight continuous hours:
Flow rate = 1000 GPM pumping water level (PWL) = 80 feet BGS
Additional tests indicate a stable flow rate of 300 GPM at 4-foot drawdown (DD) and 600 GPM at 12-foot DD
Specific capacity = 1000 GPM/80 – 50 feet (30-foot DD) = 33.3 GPM/foot of DD
Specific capacity at 300 GPM = 300 GPM/4-foot DD = 75 GPM/foot of DD
Specific capacity at 600 GPM = 600 GPM/12-foot DD = 50 GPM/foot of DD
1000 GPM at 32 psi discharge pressure at 80 feet PWL = ~154′ + 5′ (riser hf) = 159 feet TDH
600 GPM at 65 psi discharge pressure at 62 feet PWL = ~212′ + 3′ (riser hf) = 215 feet TDH
300 GPM at 89 psi discharge pressure at 54 feet PWL = ~260′ + 1′ (riser hf) = 261 feet TDH
The results from these three specific capacity tests indicate a high of 75 GPM/foot of drawdown down to a low of 33.3 GPM/foot of drawdown, or a potential drop of 56% of well production.
The data gained from the well testing can also be used to determine the efficiency of the pump itself, especially if these results and the motor’s power draw can be plotted against the original pump and brake and input horsepower curves. The relatively high percentile drop in specific capacity at increasing flow rates is indicative, but not ironclad proof, of an energy loss between the aquifer and the well due to the head loss of water entering the well (a loss in the efficiency of the well), or the aquifer itself doesn’t have the capability of yielding the required higher flow.
By themselves, the results from these tests don’t prove which could be the culprit. I have personally tested numerous wells with similar values or ratios of specific capacity. In these situations, the second test I usually conduct is water recovery testing.
A water recovery test provides the data needed to help determine if the head losses are due to the frictional losses incurred from the well screen or perforations and/or annular filter pack, or from the inherent resistance offered from the material and thickness that comprises the aquifer.
Once we are satisfied with the results from the specific capacity and pumping tests, pumping is suspended and the recovery rate of water refilling the wellbore is observed. As a rule of thumb, I consider a well structure to be inefficient if, following a period of sustained pumping, the well water level returns to the original static water level or recovers greater than 90% of the well’s drawdown in less than 5 minutes following suspension of the pumping test.
The upper 10% or so of recovery is always slower than for the lower levels of drawdown due to the smaller difference in potentiometric head, as the recovery rate slows as it nears the natural static water level of the aquifer and the greater dewatering that usually occurs throughout the upper region of the aquifer during a sustained period of pumping.
Obviously, this factor must be used cautiously and prudently since various peripheral factors have not been included. For example, when using a vertical turbine or submersible pump without a foot or riser check valve, the gain of water from the instantaneous drainage of the column/riser pipe will factor into this scenario. Particularly so for small diameter wells or one with limited drawdown as well as factoring the yield and duration of the actual well test. All the same, for a quick approximation as to the likely cause of a decline of well yield, this 90%/5-minute rule has shown to be reasonably accurate.
For our example: after eight hours of continuous pumping, pumping is immediately suspended and the well’s recovery rate is as follows (beginning SWL = 50 feet BGS):
At eight hours of pumping: flow rate and pumping water level (PWL) =
1000 GPM at 80-foot PWL (BGS)
At 5 seconds after pump shutdown: SWL = 77 feet BGS
(drawdown = 30 feet, 90% of recovery = 27 feet)
At 10 seconds after pump shutdown: SWL = 74 feet BGS
At 20 seconds after pump shutdown: SWL = 68 feet BGS
At 30 seconds after pump shutdown: SWL = 63 feet BGS
At 45 seconds after pump shutdown: SWL = 60 feet BGS
At 60 seconds after pump shutdown: SWL = 57.6 feet BGS
At 75 seconds after pump shutdown: SWL = 55.3 feet BGS
At 90 seconds after pump shutdown: SWL = 53.7 feet BGS
At 120 seconds after pump shutdown: SWL = 53 feet BGS = 90% of recovery>
(2 minutes = 90% of recovery = 80 feet PWL – 50 feet SWL = 30 feet DD × .90 = 27 feet: 80-foot PWL – 27-foot recovery = 53 feet
At 180 seconds after pump shutdown: SWL = 52.6 feet
At 240 seconds after pump shutdown: SWL = 52.2 feet
At 300 seconds after pump shutdown: SWL = 51.9 feet
At 525 seconds after pump shutdown: SWL = 50 feet BGS (original static water level)
In our example, this observation verifies the well regains 90% of the recovery water level of 53 feet just 2 minutes after pumping has ceased. This indicates the well inlet area and filter pack are offering the primary resistance to water entering the wellbore, and the aquifer has not sustained a great degree of drawdown.
Obviously, both of the previous calculations must be used with caution and factoring other elements including pumping rate, well diameter, aquifer type, thickness, head, and drawdown. Experience is also invaluable since the background gained from observing past pumping and recovery tests usually goes a long way toward accurately estimating a well’s flow rate, efficiency, and rehabilitation potential in an individual region.
Well pump testing procedures for determining performance
Although shorter in duration than a well test, performance and efficiency testing of pumping plants involves a different procedure and more observations than wells. To start, when available, obtain all background and historical data including the original pump curve and data sheet (plus past test curves if available); any data regarding past pumping and efficiency tests; past well tests that correlate to pumping tests; plus any records of previous or nearby well or pump repairs or rehabilitation. This information is invaluable when evaluating the field efficiency and possible wear of a deep well pump. We will detail the procedure for testing a vertical turbine or submersible well pump in a future column.
When and how should we rehabilitate?
The actual decision when to perform rehabilitation or repair on a well or pump can only be made after considering several factors. The primary factor obviously must be related to timing and length of shutdown.
Complete rehabilitation of a water well can be an involved process requiring as little as two to three days all the way up to two to three months. Proper timing and scheduling is essential and the work can only be performed when the client has adequate replacement facilities or can operate without the well in question for the projected time it’s unavailable.
With irrigation wells, this is usually not a critical factor; the well rehabilitation can generally be performed during the six to nine months of the irrigation offseason. But in most municipal, commercial, or industrial wells, proper timing and the planned loss of a well for any extended time is an important consideration. This becomes a critical problem when the rehabilitation runs beyond the initially planned period of shutdown.
It is important to remember rehabilitation can often take longer and more effort than originally estimated due to several reasons (mostly unexpected) and these potential delays should always be factored into any schedule.
When to actually perform the rehabilitation is a decision that must be made before the well is so far down on performance or so plugged a reasonable restoration of yield or performance is not feasible. This is extremely important in regions where the natural groundwater has a high scaling potential due to high levels of iron/manganese or calcium compounds or active biofilm action.
In some situations or localities, scale can accumulate on the interior of a well screen in layers up to several inches deep. Most scaling of this type results in a hardened, thick, and extensive layer difficult to remove, even with the powerful chemicals and acids commercially available.
To lessen this possibility, the well owner must be advised to perform routine monitoring and collect information on the well’s vital statistics. At a minimum, this should include recording the sustained capacities at each applicable pumping level, seasonal static water levels, and specific capacity monitoring.
Additional data desirable would include seasonal step-pumping and constant rate tests at a fixed duration with accurate drawdown and recovery data. Analysis and observation of any rapid or unusual condition (such as sand pumping) or sudden deviation or drop in well performance should immediately trigger well testing followed by a consideration of rehabilitation.
Well and pump maintenance
So far, this entire discussion has been slanted in the direction of well and pump rehabilitation and the procedures to help determine the viability of this process. Many cases of full-scale well or pump rehabilitation and repair can be avoided, or at least delayed, if appropriate and regular preventive maintenance measures are conducted.
It constantly amazes me a well owner will pay hundreds or even thousands of dollars to keep their vehicle in tip-top condition, but doesn’t ever consider spending any money on a regular maintenance program for the provider of their most precious resource—water.
Although it’s not possible to detail every possible method of maintenance, we can make some general recommendations.
First, for a highly valued well used for municipal, commercial/industrial, or irrigation applications, an annual or biannual evaluation of the well and well pump should be conducted. This should go along with a program of annual preventive maintenance, beginning with an initial examination of the water chemistry.
The actual frequency of the field tests should depend on the type, characteristics, construction method and materials used in the well and inlet screening, aquifer type and thickness, operating hours per year, exposure to harmful chemical or physical elements from the groundwater, type of pumping installation, and allowable downtime.
Full rate, specific capacity, and water level recovery tests can provide a baseline for future comparison for the well—and are tests normally requiring one day or less to perform.
Conducting a water quality examination (as outlined in Part 2 of this series) is a critical first step since you’ll be able to determine if the natural groundwater is encrusting or corrosive as well as the potential and degree for biofilm growth. Knowing this can aid in the future by determining the best method of maintenance and rehabilitation plus the best treatment to use for both procedures.
Typically, a shock treatment consisting of gas, liquid, or granular chlorine can be effective dissolving mild scale and biofilms on well casing and screen surfaces in most wells. However, it is vital the chlorine solution engage all wetted surfaces along with adequate contact time (up to four to six hours) to be totally effective.
All procedures for well and pump maintenance should be planned and conducted assuming the well pump remains in place. Periodic surging or alternated starting and stopping of the well and well pump or backfeeding potable water from a different well down the well or well pump during and following chemical treatment can also help in fully distributing a chemical solution as well as breaking down and removing dissolved growth from the well screen, casing, and well pump.
In addition to shock treatment using chlorine at a level between 100-500 mg/L (see Table 1), there are also numerous proprietary and other common methods such as carbon dioxide, explosive and sonic charges, and other forms of chemical treatment such as acids, dispersants, and surfactants.
In most cases, final determination of the proper method to use for a specific well and pump rehabilitation depends on the raw water quality; type and material of the well casing and inlet arrangements; degree, type, and distribution of the scaling material; and the local availability and cost for the anticipated method.
In addition, seeking out knowledge from another well contractor or manufacturer or suppliers’ representative can be worthwhile in deciding which method of treatment would likely be best for the specific application. Remember there is no such thing as a perfect chemical for every well problem. Each product has limitations and drawbacks and it is always incumbent upon you to seek out wise counsel and use your judgement as to which is best.
It’s also important to note all chemicals used in maintenance or rehabilitation must be properly pumped from the well, with the pH and possible presence of coliform bacteria checked before placing the well back into service. All residual or spent chemicals must be fully diluted or neutralized before discharge into any water body according to federal, state, and local water quality regulations. This is critical, especially for most chemical agents such as acids that inherently lower the pH or dissolved oxygen, since impacting the receiving water’s pH or DO level can have a harmful or even deadly impact on aquatic life.
As far as the well pump is concerned, the field test should include verification and plotting the field test results to the original pump curve—or if the original curve is not available, plotting a new field test curve. These tests can usually be conducted in conjunction with the well performance testing.
Future tests can then be compared to previous test curves for any changes. The parameters should include flow rate and corresponding pressure (head), well pumping water level at each flow rate, and the pump’s shutoff head (if this can be safely conducted and observed).
For electric-driven pumps, additional observations should include static and running motor voltage, motor amperage at each observed flow rate, power input (using a watt-hour meter or a power meter) at each observed flow rate, and a megohm test of the motor windings (with power and motor “off”). For engine-driven installations, pump speed and the specific fuel consumption (per hour) with a comparison to the engine’s rated performance curve is suggested.
Electrical installations using variable frequency drives should also include motor speed (tachometer or panel readout), motor’s running frequency, voltage, and amperage at each operating condition.
Many of these processes can be performed with the well pump in place, although removing the well pump usually ensures the greatest effectiveness and opportunity for rehabilitation—plus once above ground, the well pump bowl can be examined for any excessive wear or damage.
If in situ methods of maintenance or rehabilitation are contemplated, any chemicals used must not injure any pump and motor (when submersible motors are used) components or be properly blended with an inhibitor to prevent corrosion to metals used for well casing and screen and well pump/motor or riser pipe. In most cases, the general recommendations in Table 2 can be used for anticipating and scheduling major well maintenance or rehabilitation procedures.
Allow me to say I will not inflict my sole opinion of which method of rehabilitation I think is best. Even if I did, I would still be cautious of advising a “one-size-fits-all” approach. It just doesn’t work that way.
Each drilling contractor has tried and proven techniques for well and pump rehabilitation adapted to their specific region, well types, and water chemistry that have been developed through years of trial and error, success, and failure. I am in true awe of the talent and resourcefulness I see, hear, and read about every day from well drillers across the world.
I hope I have conveyed to you that periodic well maintenance should be practiced on all high value wells, and rehabilitation should be considered on a well-by-well basis and should not be approached using any unusual, unverified, or untried methods. From my experience, a systematic but flexible approach to well and pump rehabilitation will usually produce the best results at the lowest risk and cost.
Use the sciences available to you to investigate all possible causes related to the well and well pump’s physical, structural, and water chemistry conditions as well as good old-fashioned common sense before setting out to rehabilitate that next well.
Until next month, work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at firstname.lastname@example.org.