It’s important to diagnose the correct sickness before they administer the cure.
By Marvin F. Glotfelty, RG
As we slowly and not-so-surely bounce back from the COVID-19 coronavirus pandemic, there seem to be a lot of opportunities for development or expansion of residential, agricultural, and industrial facilities.
These opportunities for growth require an augmented water supply, so water well contractors are currently busy trying to keep up with the demand for new well installations.
This may not be the case everywhere, but it is definitely true in the southwestern United States where I live. In my area, demands for functioning and reliable water wells have been amplified by limited surface water supplies, as climatic change and long-term drought conditions have impacted the Colorado River and other surface water systems that serve the region.
These factors have played into a perfect storm of groundwater supply-and-demand, which is currently underway. Most drillers in the southwestern United States have several months of backlog for their drilling rig schedules, primarily due to an insufficient number of qualified drillers. I know of several drilling companies that have an available drilling rig that is sitting idle in their yard due to a lack of trained personnel to operate the equipment.
The drilling rig availability backlog has collided with enormous cost increases for almost all materials and consumables related to well construction (including casing and screen, filter pack sand, cement, diesel fuel, etc.).
The result of this “perfect storm of water well installation” is that while new well installations are happening at a high rate, those well installations are unable to keep pace with the demand for augmented water supplies. Thus, groundwater professionals are increasingly being asked to rehabilitate (structurally modify) old inactive wells to provide a near-term water supply that can sustain growing facilities until a new water well or wells can be installed to provide long-term supply.
The rehabilitation of a well—when properly conducted—can greatly improve the well’s performance, both in terms of enhancing the water production and efficiency, and also improving the pumped water quality.
But well rehabilitation is analogous to automobile mechanics or medical care, in that the fix must be predicated on a sound diagnosis of the problem. I’ve preached for years about the need to consider the purpose of a well as a basis for the new well design, and that consideration should also be applied to the design of well rehabilitation projects. The future purpose of the well to be rehabilitated may be different than the well’s original intended purpose.
In addition to the purpose of the well, it is also essential to have a good understanding of the local hydrogeology and the well’s structural details. Hydrogeologic influences to the well’s performance (such as depth-specific changes in water productivity, depth-specific variability in water quality, and vertical hydraulic gradients) and also well structure influences (such as overlapping or telescoping casings, different screen types, cascading water, etc.) will determine the success or failure of the rehabilitation effort.
In many cases, the well’s hydrogeologic and structural attributes cannot be directly measured or observed while the well is being pumped or from a video survey of the well.Therefore, these conditions must often be measured indirectly.
There are multiple tools and techniques that enable us to understand the unique attributes of each well, and some of these will be discussed. Selection of the appropriate well evaluation methods will enable the groundwater professional to determine the best approach for rehabilitating a particular well, in consideration of that well’s specific structure, specific hydrogeologic conditions, and specific operational history, so that the well can be modified for reactivation to meet a specific purpose.
The Well Diagnosis Process
A fundamental first step for diagnosing a well problem is to determine the general category of the problem. A drop in the well’s performance can usually be attributed to either an aquifer problem, a pump equipment problem, or a well problem.
Aquifer problems involve a degradation of the entire aquifer of an area, such as regional water table declines, impacts of large-scale contaminant plumes, or extensive areas of aerated groundwater. An aquifer problem may result in dropping static water levels, which in turn will cause a drop in the pumping water level and reduce the productivity from the well (Figure 1, upper left).
Aquifer problems can be addressed through sound water management practices to prevent excessive water-level declines through such measures as water conservation, reuse of treated effluent, recharge programs, etc.
Pump equipment problems involve an issue with the pump equipment, such as worn pump impellers, holes in the column pipe, improperly sized pumps that are operating off their efficiency curve, overheated motors in submersible pumps, or electrical problems. A pump equipment problem can be identified by a stable static water level and stable pumping water level, although the productivity of the well has dropped (Figure 1, lower left).
Pump equipment problems can be addressed by pulling, inspecting, and repairing the pump equipment and implementing a proper operations and maintenance program that will prevent future failures of the pump equipment.
Well problems involve a condition in the well itself, such as clogging of the well screen, sand production (from voids in the filter pack or high entrance velocities), aerated or cascading water, corroded casing or screen, collapsed casing, or dropped debris in the well. A well problem can be identified by stable static water levels but dropping pumping water levels and reduced productivity of the well (Figure 1, upper
Well problems can be addressed through various means such as cleaning the well screen, installing a well liner or casing patch, or installing an annular seal at a specific depth interval of the well. These activities are what “well rehabilitation” generally refers to.
The three types of problems shown in Figure 1 are interdependent and may ultimately result from a single cause. For example, a regional drop of the water table (an aquifer problem) can cause high nitrate or other poor-quality water to be withdrawn through a well’s screened interval and may also cause cascading water that results in damage to the pump impellers as they encounter aerated water.
This aquifer problem can be addressed by modifying the well to seal off the depth interval from which the poor-quality water is being produced and adjusting the pump setting to avoid any remaining cascading water. So, the remedy for this aquifer problem involves modification of the well conditions and the pump equipment conditions.
Well Diagnosis Tools and Techniques
Video Surveys often provide the first diagnosis of a well problem after the pump has been pulled. Well video cameras generally include both downward-looking and sideward-looking lenses, which provide a good view of the well’s attributes (well depth, dropped debris, structural damage, and scale accumulation).
However, well videos can also provide qualitative information about the well’s hydrologic conditions. Vertical hydraulic gradients are common in alluvial aquifers in some areas (Figure 2). These flow gradients can be observed in well videos by noting the movement patterns of biofilm or fine debris that are knocked free by the camera as it is lowered through the well.
If the well water is stationary, the particles will oscillate in a swimming pattern, but they will fall down the well in somewhat vertical paths. However, if water is flowing vertically through the well casing, the particles will follow a venturi pattern similar to the draining of a kitchen sink (Figure 3).
Vertical hydraulic gradients can also be observed in the sideward views of well videos where floating particles can be observed responding to the movement of water as it flows into and out of the well (Figures 4 and 5). Video surveys are commonly conducted under non-pumping (static) conditions, but they can also be run under pumping (dynamic) conditions to identify depths of localized sand invasion through the well screen.
Flow Profile Analyses are an important diagnostic technique for assessing well problems and can be performed using either spinner logging or dye tracer methods.
Spinner logs are conducted by the same geophysical logging companies that do conventional borehole logging suites, but a spinner log provides a measure of water movement inside the well, rather than assessing the characteristics of the formation that was penetrated by the well. The spinner logging tool is equipped with an impeller similar to a pipeline flow meter, so as it is lowered down the well at a constant rate of speed, changes in the flow of water are measured by the spinner tool and recorded in the logging van.
Dye tracer flow profile surveys were first developed by the U.S. Geological Survey, but the service is now available by private companies in many areas. Although the general public often thinks of wells as being a single source of water with only one set of water quality characteristics, we know most wells actually have a variety of hydrologic characteristics (both physically and chemically) at various depths (Figure 6).
Flow profiles can be conducted under static conditions or under dynamic conditions. Those two analyses provide completely different evaluations of the well, but they are both important assessments.
A static flow profile tells us whether the well is influenced by a vertical hydraulic gradient (Figure 2), which could cause water that originates at one depth of the well to migrate to other depths in the well. A dynamic flow profile tells us about the well’s response to being pumped, so we can determine the proportional contribution of groundwater that enters the well from various depths.
Due to the geologic complexities of most aquifers, the physical and chemical variability of flow from different depths in the well can be significant (Figure 6). The water quality component of a flow profile analysis involves collection of interval-specific water quality samples that are sent to a laboratory for analysis. Thus, both the amount of groundwater being pumped from each depth interval and the water quality of those respective depth intervals are determined.
With that information, the groundwater professional can do a mass balance analysis to estimate what the flow rate will be from a particular depth interval, and what the water quality from that depth interval will be.
Gyroscopic Surveys enable groundwater professionals to evaluate the constructability of the well rehabilitation design. If a well liner is to be installed, it is critical to confirm the dimensions of the well will accommodate that liner installation.
A gyroscopic survey provides a measurement of the well’s plumbness (the characteristic of being vertical and oriented towards the gravitational center of the Earth). More importantly, a gyroscopic survey also provides a measurement of the well’s alignment (the state of being arranged in a straight line or in correct relative position).
If the plumbness (also termed drift) of a well is poor, it may still be straight enough to accommodate a liner and pump. The general manager of a major pump supplier once told me that a line-shaft vertical turbine pump—if properly shimmed at the land surface—can be operated with a drift of up to 30 degrees. However, the bearings in that vertical turbine pump will wear out prematurely if the well has a dogleg. Crooked places or changes in orientation of the well’s alignment will be measured by the gyroscopic survey.
X-Y Caliper Logs also provide an evaluation of the constructability of the well rehabilitation design.
Conventional caliper logs have one or three arms that together extend outward to measure the diameter of the well. An X-Y caliper has four arms, with the two sets of arms that are oriented at 90 degrees to one another and operate independently. Thus, this log can measure the ellipticity (roundness) of the well. If an older well casing becomes out-of-round, there is an exponential drop in the casing’s collapse strength, in addition to a reduction in its cross-sectional dimensions.
The remaining wall thickness of a steel casing is also a consideration for well rehabilitation designs. That measurement can be provided by an Acoustic Televiewer Log (which measures the steel casing wall thickness using sonic wave velocities) or a Remote Eddy Log, also called a CITM Log (which measures a 4-foot-long running average of the steel casing’s wall thickness, based on attenuation of a strong magnetic field that is generated by the logging tool).
Of course, good old fashioned pumping tests are also useful for diagnosis of a well to be rehabilitated. A step-rate pumping test can be used to estimate the well’s efficiency and performance at various discharge rates.
A constant rate aquifer test can be used to identify hydrologic boundaries (such as fault lines or formation boundaries) that will alter the well’s performance after a period of time. Water quality samples can be collected after various durations of pumping with either of these tests to provide a time-series of water quality variability that can be correlated to changes in water-level drawdown or to volumes of water purged from the well.
Other diagnostic tools and techniques are available in addition to these that have been listed, but groundwater professionals should always use all the analytical techniques that are available and appropriate to adequately understand the problem with each unique well.
With proper diagnosis, the cure provided by the well rehabilitation will match the sickness of the well being addressed.
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 firstname.lastname@example.org.