Decision Crossroads for Well Rehabilitation

It’s important the correct decisions are made when diagnosing a well’s problems.

By Marvin F. Glotfelty, RG

Many groundwater professionals routinely conduct well rehabilitation projects to address problems with the water productivity, water quality, or structural stability and longevity of older wells.

Some drilling contractors and hydrogeologists conduct this type of work so frequently that they become well rehabilitation specialists and do this type of work almost exclusively.

Numerous decision points arise during diagnosing the well’s problems. Drilling contractors and hydrogeologists who undertake well rehabilitation projects frequently find themselves at a juncture of possibilities with regard to the multiple methods and techniques that can be used to understand and resolve various well performance problems.

The choices we make at each step of the well rehabilitation process constitutes a decision crossroad. Multiple decision crossroads are encountered during every well rehabilitation project, and our navigation through those conceptual intersections will determine whether we move down the correct path that leads us to a good and successful outcome, or whether we find ourselves going down an inappropriate rabbit hole that introduces pitfalls and challenges that will often doom our efforts.

In his well-renowned blues song, Robert Johnson wrote:

“I went down to the crossroads, tried to flag a ride. Nobody seemed to know me, everybody passed me by.”

Of course, Johnson was not a groundwater professional, but I’m pretty sure he faced uncertainty from time to time as we all do, and he probably had to periodically make tough choices in situations when he didn’t have information to guide his decision.

He had to pick his best pathway and deal with his uncertainty and doubt. I am no expert on the life and times of the blues great, but the words in his song provide us with a good metaphorical description of how groundwater professionals need to traverse the maze of decision crossroads that we encounter during well rehabilitation projects.

Fortunately for groundwater professionals, we have it better than Johnson in that we don’t have to face our decision crossroads alone. There are many good, published references and experts on well rehabilitation, and those information resources are available to support the difficult decisions we face during well rehabilitation projects. The information sources of the groundwater industry won’t “pass you by” when you need advice on well rehabilitation.

The dilemma faced by groundwater professionals who take on the challenge of evaluating and rehabilitating older wells is twofold.

First, the person who is evaluating a well to plan for its rehabilitation must recognize it when they encounter a decision crossroad. I have seen many cases where several potential techniques or methods were available to someone who is evaluating a well, but they automatically default to one particular approach because “we’ve always done it that way.”

Second, after the groundwater professional has recognized they’ve encountered a decision crossroad with several options available to them, they then have to make a wise selection based on experience and sound science of the most appropriate technique to evaluate the well.

Figure 1. Example of a well owner mischaracterizing the well problem. This well owner was concerned about “fluoride levels creeping up,” but the real problem was clogging of the upper screened interval, which reduced flow of the high-nitrate water in the upper aquifer.

Well Evaluation Decision Crossroads

The first step in evaluating a well’s problem is to have a discussion with the well owner about the problem being experienced. That communication will hopefully provide the groundwater professional with a good understanding of the nature of the problem to be addressed.

Defining the problem that is occurring with a well sounds obvious, but if the source of the problem is misunderstood by the well owner, they may mischaracterize that problem.

Figure 2. Video camera being lowered into a well.

For example, I once did a well rehabilitation project for a water company manager who told me his well was experiencing concentrations of fluoride (a naturally occurring contaminant) that were “creeping up.” A natural contaminant like fluoride would not be expected to increase over time, so I asked more about the well’s water quality changes and learned that during the same period when the fluoride was increasing, there was also a reduction in nitrate concentrations (Figure 1).

The problem turned out to be scale accumulation in the upper portion of the well’s screened interval, which was blocking off high-nitrate groundwater that had been diluting the high-fluoride groundwater in the lower portion of the well. By only focusing on the emerging concern rather than all changed conditions in the well, the water company manager had misinterpreted the problem and had unintentionally misstated it to me.

So, the simple process of defining the well problem is the first decision crossroad.

Proper characterization of the well problem is usually easy, but if it is not done correctly, all the subsequent actions will be a waste of time and money.

After the well problem has been clearly defined, the next step is to review all the available records relating to the well’s structure and operational history. The records review generally includes publicly available data and the well owner’s files—such as the driller’s log, as-built diagram of the well, annual pumped volumes, water table depths and drawdown amounts, geologic or geophysical data, water quality data, hydrologic records or reports, and previous video surveys of the well.

Those records can be augmented with anecdotal information such as discussions with the well driller (when possible) about their drilling method, development method, construction materials, etc.

At the conclusion of the records review, we come to the next decision crossroad.

Figure 3. Spinner logging a well being pumped through an open-ended access tube.

Is the information we’ve reviewed adequate to address the well problem? If the answer is yes, we can proceed with the rehabilitation activity that will solve the problem. If the answer is no, we need to proceed to the next analytical level, which typically involves a site visit or video survey.

A reconnaissance visit to the well site often provides information that is lacking in the well’s paperwork. For example, the site inspector can record in their field notes whether the well is currently equipped with a pump, and whether the site is currently connected to an electrical energy source. The site inspector can also note whether the wellhead shows evidence of sand production or other structural problems, and a general assessment of the site logistics (overhead power lines, fluid discharge locations, traffic control issues, sound abatement needs, etc.) can also be provided.

The field notes, GPS location, and site photographs of the inspector will assist with the next decision crossroad.

Are the records review and site visit alone adequate to determine the best rehabilitation approach for resolving this well problem? If the answer is yes, we can proceed with that activity but if the answer is no, we will usually—but not always— need to pull the pump to facilitate a video survey of the well. After pump equipment is removed from the well, it should also be inspected to identify any problems such as holes in the column pipe or worn impellers.

Once an adequate time after pump removal has transpired (one or two days) to allow the water to clarify, a video survey of the well (Figure 2) will provide a lot of additional information and improve our understanding of the well’s current conditions. The information that the video survey will provide includes well dimensions and current depth, casing and screen types and condition, current water table depth, debris or sand fill in the well, and other well attributes (such as casing corrosion, scale accumulation, and water movement from hydraulic gradients).

Figure 4. Spinner tool and example logging plot.

In some cases, the video survey may provide enough additional information for the groundwater professional to determine what the well rehabilitation activity will need to be, but in other cases, more analyses are needed. Thus, the well video survey brings us to another decision crossroad.

If the well video analysis does not provide enough information for a determination of a well rehabilitation approach, we may want to consider other analytical tools. Several other wireline tools are available to provide specific information about wells. Each of these tools are focused on a specific attribute of the well.

These other analytical logging tools include:

  • Gyroscopic survey to quantify the well’s plumbness and alignment
  • CITM (remote eddy current) log or acoustic televiewer log to quantify the remaining wall thickness of a steel well casing
  • X-Y caliper log to quantify the ellipticity of the well casing in intervals where it has become out of round and is beginning to collapse
  • Dynamic video survey where a video survey is conducted under pumping conditions (through an access tube as shown in Figure 3) to identify zones of localized sand invasion.

If any of these analytical tools address particular concerns or conditions in the well, they will provide important information, which may lead us to yet another decision crossroad.

If none of the additional wireline logging analyses are conducted, we’ve arrived at a different decision crossroad. At this juncture of the well evaluation process, we must consider whether we are addressing only a water production problem (pumping rate decline), or also a water quality problem (or both?).

Figure 5. Dye tracer flow profiling technique.

If the well problem relates only to a decrease in water production, then we can proceed with a step-rate pumping test to assess the well’s efficiency, specific capacity, sand production, etc. at various pumping rates.

If the well problem also relates to water quality concerns, then a flow profile analysis of the well would be advisable. A flow profile is an analytical process that enables us to determine the proportional flow contributions from various depth intervals of the well screen.

During this analytical process, we can also collect depth-specific groundwater samples from each of the water production zones. With the inflow rates from each production zone and the water quality from each zone, we can conduct a mass balance analysis to essentially manipulate the overall pumped water quality from the well. The flow profile analysis can be conducted in at least two ways, thus another decision crossroad.

One technique is with a spinner log analysis (Figure 3). A spinner log analysis involves equipping the well with an open-ended access tube (about 2.5 to 3.5 inches in diameter) that extends to below the pump, so it will allow a wireline spinner tool (Figure 4) to be lowered past the pump intake while it is running.

The spinner tool works like the flow meter in a pipeline, so as the spinner tool is lowered down the well at a constant rate of speed, various flow rates of groundwater passing the tool will cause it to rotate at different speeds. The data from the spinner log is recorded on a strip chart, and the sloping portions on the spinner plot (shaded areas on Figure 4) represent the depth intervals in the well that are producing water.

Analysis of the spinner log can provide quantification of each zone’s relative contribution of the well’s total water production. After the “pay zones” have been identified, a wireline sampling tool is used to collect a groundwater sample from near the top of each water production zone.

For some wells, the other technique for flow profile analysis is better suited, which is the dye tracer analysis (Figure 5). The dye tracer equipment utilizes a small-diameter plastic tubing reel to inject rhodamine dye (which is National Sanitation Foundation–certified) at various depths in the well while it is being pumped. The arrival time of the dye back at the land surface is measured precisely (at the parts per billion level) with a fluorometer. Analysis of the travel time for the dye to flow up and out of the well provides a flow profile like the spinner logging technique. At selected depth intervals, a second tubing reel equipped with a small gas displacement pump (Figure 5) is used to collect depth-specific groundwater samples.

The same flow profile information can be obtained with either the spinner log or the dye tracer approach, and either technique will enable us to characterize the flow from different zones of the well screen and the water quality attributes for each of those discrete flow zones.

This information is generally adequate to design and implement a well rehabilitation approach, but of course, there are many options for structural modification or manipulation of a well, so we have one final decision crossroad.

Well Rehabilitation Method

As with the well evaluation process described up to now, there are multiple methods and techniques for structurally modifying or hydraulically influencing a water well.

In many cases, the solution for well problems is to simply clean the well screen (generally using both mechanical and chemical means). Cleaning a well will not only improve its productivity and efficiency but may also improve its water quality and will often mitigate sand invasion problems by lowering the groundwater entrance velocities as it flows into the well.

Structural stability problems or well longevity concerns can usually be addressed by installation of a well liner. In older wells with large perforations or corrosion holes that allow sand flow during pumping, a perforated liner can be installed (which may include a pre-packed well screen with filter pack material).

In cases where water quality issues are being addressed, more complex liner installations may be required that involve pressure grouting techniques to emplace cement seals at specific depth intervals. In still other cases, a simpler approach can be used to modify wells with an inflatable or mechanical packer, or a tail pipe suspended from the pump equipment.

The alternatives for well rehabilitation are extensive, but whichever method and materials are selected for rehabilitation of a particular well, the groundwater professional should have arrived at that rehabilitation design by having navigated through all the decision crossroads that were previously encountered during the well evaluation.

If good decisions are made at each juncture of the well evaluation process, then the resulting well rehabilitation design will be appropriate and (hopefully) successful.

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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