Occam’s Razor of Water Well Design

Starting with the simplest design is often the right approach to the job.

By Marvin F. Glotfelty, RG

Figure 1. Simple generalized well design.

There is a philosophical problem-solving principle known as “Occam’s razor.” It involves seeking the simplest solution to any problem with the smallest set of assumptions.

When there are two competing ideas to explain the same phenomenon, the simpler one is preferred. In other words, Occam’s razor is the concept that the simplest explanation or solution to a problem is usually the best one.

The principle is attributed to a 14th-century English Franciscan friar and philosopher named William of Ockham, and the term Occam’s razor is derived from the process of shaving off overly-complex explanations and unnecessary assumptions until we have reached the simplest explanation or solution for a problem.

However, it is important to realize that Occam’s razor is a general guideline of logic and not a steadfast law of nature. Oversimplification of a problem does not necessarily provide a pathway to resolve it. If a complex theory provides the best explanation of the observed or measured conditions, then the complex theory should be preferred over a simple explanation that doesn’t explain the observed facts and empirical evidence.

Account for the Conditions and Use

Occam’s razor is generally applicable to water well design, but with some significant exceptions. As with many pieces of infrastructure that we design and use, the simplest design for a water well will be the best—provided that well design accommodates two things: the local hydrogeologic conditions at the well site, and the intended operational use of the well.

Appropriate well designs will vary substantially between fractured rock aquifers, aquifers composed of well-sorted sand, and aquifers consisting of a complex mixture of poorly sorted gravel, sand, and clay.

Similarly, the appropriate well designs at a given location will vary significantly, depending on whether the well is to be used as a water supply well, an aquifer storage and recovery (ASR) well, or a monitoring well.

Figure 2. Common well screen types.

A simple well design, as shown in Figure 1, is a good starting point for any water well installation project. If the hydrogeologic conditions (well location and groundwater environment) and operational requirements (intended use of the well) are not entirely addressed by that simple well design, then adjustments to the well design are necessary.

These added design elements will usually increase the complexity to the well design, but the accumulation of well design elements must continue until all the hydrogeologic and operational requirements of that particular well have been met.

The process of starting with a bare-bones simple well design and then progressively adding design elements to address site-specific needs constitutes somewhat of a “reverse-razor” approach for well design, wherein we are layering on rather than shaving off design elements until the optimum well design is achieved.

Each individual well design element should be considered individually, along with the interaction of the various design elements with one another, so that the comprehensive well design will address all site-specific concerns while remaining as simple as possible.

Principal design elements to be considered are first of all the casing/screen material. This may be PVC, low-carbon steel, high strength low alloy steel, stainless steel, or some other material that provides for the strength, corrosion resistance, and budget considerations of the particular well. Also considered should be the well screen type, the filter pack type, and the overall dimensions (depth and diameter) of the well.

Selection of the most appropriate screen for a particular well should be responsive to the hydrogeologic and operational needs for that well. Figure 2 shows some common well screen options. The specific well screen type that is selected by the well designer is contingent on the local criteria for the well that is to be installed at a specific location for a particular purpose.

Figure 3. Filter pack material options.

Another important well design element is the selection of filter pack material that is appropriate for the well’s hydrogeologic and operational criteria. For a fractured rock aquifer in a geologic environment that is unlikely to produce sand while pumping the well, a filter pack can be omitted altogether. More commonly, the grain size, grain roundness, sorting, and composition of the filter pack material constitute important well design elements that should be consistent with the local aquifer conditions and the intended use of the well.

Examples of filter pack material that are applicable for various well designs are shown in Figure 3. A more in-depth discussion of filter pack design considerations was included in my previous The Art of Water Wells column in the November 2020 issue of Water Well Journal.

Layering Design Elements

I was involved a few years ago with a well installation program for a city that exemplifies how an initially simple well design can be transformed with the addition of multiple complicating design elements to address numerous operational requirements of the project.

This project involved the design and installation of 9 ASR wells within a short time (Figure 4). To accommodate the aggressive project schedule, there were multiple design consultants, several drilling contractors, and a couple of general contractors involved.

The large cast of involved parties required heightened project coordination, but additions to the well design elements were a direct result of the complex operational needs of ASR wells and the local hydrogeology at each well site.

The ASR wells needed to be appropriate for both pumping and recharge of water at high rates (2000 to 3000 gallons per minute). For that, we needed to have stainless steel casing and screen in the saturated portions of the well (to resist corrosion and minimize scale growth), and we also chose to design the wells with a manufactured glass bead filter pack (Figure 3, Item E) to minimize biochemical clogging and to optimize redevelopment potential of each ASR well during its operational life.

The ASR wells were relatively deep and had long screened intervals to address the hydrogeologic conditions of the area, including variable depths to the water table and thick clay layers in some areas. To maintain budget control, the upper casing of each well was designed with less-expensive high strength low alloy steel, which necessitated a dissimilar metal coupling at the intersection of the two metal types in each well.

Figure 4. Aquifer storage and recovery wells from a municipal well installation program.

Although several design elements were added to the design of these ASR wells, each and every design element was essential for (1) the operational plans of the ASR wellfield and (2) the local hydrogeological conditions at the well site. These ASR wells are currently a critical part of the city’s overall water resources portfolio.

Probably the most complex well that I have ever designed is the nested piezometer shown in Figure 5. I am generally not a fan of nested wells for two reasons. First, it can be difficult to effectively and completely isolate the different screened intervals from one another. Second, the small-diameter well screens within a large-diameter borehole make it almost impossible to fully develop the piezometer wells. Thus, I favor clustered piezometers (same casing and screen diameters and depths, but in individual offset boreholes rather than all completed within a single large borehole).

In this case, a set of piezometers were needed to monitor groundwater levels at various depth intervals (depths of about 300, 940, and 1200 feet) at a municipal facility. The client wanted to have a nested well instead of clustered wells to limit the footprint of the piezometers on the land surface. The client also wanted only stainless steel or PVC material in the well screen intervals to avoid scale growth or corrosion in the future.

Based on these directives and other operational/hydrogeologic considerations, the nested piezometer design in Figure 5 was developed. This well design provided for three 3.5-inch outside diameter PVC casing strings with screened intervals adjacent to the stainless steel screens of the outer (14-inch-diameter) casing.

Figure 5. Complex well design for a set of nested piezometers.

The blank casing between the stainless steel screens of the 14-inch outer well was composed of high strength low alloy steel to reduce costs, so a total of six dissimilar metal connectors were required—one at the top and one on the bottom of each screened interval. The three screened intervals in the 14-inch outer casing were isolated from one another with bentonite annular seals outside the 14-inch casing, and since the filter pack envelope outside the 14-inch screened intervals was only about 4 inches thick, the outer screened intervals could be fully developed prior to installation of the PVC piezometer strings.

The city had on hand a supply of manufactured glass bead filter pack material left over from another well installation, so the glass beads were used for the interior filter media adjacent to each PVC piezometer screen. We were concerned about the adequate placement and hydration of the bentonite chips to facilitate a complete seal between the PVC piezometers, so we added some layers of polyacrylate grout (a liquid material that solidifies into a rubber-like material) to fill in any gaps in the seals between individual piezometers.

The nested piezometer shown in Figure 5 is definitely overly complex for almost any conventional water well application. It functions as intended, but the premise of “simpler is better” absolutely still holds true. Wells like the one shown in Figure 5 should be the exception rather than the rule.

Learn The Art of Water Wells

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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 mglotfelty@geo-logic.com.