Aquifer Complexity in Well Design

It’s important to prepare water systems that guard against complexities that can occur.

By Marvin F. Glotfelty, RG

Too often we encounter well owners or even fellow groundwater professionals who consider a well’s water
productivity and its water quality to be a singular property that will be forever attached to that individual water well.

Figure 1. Aquifer complexity. Differing groundwater flow rates from geologic strata with variable characteristics.

This mindset is founded on the oversimplified and inaccurate concept that a well receives water solely from a single water source in the subsurface and that the source will consistently provide the well with a fixed flow rate of groundwater with unchanging water quality.

In reality, hydrogeological science and multiple case studies have demonstrated that aquifers are much more complicated than this oversimplified concept would suggest. However, since groundwater professionals must often work with limited data, we can sometimes be tempted by our human nature to fall into the pitfall of oversimplification.

Layering Groundwater Flow

What we know to be the case in most aquifers is that groundwater enters the well at variable flow rates that are dependent upon the characteristics (hydraulic conductivity and thickness) of the aquifer material from which the groundwater is produced.

Figure 2. Differing horizontal and vertical groundwater flow. Groundwater has much greater flow characteristics as it moves horizontally versus vertically through the earth, due to layered geologic formations (stratigraphy) and the orientation of sediment (anisotropy) within each sediment layer.

This scenario is shown conceptually in Figure 1, and of course, as the proportional contribution of water production varies at different depths of the well, the blended water quality from those respective well depths will also be impacted.

There are exceptions to the layering of groundwater flow within wells, such as aquifers composed of aeolian or windblown sand that are massive and unlayered. Such aquifers produce groundwater almost equally at all depths of the well, but such scenarios represent uncommon outliers that are not representative of a “typical” aquifer.

We rely on the Theis Equation to evaluate and simulate groundwater flow thorough aquifers, and a couple of the primary assumptions of that equation are that the aquifer is composed of a single type of geologic material (homogeneous), and that the aquifer has equal flow characteristics in all directions (isotropic). These characteristics rarely occur in nature, although the Theis Equation remains a valid mathematical technique that is representative of groundwater flow since the scale and precision of the equation can be adjusted to provide meaningful and reasonably accurate results.

In the real world, we typically encounter aquifers that are composed of various types of sediment that have been deposited in layers (stratified). Such sedimentary formations could have been laid down by a variety of geologic environments—an ocean, a stream, or a lake—but all such sedimentary deposits will exhibit a degree of stratification. A generalized diagram of such a stratified aquifer is shown in Figure 2.

Figure 3. Impact of stratified aquifer characteristics. Example of layered sediments A, B, and C that have different specific capacity (gpm/foot of drawdown) values.

The stratification of different sediment types (sand, silt, clay, etc.) creates preferred groundwater flow paths in the horizontal direction. The horizontal movement of groundwater will have an unobstructed flow path within individual stratigraphic layers. But vertical movement of groundwater through a stratified aquifer will be hindered, as the water encounters formation boundaries that are marked by erosional surfaces and changes in the sediment characteristics.

In addition to the impact of stratification, each individual layer of sediment is generally composed of larger rock fragments that have been eroded down into smaller rock fragments. Since rocks tend to fracture along elongated cleavage planes, the eroded sediment is not composed of sphere-shaped grains, but rather, the sediment consists of saucer-shaped grains that are longer along one axis and flat along the opposite axis.

This means that the deposits of sediment will be laid down like the shingles on a roof and provide more direct flow paths in the horizontal direction rather than in the vertical direction. This unequal flow characteristic is called anisotropy and shown in Figure 2.

A photograph of an anisotropic deposit of gravel and cobbles is included in Figure 2 to illustrate this common characteristic of both coarse-grained and fine-grained sedimentary deposits. Stratigraphy and anisotropy typically result in preferred flow characteristics in the horizontal direction by several orders of magnitude (10 times to more than 1000 times greater horizontal flow).

Oversimplifying and Errors

Figure 4. Impact of aquifers with differing hydraulic heads. Example of sedimentary layers A, B, and C that have different water levels, which will impact the groundwater production from the respective aquifers.

To assess the impact of stratigraphy and anisotropy on a water well, we can consider the scenario shown in Figure 3. Let’s assume we’re pumping a well that penetrates three layered formations that each have a thickness of 100 feet. The top stratum A has a specific capacity of 10 gallons per minute per foot of drawdown (gpm/ft). The middle stratum B has a specific capacity of 8 gpm/ft, and the lower stratum C has a specific capacity of 5 gpm/ft.

If we pump the well at a flow rate that causes a drawdown of 50 feet, then stratum A would be capable of accommodating all of that drawdown by producing 500 gpm (10 gpm/ft = 500/50). Similarly, if strata B and C were individually pumped at flow rates that accommodate a drawdown of 50 feet, they would theoretically produce 400 gpm and 250 gpm, respectively, based on their individual specific capacity values.

If we apply the oversimplified concept that this well has a single set of aquifer properties, we may erroneously think that the entire 300-foot screened length in this well is producing 500 gpm with a drawdown of 50 feet, such that the inflow rate is equally spread across all of the screened interval (about 1.67 gpm per foot of screen). However, the stratification of the aquifer could be causing all 500 gpm of the flow to be entering from stratum A, so the actual groundwater inflow rate could be about 5 gpm per foot of screen, while the groundwater contributions from strata B and C are essentially zero.

This means that viewing the well and aquifer conditions in an overly simplistic manner may result in several errors. First, an oversimplification of the aquifer could cause gross miscalculations of the groundwater entrance velocities. In addition, oversimplification of the subsurface conditions could result
in a failure to identify stagnant conditions in strata B and C, which could subject that portion of the well to rapid scale and biofilm clogging, even if the well was being regularly pumped.

Another aquifer complexity that impacts the groundwater production of water wells is variability of water levels in the individual formations that have been penetrated by the well. In some areas, the stratigraphy and anisotropy of the aquifer will accommodate a vertical hydraulic gradient, as shown in Figure 4. In these areas, vertical pressure gradients may result from prolonged periods of water withdrawal from the lower portions of the aquifer, while water is being recharged in the upper aquifer.

We most commonly see this phenomenon in areas with a history of farming where deeper groundwater has been pumped to water crops and the agricultural leachate beneath farm fields percolates back down to the water table at the top of the aquifer.

Vertical hydraulic gradients impact the performance of water wells because the groundwater flow to a pumping well happens in accordance with Darcy’s Law, which states:

Q = -KiA where:

Q = discharge rate

K = hydraulic conductivity

A = cross-sectional area through which the water is flowing

i = hydraulic gradient

Consider the scenario shown in Figure 4, with a vertical hydraulic gradient causing different water levels in strata A, B, and C. Let’s assume that strata A, B, and C have identical hydraulic conductivity values and also identical cross-sectional areas through which groundwater flows to the well. Thus, the only difference between the groundwater production of each respective formation is the groundwater gradient (i) within
each individual formation.

The effective hydraulic gradient in a pumping well is the difference between the pumping water level inside the well, and the static water level outside the well. Therefore, the formations with higher static water levels (such as stratum A in Figure 4) will have the highest hydraulic gradient and will produce the most groundwater into the well. Formations with lower static water levels (such as strata B and C in Figure 4) have lower hydraulic gradients, so they will produce less groundwater.

Complexities Do Occur

Stratified aquifer conditions with different hydrologic characteristics as shown in Figure 3 or different hydraulic heads as shown in Figure 4 can generally be addressed by the groundwater professional designing the well. Unfortunately to address these complexities, the designer must be aware of them. These aquifer conditions are usually only evident if they’ve been measured on-site via analytical methods such as
depth-specific (zonal) sampling or geophysical logging.

Those analytical methods come at a cost and require time, so not all well installation programs include these analytical tasks. Thus, the site-specific complexity of the aquifer may not have been characterized during the well drilling process. Because of this, it is the responsibility of well designers to consider potentially complex aquifer conditions, even if such conditions have not been confirmed. Well designs should incorporate safeguards to address complex conditions, and well designers should communicate the possibility of associated unexpected results to the well owner.

While we don’t want to be overconservative and alarmist in our communication with well owners, groundwater professionals must remain vigilant with our responsibility to consider and communicate the potential impacts of complex aquifer conditions.

Even for projects that have budgetary constraints or scheduling limitations that hinder our ability to measure and evaluate site-specific aquifer attributes, we can still apply local experience and pragmatic common sense to produce well designs with appropriate guardrails and safety factors that account for the potential complexity of Mother Earth.

Such conservative safeguards in well design are worthwhile since we will inevitably encounter complex aquifers from time to time.

Learn The Art of Water Wells

Get The Art of Water Wells by Marvin F. Glotfelty, RG, a 2019 book from NGWA Press that is a comprehensive overview of well systems and delivers practical information applicable to real-world situations. The book is ideal for everyone working in the groundwater field. It provides practical information of water wells—covering everything from site selection to design, drilling methods, economics, and more. Click here to order it, call (800) 551-7379, fax (614) 898-7786, or email customerservice@ngwa.org.

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.