Wells

A Different Angle

By Joe Bichler, PE

Sometimes the most direct route to success is to come at it at an angle. Rural and regional water systems exploring potential water sources face numerous challenges, and one of the most basic might be how to get the water flowing from that source. Source water might be purchased from other providers. Surface water might be drawn from rivers or lakes. Or ground water might be accessed by wells. In any case, the goal is to pull as much water at the highest quality possible.

Traditionally, wells have been horizontal or vertical, with each having advantages and disadvantages in different situations. Increasingly, though, engineers have been applying a technique used for some time in the oil and gas industry: angle wells. Particularly promising applications include situations where ground water is under the influence of surface water, as when trying to tap water from beneath a river. Relatively new to water production, angle wells help balance three key constraints: transmissivity, drawdown and quality.

Transmissivity refers to how much water will pass through a soil formation. Sand and gravel are good examples of how soil formations affect how water will move through the well. Drawdown is how much water can be taken from the source. Generally, systems want the capability to draw down as much water as they need from a given source. Finally, they want to draw the highest quality water they can in order to minimize treatment costs before distribution.

Angle wells can help hit that sweet spot in a ground water source to maximize the value of the well by balancing these three constraints. And they can save money.

Vertical wells in a particular application might not provide the yield needed-at least not without multiple wells. An angle well can help increase the yield by lengthening the screen where water enters the well, potentially allowing more water to flow through the well. That can result in fewer wells and less money, including maintenance and operating costs.

Horizontal collector wells utilize a series of screens placed radially from a central collector point beneath a river bed or within an aquifer. The collector point is typically a caisson structure. The horizontal wells can also have increased screen lengths, but construction costs can be significantly higher due to the needed caisson. For certain applications, angle wells can deliver the same yields at a significantly lower price tag.
One system looked at horizontal collector wells with vertical concrete caissons and pumps under the Missouri River since the river was too shallow for direct intake. Estimated costs exceeded $3 million, so angle wells were considered as an alternative. Ultimately, the system will enjoy the desired yield at a cost of $1.2 million, which kept its customers and thus the board of directors happy.

Another promising byproduct of angle wells is they can take advantage of river bank filtration, to gain credit for removal of potentially harmful waterborne items. This has the affect of potentially reducing treatment costs.

Of course, angle wells don't fit every situation. They don't work well in soils with low transmissivity or "tight" soils, and if very large amounts of water are needed horizontal wells may be more appropriate. It all depends on the situation. This is best resolved by investigation and analysis of each situation.