Underground potable water storage can be a very effective and affordable way for a municipality to store its drinking water — if the drinking water utility is poised for the approach.
There are countless factors that a utility should consider when considering underground storage. Some factors include the constraints and management of the storage structure itself
“Long-term maintenance, initial capital costs and/or neighborhood constraints are concerns that utilities should keep in mind when deciding to use underground storage,” said Lisa Jackson, a project manager for Black & Veatch.
Placing potable water underground also introduces unique benefits and challenges to management of the water itself.
“Energy use must be considered — in most instances, re-pumping is required,” added James Winger, another project manager for Black & Veatch. “Also, for larger volume reservoirs, detention time during low demand periods must also be considered.”
Here are some of the prominent pros and cons of choosing underground water storage:
The pros
Great for limited space
Underground storage tanks leave surface land available for other uses, making it an optimal solution where space is limited.
This extra surface land can help utilities accomplish other operational tasks within a small area.
“Booster pump stations can be placed at the top of the reservoir to reduce overall footprint,” said Jackson.
Depending on the depth of the tank, the area can even be available for community use.
“They allow for other uses such as tennis or basketball courts over the top of them if they are fully buried,” said Jackson.
Maintains temperature
The surrounding materials outside the underground tank help insultate it. This makes it much easier for the utility maintain a regular internal temperature. This can resolve water management issues in particularly hot or climate climates.
Maintenance
The concrete structures of below-grade reservoirs can introduce unique maintenance benefits. The structure’s don’t require the recoating and relining that steel tanks do. In addition, work on the reservoir roof can be mor eaccessible.
“Underground concrete water storage tanks provide safer access for work on the reservoir roof where we typically have level elements and reservoir vents, sometimes mixing systems that have to come out of the top hatches,” said Jackson.
Resilient to damage
Since underground tanks can be difficult to access from the surface, underground water storage tanks have a built-in resiliency against external damage. Extreme weather events and vandalism are less likely to damage the tank.
Hidden
A well-planned community takes care to make sure that water infrastructure blends in well with its surrounding aesthetic environment. With underground water storage, the potable water tanks remain out of sight. With a smaller visual presence, water infrastructure can have an easier time fitting into planned spaces.
Larger storage
Compared to water towers, ground-level and underground water storage installations are often better suited to holding larger quantities of water. Towers can be limited to a volume that its base can withstand, while the structural concerns of underground storage are much less limiting.
“In Arizona, for example, they are very common for storage over 2 million gallons,” said Jackson. “Underground storage is generally used by municipalities requiring storage over 2 million gallons per reservoir.”
In addition, underground storage can use much wider and longer storage areas.
“Underground cast-in-place concrete storage tanks are prevalent in the Midwest,” said Winger. “Elevated tanks are limited to 3 million gallons so, if a larger storage volume is required, below grade cast-in-place concrete tanks are a viable option.”
The cons
Installation costs
“Construction costs are greater than above grade storage tanks, and they usually take longer to build,” said Jackson.
The excavation and installation of underground water tanks can be much more expensive than above-ground storage. The earthwork necessary for underground storage is a significant added cost to the base expense of the storage structure itself.
“The cost difference varies depending upon soil/foundation conditions and storage volume,” said Winger.
Maintenance
While underground storage has some maintenance benefits, it also has its own unique problems.
Underground tanks have much more limited access for inspections and maintenance, making it a generally more difficult process. This means that the necessary work to detect leaks and monitor structural integrity could require a more specialized approach, potentially raising costs and complicating operations.
Undeground tanks “often require a leak detection system that is more costly and harder to pinpoint leak source,” said Jackson.
Elevation needs
Water towers can take advantage of the force of gravity to move its water — this can help maintain water pressure throughout a distribution during power outages. The elevation of the tower is what allows for this form of water supply resiliency.
Ground-level and underground water storage installations, however, are less likely to enjoy this resiliency. They would need to be installed at an elevation higher than the community to use gravity, or would need to be able to rely on secondary power sources for pumping.
In addition, groundwater can be a limiting factor for where underground storage is viable.
“They are not recommended when there is high groundwater,” said Winger.
This can limit the depth of the reservoir if it is located in an area with high groundwater.
