If you're reading this, chances are you don't really have to think about where your drinking water is coming from, how your food was grown, or what effects such processes have had on the environment. Specifically, in reference to our drinking water, around half of the US population gets their drinking water from either public or private wells.
Water wells are used on a massive scale. They suck water from underground aquifers for use in agriculture or for drinking water. Water is, after all, essential to life, but many civilizations have developed in regions of the world that don't have access to sufficient surface freshwater.
Take, for example, California and much of the southwestern United States. While this region is one of the most densely populated in the country, it also happens to be one of the most water-scarce.
California has a massive agricultural industry too, meaning the state requires a significant amount of water on a daily and seasonal basis. For the most part, the states' water has historically come from wells drilled that pump out freshwater from confined and unconfined aquifers under the surface.
This is a fairly common practice globally, but issues arise when the amount of water pumped out of the aquifers is greater than the amount of water flowing into the aquifers. When over-pumping occurs, large swaths of soils underground that previously were saturated with water are now left dried out permanently.
All the static and dynamic forces from the land and rock above start adding up and eventually that now-dry soil starts compacting down and down. While this may not seem like a big deal on a small scale, what we've seen in California (and other parts of the world too) is the dropping of the surface elevation over a period of years, often by hundreds of feet or meters.
This dropping of the ground level is an aspect of a principle called land subsidence. Let's take a closer look at just what is occurring.
What is land subsidence?
Land subsidence can occur when significant portions of groundwater are pumped out or removed from underground rock and soil. The previously water-saturated rock and soil, which is now dry, start compressing under the forces of the rock above it. Land subsidence is a fairly slow process, and is not generally noticeable on a day to day or minute to minute basis, in the way that says, an earthquake would be.
Localized subsidence is also the principle that describes why a sinkhole or a pothole might occur, only in these cases, it's far more localized land subsidence than the kind that would drop an entire region's elevation.
While dropping the surface elevation in a region may not seem like a big deal, it's actually a rather costly one. In theory, the dropping of the ground level is fine if it occurs evenly everywhere, but that's not the case. Land subsidence is a highly variable process, based largely on the soil makeup under the surface. If one area is comprised mainly of soft clay and the other is mainly of silt, the two areas are going to compact at different rates, even if they took up the same volume when saturated with water.
Since soils are made up of different components and have different shapes and sizes of particles, this also means that different soils can take up various amounts of water. In gardening, this is one reason why you might buy specialty potting soil – to help the soil retain more or less moisture.
In geology, these different absorption rates impact how engineers design building foundations and wells for underground aquifers.
Back to the core principle here, land subsidence is a big issue because it causes the ground level to sink at highly variable and uneven rates. When structures are present on the surface, the movement can be enough to crack foundations, collapse bridges, crack underground pipes, and otherwise wreak havoc on civil infrastructure.
Humans have a pretty hard time visualizing something that happens over the span of years though, so I find that when learning about land subsidence, the most effective thing you can do is look at pictures that document its effects. Perhaps one of the most famous is that of the subsidence that occurred in the California Valley over the span of five decades. As a warning, this image is going to require some scrolling, but it hopefully underscores just how substantial land subsidence can be.
After seeing that, you hopefully have some grasp on just how substantial land subsidence from over-pumping of aquifers can be. One might next wonder how this can be tracked and prevented. While we won't go into that too much in this article, the short answer is that governing bodies track the water levels in wells across a region to monitor whether there is more or less water.
If signs of overpumping are starting to be seen, engineers can either keep pumping and deal with the effects of subsidence, or find another source for water.
The gif below is a compilation of USGS well data across the US, indicating water levels in their well network. This type of data is one of the tools engineers use to figure out how underground water is flowing, refilling, and otherwise behaving.
Engineers will also utilize tools like compaction recording devices to measure the change in sediment layer thicknesses underground. These tools, rather than measuring water level, allow engineers to measure the soil effects from well pumping in a given area. The figure below does a pretty good job explaining how this works.
The harmful effects of over-pumping
We've talked a little about how harmful over-pumping can be to buildings and infrastructure, but this isn't the only issue with overpumping, nor have we discussed the full scope of the infrastructure issues that arise.
Land subsidence has many core problems, most of which can be summed up concisely like this – land subsidence can cause:
- Changes in elevation and slope of streams and surface water systems
- Damage to infrastructures like roads, bridges, pipes, levees, and sewers
- Damage to surface buildings
- Failures of wells
- The intrusion of chemicals from agriculture
One of these problems is a change to naturally occurring geography, three are changes to infrastructure, and the last is the degradation of water quality.
Let's focus on the change to geography first. If subsidence changes the path or elevation change across a stream, it can increase its flow rate, causing more erosion in a given area, which can make it harder for certain types of fish and plants to survive, and all of this, in turn, could impact the surrounding ecosystem and even human habitation.
Changes in geography from land subsidence can affect coastal regions too. A house once built far away from high tide elevation may suddenly find itself dangerously close to the water. Keep in mind this effect is completely different from changes in tide levels caused by climate change.
Going back to the initial list of problems, we've already spent a decent amount of time going over how changes to the ground might impact, damage, or destroy surface and subsurface infrastructure, so I'm mostly going to skip over those effects for further explanation. What is a new topic though, is the discussion of how land subsidence can actually pollute subsurface water.
Specifically, overpumping from wells can cause concentrations in groundwater of pollutants like arsenic to skyrocket.
Arsenic is a naturally-occurring chemical in nature. Over time, arsenic is transported through rivers and deposited into clay. This has occurred for millions of years and over time, these clay layers get pushed deeper and deeper into the earth's crust.
What has ended up occurring is a large arsenic concentration in deep clay structures, usually far deeper than wells would ever pump. However, when wells are overpumped and the soil on top is drained of water, the well starts pulling water from the fine clays underneath, which can have high arsenic concentrations, bringing the arsenic along with it.
This increased arsenic concentration can then pollute crops and drinking water. Even low levels of arsenic, such as 10 milligrams per liter (8.3 pounds per gallon), are harmful and can cause increased rates of cancer, heart disease, and diabetes.
At the end of the day, land subsidence from over-pumping of subsurface aquifers is a major issue in water-starved regions across the world. It's also an issue that's not easily solved since, first and foremost, societies need water. It can be very difficult to switch to less water-intensive crops or growing methods, and you can't just tell people not to drink water or grow crops.
Land subsidence is just one of the fascinating issues that civil and environmental engineers work to solve on a daily basis, and it's one that crucial to the future of many regions across the world.