As the outlook for drought in the western United States grows darker, attention is again turning to groundwater - literally, water stored in the ground. It is the most abundant and reliable source of fresh water on Earth, but it is not unlimited.
Wells that people drill to access groundwater provide nearly half of the water used for irrigated agriculture in the United States and supply drinking water to over 100 million Americans. Unfortunately, the widespread pumping of water is causing groundwater levels to drop in some areas, including much of California's San Joaquin Valley and the Kansas High Plains.
In a recent study, scientists mapped the location and depth of wells in 40 countries around the world and found that millions of wells can dry up if the water table drops by just a few meters. While different countries have different solutions, we believe that the most important thing to protect wells from depletion is the rational use of groundwater - especially in countries like the United States that use large quantities of it.
Which countries use the most groundwater today
People have been digging wells to obtain water for thousands of years. Examples include wells 7,400 years old in the Czech Republic and Germany, 8,000 years old wells in the eastern Mediterranean and 10,000 years old wells in Cyprus. Today, wells provide 40% of the world's irrigation water and supply billions of people with drinking water.
Groundwater flows through tiny spaces in sediment and underlying bedrock. In some places, called discharge zones, groundwater rises to the surface into lakes, rivers and streams. Elsewhere, called recharge zones, water seeps deep into the ground, either from precipitation or from leakage from rivers, lakes and streams.
Consequences of lowering the groundwater level
A drop in the water table can have many undesirable consequences. The surface of the earth sinks as the subterranean layers of clay are compacted. The intrusion of seawater can pollute groundwater supplies and make them too salty for use without energy-intensive treatment. River water can seep into aquifers, leaving less water on the surface.
Groundwater depletion can also cause wells to dry out when the top water table - the so-called water table - sinks so far that the well is not deep enough to reach it, leaving the well literally dry and high. Until recently, however, little was known about how vulnerable wells around the world are to depletion due to declining groundwater levels.
There is no global well construction database, so over six years we have collected 134 unique well construction databases in 40 different countries. In total, we analyzed nearly 39 million well construction records, including the location of each well, the reason for its construction, and its depth.
The new results show that wells are vital to human livelihoods, and considering the depth of the wells helped us see how vulnerable wells are to drying out.
Millions of wells at risk
The new analysis led to two main conclusions. First, up to 20% of wells worldwide are located no more than 16 feet (5 meters) below the water table. This means these wells will dry up if the water table drops by just a few feet.
Second, scientists have found that in some places where the water table is dropping, new wells are not dug much deeper than old ones. In some areas, such as eastern New Mexico, new wells are drilled no deeper than old ones because deeper rock layers are impenetrable and contain salt water. In these areas, the likelihood of new wells running dry is at least as likely as old wells.
Subsea wells are drying up in the USA
In some places, including the western United States, wells are already failing. In previous studies, we estimated that 1 in 30 wells are drying up in the western United States, and 1 in 5 in parts of California's southern Central Valley.
Wells are already running low in Central Valley and southeastern Arizona. Outside the Southwest, wells dry up in states as diverse as Maine, Illinois, and Oregon.
What to do if the well runs out
How can households adapt to the situation when the well runs out? Here are five strategies, each with its own drawbacks.
- Dig a new, deeper well. This option is only possible if fresh groundwater occurs at great depths. In many aquifers, deeper groundwater tends to be saltier than shallower ones, so deep drilling is only a temporary solution. And since new wells are expensive, this approach favors wealthier groundwater users and raises questions of equity.
- Sell the land. This option is often considered if construction of a new well is not affordable. It can cost tens of thousands of dollars to drill a new utility well in the Southwest United States. But selling a property that doesn't have access to a reliable and convenient water supply can be challenging.
- Removal or delivery of water from alternative sources such as nearby rivers or lakes. This approach is only possible if surface water resources are not yet reserved for other users or are too far away. Even if nearby surface water is available, purifying its quality to make it safe to drink can be more difficult than purifying water from a well.
- Reduce water consumption to slow down or stop the decline in the water table. This could mean switching to less water-intensive crops or introducing irrigation systems that reduce water losses. Such approaches can reduce farmers' profits or require upfront investment in new technologies.
- Limit or avoid activities that require large amounts of water, such as irrigation. This strategy can be challenging if irrigated land yields higher yields than non-irrigated land. Recent research indicates that some land in the central US is not suitable for irrigated agriculture.
Techniques for protecting wells from depletion
Households and communities can take active steps to protect wells from depletion. For example, one of us is working closely with Rebecca Nelson of Melbourne Law School in Australia to map groundwater abstraction permits - the process for obtaining groundwater abstraction permits - throughout the western United States.
State and local authorities can allocate permits for groundwater abstraction in such a way as to help stabilize falling groundwater levels in the long term, or in such a way as to determine the priorities of certain water users. Adopting and enforcing policies to limit the depletion of groundwater can help protect wells from depletion. While limiting the use of such an important resource as water can be difficult, we believe that in most cases, simply deepening drilling is not a sustainable development path.