THE WATER CYCLE AND GROUNDWATER
Water is created and destroyed in natural chemical reactions within plants and animals. However, most water sticks around. It changes phases through the water cycle; it can become polluted with salt, toxic chemicals, or pathogenic organisms. However, it generally doesn’t go away, globally speaking.
The water, or hydrologic, cycle describes how water moves through the atmosphere, on the Earth’s surface, and underground.
As “surface water” in the lakes, streams, rivers and oceans warms from the sun’s electromagnetic radiation, some evaporates into the atmosphere.
This water vapor in the atmosphere condenses into rain and snow, called precipitation. The precipitation falls on the Earth, eventually feeding into streams, lakes, and oceans. Some of the water seeps into the ground and collects in underground aquifers as groundwater. About 20 percent of the U.S. water supply comes from groundwater.
Groundwater can resurface from springs or it can discharge into lakes, streams, rivers, and oceans. High pressures deep inside the Earth can force groundwater up through artesian wells, or groundwater can be pumped up or pulled up in old-fashioned buckets from wells. (“Artesian” means that there’s sufficient water pressure that the groundwater need not be pumped).
Humans use water from the surface sources (lakes, rivers, oceans), we collect rainwater and snowmelt, and we also use groundwater. Most of this water gets discharged back out into waterways or oceans. However, water used in homes and businesses is sent to municipal water treatment, after which it is discharged into waterways, returning to the water cycle.
GROUNDWATER AND DEPLETION
Groundwater isn’t as free-flowing as surface water. Predicting and modeling how it flows is wildly complex, factoring for what’s dissolved in the water and what materials it’s moving through, in three dimensions. What is easy to say is groundwater moves slower than surface water, and it gets recharged more slowly. Because modeling is complex, and tracking depletion involves drilling wells, it’s far more difficult to gauge groundwater depletion than water shortages on the surface.
When groundwater is depleted, it is still there, just lower down, as many as several hundred feet lower in extreme cases. However unseen it is, groundwater depletion – and the lowering of the water table – is very serious for several reasons.
Trees and plants rely on groundwater, and if they cannot reach water with their roots in regions where it doesn’t rain all year long, they can die, and with them all the life that depends upon them.
For people who rely on well-water, depletion can be equally disastrous. As the depth needed to reach the water increases, the amount of energy required to pump it out also increases. Lowering the water table can pollute the water, as saltwater zones can underly freshwater zones.
And even for those who depend on surface water, which is all of us, groundwater depletion can have its effect because ground water feeds surface water and vice-versa. Groundwater depletion can reduce the amount of water in streams and lakes, even if the effects take years to become obvious.
ARE WE SINKING?
As the water table lowers from groundwater depletion, the materials within the ground dry out and the ground can actually collapse in on itself, either suddenly or slowly over time, a phenomenon called subsidence. The most dramatic incidents of subsidence are sinkholes, but most of the sinking is happening imperceptibly slowly. This sinking is why some regions of the Netherlands came to be below sea level; centuries of pumping water out of the peat-based soils shrank them, and the land — protected from flooding by the North Sea and Rhine River waters behind dikes — sunk lower and lower.
Today, subsidence from pumping of water has been recorded all over the United States, but the Santa Clara Valley in California was the first area in the country where land subsidence from human use of groundwater was recognized and the first place that organized remediation to stop the subsidence in 1969, according to a report by the U.S. Geological Survey.
While today the region is best known for its Silicon Valley technology, in the late nineteenth century, Santa Clara was full of fruit orchards irrigated with groundwater, much of it from artesian wells, meaning that the wells filled themselves with the pressure of the water created by confined aquifers. Constant reliance on this easy source of groundwater meant by 1930, wells that formerly filled themselves had to be pumped, and by 1964 one well in downtown San Jose had sunk well over 200 feet below the surface. As water was permanently removed from the ground, the ground shrank, and by 1984, downtown San Jose had sunk quite substantially, to just 84 feet above sea level from 98 feet above sea level in 1910.
For more about water use and energy see here.