The water inside an aquifer pushes against the sediment grains, holding them apart. When people dig a well to remove some of that water, they suck the water out like a straw. Just like ice cubes in a cold drink, the pieces of sediment clump together as the liquid is removed. This causes the land surface above to sink. When the sediments are large, like sand or gravel, the pores between them don’t close up completely. These areas can refill when more water is added. But in areas with clay and silt, the pores are tiny. When water is removed, the grains stick together, forever closing those pores to water. In these areas, the drop in land surface is permanent.
Ground water is often removed to water crops, supply water to people living in large cities, and supply water to factories. Over the last century, areas like California’s San Joaquin Valley have experienced extreme sinking. The land dropped nine meters (nearly 30 feet) in just 52 years because groundwater was used to water crops. Removing oil and natural gas has a similar effect on the land.
When wetlands are drained, they expose soil that is rich in organic matter. Once these soils are exposed to the air, that carbon begins to break down. This process creates carbon dioxide that is lost to the atmosphere. The layers of soil decrease over time, causing the land to sink. These areas are often drained to allow people to construct buildings. The weight of these structures presses down on the soil, compacting it even more. A number of large coastal cities may be underwater by 2100 due to a combination of land sinking and sea level rise.
