Would you believe that 80 percent of the world’s groundwater resources are managed sustainably? It’s true! Sadly, the good news ends there.

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A new global groundwater assessment reveals that many key aquifers are being depleted faster than they can be replenished, more than offsetting sustainable management elsewhere. Globally, humans are consuming 3.5 times more groundwater than aquifers can support, the study states.

Much of the water extracted in the high-stress areas is used to irrigate farmland, and dry parts of North America and Asia fare the worst. Some parts of India, for instance, are using 54 times more water than the local aquifers can sustain.

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In the U.S., worrisome areas include parts of the High Plains, especially in the Texas Panhandle and western Kansas, and the Central Valley of California.

In the worst cases, groundwater extraction has already caused a permanent reduction in the storage capacity of the aquifer—meaning that the aquifers can never recover even if pumping stops.

This is “groundwater mining” in the truest sense of the term. It happens when groundwater is stored in layers of unconsolidated sand and silt. As water is pumped out, the weight of the overlying rock and soil squeezes out the empty space left between grains of sediment.

This compaction caused the surface of the land to subside—which is exactly what happened at the site of this U.S. Geological Survey photograph of California's agricultural San Joaquin Valley (left).

Years of pumping groundwater for irrigation compacted the underground layers of sediment, and the land surface dropped accordingly. The top sign shows where the land surface was back in 1925. Compare that to where the man is standing (circa 1977). You can think of the elevation difference between the two signs as groundwater storage capacity lost forever.

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Much of California’s groundwater is used for agricultural irrigation. (California Department of Water Resources)

San Joaquin Valley c. 1977 (U.S. Geological Survey)