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The US’s��thirst for water – whether to drink or to grow crops – is encouraging an unsustainable trend towards ever deeper groundwater wells. That is the warning from US researchers who have produced the first map of the country’s wells, spending four years painstakingly talking to scores of public authorities to unearth the data.

Groundwater provides drinking water for 120 million Americans and around half of the country’s irrigation needs. But demand in places such as California’s central valley is drying up existing wells, and engineers are drilling deeper ones to replace them.

No one knew how many and how deep these new wells were.��But a mapping exercise by Debra Perrone and Scott Jasechko at University of California, Santa Barbara found there are 11.8 million wells in total, and they are becoming deeper over time. “We wanted to make the invisible visible,” says Perrone.

Simply drilling deeper is “an unsustainable stopgap to groundwater depletion”, Perrone and Jasechko argue, for four reasons: cost, energy for pumping, geology, and water usually becoming saltier the deeper you go. Perrone suggests that the alternative is to improve governance around groundwater use, and to look closer at which legal controls around withdrawals are most effective. In those cases where deeper drilling has to go ahead, water quality must be protected, she adds.

Journal reference:��Nature Sustainability,��