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The rapid expansion of data centers in Northern Virginia is adding pressure to the Potomac River, the primary—and sometimes only—source of drinking water for communities near Washington.

The Potomac River basin contains 290 data centers, the world’s largest concentration, according to Michael Nardolilli, executive director of the Interstate Commission on the Potomac River Basin. Much of that development is concentrated in Loudoun and Prince William counties.

The commission evaluates the river’s long-term ability to supply Fairfax Water, WSSC Water and the Washington Aqueduct. Its latest analysis warns that data center growth could make it harder to meet the region’s drinking water needs, particularly during hot, dry periods.

Water demand is difficult to track

Although data centers are frequently scrutinized for their electricity consumption, they can also use substantial amounts of water to prevent computer equipment from overheating.

Cooling systems generally rely on either air or water. Water-based systems often release water through evaporation, meaning it is not returned to the river or water system.

Exactly how much water individual facilities consume remains unclear. Nardolilli said data center operators commonly negotiate nondisclosure agreements that classify operating information as proprietary.

That leaves the commission relying on publicly available information and estimates when projecting the basin’s future water supply.

Summer creates a ‘perfect storm’

The greatest concern is not simply annual water use but the overlap of peak demand and low river levels.

Data centers require more cooling water during hot weather. At the same time, household outdoor use rises as residents water lawns, fill swimming pools and pursue other recreational activities. Summer is also when the Potomac’s flow is typically at its lowest.

Nardolilli described the convergence as a “perfect storm” of greater demand from data centers and other customers just as less water is moving through the river.

Data centers account for about 1% of total water withdrawals in the Washington metropolitan area, according to the commission. Their share is much larger when measuring consumptive use—water that is not returned to the system—reaching 9% annually and as much as 12% during summer.

Future strain on the Potomac could depend heavily on whether advances in cooling technology reduce the amount of water data centers need in the years ahead.