People are being asked to boil their water before drinking it in all of Washington, DC and Arlington County in Virginia due to an increase in algae blooms in the Potomac River, as the nation’s capital prepares for its annual Fourth of July celebrations.

The Washington Aqueduct is sourced from the Potomac River and serves as the public water supply for the D.C. area, Arlington County and other portions of Northern Virginia.

Local water systems issued a boil-water advisory affecting all of the District of Columbia and almost all of neighboring Arlington County because of impurities found in the Washington Aqueduct that were blamed on algae blooms. Notices were sent to residents in both jurisdictions via emergency cell-phone notices that began appearing after 11 p.m.