As Pennsylvania’s salt lines move, the Philadelphia Water Department assures residents that saltwater in the drinking water is unlikely, reports Anthony R. Wood for the Philadelphia Inquirer.

However, a dry spell over the past three months has pushed the Delaware River’s salt line—the border where freshwater meets saltwater—farther north than usual.

As of Monday, the salt line had advanced near Philadelphia International Airport, about 20 miles north of its typical position for this time of year. This is 20 miles south of the Baxter water treatment plant, which supplies drinking water to 60% of the city. This phenomenon occurs during droughts when reduced river flow allows saltwater to creep upstream.

Rain from Wednesday night into Thursday may provide temporary relief, but it won’t fully resolve the issue, according to David Ondrejik, a hydrologist at the National Oceanic and Atmospheric Administration.

The Delaware River Basin Commission monitors the salt line, which has been tracked since 1963. In extreme cases, such as the drought of 1964, the salt line reached as far north as the Benjamin Franklin Bridge, causing millions of dollars in industrial damages.

To combat the salt line’s advance, freshwater is released from upstream reservoirs, a safeguard against water quality threats. While current conditions are concerning, the Philadelphia Water Department states that any immediate risk to drinking water remains remote.

Read more about the Pennsylvania salt lines in the Philadelphia Inquirer.