A pipeline leak near Williston, North Dakota, that began January 6 has spilled 3 million gallons of brine — a byproduct of hydraulic fracturing. The leak has reached the Missouri River, the Associated Press reported on Friday.

It’s the largest saltwater spill in the state’s history. Brine is considered toxic; it is saltier than seawater and often contains other fracking fluids and petroleum.

The leak contaminated two creeks near Williston: Blacktail Creek and the Little Muddy River. The Little Muddy River empties into the Missouri River, one of the town’s sources of drinking water.

State health official Dave Glatt told the Associated Press that given the size and volume of the Missouri River, the contaminants were quickly diluted. But Karl Rockeman, the director of water quality at the Department of Health said “high readings” of contamination were found where the Little Muddy meets the Missouri, the Williston Herald reported.

The number of spills from North Dakota’s booming oil industry has risen steadily since 2006, the New York Times reported in November. A Times investigation found that 18.4 million gallons of oil and chemical substances have leaked into the North Dakotan air, water and soil between 2006 and October 2014. The Summit Midstream leak follows on the heels of a 50,000 gallon oil spill in the Yellowstone River near Glendive, Montana — the second oil spill in the river in four years.

Clean-up has already begun, but contaminated water trapped in ice may slow down the process. Oil and fracking spills have already proven difficult to clean up in North Dakota; a spill near the town of Alexander in 2006 is still being cleaned up the Associate Press reported.

The saltwater spill has prompted North Dakota Democrats to call for increased monitoring and regulation of the industry.

“It should not take a 3 million gallon spill to realize that this monitoring is needed,” Senate Minority Leader Mac Schneider, D-Grand Forks, told the Associated Press. “If North Dakota does not get this under control, the feds are going to step in and do it for us. And nothing is going to slow the oil industry down like the federal government. We want to protect our environment first and foremost but this also will be good for industry in the long run.”