Contamination levels have started to drop along waterways affected by a massive saltwater spill in western North Dakota’s oil patch, but they remain high near the site of the pipeline breach, government regulators and company officials said Tuesday.

The leak detected last month spilled nearly 3 million gallons of saltwater brine from a four-inch pipeline north of Williston. The wastewater – a byproduct of intensive oil drilling in the Bakken region of North Dakota and Montana – primarily contaminated Blacktail Creek, but also flowed into the Little Muddy River and the Missouri River.

Saltwater produced by oil drilling can be more than 10 times saltier than seawater and can contain heavy metals and hazardous wastes. The salt itself can sterilize soils and make them unsuitable for plants.

Pipeline owner Summit Midstream Partners LLC said in a statement Tuesday that it was making “significant progress” in the cleanup, but the Texas-based company offered no timeline for when the work may be done. Some previous saltwater spills have taken years to clean up, including the still-ongoing cleanup of a million-gallon spill in 2006 in nearby Alexander.

Elevated chloride levels initially detected along the Missouri and Little Muddy rivers were returning to normal levels, said Dave Glatt, chief of environmental health for the North Dakota Department of Health. Glatt said the improvement was aided by the removal of contaminated water and ice from Blacktail Creek – which was nearly pumped dry – and the construction of trenches to capture some of that saltwater before it reached the creek.

The Missouri River is about 25 miles downstream from the spill site.

Almost 6 million gallons of water have been removed from Blackwater Creek since the spill was discovered on Jan. 6, although much of that was fresh water. Unknown is how much of the contamination was absorbed into the soil around the spill site, and whether that will be released slowly over time.

Officials still are working on a long-term remedy. That could include digging up the soil around the source of the spill, further steps to contain and treat the contaminated water, or a combination of the two, said Steven Way, the on-scene coordinator for the spill for the U.S. Environmental Protection Agency.

Way said completing the cleanup could take “somewhere between many months and potentially years.”

Summit Midstream Partners LLC is the parent company of the pipeline’s operator, Denver-based Meadowlark Midstream Co. LLC. Summit, with $242.81 million in annual sales, said it operates more than 2,300 miles of natural gas pipelines in several states, including 850 miles in North Dakota. It also has operations in West Virginia, Ohio, Texas, Colorado, Wyoming and Utah.

Summit entered the North Dakota market in 2013 and has since invested more than $800 million in its operations in the state. Further expansions are planned, and Summit CEO Steven Newby said during a Nov. 7 earnings call that the company is investing $2 billion in its operations over several years.