MANDAREE (AP) – Officials are assessing a spill of oil-drilling saltwater from a North Dakota pipeline to ensure none of the brine affected the lake an American Indian reservation uses for drinking water, the Environmental Protection Agency said Thursday.

In its first public statement in the two days since the spill was detected, the agency said it had no confirmed reports that the saltwater had reached Bear Den Bay. It leads to Lake Sakakawea, which provides water for the Fort Berthold reservation occupied by the Mandan, Hidatsa and Arikara tribes in the heart of western North Dakota’s booming oil patch.

EPA said most of the spill was pooled on the ground, soaked into the soil and held behind beaver dams. It said the leak involved an estimated 24,000 barrels, or 1 million gallons, of saltwater and condensate, which are byproducts of oil and gas production.

Cleanup at the reservation site continued Thursday and was expected to last for weeks, said Miranda Jones, the vice president of environmental safety and regulatory at Houston-based Crestwood Midstream Partners Inc.

Jones said the leak at the underground pipeline, owned by Crestwood subsidiary Arrow Pipeline LLC, likely started over the Fourth of July weekend. The pipeline was not equipped with a system that sends an alert when there is a leak, she said, and the spill was only discovered when the company was going through production loss reports.

“This is something no company wants on their record, and we are working diligently to clean it up,” Jones said.

Although the EPA said additional assessment activity was being conducted, company and tribal officials said the spill had been contained and did not affect the lake.

“We have a berm and a dike around it, around that bay area, to keep it from going into the lake,” said Three Affiliated Tribes Chairman Tex Hall.

Saltwater is a naturally occurring, unwanted byproduct of oil and natural gas production that is between 10 and 30 times saltier than sea water. The state considers it an environmental hazard.

The briny byproduct also may contain petroleum and residue from hydraulic fracturing operations.

Kris Roberts, an environmental geologist with the North Dakota Department of Health, said damage from the toxic spill could be seen when he visited the site on Tuesday.

“We’ve got dead trees, dead grasses, dead bushes, dying bushes,” he said.

Karolin Rockvoy, a McKenzie County emergency manager, said it was apparent from looking at vegetation that the spill went undetected for some time.

The number of saltwater spills in North Dakota has grown with the state’s soaring oil production. North Dakota produced 25.5 million barrels of brine in 2012, the latest figures available. A barrel is 42 gallons. There were 141 pipeline leaks reported in North Dakota in 2012, 99 of which spilled about 8,000 barrels of saltwater. About 6,150 barrels of the spilled saltwater was recovered, state regulators said.

Fort Berthold Indian Reservation plays a key role in the state’s oil production, the second-highest in the nation. The reservation currently represents more than 300,000 of North Dakota’s 1 million barrels of oil produced daily, according to the state’s Department of Mineral Resources.

In 2006, a broken oil pipeline belched more than a million gallons of saltwater into a northwestern North Dakota creek, aquifer and pond. The cleanup efforts are ongoing at that site, which has been called the worst environmental disaster in state history.

The ruptured pipeline allowed saltwater to spew unnoticed for weeks into a tributary of the Yellowstone River near Alexander and caused a massive die-off of fish, turtles and plants.

That spill came during the infancy of North Dakota’s oil boom. Now, a network of saltwater pipelines extends to hundreds of disposal wells in western North Dakota, where the brine is pumped underground for permanent storage.

Proposed legislation to mandate flow meters and cutoff switches on such lines was overwhelmingly rejected last year in the Legislature.