Wastewater spills from oil and gas production in the United States have released more than 175 million gallons of contaminated water over a five-year period, a new analysis reveals.

The Associated Press (AP) examined data from leading oil- and gas-producing states relating to incidents involving ruptured pipes, overflowing storage tanks and even deliberate dumping. From 2009 to 2014 there were 21,651 individual spills, and the true number is likely to be higher because many releases go unreported.

Spills of oil and gas wastewater can harm the land, kill wildlife and pose a threat to freshwater supplies.

As the news agency explained, oil spills tend to get more attention, but wastewater spills can be more damaging to the land. That’s because while spilled oil is eventually degraded by microbes present in the soil, salty wastewater from oil and gas drilling can dry up the land. Trees and plants die, and crops cannot take root.

What’s more, the wastewater often contains heavy metals such as arsenic and mercury which can poison cattle that drink the liquid or eat tainted grass.

“Oil spills may look bad, but we know how to clean them up,” said Kerry Sublette, an environmental engineer at the University of Tulsa. “Brine spills are much more difficult.”

The AP report is based on data from Texas, North Dakota, California, Alaska, Colorado, New Mexico, Oklahoma, Wyoming, Kansas, Utah and Montana, which together account for more than 90 percent of U.S. onshore oil production.

The researchers also highlighted how spills have increased in quantity and volume as oil and gas production has increased. There were 2,470 reported spills in the 11 states in 2009, but by 2014 the total had reached 4,643. Meanwhile, the amount of wastewater spilled doubled from 21.1 million gallons in 2009 to 43 million in 2013.