New research suggests that thousands of oil and gas wells in the United States may pose a threat to underground drinking water sources because they are not deep enough.

Stanford University environmental scientist Rob Jackson and colleagues analyzed hydraulic fracturing depths and water use for around 44,000 wells. They found that the average fracturing depth across the country was 8,300 feet, but at least 6,900 wells were fracked less than a mile (5,280 feet) from the surface. At least 2,600 wells were fracked at depths shallower than 3,000 feet, particularly in Texas, California, Arkansas and Wyoming, and some were as shallow as 100 feet.

This is despite the fact that many reports that describe fracking as safe for drinking water only if it occurs at least thousands of feet to a mile underground, Jackson said.

The study, published in Environmental Science & Technology, also revealed that at least 2,350 wells less than one mile deep had been fracked using more than 1 million gallons of water each.

According to the researchers, such high-volume fracking at shallower depths poses a greater potential threat to underground water sources because there is so little separation between the chemicals pumped underground and the drinking water above them.

“The places where hydraulic fracturing is both shallow and water-intensive may need additional safeguards,” Jackson suggested.

These safeguards could include a mandatory registry of locations, full chemical disclosure and, where horizontal drilling is used, predrilling water testing to a radius 1,000 feet beyond the greatest lateral extent.