AUSTIN, Texas — Texas allowed the drilling of oil and natural gas injection wells in some areas near drinking water sources starting more than three decades ago, but state regulators recently assured the federal government the effort posed “little to no risk” to the subterranean reserves, according to a report released Friday.

Clean Water Action, an environmental advocacy group, contends in its report that the nation’s top oil-producing state doesn’t really know how many injection wells are affecting drinking water or the full impact. That’s because the state still hasn’t properly implemented federal aquifer safeguards, 34 years after telling the U.S. Environmental Protection Agency that it would do so, the group said.

“We actually don’t know the full extent of the problem,” said John Noel, primary author of the report. “They’ve permitted tens of thousands of injection wells without following federal guidelines.”

Injection wells take various forms and are used for production and waste disposal in oilfields. In addition to exploratory drilling, oil producing operations can produce large amounts of briny fluid and other waste, and oilfield operators often dispose of the waste by injecting it back underground.

In 1982, the EPA and Texas’ Railroad Commission, which oversees oil and gas production, agreed that many existing injection wells would be exempted from federal rules designed to protect underground sources of drinking water.

Excerpts of letters from 1982 released Friday as part of the report show that the commission said it would provide federal officials with a map of existing exempt areas already being used by oil producers and pledged to secure EPA approval before sanctioning new exempted areas or allowing existing ones to get larger.

Noel’s report says Texas never produced such a map and that the Railroad Commission doesn’t have a full record of the exemptions granted. The commission now says it is in the process of producing a more complete inventory.

The report references a letter the Railroad Commission wrote to the EPA in March in which the Texas agency said that since 1982, it had issued permits for injection wells in “a handful of fields” that could affect underground water reserves.

“These injection wells pose little or no risk to underground sources of drinking water because of the characteristics of the reservoir, completion of wells in the area and the conditions included in the injection well permits,” that letter states. “This example illustrates that the commission has an effective program that ensures projection of underground sources of drinking water.”

Railroad Commission spokeswoman Ramona Nye said Friday that the agency “remains in compliance” with the 1982 EPA agreement. She said the commission will use grant funding to verify that permits issued since “do not authorize injection into underground sources of drinking water in zones that were not approved by the EPA.”

“The commission’s highest priority is protecting the public and the state’s natural resources by continuing to ensure we have strong rules, permitting procedures and enforcement processes in place to protect the state’s drinking water,” Nye said in an emailed statement.

But Noel’s report says the problem is potentially far more serious given the number of injection wells approved since 1982. The commission says Texas has more than 54,700 permitted oil and gas injection and disposal wells, with approximately 34,200 active as of July 2015.

The report concludes that Texas “routinely prioritized the concerns of the oil and gas industry over the long term drinking water needs of Texas residents.”

Texas has more injection wells than any other state, but similar issues have surfaced elsewhere. An Associated Press analysis in 2015 cited more than 2,000 permits California had given oil companies to inject into federally protected drinking water reserves.

Noel called the Texas findings “the early stages of what happened in California.”