SAN ANTONIO, Texas (Reuters) - A 1,300-acre dump to bury low-level radioactive waste has opened in a remote corner of west Texas, the fourth U.S. site to allow such waste, despite concerns about water seepage at the site, which sits above the huge and vital Ogallala aquifer.

In a letter to Waste Control Specialists LLC of Dallas, the Texas Commission on Environmental Quality granted approval to the company's dump near Andrews, Texas, about 45 miles northwest of Midland and close to the New Mexico border.

"Initially we will be disposing of Texas waste - medical waste - it is coming in from destinations around the state," Chuck McDonald of dump owner Waste Control Specialists said in an interview.

McDonald said waste from as many as 38 other states will soon be trucked in to the site.

"That will probably happen in the next couple of months. We will see limited amounts of waste from outside the state," he said.

The Texas Commission said the majority of space at the site is reserved for waste generated within Texas.

According to the federal Nuclear Regulatory Commission, low-level radioactive waste typically consists of contaminated clothing, rags, mops, filters, reactor water treatment residues, equipment and tools, luminous dials, medical tubes, swabs, needles, syringes and laboratory animal carcasses.

An agency spokesman said Texas has authority over its new dump site. The only three other low-level radioactive waste disposal facilities in the United States are in Barnwell, South Carolina; Richland, Washington; and Clive, Utah.

There is still no long-term U.S. repository for high-level nuclear waste, such as spent fuel rods from nuclear plants, which are stored in pools or dry concrete casks on-site.

Environmental activists have raised concerns that the Texas dump could poison underground water supplies, including the southern portion of the Ogallala aquifer and the Dockum aquifer.

"It could affect the Ogallala Aquifer, which underlies eight states ranging from Texas all the way to the Canadian border," said Tom Smith, Texas Director of the consumer advocacy group Public Citizen.

Potential risks to the aquifer in Nebraska have been a key sticking point in the dispute over possible routes for TransCanada Corp's Canada-Texas Keystone XL oil pipeline.

The dump's owner has said the dry clay in the soil will prevent contamination of underground water supplies, Smith said.

But Texas state Representative Lon Burnam, a Democrat, said he received documents under the Freedom of Information Act which indicate that the dump may not be as safe as officials have claimed.

Burnam said groundwater is seeping into the perimeter of the site, which he said could be from the aquifer.

"They have pumped out 23,000 gallons (87,000 liters) of water that is inside the perimeter since November, and they don't seem to be able to reduce the water level," he said. "That means the water is being replaced from underground."

WATER SEEPAGE

Burnam suggested support for the dump in the Republican-dominated Texas legislature may stem from its influential majority owner, Harold Simmons, who is a big contributor to Republican office-holders, he said.

The Center for Responsible Politics lists Simmons as the second-largest donor to Republicans in the country.

Burnam has asked the Texas Attorney General to waive confidentiality rules so he can release documents that he said would show that the dump poses a danger to underground water supplies.

The Texas Commission said in its approval letter that it is closely monitoring the water seepage issue.

"It is important to be aware that saturated conditions do not exist within 100 feet of the disposed waste," commission Deputy Director Brent Wade wrote in the letter, which was posted on the commission's website.

Environmental groups have also expressed concerns about possible contamination from wind-blown gas or material from the site, or if trucks carrying radioactive waste to the dump spilled their loads due to accidents.

The commission denied a request by the Sierra Club for a hearing on the dump's safety before an administrative law judge, saying the environmental group's members would not be impacted though some lives within a few miles of the site, Sierra Club lawyer Marisa Perales said.

The group has appealed the decision, and a hearing is scheduled for May 8 in Austin, Texas.

"We felt there was enough doubt, and enough concerns, that they'd take another look," Sierra Club spokesman Cyrus Reed said. "We never got that chance."

(Editing By Andrew Stern, Greg McCune and Paul Simao)