Updated 12/18/14:

Groundwater under the West Lake Landfill in Bridgeton is contaminated with unhealthy levels of radium.

That’s according to a U.S. Geological Survey report, released on Wednesday by the federal Environmental Protection Agency.

This past spring, the EPA asked the U.S. Geological Survey to review data from groundwater samples collected at the West Lake Landfill between 2012 and 2014, to try to figure out whether the water is being contaminated by radioactive waste.

The USGS analysis confirms that some groundwater samples taken from wells at the landfill have more than 5 picocuries per liter of radium ― the federal safe drinking water limit.

But EPA Regional Administrator Karl Brooks said the contaminated groundwater is not used for drinking.

And Brooks said it’s still unclear whether that contamination is coming from the landfill’s radioactive waste ― also known as radiologically-impacted material, or “RIM.”

“The USGS recognized they don’t have enough data yet," Brooks said. "And obviously then that means that EPA doesn’t have enough data to say that the contamination is either from the RIM, or from leachate generated by the site, or from any other particular origin. That’s one of the questions to be determined.”

Some radium occurs naturally in groundwater in Missouri and throughout the U.S. ― sometimes in concentrations higher than the federal drinking water standard.

Aside from that, the USGS cited three possible sources of the radioactivity in the groundwater under the West Lake Landfill:

The leaching of radium from the radioactive waste that was dumped at the landfill illegally in the 1970s; The leaching of radium from other, non-radioactive wastes at the site; The mobilization of radium from rocks, soils, or sediments by the waste liquid (leachate) inside the landfill.

But with limited data on background levels of radiation in groundwater ― and without knowing the full extent of the landfill’s radioactive waste ― the USGS couldn’t definitively attribute the groundwater contamination to any particular source.

The report did say that one well, PZ-101-SS, which had particularly high concentrations of radium, had the "greatest potential" to have been contaminated by the radioactive waste (it's shown in red in the figure, above).

The USGS also said that surface runoff (not groundwater) could have transported radioactive material to "lower areas of the South quarry."

That's where a subsurface fire has been smoldering for the past four years.

That surface runoff would have happened back in the 1970s, when the mixture of radiative waste and soil was first dumped in the north part of the quarry, in the areas circled in pink in the figure. At that time, rain could possibly have washed some of the contaminated soil ― and radium ― down into the south quarry, which at the time was just a big open pit.

That's one theory the USGS is proposing to explain the higher levels of radium in two wells in the southern part of the quarry (labeled PZ-104-SD and MW-1204, and shown in orange in the figure). But it's just a theory.

Credit Sarah Skiold-Hanlin, St. Louis Public Radio This radiation warning sign is posted on the West Lake Landfill perimeter fence in Bridgeton, Mo.

Missouri Attorney General Chris Koster, who sued landfill owner Republic Services over environmental contamination at the site, declined to comment about the groundwater report at this time, as did the Missouri Department of Natural Resources.

A spokesperson for Republic Services, Russ Knocke, provided the following statement: "The report should allay the worst fears being spread by alarmists," Knocke said. "It should reassure the community that they are safe from and not being exposed to any risk from groundwater beneath West Lake Landfill. We also hope it provides peace of mind that the site is closely monitored by leading scientific experts."

The EPA's Karl Brooks said regional staff would work with the agency's Office of Research and Development, the USGS, and the Missouri Department of Natural Resources to get a better understanding of how groundwater moves around the landfill site.

According to the USGS report, the natural flow of groundwater is toward the northwest, into the Missouri River.

But Republic Services is pumping wastewater out of "leachate risers" throughout the landfill (those are marked with white stars in the figure). That pumping has the effect of changing the natural movement of groundwater, making it flow into the landfill, instead of out. The USGS said that means it's unlikely that groundwater contaminated with leachate is moving much beyond the landfill's borders. But more groundwater wells would need to be dug and sampled outside the landfill to know for sure ― for example, between the landfill and the Missouri River.

Karl Brooks said that once EPA scientists have reviewed the USGS report, the agency could decide to ask for more groundwater sampling.

He also confirmed that the EPA would need to do more radioactivity testing at the landfill to determine the extent of the radioactive contamination in the waste. He said that needed to happen both in preparation for building the planned firebreak, and before deciding what to ultimately do with the waste.

But he wouldn't say whether that sampling would encompass the entire area north of the subsurface fire.

Follow Véronique LaCapra on Twitter: @KWMUScience