The wall of sand that flushed down Prickly Pear Creek into East Helena this week could be a portent of future high-water events.
While a definitive cause for the sand flow remains unclear, state and federal officials say they believe that last year’s flooding in June, along with the cessation in November of the historic diversion of the creek into the man-made Upper Lake at the former Asarco lead smelter, changed the dynamics of the stream.
Betsy Burns, the U.S. Environmental Protection Agency’s program manager for the cleanup of the Asarco site, said the spring flooding flushed a lot of sediment, sand and debris into Prickly Pear Creek. Then heavy snows this winter melted during the recent warm weather, and she theorized those flushed more sand and sediment downstream last week.
In years past, when the stream was diverted into Upper Lake, most of that sand and sediment would settle out in the lake. But after the concrete and wood diversion structure was closed in November, the creek now just runs straight into East Helena, eventually flowing into Lake Helena.
Jim Ford is the project manager for the Montana Environmental Custodial Trust (MECT), which is working with the EPA on the Asarco cleanup. He said they’re not sure if surges of sand will continue to find their way into Prickly Pear Creek whenever it receives high flows.
“I don’t know if this is a unique event or something that will be regularly occurring,” Ford said on Friday. “The dam and Upper Lake were, in effect, sediment barriers. Upper Lake is full of sediment and Asarco had problems with it building up below the dam. They were fined in the ’70s for stirring it up and discharging it.
“So now that we’re not diverting the water it’s just flowing down the creek.”
Some East Helena residents said they thought the EPA had flushed the sediments out of Upper Lake, but both Burns and Ford, along with Cindy Brooks, president of the MECT, said that wasn’t the case.
“This most recent sand and sediment event observed in the East Helena stretch of Prickly Pear Creek, we believe, likely resulted from the sand and sediment deposited in the creek in June during the high flow event,” Brooks said. “The big snow in January, followed by the warm weather leading to higher flows, possibly caused this.”
Travis Erny, an environmental enforcement specialist with the state Department of Environmental Quality, said he also didn’t find any evidence that the Asarco cleanup had an impact on the sand in the creek.
“Closing that diversion may be contributing to the changing dynamics of the stream, but I didn’t see any evidence they were putting sand into the creek,” said Erny.
He added that sand isn’t apparent in Prickly Pear Creek upstream of the Asarco site, possibly because the gradient is steeper and the stream runs faster than it does in East Helena, where it flattens out. So it could be picking up sand and sediment from even further upstream, but they weren’t settling out onto the gravel streambed like they did in East Helena.
“Near Ash Grove, the creek is a completely different animal than in town,” Erny said. “Upstream, it is steeper with greater flows. As it goes past Asarco it slows down and becomes more winding. You can’t compare the two.”
The sand flows first became apparent last weekend, but had ebbed by Friday. However, the creek bottom in East Helena remains covered with a layer of sand.
Erny added that while MECT and the EPA were digging test pits during the past few weeks, there is no evidence that they were in or near the creek. Ford said they were at least 150 feet from the creek, analyzing the types of soils and the groundwater level.
“I’m not sure how those efforts got interpreted that we were digging in the creek or digging stuff up behind the dam,” Ford said. “That’s not allowed without a permit, and we wouldn’t dig in the creek without precautions in place.”
Asarco had diverted the creek into Upper Lake for decades in order to use the water during the lead smelting process. While the company initially dumped the process water — which had high levels of arsenic and other contaminants — back into Prickly Pear, it later created what’s known as Lower Lake. Water in both of the unlined ponds then seeped into the soil, recharging the groundwater.
Brooks said the two lakes haven’t been used for a decade, after Asarco shut down the plant in 2001. As part of Asarco’s bankruptcy proceedings, her company was hired to manage the $100 million Asarco put into a trust to clean up contaminated soils and groundwater left after more than a century of lead smelting.
Part of that contamination includes two plumes carrying arsenic and selenium off the plant site toward Canyon Ferry Road. Brooks’ company, working in conjunction with the EPA, is currently lowering the water level of the two lakes, which they hope will lower the groundwater underneath the plant and slow down the flow of the plumes.
While some of the water level is being lowered by seeping into the ground, in December the gates to the dam were fully opened. Ford said they’ve tested the discharged water for turbidity, and didn’t see any difference in Prickly Pear Creek upstream or downstream until the last two weeks, when the turbidity almost tripled both above and below the Asarco plant site.
He added that there hasn’t been any water in Upper Lake since December, and they plan on removing the diversion structure and rerouting Prickly Pear Creek away from the nearby slag piles, possibly during the summer of 2013. During last June’s flooding, concerns were raised that the creek was undercutting the slag piles, making them unstable.
Ford said that while the design work isn’t done yet, he anticipates that during high-water events — like a 25- or 50-year flood — the former Upper Lake will be able to hold back some water.
“During a flood, water could spill over, filling not just Upper Lake but Tito Park and Lower Lake,” Ford said. “That would add about eight hours of protection for the town of East Helena. So it wouldn’t be fully protective, but it would add some time for them.”
Reporter Eve Byron: 447-4076 or firstname.lastname@example.org
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