BOISE -- The maker of Roundup weed killer is investigating the engineering and safety of the water treatment system at its new phosphate mine in southeast Idaho after an earthen holding pond recently sprung a leak, sending sediment and millions of gallons of water into an adjacent wetland.



Monsanto Co. reported the March 29 failure to state and government regulators, touching off an internal review of the network of ponds designed to manage and collect storm water and snowmelt at the Blackfoot Bridge Mine near Soda Springs.



The breach occurred at one of several ponds filling with spring runoff, and testing showed the water was free of contaminants. Still, the failure drained an estimated 3 million gallons of water and created a sediment plume that stretched for 100 feet.



The U.S. Bureau of Land Management has given Monsanto 60 days to investigate and submit a corrective action plan that includes a strategy for redesigning the holding pond and appraisal of other water holding structures on site.



"We are already moving forward on a complete redesign on the water management system," Monsanto spokesman Trent Clark said Monday.



Blackfoot Bridge, in southeast Idaho's phosphate patch, was permitted in 2011 and is on track to begin operations later this year. The mine has a 17-year life expectancy and during that period will supply Monsanto with a key ingredient in its Roundup weed killer product.



During the design and environmental review process, Monsanto officials touted the project for incorporating state-of-the-art design in all facets of protecting the environment and the Blackfoot River that flows nearby. Last month's breach was limited to a specific component in the holding pond and has done nothing to shake the company's overall confidence in the design and safeguards, said Randy Vranes, the mine designer for Monsanto.



"Preliminary observations suggest that the breach ... was limited to a specific type of spillway, and the cause is un-related to any of the new or innovative technologies applied at Blackfoot Bridge," Vranes said in an email. "Based on our experience ... however, we are taking a close look at the whole system to re-confirm its reliability."



Jeff Cundick, minerals chief for the BLM office in Pocatello, said the agency is withholding judgment until it can review the company's response plan.



"We've looked at the situation, and although it's unfortunate, as far as BLM's rules and regulations, we don't see a violation at this point," Cundick said.



The biggest environmental threat from phosphate mining comes from selenium, a byproduct created when water interacts with mine waste rock. The Blackfoot Bridge Mine plan includes a massive waste pit that will be capped by geosynthetic cap -- estimated to cost $35 million -- designed to protect interaction between the waste and rain and spring runoff. The pit also has a plumbing system intended to capture any tainted water and a series of treatment ponds for water with testing positive for selenium and other toxins.



That network of treatment ponds is separate from the pond that failed. Company and government regulators say a series of breakdowns would have to occur before contaminated water would ever find its way into the clean water holding ponds.



But for environmentalists watchdogging phosphate mining and the industry's legacy of pollution in the region, the pond failure is a signal that even the latest and best technology can falter in the field.



"It only goes to show that no matter how well-designed, how good the intentions are, things like this inevitably happen," said Marv Hoyt, with the Greater Yellowstone Coalition, a group that worked during the permitting process for tougher safeguards. "This was a relatively minor incident. But our concern is that this never happen again."



-- The Associated Press

