An Asarco mining operation has agreed to pay a $170,000 fine to settle charges that it released contaminated wastewater in 2006 from its open-pit copper mine near Marana, the Arizona Department of Environmental Quality said Tuesday.
Asarco, which did not admit wrongdoing as part of the settlement, disputed the state’s description of the hazard posed by the releases.
Silver Bell Mining, which is majority owned and operated by Tucson-based Asarco LLC, was fined for releasing 340,000 gallons of wastewater containing sulfuric acid and heavy metals into dry washes in three separate incidents between October and December 2006, state regulators said.
State regulators said the pollutants seeped into the soil, which endangered the groundwater in the aquifer below the mine and exceeded Arizona surface water-quality standards.
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But Asaco said in a news release that that one spill — described by ADEQ in a press release as containing "90,000 gallons of sulfuric acid" — was in fact a very dilute solution containing only 0.6 percent sulfuric acid, with the remainder water.
Another spill involved a copper leach solution, and the third involved stormwater, the company said, adding that none of the spills left the mine property.
ADEQ Acting Director Patrick J. Cunningham said the leaks could have been minimized had they been immediately discovered and the company put public health and safety at risk.
Asarco said the mine’s routine groundwater monitoring at the locations nearest to the spills confirmed that ground water standards were not exceeded.
Soil sampling showed that all metals were substantially below acceptable levels, and ADEQ agreed that no further actionwas required, the company said.
“We responded diligently to the spills and are confident that the issues that caused the spills have been resolved,” Tom Phillips, Silver Bell’s general manager, said in a prepared statement.
ADEQ noted that in response to the release from the leaking wastewater impoundment, the company installed a second liner with a leak detection sump between the two liners and has begun monitoring weekly for the presence and amount of solution detected from between the two liners.
Under terms of the settlement, the company has agreed to increase the frequency of its inspections to monitor for leaks from its dams, ponds and impoundments above what is required in its aquifer-protection permit.