The Environmental Protection Agency's human health risk assessment for the Iron King Mine-Humboldt smelter sites suggests that those lands in Dewey-Humboldt with the largest concentrations of toxic arsenic and lead should be cleaned up to safeguard public health over the long term.
This assessment, released this past week, included testing soil samples from 65 off-site parcels in the town. Of those parcels, data shows that 23 of them held a cancer risk greater than 1 in 10,000, while 36 exceeded the 99th percentile for lead.
EPA officials say that means many of those properties contain levels of arsenic and lead that could increase the potential for health problems over time.
It is in these areas where the EPA will consider any number of options for cleanup.
However, for example, at the Humboldt Elementary School playground, where some of the testing occurred, officials say they do not need to evaluate that parcel any further because of its low contaminant levels.
Most of the off-site groundwater locations that the EPA tested showed elevated concentrations of naturally occurring arsenic that were not tied to the mine or smelter.
But many of the locations exceed the EPA's maximum contaminant level for arsenic, which is based on drinking water standards that apply to public water systems.
Results show that the levels of arsenic in wells tested at the mine and smelter are tied to their proximity to ore deposits or residual mine material. The EPA is recommending an evaluation of alternatives for cleanup there.
However, the report says elevated arsenic concentrations in private and municipal wells in Dewey-Humboldt and the surrounding area come from contact with natural geologic formations and not from past mining and smelting operations.
"The variability and magnitude of arsenic concentrations in the vicinity of the site are similar to those throughout Arizona," the report adds.
The report shows that groundwater downgrade from the mine has sulfate-dominated total dissolved solids (TDS) because of contact with tailings or from rocks high in sulfate. EPA officials say these wells should be evaluated for cleanup.
At the smelter site, chloride-dominated TDS is impacting the groundwater there, whether it's from rocks or past smelting operations. While EPA officials believe this chloride is unlikely to harm people, they say the source of the chloride should be determined.
In a separate ecological assessment, the EPA found that highly elevated concentrations of metals at the mine and smelter sites pose risks to plants, invertebrates, mammals, birds, fish, reptiles and amphibians. Officials are recommending an evaluation for cleanup at these sites.
However, the report shows that habitat in the nearby Agua Fria River "appears relatively healthy."
Therefore, the EPA suggests that containing the contaminants at the mine and smelter would prevent further migration of toxins. Those toxins that are along the river now would dissipate over time, officials add.
The report gives 11 recommendations to consider in the cleanup process, including, but not limited to:
Repairing and stabilizing the western dam of the mine's tailings pile.
Conducting an engineering evaluation of the smelter's slag pile, large portions of which have cleaved off into the Agua Fria River.
Sampling the soils of additional residential parcels in the vicinity of the mine and smelter to determine which yards require cleanup. (EPA will conduct residential yard sampling for free to those who live near the mine or smelter.)
Compiling an inventory of wells near the mine and smelter for further testing.
Encouraging private well owners in town to get their wells tested for arsenic. The EPA found that many private wells near the sites have arsenic concentrations above the maximum contaminant level.
Suggesting that landowners at the mine and smelter wet the tailings or ash piles before high-wind events to control particulates from migrating.
Avoiding contact with soil and water from the mine, smelter and Chaparral Gulch because of the high content of arsenic and lead in those places. Arsenic can enter the body through breathing and/or ingesting contaminated soil.
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