Radioactive Chemicals on
Phelps Dodge Sierrita Mine Site
Uranium is present with other metals in the mining district of Pima County south of Tucson. Uranium was mined at the Twin Buttes mine until 1985, when the price of uranium dropped. Uranium is present as a by-product in the operations of Duval/Sierrita mine. During our last trip to ADEQ in mid-July we finally acquired groundwater monitoring data for the Duval/Sierrita mill site and some monitor wells.
Uranium and its radioactive decay products, most notably radium-226 and radon gas, are showing up in the monitor wells on-site, especially in the milling area, at levels that are much higher than considered acceptable in drinking water. Uranium is a natural element in our vicinity, but its levels in soils and groundwater are increased many hundreds of times over what is natural by the mining and milling process. Once they are milled into a fine powder, uranium, other radioactive substances, and certain heavy metals are present in the mine wastes and mill tailings; therefore, they can easily leach into the soil and eventually into the water table. In addition, the wells along the tailings dam, which are closer to Green Valley than the mill, show uranium levels that are above the accepted federal drinking water standard. This is a further indication that uranium and its radioactive decay products are in the tailings slurry. Unfortunately, the data I have of the composition of the tailings slurry does not include data for radioactive substances. This data indicates that we need to have an analysis of the tailings content and a land survey in the vicinity. As we know dust from the dams and tailings are an ever-present reality on windy days.
Then, there are “indicator” monitor wells off the mining property. [See Monitor Well Map.] These wells show high levels of uranium and its decay products, however, except for some unusually high readings for radon, the levels are within the levels accepted by EPA.
Monitor Well No. BW-3: On-site, South of milling area near Duval Canal
Monitor Well No. MH-21: On site, Northeast of mill area at Demetrie Wash
Monitor Well No. MH-15W: On site boundry, east side of tailings pond
Monitor Well No. MH-11, 12, 13: Three monitor wells on public state land between tailings dam and Green Valley
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