The site of the Hermosa Project in the Patagonia Mountains.

State regulators have tightened up the limits on a permit modification that South32 is seeking for its Hermosa mining project in the Patagonia Mountains.

The modification to permit AZ0026387, if it’s approved by the Arizona Department of Environmental Quality, will let the company dump millions of gallons of treated wastewater into Harshaw Creek, south of Patagonia. But an update to the modification puts stricter limits on the levels of various substances in that water, such as lead, mercury and iron.

An ADEQ spokeswoman said in an email that the department “changed the draft permit to include the more protective water-quality based limits based on comments received during the first public notice period.” She added that the earlier proposed modification set limits based on the technology used, rather than water quality.

The changes impact the maximum allowable levels of 16 different substances. For arsenic, the limit was halved, from 246 micrograms per liter to 123 micrograms per liter. For zinc, the limit was reduced from 750 micrograms per liter to 130 micrograms per liter.

Other limits remained the same. The maximum total amount of suspended solids present in the discharge is still set at 20 milligrams per liter. And the total discharge allowed under the modified permit is also unchanged: 6.48 million gallons per day.

The discharge permit, along with an Aquifer Protection Permit that South32 is also seeking from ADEQ, will allow the company to suck water out of the ground in the area where it’s hoping to eventually mine silver, lead, zinc and manganese. The company is seeking both permits through its subsidiary Arizona Minerals.

The permit modification created controversy earlier this year, with the Patagonia Town Council and U.S. Rep. Raul Grijalva, a Tucson Democrat who represents Santa Cruz County in Congress, asking regulators to go through a new permit process rather than modifying the company’s existing permit to allow the Harshaw Creek discharge.

“Harshaw Creek merges with Sonoita Creek, which runs through our town, (and) may eventually affect the underground water quality that our town wells draw, in supplying water to our 874 citizens,” the town council wrote in a March letter to ADEQ.

Public comment periods for both the modified discharge permit and the Aquifer Protection Permit, P-51235, came to a close at the end of last month. The ADEQ spokeswoman said the department could make a final decision on both permits “as early as this month.”

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