Summary

EPA is issuing a notice of proposed action under the Clean Water Act. EPA is proposing to issue a National Pollutant Discharge Elimination System (NPDES) permit renewal (No. AZ0024601) to Cyprus Tohono Corporation (CTC) for the discharge from an inactive mine located 32 miles south of Casa Grande on Hwy 15, Arizona, on lands leased from the Tohono O’odham Nation. The CTC mine site is currently in care and maintenance status with no active production occurring. Although CTC has not discharged treated mine pit water as authorized by the permit since March of 2005, the company intends to maintain an active NPDES permit in the event it becomes necessary to begin treating and discharging water once again. The proposed permit requires that CTC notify EPA and the Tohono O’odham Nation’s Environmental Protection Office at least 60 days prior to the commencement of discharge of treated pit water.

Effluent from the CTC mine site consists of treated pit water. The total volume of the pit is estimated at 110 million gallons. During the 2003 initial permit term, all of the pit water was treated by an onsite treatment system, with 80-90% of the treated wastewater discharging through Outfall 001 at a daily average flow of 1.37 million gallons per day. The remaining 10-20% of the wastewater was recycled back through the heap leach pad evaporative system. Data characterizing the current pit lake water quality was provided in the permit renewal application based on sample analysis conducted on January 18, 2018. Discharges in the future, if they occur, would be expected to have similar characteristics to that observed during the initial permit term. Outfall 001 from the Cyprus Tohono mine discharges to an unnamed ephemeral tributary to the Santa Rosa Wash on the Tohono O’odham Nation. This is an ephemeral wash that only flows during a storm event.

The Tohono O’odham Nation does not have EPA-approved water quality standards for discharges to waters located on the Nation. However, the Santa Rosa Wash, at the point where it leaves the boundary of the Nation, is a water for which the state of Arizona has established water quality standards. Therefore, Arizona water quality standards applicable to the Santa Rosa Wash and its tributaries apply to the discharge at the point where the discharge enters Arizona waters. The proposed permit establishes discharge limits and monitoring requirements to protect applicable Arizona water quality standards and designated beneficial uses of the river and not contribute to any further degradation.