South 32 representative outlines treatment upgrades after antimony exceedance; town seeks clearer monitoring

Pat (South 32 representative) told the Patagonia town council that a single grab sample taken at the plant on a November monthly test measured 6.23 parts per billion (ppb) antimony — just above the aquifer protection permit (APP) discharge limit of 6 ppb — while an 8-hour composite from the same day measured 5.73 ppb.

Pat said South 32 immediately ordered a retest, shut two wells feeding the plant and submitted a 30-day report to the Arizona Department of Environmental Quality (ADEQ). "We asked our lab to retest the sample, because it was such an outlier," Pat said. He described the 6.23 result as isolated amid other samples that were well under the limit and said December sampling returned values below 5 ppb.

The company representative emphasized that the APP limit (6 ppb) is based on the U.S. Environmental Protection Agency's long-term drinking-water standard and noted the difference in sampling approaches: surface-water sampling uses an 8-hour composite while APP groundwater sampling is a single grab sample. "A grab sample is an acute point in time single instance," he said, adding that averages are more relevant to long-term human-health risk assessments.

To address the change in influent water chemistry, Pat said South 32 has tested and will implement a second phase of treatment upgrades to increase antimony removal efficiency and will not switch the two wells back on until the modifications are proven. "We will make sure it's working before we switch those other 2 wells on from a compliance standpoint," Pat said. He said bench-scale tests and pilot runs confirm the upgraded design can reduce antimony to well below the 6 ppb limit even with influent concentrations as high as 15 ppb.

Pat also said South 32 engaged an independent PhD toxicologist to review discharge data and will make her report publicly available. "Everything I'm going to present that has numbers, data to back up ... is coming either from a study, a document, a regulatory analysis that is publicly available," he said.

Residents at the meeting pressed the company for plain-language answers about safety. "What I'm listening for is to understand, is this water safe?" said Anna Darian (Para), who asked the council to ensure monitoring and reporting allow residents to make informed decisions about drinking water and property. Pat responded that monitoring results and the toxicologist's report will be made available and that South 32 will commit to a groundwater and surface-water monitoring program in the final environmental impact statement (EIS).

Pat also summarized long-term data: South 32 said downstream surface-water samples averaged about 1–3 ppb in multiple locations and that the company's annual average discharge concentration for 2025 was about 3.2 ppb, roughly half the 6 ppb benchmark. He said the anomalous community well with elevated levels will be investigated with the well owner.

The presentation included technical details about plant design and treatment approaches. Pat described chemical precipitation with sand-ballasted clarifiers as the plant's primary technology and said the team will increase reagent dosing, recycle solids and adjust pH to improve removal rates. He warned that transplanting another plant's front-end design (referenced by some commenters) could compromise the plant's selenium-treatment circuitry and that solutions must be tailored to local water chemistry.

The company also committed to including specific South 32 and community wells in a monitoring program to be memorialized in the final EIS and said it will make new-method testing results public once the revised sampling is in operation.

The council and residents asked follow-up questions about sampling locations, what constitutes human-health risk under long-term exposure assumptions, and whether antimony concentrations correlate with mining depth; Pat said his team has modeled deposit distribution and sees no simple correlation with depth. He said observed inflows have been lower than initial permitting assumptions — roughly 1,200–2,000 gallons per minute observed versus 4,500 gpm expected — and that models are being recalibrated.

The town council did not take formal regulatory action at the meeting; public commenters sought stronger permit conditions and, in one case, a temporary halt to operations pending amended permits. Pat repeatedly pointed to the forthcoming final EIS commitments and the planned monitoring program as steps to address community concerns.

The council thanked Pat for the presentation and moved on to routine town business.