Get Full Government Meeting Transcripts, Videos, & Alerts Forever!
Get email alerts on the Water Quality Antimony topic
No spam. Unsubscribe anytime.
South32 presents water‑monitoring update, says treatment upgrades will lower antimony after single exceedance
Summary
South32 told Patagonia’s council it found one grab sample slightly over the 6 ppb aquifer-protection limit but said most samples were lower and that additional treatment upgrades and monitoring commitments in the final EIS will be implemented before two wells are returned to service.
South32 representative Pat provided a detailed update to the Town of Patagonia Council on water‑monitoring and treatment work after a November sample registered 6.23 parts per billion (ppb) antimony — slightly above the Aquifer Protection Permit (APP) limit of 6 ppb. Pat said the mine operator immediately retested the outlier, shut two wells feeding the plant, and is installing a second phase of treatment upgrades that South32 expects to have operational in February before switching those wells back on.
Pat explained differences between Arizona permits and sampling methods, saying surface‑water composites average eight hours while APP samples are single “grab” measurements taken at a single point in time. “We shut that well down in February,” Pat said, describing a precautionary approach. He added the site has seven feed wells and that the outlying 6.23 ppb reading was surrounded by multiple samples below the APP limit; the company has reported the exceedance to ADEQ and submitted a 30‑day report. Pat said baseline averages for 2025 were about 3.2 ppb and surface‑water downstream averages 2.5 ppb across multiple locations.
On treatment changes, Pat said the plant uses chemical precipitation and sand‑ballasted clarifiers and can be adapted by increasing reagent dosing, recycling solids and adjusting pH to raise removal efficiency. He said bench‑scale tests show the modifications can remove antimony to well below 6 ppb even when influent reaches higher concentrations; “we think we can go even up as high as 15 parts per billion … and still maintain below the limit,” he said. Pat also said South32 has engaged external toxicologists and process engineers, will make the human‑health assessment available, and will commit to a water‑monitoring program in the final EIS that will include community and South32 wells and multiple downstream surface stations.
The presentation addressed community concerns about livestock and sediment leaching, noting aquatic organism thresholds are far higher than the discharge rates observed and that surface‑water monitoring across multiple downstream locations has generally shown 1–3 ppb with no increasing trend. Pat said the Forest Service evaluated monitoring included in the draft EIS and that South32 will commit to further monitoring and transparency in the final EIS.
