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Hydrogeologist warns of antimony and other metals in Harshaw Creek discharge; urges frequent testing and compliance wells
Summary
Patagonia resident hydrogeologist Chris Gardner presented data showing antimony concentrations at or above drinking-water limits downstream of the South32 Hermosa mine outfall and asked the council to press ADEQ to require more frequent monitoring, surface-water sampling and point-of-compliance wells.
Chris Gardner, a hydrogeologist with Friends of Sonoita Creek, told the Town of Patagonia council that public data show antimony concentrations in discharge from the South32 Hermosa mine that exceed Arizona drinking‑water standards and that existing permit monitoring may miss spikes.
"From leachate samples of the waste rock we saw 4 to 13 parts per billion antimony," Gardner said, citing a figure from a South32 2017 application and noting that the EPA/Arizona drinking-water limit is 6 parts per billion. He showed a late‑2024 spike and said a groundwater sample taken under ADEQ supervision returned 12 parts per billion in a residential well downstream.
Gardner told the council the difference in sampling methods matters: ADEQ composite (ASIPTI) samples represent an eight‑hour average, while the groundwater/Aquifer Protection Permit (APP) grab samples can miss short‑term peaks. He said the APP’s historical monitoring did not require antimony analysis in all discharge samples and urged the department to reopen the APP to require daily or weekly discharge monitoring, quarterly surface‑water monitoring and a point‑of‑compliance monitoring well in the areas of concern.
Gardner also described treatment options used elsewhere, pointing to a Park City, Utah, drinking‑water plant that uses a titanium‑dioxide absorptive medium to remove antimony after conventional metal removal steps. "Antimony can be treated," he said, noting the cost and maintenance implications of the specialized media.
Gardner asked the town to require South32 to provide all water‑level and water‑quality data for wells along Harshaw Creek and to use any community benefits agreement to secure protections for drinking water, including municipal or individual treatment systems and relocation of supply wells if necessary. He repeatedly framed his recommendations as requests for more monitoring and third‑party verification rather than definitive proof of source attribution.
The mayor and members of the council listened during the presentation; council did not take regulatory action at the meeting but approved an item later in the agenda authorizing routine town business. Gardner’s presentation concluded with a list of monitoring and permitting changes he recommended the town pursue or request from ADEQ and the Forest Service.
