The Coronado National Forest (CNF) and its subcontractors completed the earthwork and mine-closure portions of the Mansfield Canyon Remediation Project (reported by PRT in Nov. 2019) at the beginning of 2020. The biological portion of the work – remediation via planting – has now been carried out by Borderlands Restoration Network (BRN), under Native Plant Plant Manager Francesca Claverie.
BRN’s work focused on disturbed areas at the tailings encapsulation site along Temporal Road and at the mine sites themselves, a total of six replanted areas. The replanted areas are fenced to prevent grazing, and have ‘straw snakes,’ cylindrical straw-filled wattles, staked into the ground roughly along horizontal lines, to decrease soil erosion.
The disturbed ground of the project was graded to resemble original landforms, and Claverie believes plant ‘volunteers’ from surrounding areas are likely to show up in the enclosures as well, perhaps even oaks and juniper trees. The long-term goal of the project is to return disturbed areas to nature, making them less toxic as well.
The Hosey Mine, near the head of the Mansfield watershed, is on a ridge about 1/2 mile west, below which water flows to Josephine Canyon. The mine is in a wash accessed by a branch road from Forest Service Road 72A, numbered 4901, and gated on a ridge, about 700 ft. vertically above the mine site. From about 1906-1935 the several mines of the “Hosey Group” produced both lead and copper, with secondary minerals being gold, silver, barium and zinc. Besides zones of gray and rusty red rock, the site has abundant yellow sulfite/sulfide coloration in the ground and water. The tailings, containing high levels of heavy metals such as antimony, arsenic and lead, were largely repositioned to the Temporal Road encapsulation site, for a total relocated tailings volume of about 54,000 cubic yards.
At the Dixie Mine, above Piper Gulch, similarly toxic tailings were largely encapsulated onsite, and the main shaft was provided with a bat-friendly wire closure bolted to the rock surrounding the opening. The closure is woven in place and bolted to the rock wall. Other mine openings at this site were filled, and the access road is blocked by large boulders and a pile of tree stumps. There is no seepage immediately visible below this mine.
A third mine in Mansfield Canyon, ‘Site 7’, about a mile below the Hosey Mine, was added to the project before the work began. At that site, at least two tunnels were filled, and a long slope down to the wash was revegetated and enclosed.
The purpose of these Mansfield projects was to reduce the amount of soluble toxic metals like lead and arsenic flowing from these mines into the Sonoita Creek watershed, Lake Patagonia and the Santa Cruz River. However, the remediation proposal did not include an ongoing monitoring program to determine the downstream water-quality results. At both the Hosey Mine and “Site 7,” seepage near creekbed level continues, but at this time does not have enough flow to reach about four miles downstream to the wash at Temporal Gulch, which joins Sonoita Creek about a mile below Patagonia.
Using conventional acid-base test strips, pH testing of water in Temporal Gulch (about a half mile above the AZ trailhead) showed its water to be normal at the neutral pH of 7.0. Testing for the first hundred yards or so of flow at the Hosey site revealed increasingly acidic pH levels. Going downstream from seepage beginning at a filled tunnel, readings were 4.0, 3.5, 3.0 and 2.5, with readings less than 3.5 labeled as “ultra acidic.” These values show that the acid mine drainage picks up acidity as it flows over the sulfate/sulfite-containing bed of the stream, likely through the formation of sulfuric acid which in turn dissolves and transports other minerals from rocks it passes over and through.
Dissolved minerals in acid mine drainage, especially heavy metals, are the primary environmental concern in historical mining areas, since most are toxic to life forms ingesting them, or trying, like fish and frogs, to live in them. Determining the mineral content of the Mansfield Canyon waters would require full lab analysis of samples at many different flow conditions.