Acid drainage from abandoned metal mines in the Patagonia Mountains of southern Arizona

Abstract

The Patagonia Mountains were mined intermittently from the 1600's to 1949 primarily for silver, lead, and zinc. A baseline study in the ephemeral streams revealed pH measurements ranging from 2.9 to 4.2 directly below three sampled mines. Drainage from tailings contained high concentrations of sulfate, iron, and manganese, while recommended levels for zinc, copper, arsenic, lead, chromium, and cadmium were also exceeded. Water quality improved to varying degrees downstream. However, lead, manganese, iron, and arsenic remained above drinking water standards 2.6 km downstream from a major mine. A 9-hour, 2.3 cm. winter rainstorm was estimated to have released 718 kg of sulfate, 60 kg of iron, and 28 kg of manganese from the study watershed. Most soils analyses of tailings were qualitatively comparable to downstream water chemistry. Trench Mine tailings and drainage contained the highest sulfate and metal levels; reclamation possibilities and a more complete ground water study were considered.