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Source Chemistry and Characteristics of Intermittent Stream Waters Having Low pH and Elevated Metal Concentrations, Patagonia Mountains, Santa Cruz County, Arizona

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GRAY, F.¹, CHAFFEE, M.², WIRT, L¹, LJCHTE, F.², and CARUTHERS, K.
¹ U.S. Geological Survey, 520 N. Park Avenue, Suite 355, Tucson, Arizona 87519, USA
²U.S. Geological Survey, Federal Center, MS 973, Denver, Colorado 80225, USA

Areas selected for this study include the drainage basins for Harshaw Creek, Mowry Wash, Sonoita Creek, and the upper part of the Santa Cruz River basin in the San Rafael Valley. Numerous mines, prospects, waste dumps, and tailing piles are abandoned within these basins, but large parts of the region are mineralized and remain largely undisturbed. Altered but unmined exposures, such as those areas around Red Mountain and the northwest slope of the Patagonia Mountains, contain extensive quartz-sericite-pyrite and clay alteration zones. The areas around Red Mountain and the 4-Metals and Sunnyside mines contain significant deposits of ferricrete that are related to low-pH fossil streams. Both mined and unmined areas produce waters with pH values as low as 2.7, and elevated concentrations of metals. Preliminary values of dissolved trace metals in streamflow range from 0.7 to 12.4 mg/L^-1 of Cu, 0.33 to 11.4 mg/L¹ As, 0.2 to 10.4 mg/L¹ Pb, and 2.1 to 60 mg/L^-1 of Zn. Mine tailings are a significant source of suspended -sediment during infrequent storms and contain potentially acid-generating materials; however, springs and seeps related to fractures in the mineralized outcrops may also contribute to the dissolved chemistry.

During typical, prevailing, low-flow conditions, acidic streamflow at higher elevations is intermittent and does not connect to larger drainages downstream. For example, on July 23, 1997, a pH value of 3.4 was measured from a small perennial reach in upper Flux Canyon. Streamflow continued approximately 0.8 kilometers before disappearing beneath the streambed. Base flow reappeared 4 kilometers downstream having a pH value of 7.3.

1. Continued investigations will attempt to address whether this and other observations of large variations in pH in other drainages can be attributed to chemical reactions along the flowpath, such as buffering from carbonates,
2. dilution from other groundwater sources, or
3. the possibility that water moving downgradient from the upper to the lower reaches is not hydrologically connected except during periods of runoff. Further studies will evaluate whether acidic waters from mineralized and mined areas are neutralized downstream primarily by secondary calcite or detrital volcanic materials present in active streambed alluvium and shallow basin-fill deposits. During low-flow conditions, acid-base redox reactions involving manganese and iron, and sorption reactions promote the neutralization of low-pH waters. Buffering reactions appear to be more effective in an arid climate than in other study locations having greater precipitation.

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