INFLUENCE OF HYDROLOGIC CONDITIONS ON HISTORICAL WATER QUALITY IN A STREAM AFFECTED BY NATURAL AND MINING-RELATED SOURCES, 1965-2001

Historical water quality samples from the Red River, New Mexico, were evaluated to determine influences on water quality from 1965 to 2001. The flow regime of the Red River was found to have a major effect on water quality. The highest sulfate and zinc concentrations and lowest alkalinity values were associated with the early part of the rising limb of summer thunderstorm events and early snowmelt runoff when sulfide oxidation products (soluble salts and concentrated water) are flushed from mineralized areas of the Red River before being diluted by runoff. These transient events can be difficult to capture as part of planned sampling programs but were observed in some of the data.

A trend of increasing sulfate concentrations and loads over long time periods also was noted at the Questa Ranger Station gage on the Red River, possibly related to mining activities, because the same trend was not apparent for locations upstream from the mine. This trend was only apparent when the dynamic events of snowmelt and summer rainstorm events were eliminated and only low-flow concentrations were considered. Sulfate concentrations and loads and zinc concentrations downstream from the mine were uniformly higher, and alkalinity values were consistently lower, than those upstream from the mine, suggesting that additional sources of sulfate, zinc, and acidity enter the river in the vicinity of the mine. During storm events, alkalinity values dropped both upstream and downstream from the mine, indicating that weathering of natural sources, most likely hydrothermal scar areas, caused short-term changes in the buffering capacity of the Red River.

The major element water chemistry of the Red River is controlled by the dissolution of calcite and gypsum and the oxidation of pyrite, and the river is generally not well buffered with respect to pH. During higher flow periods, Red River water is diluted by calcium-carbonate waters, most likely from unmineralized Red River tributaries and areas upstream from scars. The effect of pyrite oxidation on Red River water chemistry was more pronounced after the early 1980’s. The water-quality results demonstrate that hydrologic conditions must be distinguished when interpreting water chemistry in naturally mineralized or mined drainages.