LIMITATIONS TO STREAM RECOVERY DUE TO ALUMINUM PRECIPITATION AFTER REMEDIATION OF ACIDIC MINE DRAINAGE

Leaching of aluminosilicate minerals by acidic water, derived from mining activity or natural weathering of pyritized rock, results in the discharge of aluminum-rich ground water to streams. Once in streams, the behavior of aluminum is strongly affected by pH. At a pH less than about 5.5, the aluminum will remain dissolved; at a pH greater than about 5.5, aluminum will precipitate. This may be related to the first hydrolysis constant of aluminum. Remediation of acidic drainage may result in an increase of pH to allow the precipitation or sorption of metals from solution. After removing metals, streamwater may have a higher pH. Aluminum in an acidic ground-water plume may continue to enter the stream and precipitate if pH is above 5.5, causing a physical toxicity to fish and coating the stream substrate. Changes in the stream chemistry of Mineral Creek, in the San Juan Mountains of southwestern Colorado, demonstrate possible changes from remediation schemes that employ pH perturbation of acidic waters. Along a 15-kilometer study reach, aluminum-rich, surface and subsurface inflows enter the stream, and instream pH fluctuates above and below pH 5.5 several times. Dissolved and colloidal phases of aluminum were distinguished by ultrafiltration. Each time the pH of the stream was less than pH 5.5, dissolved aluminum was the predominant phase; when inflows caused the pH to rise above pH 5.5, a rapid transition of the instream aluminum to the colloidal phase occurred. Mass-transfer and rates of reaction were quantified in the context of a hydrologic-tracer experiment. Rates of formation and dissolution of colloidal aluminum were consistent with laboratory and field kinetic studies. The reaches where colloidal aluminum formed are analogs to the situation that may occur when subsurface sources of acidic, aluminum-rich water continue to enter a stream after remediation of acidic drainage.