ESTIMATING PRE-MINING WATER QUALITY USING FERRICRETE

Mine remediation efforts in mountainous watersheds affected by sulfide mineralization and historic disturbance ultimately encounter the question of background (pre-mining) water quality. Ferricrete terraces mapped and radiocarbon-dated as Holocene age, discovered along many of these streams appear to be the result of acid rock drainage from sulfide mineral weathering, and suggest that streams in these watersheds may not have met aquatic or human health, water quality criteria prior to mining. Our method, developed at Fisher and Daisy Creeks on the Gallatin National Forest near Cooke City, Montana uses the geochemical characteristics of ferricrete coupled with the geochemistry of modern metal precipitates to predict pre-mining water chemistry in streams. Based on an empirical trace metal relationship between the terrace deposits of ferricrete in Fisher and Daisy Creeks and the modern iron oxyhydroxide precipitates presently forming in the streams, the longitudinal dissolved metal and pH continuum that existed along these streams before any mining disturbance occurred can be reconstructed. For each hydrochemical variable (Cu, Zn, Al, Fe, Fe/Cu), an equation is generated that describes the longitudinal relationship between aqueous metal concentration or pH and modern precipitate metal concentration along a stream reach. Curves representing water column/precipitate metal and pH relationships, primarily exponential and power functions, show reasonable R² values. For each ferricrete sampling location along the stream, the precipitate metal value is substituted by the ferricrete value in the equation to calculate the composition and pH of the paleo-stream. Using these methods at Fisher and Daisy Creeks, we estimate a pre-mining stream pH of 4 with associated high levels of dissolved metals. The pre-mining water quality estimation methods presented here have the potential for use in monitoring the progress of remedial efforts at mine sites associated with ferricrete deposits. A reasonable remediation objective for impacted streams would be realized when trace metals in streams correspond to patterns found in the adjacent ferricrete geologic record or when stream pH corresponds to the predicted pH.