Historic and current mining activities have contaminated stream sediments around the world with toxic heavy metals. A general lack of premining baseline data makes it difficult to quantify the extent of contamination and to set realistic remediation goals. These problems can be solved by modeling the downstream dispersion of metal anomalies based on dilution mixing of anomalous and tributary sediments. The model allows calculation of the dispersion curves of metals in stream sediments both before and after mining and also allows the quantification of any anthropogenic exaggeration of an anomaly. Mining activities were found to amplify naturally occurring metal anomalies up to 3 orders of magnitude, extending downstream dispersion trains from a natural limit of around 20 km to as much as 500 km. The dilution mixing model provides a useful tool for calculating premining dispersion trains and quantifying the effects of mining on a river basin. Such information is relevant to the understanding, litigation, and remediation of contaminated rivers around the world.

Historical mining practices in Butte, MT, resulted in deposition of tailings as overbank deposits in the Clark Fork River. This study used pore water chemistry, electron microprobe analysis, and geochemical modeling to compare the chemistry of these acid-generating deposits 5 years after liming with naturally revegetated and barren tailings. The sequence of weathering reactions is predictable, covering a continuum from acidic soils through the lime-amended soils to the naturally revegetated overbank tailings deposits. For example, sulfides and sulfates predominated in the untreated tailings mineral assemblage, while liming facilitated alteration of pyrite to ferrihydrite that sequestered weight percent concentrations of As, Cu, Pb, and Zn. Collocated pore waters collected using suction lysimeters installed in the surficial treated material (<30 cm deep) were alkaline (pH 7) and low in metals as compared to the pore water in unlimed barren tailings (pH <4.8). Geochemical modeling using MINTEQA4 was consistent with the electron microprobe results in that pore water was predicted to be oversaturated with respect to jarosite and gypsum under acidic conditions and with respect to calcite, gibbsite, ferrihydrite, and plumbogummite with increasing pH. This study demonstrates that lime amendment represents a viable long-term method to mitigate acid generation in tailings.

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Identifying and quantifying the contributions of multiple sources of trace elements to stream sediments in a basin containing several possible inputs presents a unique problem related to the investigation of rivers impacted by industrial activity. A multi-source dilution±mixing model was developed and applied to determine the relative contributions to As, Cu and Pb burdens in the Clark Fork River, Montana, a recipient of historical mine wastes as a result of over a century of mining and milling operations. The results identi®ed the Flint Creek drainage as a major source of anthropogenic As (47%) and Pb (35%) to sediments of the Clark Fork River and the Milltown Reservoir, in addition to the major sources associated with mining operations in Butte, MT. The Little Blackfoot River also contributes anthropogenic As (3%) and Pb (4%) to the Clark Fork River, while minor inputs of Cu (1%) and Pb (2%) emanate from the Blackfoot River. The model allows source quanti®cation, and an understanding of the fate and transport of mine wastes in a basin, allowing identi®cation and eventual prioritization of sites destined for remediation.# 2000 AEHS

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