Date of Award

Summer 2017

Degree Type

Non-Thesis Project

Degree Name

Master of Science (MS)

Department

Geosciences

Committee Chair

Chris Gammons

First Advisor

Glenn Shaw

Second Advisor

Alan English

Abstract

This study was conducted to identify environmental tracers using water chemistry and stable isotopes in the Moulton Road area adjacent to the Yankee Doodle tailings impoundment north of Butte, Montana. The water level of the tailings lake is being raised as tailings slurry is pumped up to the impoundment, and numerous residential wells are located in close proximity. Although it has an alkaline pH (> 9), the water in the tailings lake is much higher in some chemical constituents than the wells and some of these constituents could be used as tracers to determine if lake water is migrating out of the impoundment due to the rising level. Tracers examined in this study include the O- and H-isotopic composition of water and the concentration and S- and Oisotopic compositions of dissolved sulfate. Although water in the tailings lake is partly evaporated, the contrast between the isotopic signature of the lake and the groundwater wells is relatively small, making it a less powerful tracer. On the other hand, sulfate concentrations are more than an order of magnitude higher in the lake than in the groundwater. Whereas the Sisotope compositions of dissolved sulfate in lake and groundwater samples are similar, the Oisotope compositions of sulfate for the two end members are very different (-2.8 and -14.0 ‰, respectively). A conservative mass balance model was used to predict changes in the concentration and isotopic composition of sulfate in background groundwater as water sourced from the tailings lake is mixed in. As little as 5% mixing of lake water is enough to cause a substantial shift in the δ18O of dissolved sulfate in a long-term groundwater monitoring well.

This study also reports new data on the geochemistry of the active tailings slurry, the shallow sediment in the tailings lake, and the shallow sediment pore water of the lake. Acid-base accounting tests show that the solid fraction of the tailings slurry is net acidic, and will generate acid leachate if allowed to completely oxidize. The shallow sediment in the lake is also net acidic, but has somewhat higher neutralization potential compared to the slurry due to precipitation of calcite in the lake. Results from sediment pore water samplers (peepers) and piezometers show very little change in the chemistry of the pore water as the sediment is buried, at least to a depth of 8 feet. Vertical gradients in the pore waters are directed upwards, consistent with an increase in pore-water pressure as the tailings sediments consolidate.

Based on the results of this study it is recommended that δ18O-sulfate should be added to the list of analytes obtained from long-term monitoring wells in the Moulton Road area. This parameter is the most powerful tracer of potential mixing of lake water and background groundwater.

Comments

A non-thesis research paper submitted in partial fulfillment of the requirements for the degree of Masters of Science in: Geosciences: Hydrogeological Engineering Option

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