Seepage Geochemistry and Mineral Dissolution at Clark Canyon Dam, Pick-Sloan Missouri Basin Project, East Bench Unit, Montana. Technical Memorandum 86-6829010

摘要

This report summarizes an evaluation of soluble mineral dissolution in the foundation, abutments, and embankment of Clark Canyon Dam, Montana, USA. Seepage and reservoir samples were collected in October 2005 (reservoir elevation 5,515 feet - 1,681 miles), and in May 2006 (reservoir elevation 5,535 feet - 1,687 miles), to represent seepage conditions at low and high reservoir surface elevations. Mineral dissolution was evaluated by comparing the chemistry of the reservoir and seepage samples from wells, toe drains, and surface seeps. Mineral saturation indices for reservoir and seepage water were calculated using the PHREEQE chemical equilibrium model. Void formation rates were estimated for soluble minerals using concentration difference data for the toe drain outlets and other measured seeps. The results of this assessment suggest that dissolution of calcite, gypsum or anhydrite, and amorphous silica is an active process at Clark Canyon Dam at both low and high reservoir elevations. The greatest seepage volumes and void formation rates appear to follow the old Beaverhead River channel that lies beneath the central section of the dam. The void formation rates suggest that seepage flow will increase over time and future monitoring of mineral dissolution would be best accomplished by enhanced frequency and accuracy of seepage flow measurements and water levels in piezometer wells.