Mechanisms Causing Ferric Staining in the Secondary Water System of Brigham City, Utah

• Main Author: Wallace, Robert Derring
• Format: Others
• Published: BYU ScholarsArchive 2007
• Subjects: anaerobic; hypolimnion; epilimnion; stratification; iron-staining; reddish-brown staining; hypereutrophic; mesolimnion; dissolved oxygen; benchscale models; iron reduction; manganese; redox potential; Mantua Reservoir; dissolved iron; precipitate; reoxidation; aerobic; PHREEQC; benthic sediments; seasonal fluctuations; iron-rich sediments; calibration curve; spectrophotometer; ICP; Beer's Law; EPA Methods; thermal stratification; Civil and Environmental Engineering
• Online Access: https://scholarsarchive.byu.edu/etd/903; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=1902&context=etd

Water from Mantua reservoir has, during some years, exhibited reddish-brown staining when used by Brigham City for irrigation. I propose that seasonal fluctuations in the reservoir chemistry create an environment conducive to dissolving iron from the iron-rich sediments, which subsequently precipitate during irrigation, resulting in a staining event. These conditions are produced by chemical and biological decomposition of organic matter, coupled with isolation of the hypolimnetic waters, which results in seasonal low concentrations of dissolved oxygen in these waters. Under these specific circumstances, anaerobic conditions develop creating a geochemical environment that causes iron and manganese reduction from Fe(III) to Fe(II) and Mn(IV) to Mn(II), respectively. These reducing conditions facilitate reduction-oxidation (redox) chemical reactions that convert insoluble forms of iron and manganese found in the reservoir sediments into more soluble forms. Consequently, relatively high amounts of dissolved iron and manganese are generated in the bottom waters immediately adjacent to the benthic sediments of the reservoir. Water withdrawn from a bottom intake pipe during these periods introduces iron-rich water into the distribution system. When this water is exposed to oxygen, reoxidation shifts redox equilibrium causing precipitation of soluble Fe(II) and Mn(III) back to highly insoluble Fe(III) and Mn(IV). The precipitant appears on contact surfaces as the aforementioned ferric stain. This research focuses specifically on the iron chemistry involved and evaluates this hypothesis using various measurements and models including field data collection, computer simulations, and bench-scale testing to validate the processes proposed.