Methylisoborneol (MIB) and geosmin are cyanobacterial metabolites that occur at nanograms per liter levels in surface water supplies and are responsible for many taste and odor complaints about the aesthetics of drinking water. This study evaluated three water supply reservoirs with bottom-release (hypolimnion) outlet structures in Arizona. MIB concentrations were always higher than geosmin concentrations, but both followed similar seasonal trends. MIB concentrations increased from spring to late summer, and stratified vertically with depth in the water column; the highest concentrations were always in the upper 10 m of the water column. Thermal destratification in the autumn increased MIB concentrations released from the outlet of reservoirs and impacted downstream utilities for several months. By winter of each year MIB concentrations were non-detectable. Mass balance analyses on MIB indicated that in-reservoir reactions were more important in changing MIB concentrations than conservative hydraulic "flushing" of the reservoir. Maximum net loss rates for MIB in the field (RF,max) were on the order of 0.23-1.7 ng/L-day, and biodegradation appeared more important than volatilization, photolysis or adsorption. Using lake water in laboratory experiments, bacterial biodegradation rates (RL) ranged from 0.5-1 ng/L-day and were comparable to RF,max values. Based upon these rates, MIB concentrations in a reservoir would decrease by approximately 30 ng/L over a period of 1 month. This was the magnitude change in MIB concentrations commonly observed after autumn thermal destratification of the reservoirs.
Bibliographical noteFunding Information:
This research was supported by the City of Phoenix (Arizona) in cooperation with the Salt River Project and Central Arizona Water Control District. Post-doctoral researchers (Darlene Bruce, Qiang Hu, Mario Esparza-Soto), graduate students (Tom Dempster, My-Linh Nguyen, Kirsten Hintze, Michelle Cummings, Samanth Dawson), and technicians (David Lowry, Marisa Masles) assisted in sample collection and analysis. Helpful comments from an anonymous reviewer are greatly appreciated.
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- Drinking water
- Taste and odor