December 8, 2006
Water quality in the Clark Fork River below Milltown Dam was good most of the month, although turbidity (the “cloudiness” of the water) spiked during the second week of November when heavy rain, small-stream flooding in the Blackfoot River, and exposed sediment in the reservoir resulted in a muddy river below the dam. Although turbidity increased during this period, arsenic and metals did not; they remained at low background levels.
During the month of November, Envirocon conducted a dewatering test in the reservoir area and began pumping contaminated discharge water into the Clark Fork above the dam. Although high in arsenic, the volume of this discharge was small compared to flow in the river, and arsenic levels remained low at the monitoring station below the dam. All dissolved arsenic and metal concentrations were well below warning limits and applicable standards.
The reservoir level was drawn down 11.5 feet at the beginning of the month, but rose about 4 feet during the rainy spell in the second week, causing water to flow over the dam’s spillway again. After the storm, the level gradually fell until it was back to 11.5 feet of drawdown by the end of the month.
Turbidity below the dam was monitored three times per day during the rainy period, and daily for suspended sediment, dissolved arsenic and dissolved copper. Upstream sites on the Blackfoot River and on the Clark Fork at Turah were also monitored daily during this period. The rest of the month, turbidity was measured daily below the dam and all sites were monitored weekly for suspended sediment, metals, and arsenic.