March 29, 2007
This monthly update comes courtesy of Peter Nielsen of the Missoula Valley Water Quality District.
The photo above shows the reservoir sediments from the bluff above the dam on the south side of the river. Major construction activity at the site includes construction of flood control berms along the Clark Fork and Blackfoot Rivers, which will be completed by the end of April to protect contaminated sediments from erosion during spring runoff. In this photo, the first loads of rock riprap for the flood berms are being delivered to the site by the trucks in the foreground. At the top left of the photo is the bypass channel excavation test pit, and at the top right is the sediment stockpile area which contains 40,000 cubic yards of contaminated sediments removed so far. This material is protected by a flood berm, and is planned to be shipped to the BP waste repository near the former Anaconda Smelter this fall.
This photo show exposed reservoir sediments about one mile upstream of the dam along the Clark Fork, near the old railroad bridge crossing. These sediments will be subject to scouring during spring runoff this year. Sediment, copper and arsenic concentrations will be monitored daily when turbidity levels are elevated, and if excessive scouring causes water quality standards to be exceeded, the reservoir water level will be increased to reduce sediment scour. As river flows increase, the reservoir level will also rise, helping to limit sediment scour.
This photo shows placement of rock riprap for the clerk Fork River flood berm. The upstream face of the Milltown Dam is in the background. Turbine units are being removed from the penstocks, in the powerhouse, which are large pipes that bring the flow of water from the reservoir through the powerhouse. This will allow more water to be channeled through the powerhouse this spring, reducing the water level rise in the reservoir during spring runoff.
This photo shows the discharge pipe from the dewatering pumps for the bypass channel test pit. The pumps have been turned off this week, and discharge to the river has stopped for the time being. Discharge may resume again when bypass channel construction resumes later this year. The pumps had discharged about 400 gallons per minute of contaminated water from the shallow aquifer under the reservoir, to dewater the excavation for the bypass channel test pit.
Monitoring results from groundwater monitoring wells in the reservoir area have shown a significant drop in arsenic, iron and manganese in the polluted aquifer since the project began in June, 2006. December, 2006 results show arsenic levels in monitoring wells near the reservoir in the arsenic plume have dropped by at least 50% as compared to the June results. Iron and manganese levels have dropped even more significantly. Arsenic levels in one of the most contaminated wells dropped from 339 parts per billion to 18 ppb in December. Another well went from 217 ppb to 41 ppb in December. While these results are promising, many wells remain within the range of variability over the last ten years of monitoring and it is too early to pronounce a long-term trend for restoration of drinking water quality. Wells on the west side of the reservoir have not shown improvement, and some have shown some decrease in water quality although quality remains similar to previous monitoring and below drinking water standards. Restoration of drinking water quality in the 325 acre arsenic plume area is one of the primary objectives of the Milltown project, and these initial results are very promising. The short-term improvements in water quality are likely the result of lower water level in the reservoir, reducing the water pressure through contaminated sediments into the aquifer below. Also contributing to the improvement may be the pumping of contaminated water from the bypass channel test pit and removal of some of the most contaminated sediments from the pit where they were in contact with ground water. As water levels rise with spring runoff this year, arsenic levels in the aquifer may increase again.