June 17, 2006
From Peter Nielsen, Missoula County’s point person for all things Milltown, here’s a good summary of the first two weeks of the drawdown at the reservoir:
The Milltown Reservoir drawdown was initiated on June 1 in preparation for the removal of the Milltown Dam and cleanup of contaminated sediment at the Milltown Superfund site. The reservoir will be drawn down by about 8 to 10 feet below normal full poll elevation this summer. It will remain drawn down until next May, when spring runoff will partially refill the reservoir. Following spring runoff next year, the reservoir will drop again during the summer to 8 – 10 feet below normal full pool elevation.
Beginning in August of this year, work will be initiated on upgrades to the Interstate 90 bridges on the Blackfoot River arm of the Reservoir. The Highway 200 bridge and railroad bridge will also have work done to fortify them this fall and winter. This work must be completed prior to October 2007 when the reservoir will be drawn down an additional 7 feet to prepare for removal of the Milltown Dam (Stage 2 drawdown). The pedestrian bridge in Milltown will also be replaced with a new pedestrian bridge in 2007.
While the reservoir is drawn down this year and next year, Envirocon (Atlantic Richfield’s contractor) will build roads, berms, a rail spur and a bypass channel on the Clark Fork arm of the reservoir. In fall 2007, the Clark Fork River will be diverted into the bypass channel, prior to the Stage 2 drawdown. The Milltown Dam spillway is scheduled for removal in early 2008. Following its removal, the reservoir will drop the final 12 feet and the river will be restored to its final channel elevation.
The reservoir water elevation was drawn down by 6.4 feet this afternoon. The water level in the reservoir is currently controlled by river flows, and will continue to drop gradually as the river drops through the remainder of June and into July. It only dropped about 0.16 feet in the last 24 hours, and the rate of drawdown may be as little as 0.1 to 0.4 feet per day in the coming week, depending on river flows.
So far the drawdown has gone well, and downstream water quality has been similar to that which routinely occurs during spring runoff. River water quality is being monitored closely, and the results are available daily at the following website:
Turbidity levels in the river increased above 12 turbidity units last week, requiring daily sampling of the river for suspended solids, metals and arsenic. Due to heavy rainfall in the Clark Fork headwaters last weekend, river flows increased along with turbidity, arsenic and metals upstream and downstream of the Milltown Dam. Contaminant levels upstream of the dam on the Clark Fork were higher than those detected downstream. For example, arsenic levels were as high as 13 parts per billion upstream of the dam this week, while arsenic concentrations were as high as 6 parts per billion downstream.
Water quality in the river has cleared up significantly Thursday and Friday. Turbidity levels in the river were down to 7.5 units this morning. Suspended solids, copper and arsenic concentrations downstream of the Milltown Dam are at fairly typical levels for this time of year during spring runoff, and well below the warning limits established for the project. Dissolved arsenic was at 4 ppb yesterday (the warning limit is 8 ppb) and the copper concentration was 4 ppb (with its warning limit at 20 ppb.) Even though the dissolved arsenic below the dam was somewhat elevated, the data for the CFR at Turah showed a value of 8 ppb for dissolved arsenic.
Domestic well and monitoring well sampling was conducted in late May and the first week of June, and results will be posted on the ftp site in the near future. All wells in the monitoring program will be monitored again at the end of June. The monitoring well network includes private and public supply wells, and monitoring wells surrounding the reservoir and extending downstream into Missoula. Additional wells may be added to the monitoring well network in the unlikely event that arsenic levels in the river or in wells increase significantly.
Contingency plans have been finalized for domestic well contamination, exceedance of standards in the river, and for cleanup of irrigation intakes. These contingency plans and the remedial action monitoring plan can be downloaded from another web site:
Other design documents are also available at this website