October 2, 2008

Aerial view of Duck Bridge

A recent Missoulian article featured coverage of Mountain Water’s concern about higher than expected scouring from last spring’s breach of the Milltown Dam. Mountain Water, the company that owns Missoula’s water system, sent EPA officials a letter outlining its concerns. “In light of EPA’s failure to accurately predict the sediment scouring rate, MWC submits that the river protection mechanisms need to be reanalyzed to ensure that no further events like this happen again,” Mountain Water’s attorneys wrote. “Unpredicted actual releases require that the monitoring and modeling program also be redone.”

Following the breach in March, more heavy metals contaminated sediment than expected flushed downriver with higher than average flows from the area above Duck Bridge (pictured above.) UM scientists documented a spike in metals in May, though subsequent analysis by the EPA (PDF) and Missoula County have shown metals levels well below residential and recreational human health standards along the Clark Fork River from Milltown to Thompson Falls.

The EPA will present additional information on sediment sampling at meetings scheduled for Oct. 14 in Bonner and Oct. 15 in Thompson.

Missoula County’s Peter Nielsen also responded in an email note included below:

This information may help answer questions you have received as a result of several recent news accounts. In response to some of the questions we have been hearing recently:

1. Water quality in the river today is very good, and it can be used for public recreation without concern for impacts to human health. Concentrations of arsenic and metals in the river currently meet drinking water standards. Recreational exposure to Clark Fork River Water does not pose a threat to humans or pets.

2. Sediment deposited along the river below the former dam site following the dam breach contained elevated levels of arsenic and metals. The samples recently reported in news accounts were collected in May, prior to spring runoff. UM samples collected in June, near the end of spring runoff, showed greatly reduced concentrations of arsenic and metals. The sediment samples collected by the UM Geology Department in May, recently reported in news accounts, were sieve samples, which report the concentration of metals in the fine sediments (63 microns and less). This type of analysis is appropriate for many purposes, including assessment of potential affects on aquatic life. But this technique is not appropriate for assessment of potential human health effects. It exaggerates the concentrations of metals in the total volume of sediment deposited along the river, because it only accounts for the most contaminated portion of those sediments. Bulk samples, including sand and other coarse grained sediments which are much less contaminated, are appropriate for assessing human health impacts from the sediments.

3. Bulk Sediment Sampling conducted following spring runoff indicates very low levels of arsenic, copper and other metals. Sediment samples collected by the Health Department, DEQ and EPA show levels of arsenic far below recreational risk based concentrations and below levels expected to impact aquatic life. This includes sediment samples collected in the Thompson Falls Reservoir. Arsenic levels in sediments are below even the most stringent residential cleanup guidelines for contaminated soils. Exposure to sediments along the Clark Fork River below the former dam does not pose a risk to human health. Contamination levels are not now as high as they are in the Deer Lodge Valley, as suggested in the newspaper. Not even close.

4. More sediment was eroded from the former reservoir this year than predicted based on computer modeling. This was due to a number of factors, including higher than normal and longer than normal spring runoff, and the inability of the computer models to accurately predict the rate of lateral erosion in the river’s banks upstream of the sediment cleanup area. However, recent press accounts have exaggerated the volume of sediment eroded. The U.S. Geological Survey recently released its data on suspended sediment loading in the river. The total volume of sediment eroded from the project area this year was not as high as alleged in the press accounts. A total of 371,000 tons of sediment were eroded from the project area this year, and 150,000 tons were eroded last year, for a total for the two years of 527,000 tons. The computer model predicted a total of about 600,000 tons during the remediation project. The total amount eroded includes sediments from the Blackfoot arm of the reservoir, which are not contaminated by mine waste. We don’t know how much sediment will be eroded next year, which will depend largely on how high the river gets during spring runoff. It is likely that the total amount eroded will exceed the original computer model prediction. We believe it is reasonable to take appropriate measures to limit the rate of erosion in future years, through such measures as sediment removal and bank stabilization.

5. EPA and the State of Montana planning some additional sediment removal and bank stabilization in the upper reservoir area prior to next spring’s runoff season. The State Natural Resource Damage Program has planned some sediment removal in the upper reservoir area to achieve a smooth transition in river grade from the river upstream to the area near the former dam where 20 to 25 feet thick sediment deposits are currently being removed. The Health Board, Health Department and Missoula County have supported additional sediment removal and stabilization as a means to limit the rate of erosion of sediments in the upper reservoir and potential impacts on aquatic life downstream of the former dam. We encourage EPA and the State to complete this work before next year’s spring runoff.

6. Surface Water Quality standards for dissolved arsenic and total suspended sediment were exceeded for one day this year, immediately following the breach of the dam in March. The dissolved copper standard was not exceeded. Arsenic, copper and suspended sediment concentrations have historically been higher than the levels recorded this year, especially during events such as the 1996 ice jam. Total suspended sediment and total arsenic and copper levels (metals attached to sediment particles and dissolved in the water) were higher than normal during much of the spring and summer, during the above average and longer than normal period of spring runoff this year.

7. Arsenic concentrations in many of the monitoring wells near the former reservoir have dropped significantly since the dam removal. Of the 10 contaminated wells in the arsenic plume, all but one is substantially cleaner since the start of the project. In several cases, arsenic is now 10 times lower than it was in recent years, and two of these wells now meet drinking water standards. The project appears to be well on its way to achieving its primary goals, or cleaning up a plume lf groundwater contaminated with a substance know to cause cancer in humans. This was the largest plume of contaminated groundwater in Missoula County, and it is now shrinking. Arsenic levels in uncontaminated early warning wells have not increased since the dam removal. Arsenic levels in several domestic wells have dropped since the dam was removed, and none of the domestic wells show increased concentrations. Wells in East Missoula and the Missoula Valley remain at background levels for arsenic, as they were prior to the project. This includes wells owned by the Mountain Water Company, which have been monitored since the project began several years ago.

8. Several wells near the Thompson Falls Reservoir have arsenic levels exceeding the federal drinking water standard, but this has been the case since long before the Milltown Dam project began. The City of Thompson Falls water supply has not been affected. Arsenic levels are slightly above the new federal drinking water standard, and not as high as those seen near the Milltown Reservoir. The Thompson Falls Reservoir was built in 1914, six years after the Milltown Dam, and it is essentially full of sediments from the Clark Fork, Bitterroot and Flathead watersheds. The Dam is owned by PPL Montana, and the reservoir is listed as a State Superfund site. This is one reason that PPL Montana has taken the position it has on the Milltown Dam removal, to limit its potential liability for the Thompson Falls Reservoir and any cleanup or removal that might be required there in the future. PPL’s spokesman claimed that its primary interest it to protect the health of the residents of the Clark Fork Valley. I believe that it is more likely that PPL’s primary interest it to make money selling electricity, and to avoid spending money cleaning up problems at the dam and reservoir that it owns. If PPL is really interested in public health, why has it done nothing before to address the arsenic contamination of wells surrounding the reservoir it owns?

9. Fish population data indicates an approximate 60% reduction in trout numbers between the former dam the Bitterroot River this summer, with a 50% increase in numbers upstream of the former dam near Turah. Fish movement upstream of the former dam has been documented through radio telemetry. Fish populations in the lower Clark Fork River have not been impacted by the Milltown project. Mortality of caged fish below the dam was slightly higher than the combined mortality on the Clark Fork upstream at Turah and the Blackfoot upstream of Bonner this year. But mortality for some reason was actually higher on the Blackfoot River this year. Fish mortality was higher in 2006 and 2007, before the dam breach, due in part to high water temperatures and low stream flows.

10. Groundwater elevations did not drop as predicted this year following the dam breach. Computer modeling done by the University of Montana Geology Department predicted significant impacts on local drinking water wells, but fortunately these impacts did not occur. Nonetheless, EPA has replaced more than 70 local drinking water wells and pumps for local residents. We appreciate EPA stepping up to the plate and taking this responsibility. Computer models are seldom precisely accurate in their predictions. In this case, unlike the sediment erosion model, the impacts were much less than predicted.

11. Short-term impacts to aquatic organisms may have occurred this year between the former dam and the Bitterroot River. Aquatic macro invertebrate data should be available within the next two weeks. Aquatic organisms may be affected by metals contamination and physical factors such as smothering of river bed gravels by sediments. These impacts may continue for several years. Additional sediment removal and stabilization in the former reservoir would reduce potential impacts to aquatic life.

12. Contingency plans exist for protection of drinking water supplies, surface water quality and cleanout of irrigation ditch intakes. EPA was responsive to local concerns, including those expressed by the Health Department, in crafting these contingency plans. Sediments have been removed from irrigation ditches and trucked back to the former reservoir site for shipment to the waste repository. Sediments removed were sampled, and were low in arsenic and copper concentrations. No water quality impacts are expected from sediment accumulation in irrigation ditches. It is safe to use water in the ditches for irrigating vegetable gardens.

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