Sediment and Arsenic Quantity

In the water quality section, we look at how much sediment and arsenic were suspended in the river after the dam breaching and during this year’s high flow.  But what about the total amount of sediment that was delivered downstream? This is known as total load – it’s the product of concentration (in milligrams per liter) times the amount of flow in the river (cubic feet per second), and is typically expressed as tons per day or tons per year.  In this case, it’s been calculated as the total tons of material that flushed downstream from March 28th to June 30th, covering the period from the breach through high flow.  

Sediment Quality

And once all that sediment settles out of the water, how does it affect beaches along the river, habitat for insects and fish, and the reservoir at Thompson Falls?

Sediment Quantity: Suspended Sediment Load

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The chart above refers to suspended sediment load.  This spring, Milltown contributed roughly double the normal load of sediment downstream to the reservoir at Thompson Falls.  Each wedge of the pie shows the contribution of sediment from each tributary and from the Milltown project area (in yellow).  Of the total amount that scoured from Milltown, about half came from clean parts of the remedial project area (RPA) – namely sediments in the Blackfoot channel.  The other half came from the area above the RPA (the upper reservoir) and these sediments were not as clean (see the section on sediment quality).

Arsenic Quantity
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Similar to suspended sediment, Milltown delivered almost double the normal amount of arsenic to the Thomson Falls reservoir this spring (Milltown contribution in yellow).  Most of this came from the upper reservoir, above the bermed and bypassed work area.  On a typical year, 10 tons of arsenic flow into and settle in the Thompson Falls Reservoir.  Most of this comes from the Clark Fork River above and below Milltown, but about 30% comes from the Blackfoot, Bitterroot and Flathead.  This year over 26 tons of arsenic were delivered to Thompson Falls, with 12 of those tons from Milltown.  Essentially, this adds an extra year’s worth of arsenic to the 94 years of deposition since the Thompson Falls dam was constructed in 1914.

Sediment Quality

Samples of sediment (silt and sand) collected from Clark Fork River beaches from East Missoula to Thompson Falls contained arsenic in concentrations far lower than EPA standards for soils in yards, parks or recreational areas:

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What’s the difference between these results and those of University researchers?

Scientists at the University of Montana collected samples of very-fine-grained sediment from the bed and banks of the river this spring and summer.  They sieved samples with river water to separate out only the smallest particles – fine silts and clays.  For research purposes, this is the best way to sample because metals are more concentrated in this fraction, and sieving minimizes the confounding effects of different grain sizes in a sample.  With these results they can more readily compare trends over time and between locations, but since this type of sampling is not representative of all the sediment in the river, it is not used for assessment of human health.  What did they find?  In early May, after the breach and before peak runoff, concentrations of arsenic in silts and clays were much higher than sampling of this fraction in previous years – over 300 parts per million.  Additional sampling in mid-May and early June showed that high flows mobilized tons of cleaner sediment, effectively diluting the Milltown contaminants. By early June, concentrations of arsenic in silts and clays had decreased to about 70 parts per million – close to historic values.  Ditto for copper.  The high concentrations detected in early May were back to normal by early June.