| Abstract/Description or Keywords |
Serpentinitic sediments rich in chrysotile asbestos, from a natural landslide in the Sumas River Watershed in Washington State, are deposited on agricultural land during flooding events, creating a health concern for the rural population. The sediments have a chemical composition of high magnesium (Mg), nickel (Ni) and chromium (Cr) and high pH values compared with non-serpentinitic sediments that originate from the agricultural lowland. As the sediments are transported from the headwater to the mouth, the effects of mixing with non-serpentinitic water from agricultural drainage decrease the pH, increase the contents of organic matter, nitrate and zinc (Zn), and impact the sediments that originated from the landslide. A comparison of bed and suspended sediment showed that there is a clear seasonal effect with lower Mg and Ni values during low flow for both types of sediments. Bed sediments were consistently higher than suspended sediment in Mg and Ni. Sediments emerging from the landslide have positive zeta potentials but disperse very rapidly and become negatively charged. Once the sediment interacts with agricultural sources downstream the zeta potential becomes more negative, causing the sediment to stay suspended. It is postulated that at high pH, Mg is leached from the brucite layer of chrysotile and precipitates as Mg carbonate, which enriches the bed sediments in Mg and associated trace metal concentration. |
