Dames & Moore, 1999 - USDA Forest Service

I
I
tailings piles difficult. Likewise, the presence of iron oxyhydroxide precipitates or "flocculent" in slow
moving reaches of Railroad Creek may suffocate invertebrates and developing fish eggs andlor alevins. The
precipitation of metals on creek sediments may also reduce the productivity of reaches of Railroad Creek
where cementation is found. It is also possible that the ecological effects found on invertebrate populations
may be explained by adverse effects on periphyton, food source of invertebrates, and unsuitable habitat
which could have implications for trout populations. This receptor was not addressed because of the lack of
available toxicity data and the probable lack of impact on selection of the appropriate remedial solution.
Leland et al. (1984) showed that the standing crop of periphyton was not reduced in an experimental stream
chronically exposed to copper, but that the species composition changed. The impact of this effect on
invertebrate and fish population is unknown.
Although it was assumed that flocculent may be a source of toxicity to benthic'invertebrates, the potential
toxicity of flocculent is not well understood. Most reports of flocculent suggest that it produces adverse
effects by suffocation, not toxicity. This conclusion is supported by the use of metals precipitation in
settling ponds as a method for reducing the toxicity of metals at mining sites. At the Warm Springs Ponds
near Butte, Montana, metals from Silverbow Creek are precipitated by additions of lime for pH adjustment
and the toxicity of Silver Bow Creek is substantially reduced. Metals are incorporated into flocculent which
settles out and is incorporated into the sediments in the ponds. Biological studies in the ponds revealed that
invertebrate density and diversity immediately downstream from the lime addition system ranked among the
highest found (ARC0 1997). This strongly suggests that metal precipitates such as flocculent are not toxic.
7.2.6 Conclusions
The ERA included a tiered risk characterization that included the evaluation of a list of species observed
on-site (using the guild approach), site-specific potential compounds of concern, and potential exposure
pathways.. The receptors of concern (ROC) evaluated included aquatic and terrestrial invertebrates,
plants, and animals. Specific ROC include: (I) Aquatic: Mink, Osprey, Trout, Dipper, Caddisfly, and
Periphyton, and (2) Terrestrial: Mink, Red-Tailed Hawk, Bat, Dusky Shrew, Earthworm, Deer Mouse,
Mule Deer, American Robin, and GrassesRorbes.
The risk characterization proceeded from the worst-case exposure scenario to more reasonable exposure
scenarios. The conclusions for potential risks to aquatic and terrestrial ROC are described below.
.7.2.6.1 Aquatic Exposure Pathway and Receptors of Concern
Hazard Quotients calculated in the risk characterization to aquatic ROC proceeded from the worst-case
exposure scenario, the water quality data from the South Bank of Railroad Creek, to the reasonable
exposure scenario, median water quality in the mainstream of Railroad Creek.
Trout
An intermediate potential risk for adverse effects (HQ>1 but