Mine tailings are the "residue" from the extraction of metallic silver from the host rock. In the Cobalt area, the silver ore contains arsenic and mercury, and these "waste" minerals are discarded into nearby lakes. These heavy metals are leaking into the surrounding lakes to this day, even though some of the mines had been shut down several decades ago, as in the cases of Peterson and Crosswise lakes.
Arsenic commonly occurs with silver as cobaltite (CoAsS), also known as Cobalt Bloom, but it does appear in other minerals such as realgar and orpiment. The acceptable level of arsenic for drinking water and aquatic life is 50 ppm within the environment. Arsenic is initially taken up into the food chain by the roots and leaves of plants. Arsenic mainly enters other organisms by way of ingestion of these plants, but it can also enter through inhalation or through diffusion. Lower forms of aquatic organisms tend to contain higher concentrations of arsenic. Arcellaceans happen to be highly resistant to elevated levels of these heavy metals.
Mercury commonly occurs in solid solution along with silver as cinnabar (HgS). Mercury is used as an amalgam in the recovery of silver and it occurs in great concentrations in mine tailings. The acceptable level of mercury within the environment for drinking water and aquatic life is 0.1 ppm. Once mercury is released into the environment it is available for consumption by organisms. Mercury is notorious for its ability to bioaccumulate in the environment. An organism on the bottom of the food chain will injest mercury.
This "bug" then is consumed by another organism higher up on the food chanin and the mercury is concentrated in the subsequent predator. Through this process the average mercury content in a food chain continues to increase. Fish in mercury polluted lakes can have very high concentrations of this toxic metal, even if microorganisms at the bottom of the ladder have relatively low concentrations. Bioaccumulation is a very big rpoblem for those who fish near certain old mine sites.