Molybdenum is an essential trace metal for virtually all living things. Although it does not exist naturally in the metallic state, it combines readily with other elements including oxygen, nitrogen and sulfur compounds. The molybdate anion is the predominant form occurring in soils and natural waters (Fairhall et al., 1945). Generally, the recommended daily allowance of molybdenum is acquired through foods grown above ground, such as legumes, leafy vegetables, and cauliflower. Molybdenum is beneficial to persons with sulfite sensitivity, asthmatics, and those intolerant to intravenous sulfur containing amino acids.

As no data is available on the genotoxicity of molybdenum compounds, there is no evidence to support the carcinogenicity of molybdenum. It has not been classified for toxicity by The U.S. Environmental Protection Agency (EPA), The International Agency for Research on Cancer (IARC), The National Institute for Occupational Safety and Health (NIOSH), or The U.S. Occupational Health and Safety Administration (OSHA). The World Health Organization (WHO) has established a drinking water standard for molybdenum of 0.02 mg/day (WHO, 1993).

At the Monticello Mill Tailings Site, the concentration of molybdenum in the shallow alluvial groundwaters slightly exceeds the remediation goal (100 μg/L) (Table 4 of the PHA [pg. 57]). Molybdenum contamination is confined to one location upgradient to the permeable reactive barrier. No human exposures or health effects are likely at this location.

It should be noted that ruminant animals such as deer and cattle can develop a condition known as molybdenosis due to high exposure to molybdenum. Molybdenosis in ruminants is characterized by loss of pigment around the eyes and persistent diarrhea. The condition is easily treated by addition of dietary copper to feed. As the background levels in the area are naturally high, so is the expression of molybdenosis in the areas ruminant populations. As such, its presence cannot be definitively related to the Monticello Mill Tailings Site (Graham et al., 2009).