5.2.1. Production

Molybdenum is a naturally occurring trace element that can be found extensively in nature. Biologically, it plays an important role as a micronutrient in plants and animals, including humans. It is also used widely in industry for metallurgical applications (USGS 2015a).

Molybdenum does not occur naturally in the pure metallic form, but is in minerals, principally as oxide or sulfide compounds (Barceloux 1999; EPA 1979). Important naturally occurring molybdenum compounds are the minerals molybdenite (MoS₂, the predominant source), powellite, wulfenite, ferrimolybdite, and ilsemannite. Molybdenum may also form molybdate, a tetrahedral poly atomic anion, or other isopolyanions, which can form salts used in industrial applications. The earth’s crust contains an average of 0.0004% (4 ppm) of molybdenum (Sebenik et al. 2012). Deposits that are economically feasible for mining contain ≥200 ppm of molybdenum, with lower concentrations obtained as a byproduct of copper mining (EPA 1979).

Molybdenite (MoS₂) is the principal mineral from which molybdenum is obtained. Mining and milling of crude ore produce molybdenite concentrate containing ≥90% of MoS₂, almost all of which is converted to technical-grade molybdenum trioxide. Molybdenum trioxide is the base material for the production of a variety of chemical compounds, ferromolybdenum, and purified molybdenum (EPA 1979).

Roasting molybdenite concentrate in a multiple hearth furnace at temperatures up to 600°C produces technical-grade molybdenum trioxide. This can be further purified by sublimation or selective recrystallization at about 1,000–1,100°C (Sebenik et al. 2012).

Worldwide mine production of molybdenum was estimated to be 258,000 metric tons in 2013, with approximately 92% produced, in descending order, by China, the United States, Chile, Peru, Mexico, and Canada. The United States accounted for 24% of world production with 60,700 metric tons in 2013, down slightly from 61,500 metric tons in 2012. Primary molybdenum operations accounted for 53% of total U.S. molybdenum production, while byproduct production made up 47% of the total in 2013. All U.S. molybdenum concentrates and products are from the mining of ore (USGS 2015a). U.S. production of molybdenum increased roughly 8% in 2014 to 65,500 metric tons (USGS 2015b). U.S. production of molybdenum for 2018 was 41,900 metric tons (USGS 2019). The USGS Mineral Industry Survey for molybdenum reported that domestic production for the first 3 months of 2019 was 3,620 metric tons (January), 3,420 metric tons (February) and 3,650 metric tons (March) (USGS 2019).

Table 5-1 contains a list the number of facilities per state that produced, processed, or used molybdenum trioxide in 2017, as well as information on the amount of molybdenum trioxide on site and related activities and uses (TRI17 2018).

Table 5-1

Facilities that Produce, Process, or Use Molybdenum Trioxide.

Molybdenum-99 (⁹⁹Mo) is a radioactive form of molybdenum and the only molybdenum radioisotope of commercial importance. It is produced in nuclear reactors, and then processed, packaged, and shipped to medical facilities throughout the world, where the ⁹⁹Tc progeny into which it transforms is eluted and injected into patients for imaging purposes (e.g., cardiac stress tests).

⁹⁹Mo was produced in one of eight nuclear reactors (mainly at the Chalk River complex in Canada) using highly enriched uranium, and then commercialized at five processing facilities and six generator manufacturing facilities. The availability of those reactors was reduced by the closure of the Chalk River facility, and this impacted the supply stream. The United States has established a high national priority on assuring an adequate supply of ⁹⁹Mo and urged manufacturers to switch from using highly enriched uranium (HEU) to low enriched uranium (LEU) to reduce the use of HEU for civilian applications (Ballinger 2010; The White House 2012; USNRC 2015; Van Noorden 2013). At a NAS symposium in 2017, several companies discussed their plans to produce ⁹⁹Mo in the United States (NAS 2018).

Currently, ⁹⁹Mo can be produced by placing HEU or LEU targets in an operating nuclear reactor and allowing the neutron flux to produce ⁹⁹Mo and its radioactive precursors. The quantity of ⁹⁹Mo peaks after approximately 6 days, at which time, the target is removed, processed, and prepared for shipment. New facilities for producing ⁹⁹Mo from LEU in the United States are being planned (Welsh et al. 2015).

5.2.2. Import/Export

Molybdenum-containing exports rose from 49,900 metric tons in 2010 to 55,300 metric tons in 2014, while imports for consumption rose from 19,700 metric tons in 2010 to 23,600 metric tons in 2014 (USGS 2015b). Imports of molybdenum (excluding ore) were 22,190 metric tons in 2018 and exports totaled 44,700 metric tons (USGS 2019). These data, along with U.S. production volumes, are summarized in Table 5-2.

Table 5-2

Molybdenum U.S. Production, Import, and Export Data from 2010 to 2014 and 2018 in Metric Tons.

5.2.3. Use

Molybdenum is used primarily in metallurgical applications, including as an alloying agent in cast iron, steel, and superalloys to enhance properties such as hardenability, strength, toughness, and wear- and corrosion-resistance. Molybdenum is commonly used in combination with other alloy metals like chromium, cobalt, manganese, nickel, niobium, and tungsten. The leading form of molybdenum used by industry, particularly in stainless steel production, is molybdenum trioxide (USGS 2015a).

Molybdenum is used significantly as a refractory metal and molybdenum compounds in a variety of non-metallurgical chemical applications, such as catalysts, lubricants, and pigments. For example, MoS₂ is used along with cobalt during the desulfurization process of petroleum (Sebenik et al. 2012). Most molybdenum nitride catalysts are nitrogen deficient due to thermodynamically unfavorable conditions at atmospheric pressure; however, molybdenum nitride was recently produced in a high temperature and pressure environment by solid state ion exchange. Testing found its catalytic activity to be 3 times that of MoS₂ and its selectivity to hydrogenation to be 3 times that of MoS₂ for hydrodesulfurizing dibenzothiophene (Wang et al. 2015). As green technology is becoming more popular, molybdenum has become increasingly important in areas like biofuels, catalysts, ethanol, solar panels, and wind power (USGS 2015a).

A radioactive isotope of molybdenum, ⁹⁹Mo, is used as a source to produce the metastable radioisotope technetium-99m (^99mTc), which is used in the vast majority of medical imaging tests performed today (Doll et al. 2014; Parma 2009; Richards 1989). It was estimated that 85% of all medical radioisotope procedures use ^99mTc and that about 50,000 ^99mTc-based diagnostic procedures are performed in the United States each day, resulting in about 13 million procedures annually (Parma 2009).

Molybdenum concentrate produced by U.S. mines is roasted, exported for conversion, or purified to lubricant-grade molybdenum disulfide. Purified MoS₂ is used directly as a solid or in coatings that are bonded onto the metal surface by burnishing, vapor deposition, or bonding processes that use binders, solvents, and mechanochemical procedures (Stiefel 2011).

Metallurgical applications accounted for about 87% of total molybdenum use in 2013. The principle non-metallurgical use was in catalysts, primarily catalysts used in petroleum refining. Molybdenum compounds are also used to produce pigments (USGS 2015a).

5.2.4. Disposal

Recycling is the most environmentally acceptable means of disposal for stable molybdenum (USGS 2015b). Because molybdenum is difficult to remove from waste water, it often is adsorbed to biosolids in municipal waste water treatment facilities. Biosolids are beneficial and are often used as fertilizer or compost for agricultural applications. In the United States, the land application ceiling limit for molybdenum in biosolids is 75 mg/kg (EPA 2018b).

A ^99mTc generator containing a depleted uranium shield or sufficient residual ⁹⁹Mo radioactivity to be considered radioactive can be disposed of by shipping to an authorized licensee following USNRC agreement state requirements along with those of the Department of Transportation (USNRC 2015). If the ⁹⁹Mo is allowed to decay sufficiently (typically ≥10 half-lives) and the internal shield is lead or tungsten, then disposal should follow state and local requirements.

5.3.1. Air

Estimated releases of 83,484 pounds (∼37.87 metric tons) of molybdenum trioxide to the atmosphere from 188 domestic manufacturing and processing facilities in 2017, accounted for about 4.23% of the estimated total environmental releases from facilities required to report to the TRI (TRI17 2018). These releases are summarized in Table 5-3.

Table 5-3

Releases to the Environment from Facilities that Produce, Process, or Use Molybdenum Trioxide.

The primary source of molybdenum emissions to the atmosphere is coal combustion. In 1970, it was estimated that 550 metric tons of molybdenum were released via coal combustion in the United States, in comparison to 900 metric tons estimated from all air pollution sources (EPA 1979). A total of 909 metric tons of molybdenum can be emitted from a single 1,000 megawatt power plant per year (EPA 1979). Historical concentrations of molybdenum in fly ash from coal combustion were reported to range from 7 to 160 mg/kg (Barceloux 1999). Advances in sorbent and air pollution control technology such as fabric filters and electrostatic precipitators in power plants have resulted in a reduction of atmospheric emissions of molybdenum and other metals as compared to emissions from decades ago (Cho and Wu 2004; EPA 2009a). A report from the EPA, which compiled data on 73 coal combustion residues (CCR), typically found molybdenum levels of 8–30 μg/g (mg/kg) in fly ash and scrubber sludges and about 1–10 μg/g (mg/kg) in gypsum (EPA 2009a). The study reported that no correlation was observed in molybdenum content and coal type or air pollution control system employed.

5.3.2. Water

Estimated releases of 38,868 pounds (∼17.63 metric tons) of molybdenum trioxide to surface water from 188 domestic manufacturing and processing facilities in 2017, accounted for about 1.97% of the estimated total environmental releases from facilities required to report to the TRI (TRI17 2018). This estimate includes releases to waste water treatment and publicly owned treatment works (POTWs) (TRI17 2018). These releases are summarized in Table 5-3.

Per year, it has been estimated that natural processes result in the release of 3.6×10¹⁰ g of molybdenum into surface waters (EPA 1979).

Aqueous effluents from industries with a high presence of molybdenum, including molybdenum mining, milling, and smelting; uranium mining and milling; copper mining and milling; shale oil production; oil refining; and coal-fired power plants, contain molybdenum at concentrations ranging from 100 to 800,000 μg/L (EPA 1979). Molybdenum levels in leachate samples obtained from a landfill located in Caledonia, Wisconsin ranged from 1.28 to 16 μg/L (WDNR 2013).

Effluent concentrations of molybdenum from three molybdenum mining and milling operations (two in Colorado, one in New Mexico) ranged on the order of 1,000–10,000 μg/L. In 1972, a mine in Colorado released approximately 100,000 kg of molybdenum into a receiving stream. Releases of molybdenum from coal power plants to surface waters in the United States average about 1,800 metric tons/year. A uranium mill in Colorado reported leaking of the tailings ponds containing 860,000 μg/L molybdenum in 1965. Some uranium operations in New Mexico reported as much as 1,000 μg/L molybdenum in aqueous effluents. Copper milling operations have reported molybdenum effluent concentrations as high as 30,000 μg/L (EPA 1979).

Frasacoli and Hudson-Edwards (2018) compiled monitoring data on molybdenum levels in mining-affected areas in different parts of the world, which included groundwaters, nearby rivers, and tailing pore water. The largest levels of molybdenum were observed in mine waste from a coal mine located in Poland (2,332,000 μg/L). Groundwater from an area near 13 nonactive mines in Mexico ranged from <5 to 150 μg/L. Tailing pore water from an inactive mining operation in Manitoba, Canada had molybdenum levels of <5–1,100 μg/L. Molybdenum concentrations from tailings channel water from an active copper mining facility in Chile ranged from 2,670 to 3,900 μg/L. Mine drainage samples obtained from an operational mine in Peru had molybdenum levels of 0.001–13.9 ppm (1–13,900 μg/L) (Skierszkan et al. 2016).

5.3.3. Soil

Estimated releases of 1,526,424 pounds (∼692.37 metric tons) of molybdenum trioxide to soil from 188 domestic manufacturing and processing facilities in 2017, accounted for about 77.41% of the estimated total environmental releases from facilities required to report to the TRI (TRI17 2018). An additional 186,213 pounds (∼84.46 metric tons), accounted for about 9.44% of the total environmental emissions, were released via underground injection (TRI17 2018). These releases are summarized in Table 5-3.

Metals, such as molybdenum, may leach into soil via municipal solid waste incineration bottom ash (IMOA 2015).

5.4.1. Transport and Partitioning

Air. Molybdenum released to the air by industrial processes will be subject to atmospheric deposition (IMOA 2015). Deposition from the atmosphere is only a minor source to terrestrial and aquatic environments (Fitzgerald et al. 2008).

Water. Molybdenum can be leached into the aquatic environment near industrial use areas via direct release or atmospheric wet deposition by rain (IMOA 2015). The pH of water, along with the composition and redox conditions of the sediment, greatly affect the speciation and adsorption behavior of molybdenum in natural waterbodies. Molybdenum accumulation in the sediment phase is favored under conditions of low pH and in sediments with low redox potential and high iron and organic matter content (Fitzgerald et al. 2008). In more favorable reducing geochemical conditions, solid-phase iron and manganese oxyhydroxides tend to undergo dissolution, and sorbed molybdenum may be released back into the water phase.

Sediment and Soil. In a seasonally anoxic basin, the distribution of molybdenum in the pore water of sediments was relatively uniform. In a perennially oxic basin, however, there was a redistribution of molybdenum in the sediment-water interface subsequent to deposition. This was determined to be a consequence of adsorption of molybdenum to iron oxyhydroxides at a rate of 36 cm³/molecule-second in the first 1–2 cm depth (IMOA 2015).

Geological uplift and atmospheric deposition result in the molybdenum enrichment of surface soils (IMOA 2015). Molybdenum concentrations are found to be the highest in the topsoil layer, due to strong binding to natural organic matter. Goldberg et al. (2002) studied the adsorption potential of molybdenum as a function of pH on 36 surface and subsurface soil samples from 27 soil series belonging to six different soil orders, which provided a wide range of soil physical-chemical characteristics such as organic carbon content, cation exchange capacity, and iron content. In general, maximum adsorption occurred under acidic pH conditions (pH 2–5) in which molybdenum adsorbed to oxyhydroxide mineral surfaces and sorption decreased rapidly from pH 5 to 8 and was minimal in all soils at pH >9. Skierszkan et al. (2016) studied the stable isotopic composition of molybdenum and zinc in mine wastes and noted a large variation in δ⁹⁸Mo (a measure of how the isotopic composition in the liquid or solid waste differed from a National Institute of Standards and Technology [NIST] standard solution) as a function of adsorption. At lower pH, adsorption of molybdenum is greatest, and the molybdenum isotope profile shifts toward heavier isotopic composition, as the adsorption process preferentially removes lighter isotopes. In contrast, zinc has the opposite behavior as it is more mobile under acidic conditions and adsorption is enhanced under alkaline conditions with lighter zinc isotopes more prominent. These results suggest the possibility of using isotopic composition as a method to understand attenuation mechanisms such as adsorption and molybdate precipitation during the weathering process.

Other Media. As reviewed by Regoli et al. (2012), the bioaccumulation factor (BAF) ranged from 30.1 to 71.6 (average of 49) in fish exposed to molybdenum levels <65 ug/L. At higher molybdenum levels (up to 766 μg/L), the BAF ranged from 0.4 to 9.9 (average 1.4). A laboratory study in rainbow trout found a similar inverse relationship between molybdenum concentration in the water and bioconcentration factor (BCF) (Regoli et al. 2012). A 60-day exposure to 880 μg/L resulted in tissue levels below the limit of detection. Exposure to 11,100 μg/L for 28 days resulted in whole-body molybdenum levels of 0.53 mg/kg fish; the calculated average BCF was 0.05. In another study, fish in a creek near a molybdenum tailings pile had measured BCFs of <100 after a 2-week exposure (CCME 1999). The accumulation data show that the BAF decreases with increasing molybdenum levels. At low molybdenum concentrations, there is an active accumulation of essential metals in organisms (and often non-essential metals via the same uptake mechanisms) to ensure that metabolic requirements are met. This active uptake process decreases when organisms are exposed to higher metal concentrations. At higher concentrations, organisms with active regulation mechanisms are even limiting their uptake by excretion of excess metals. EPA published a framework for metals risk assessment that discusses the difference in interpreting BCF and BAF values for organic versus inorganic compounds (EPA 2017a). It was generally concluded that the most recent scientific data on bioaccumulation do not currently support the use of BCF and BAF values when applied as generic threshold criteria when assessing the hazardous potential of metals. Moreover, single-value BCF/BAF data are most applicable to site-specific assessments; for more general regional or national assessments, the media chemistry and metal concentrations for a particular species should be considered for BCF/BAF studies.

5.4.2. Transformation and Degradation

As a naturally occurring trace element, molybdenum can be found extensively in nature. The predominant form of molybdenum in natural waters is as the molybdate anion, [MoO₄]²⁻ (Barceloux 1999), while naturally occurring molybdenum salts are the dominant form in dry environments (EPA 1979).

Air. No information regarding the chemical forms of molybdenum in the atmosphere and their transformations could be located. It is generally assumed that metals, especially those from combustion sources, exist in the atmosphere as oxides since metallic species are readily attacked by atmospheric oxidants.

Water. The speciation of molybdenum in aqueous media as a function of pH and molybdenum concentration, has been thoroughly investigated and reported upon in open literature. As discussed in Chapter 4, at pH >6.5, the sole molybdenum species is the molybdate anion, [MoO₄]2⁻ (Cruywagen 2000; Cruywagen et al. 2002). Molybdenum compounds transform rapidly into the [MoO₄]2⁻ ion under environmentally relevant conditions (Greenwood and Earnshaw 1997). In low redox environments, the molybdate anion can be reduced to molybdenum disulfide or molybdenite (Fitzgerald et al. 2008).

Sediment and Soil. Molybdenum is found naturally in soil as the minerals molybdenite, powellite, wulfenite, ferrimolybdite, and ilsemannite (EPA 1979; Fitzgerald et al. 2008).

The predominant form of molybdenum in wet soil is as the molybdate anion, [MoO₄]²⁻ (Barceloux 1999).

Other Media. No data for the degradation of molybdenum in other media were located.

5.5.1. Air

Molybdenum concentrations in ambient air have been reported to range from below detection limits to 0.03 mg/m³ (EPA 1979). Concentrations of molybdenum in ambient air of urban areas, 0.01–0.03 μg/m³, are higher than those found in rural areas, 0.001–0.0032 μg/m³ (Barceloux 1999). Data from the EPA Air Quality System Database reported 24-hour concentrations of molybdenum at several locations in the United States for 2018 (EPA 2018c). These data are summarized in Table 5-7.

Table 5-7

24-Hour Molybdenum Concentrations (μg/m³) in Air Samples (2018 Data).

5.5.2. Water

It has been reported that concentrations of molybdenum are generally <1.0 μg/L in surface waters (USGS 2006) and 1.0 μg/L in drinking water (USGS 2011). Groundwaters contain about 1.0 μg/L (USGS 2011). Smedley and Kinniburgh (2017) compiled ranges of molybdenum levels in rain water, stream water, rivers, lakes, estuaries, and oceans. Most surface water levels were <1 μg/L; however, there was wide variability, with levels tending to be higher with increasing salinity of the body of water (for example, molybdenum levels in the Salton Sea, California were reported as high as 37 μg/L).

A USGS study of surface water from 51 of the nation’s major river basins was conducted from 1991 to 2002 (USGS 2006). The median concentration of molybdenum in 2,773 surface water samples was <1.0 μg/L, with a maximum concentration of 157 μg/L. There were eight samples (approximately 0.29% of the total) that exceeded the health-based screening level of 40 μg/L for molybdenum.

In a study of surface waters collected from 197 sampling stations in Colorado, molybdenum was found at concentrations <10 μg/L in 87% of the 299 samples. Samples that contained concentrations >5 μg/L were concluded to be the result of proximity to mineralization or mining and milling operations (EPA 1979). However, another study comparing surface waters draining highly mineralized areas to those with baseline molybdenum areas found that molybdenum mineralization did not contribute significantly to concentrations in surface waters. The waters from streams draining the highly mineralized areas rarely had molybdenum concentrations above 1–2 μg/L (EPA 1979).

Huang et al. (2010) discussed surface water concentrations of metals including molybdenum in the Gyama Valley, an area impacted by four metal mining operations. Molybdenum concentrations ranged from <0.6 (detection limit) to 10.4 μg/L in the Gyamaxung-chu stream/river.

DOI (1967) collected river and lake water samples from 100 sampling stations around the United States from 1962 to 1967. The samples were taken from areas susceptible to contamination, including highly populated areas, industrial areas, recreational use areas, and state and national boundaries. Molybdenum was detected in the water samples at maximum concentrations >100 μg/L at 38 of the sample sites, while 26 sites had mean molybdenum concentrations >50 μg/L.

Molybdenum levels of 9.3–10.4 μg/L for open oxic seawater and 0.67–3.74 μg/L in euxinic waters of the Black Sea were reported (Smedley and Kinniburgh 2017). Kulathilake and Chatt (1980) reported the molybdenum concentration in the Atlantic Ocean as 7.2–7.9 μg/L. Another study reported that the molybdenum concentration in the North Atlantic ranged from 0.5 to 1.0 μg/L (Chan and Riley 1966). In the Pacific Ocean, measured molybdenum concentrations included 8.8 μg/L in the Eastern Pacific (Kiriyama and Kuroda 1984) and 1.5 μg/L in the Western Pacific (Nakata et al. 1983). Kawabuchi and Kuroda (1969) reported a mean molybdenum concentration of 7.7 μg/L in Tokyo Bay. Molybdenum concentrations measured in the English Channel ranged from 12 to 16 μg/L (Chan and Riley 1966), while the Irish Sea was reported to have a mean molybdenum concentration of 8.4 μg/L (Riley and Taylor 1968).

A comprehensive groundwater monitoring study conducted from 1992 to 2003 by the USGS of 5,183 monitoring and drinking-water wells representative of over 40 principal aquifers in humid and dry regions and in various land-use settings reported that the median concentration of molybdenum in 3,063 samples was 1.0 μg/L, with a maximum value of 4,700 μg/L (USGS 2011). Approximately 1.5% of the groundwater samples had molybdenum levels exceeding the health-based screening level of 40 μg/L (USGS 2011). Levels of molybdenum tended to be greatest in glacial unconsolidated sand and gravel aquifers as compared to other major aquifer groups in the study.

A report issued by the Wisconsin Department of Natural Resources found elevated levels of molybdenum in private supply wells and groundwater monitoring wells near the We Energies Oak Creek power plant located in Caledonia, Wisconsin (WDNR 2013). Molybdenum levels in 21 private well samples exceeded the state of Wisconsin groundwater enforcement standard of 40 μg/L. It was not determined whether the elevated levels of molybdenum were naturally occurring or were a consequence of the activities of the power plant and the coal ash fill areas located nearby the plant.

In January of 2017, the EPA published the final results of the third Unregulated Contaminant Monitoring Rule (UCMR 3) program. Molybdenum levels >1 μg/L were measured in 25,377 out of 62,981 analyzed drinking water samples, and 151 samples had levels greater than the health-based screening level of 40 μg/L. In 40 of the 4,922 public water systems tested, at least one measurable level above 40 μg/L was found (EPA 2017b). Concentrations as high as 1,400 μg/L have been detected in drinking waters in areas impacted by mining and milling operations (USGS 2011).

In a study of finished drinking water supplies from the 100 largest cities in the United States in 1964, median and maximum molybdenum concentrations of 1.4 and 68 μg/L, respectively, were reported (USGS 1964). Another study reported a mean molybdenum concentration of 8 μg/L in samples collected from 161 drinking water sources from 44 states in the United States (Hadjimarkos 1967). Molybdenum levels measured onsite at 12 public water facilities across England and Wales ranged from below the detection limit (0.03 μg/L) to 1.51 μg/L over an 18-month collection period (Smedley et al. 2014). Corresponding molybdenum levels in tap water from 24 residences in three towns (North Wales, the English Midlands, and South East England) served by these public water facilities ranged from <0.03 to 1.00 μg/L. The study indicated that there was little variability in molybdenum concentrations when comparing levels in tap water versus respective water supply facilities, construction ages of the residences (i.e., new homes versus older homes), and pre-flush versus post-flush tap samples, suggesting that water distribution pipework has a negligible effect on supplied tap water levels of molybdenum.

Drinking water may also be affected by industrial contamination, as water treatment facilities are ineffective at removing molybdenum from source waters. In tap waters samples collected in 1971 from Golden, Colorado, a community that derives its water supply from a stream draining a molybdenum mine and mill, the mean molybdenum concentration was reported to be 440 μg/L. However, after the mine closed in 1974, the mean concentration in drinking water samples decreased to 150 μg/L by January 1975, 60 μg/L by June 1975, and 30 μg/L by 1977 (EPA 1979).

5.5.3. Sediment and Soil

Globally, most soils contain molybdenum at concentrations between 0.6 and 3.5 mg/kg, although total concentrations in soils can vary widely depending on geological composition, soil horizon, or industrial contamination. Statistical analysis of 4,841 samples of soil collected from a depth of 0–5 cm in the conterminous United States showed molybdenum levels ranging from <0.05 to 75.7 mg/kg (USGS 2014). The 5^th, 25^th, 50^th, 75^th, and 95^th percentile concentrations were 0.24, 0.51, 0.78, 1.14, and 2.27 mg/kg, respectively (USGS 2014). From 4,780 samples of C horizon (substratum) soils in the United States, the molybdenum levels were reported as ranging from <0.05 to 94.7 mg/kg and the 5^th, 25^th, 50^th, 75^th, and 95^th percentile concentrations were 0.20, 0.51, 0.83, 1.27, and 2.88 mg/kg, respectively (USGS 2014). A review of 25,673 deep soil samples from the British Geological Survey reported molybdenum concentrations of <0.6–885 mg/kg, with a median value of 1.4 mg/kg (Smedley and Kinniburgh 2017). The Forum of European Geological Surveys (FOREGS), under the International Union of Geological Sciences/International Association of Geochemistry (IUGS/IAGC) Global Geochemical Baselines Programme, collected 840 topsoil samples from 26 European countries and reported molybdenum concentrations ranging from <0.1 to 21.3 mg/kg (mean 0.943 mg/kg) (FOREGS 2005).

Above average molybdenum soil concentrations may occur in areas containing molybdenum-rich rock formations or in areas of industrial contamination. Natural sources sampled, including soils covering a mineralized area, soil derived from a marine black shale, alluvial soils on the eastern footslopes of Sierra Nevada, and soils formed from volcanic ash in Kauai, Hawaii, contained mean molybdenum concentrations of 76, 12, 17.4, and 14.9 mg/kg, respectively. Soils sampled near industrial contamination, such as soils downstream from a molybdenum mine and mill in Colorado, soil irrigated with water contaminated by a uranium mill, and soils 2 miles from a molybdenum smelter in Pennsylvania, had mean molybdenum concentrations of 59, 61, and 29 mg/kg, respectively (EPA 1979).

Typical molybdenum concentrations found in stream sediments were reported to range from 1 to 5 mg/kg (EPA 1979). Sediments in streams that drain water from natural deposits of molybdenum in the United States have been reported to have molybdenum concentrations ranging from 10 to 200 ppm (10–200 mg/kg). Another study reported molybdenum levels of up to 300 mg/kg in sediments derived from black marine shales in England. Stream sediment collected from water below a molybdenum mine and mill in Colorado had molybdenum concentrations ranging from 50 to 1,800 mg/kg (mean of 530 mg/kg). Molybdenum content in stream sediments have been shown to reflect mineralization, as the concentration increases with decreasing sediment grain size (EPA 1979). FOREGS collected 848 freshwater sediment samples from 26 European countries and reported molybdenum concentrations ranging from 0.12 to 117 mg/kg (mean 1.34 mg/kg) (FOREGS 2005). An analysis of 65,477 stream sediments in the British Geological Survey G-Base reported a range of molybdenum concentrations of <0.1–309 mg/kg, with a median of 0.4 mg/kg (Smedley and Kinniburgh 2017). Sediment samples collected from river/streams in Tibet close in proximity to mining operations had molybdenum levels of 9.1–20.8 mg/kg (Huang et al. 2010).

5.5.4. Other Media

In a study detecting and comparing trace elements in the milk of guinea pigs (n=87), dairy cattle (n=48), horses (n=35), and humans (n=84), the average molybdenum concentrations measured were 26, 22, 16, and 17 μg/L, respectively (Anderson 1992). Average concentrations of molybdenum detected in six kinds of milk, including cow’s milk-based formula, breast milk, soya milk, bottled milk, dried milk, and evaporated milk, were 18, 4, 160, 34, 35, and 29 μg/L, respectively (Biego et al. 1998). Most of the molybdenum is in the cream fraction (Archibald 1951).

Food derived from aboveground plants, such as legumes, leafy vegetables, and cauliflower generally has a relatively higher concentration of molybdenum in comparison to food from tubers or animals. Beans, cereal grains, leafy vegetables, legumes, liver, and milk are reported as the richest sources of molybdenum in the average diet (Barceloux 1999).

Typical concentrations of molybdenum in plants are 1–2 mg/kg; however, a range of tenths to hundreds of mg/kg have been reported (EPA 1979). Tobacco contains molybdenum concentrations of 0.3–1.76 mg/kg (Barceloux 1999).