Molybdenum in British drinking water

Variation in concentrations of molybdenum in British groundwater (data from BGS groundwater database, including groundwater chemistry studies in the Baseline England & Wales and Baseline Scotland series).

BGS, in collaboration with the Centre for Ecology and Hydrology (CEH), has carried out an investigation of the occurrence of molybdenum in British drinking water. Our reconnaissance investigations showed that molybdenum concentrations were less than 2 μg/L in all analysed samples and are unlikely to pose a problem for water supply in Britain.

Drinking water in Britain is taken from both groundwater and surface water sources. Our study involved assessing the concentrations and distributions of molybdenum in these sources as well as in treated water used for public drinking water supply.

Assessments have also been made of the occurrence of molybdenum in sediments, rocks and soils.

Is molybdenum harmful to health?

Molybdenum is considered an essential trace element for human health, but as for many elements, exposure to high doses can be detrimental.

In 1993, the WHO introduced a health-based guideline value for molybdenum in drinking water of 70 µg/L.

The WHO retains a health-based value of 70 µg/L in the 2011 (fourth edition) guidelines but no longer lists a formal guideline value for molybdenum on the grounds that concentrations in drinking water are usually much below this value.

Molybdenum in the rocks, minerals and soils of Britain

BGS datasets indicate that concentrations of molybdenum in rocks, sediments and soils in the UK are typically less than 10 mg/kg.

Concentrations greater than 20 mg/kg can be found in some fine-grained sediments, especially those containing sulphide minerals. Relatively high concentrations are also found in some ironstones and granites. Concentrations of molybdenum in 65 000 stream sediments from England and Wales (G-BASE data) range up to 309 mg/kg, but with a 90th percentile value of just 2.9 mg/kg.

Monitoring for molybdenum

A water supply reservoir in northern England.

Samples of treated drinking water were taken from 12 public supply sources across England and Wales, monitored over an 18 month period. For all sources sampled, observed concentrations were less than 2 µg/L. For three of the 12 sources, concentrations of molybdenum in all samples were <0.03 µg/L.

Concentrations of molybdenum in untreated surface waters and groundwaters in Britain are also usually very low:

analyses of some 11 500 streamwater samples from the G-BASE dataset have a range of <0.05–230 µg/L, although the 10–90th percentile range is much narrower, 0.08–2.45 µg/L, with a median value of just 0.57 µg/L
analyses of water samples taken from rivers as part of the Land Ocean Interaction Study (LOIS) typically have median concentrations <1 µg/L, although higher concentrations are observed in some urban rivers, with median values of the order of 3–10 µg/L. Molybdenum concentrations under low-flow conditions in some urban rivers are relatively high (10s of µg/L). Potential urban sources of molybdenum in river water include mine drainage and industrial effluents
concentrations in British groundwater samples have been found in the range <0.1–120 µg/L but with a 10–90th percentile range of 0.1–1.4 µg/L and a median of just 0.10 µg/L (1700 samples). Relatively high concentrations characterise mine-drainage waters (0.60–6.0 µg/L, median 1.4 µg/L)

Molybdenum unlikely to pose a problem

The ranges of molybdenum observed in groundwaters and surface waters and treated drinking water indicate that molybdenum concentrations in Britain rarely occur in excess of, or approaching, the WHO health-based value of 70 µg/L.

Results suggest that molybdenum is unlikely to pose a significant problem for the British water-supply industry and support the WHO conclusion that molybdenum in drinking water does not approach the concentrations likely to be detrimental to health.

The initial funding for the project was provided by Defra (2006–2008), with ongoing studies funded by NERC.