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Molybdenum distributions and variability in drinking water from England and Wales

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This survey measured molybdenum in treated water from 12 public supply works across England and Wales over 18 months and in morning and flushed samples from 24 household taps. Every result was below 2 µg/L, less than a tenth of the World Health Organization health-based value of 70 µg/L. Household plumbing had a negligible effect on molybdenum, in contrast to copper, zinc, nickel, lead and cadmium.

Key numbers

  • Highest treated-water concentration: 1.51 µg/L, Homesford mine-drainage groundwater source (two rounds) (Table 2, p. 6409).
  • Other treated groundwater sources: Great Wratting 0.915 to 1.06 µg/L; Candlesby 0.823 to 0.914 µg/L; Catterick 0.14 to 0.161 µg/L; Brockhill below 0.03 µg/L in all rounds (Table 2, p. 6409).
  • River sources: Bray 0.587 to 1.05 µg/L; Mythe 0.613 to 0.805 µg/L; Little Eaton 0.303 to 0.778 µg/L; Acomb Landing 0.126 to 0.364 µg/L. Upland reservoirs Loxley and Mynydd Llandegai below detection; Ogston 0.120 to 0.937 µg/L (Table 2, pp. 6409 to 6410).
  • Tap water (n = 24 taps, pre-flush and post-flush): Haverhill 0.911 to 1.00 µg/L; Mickleover 0.522 to 0.557 µg/L; Bangor all below 0.03 µg/L (Table 3, pp. 6411 to 6412).
  • No significant difference (p > 0.05) in molybdenum between old and new houses, between pre-flush and post-flush samples, or between taps and source water, except a 0.03 µg/L source-to-tap difference at Mickleover (Results, p. 6408).
  • In the same taps, one pre-flush sample had nickel at 26 µg/L and two had lead at 199 and 11.2 µg/L, above national limits; all post-flush samples complied (pp. 6411 to 6412).
  • Detection limit 0.03 µg/L (0.06 µg/L for two diluted samples); reference water SLRS-4 measured 0.19 ± 0.01 µg/L vs certified 0.21 ± 0.02 µg/L (p. 6407).
  • Health context cited by the authors: adult daily requirement estimated at 75 to 250 µg; WHO health-based value 70 µg/L, no longer a formal guideline value (p. 6404; secondary).
  • Compiled British data (Table 1, p. 6405; secondary): British groundwater median 0.20 µg/L (range below 0.02 to 89.2, n = 1,735); small headwater streams in England and Wales median 0.57 µg/L (range below 0.05 to 230, n = 11,562); River Aire median 20.7 µg/L (range 0.32 to 70.3).

Methods (brief)

Purposive sampling targeted sources thought most likely to have high molybdenum (mineralized catchments, industrial rivers, selected aquifers). Samples were acidified, heated at 80 °C and analyzed by ICP-MS (Perkin-Elmer DRCII), with blanks, duplicates and a certified river-water reference material. Linear models tested differences between sites, rounds, houses and flush states.

Evidence fitness

Strong, directly measured drinking-water data for England and Wales with documented quality control. Sampling was purposive toward higher-risk sources, so national averages may be lower. Supports statements on tap-water levels, pipework effects and comparison with the WHO health-based value. The study was commissioned and funded by Defra through the Drinking Water Inspectorate.

Limitations

Twelve supply works and three towns only; taps were chosen within a single street per age class, not at random. No food or total-diet data. Literature values in Table 1 are compiled from other studies.

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