Occurrence of molybdenum in British surface water and groundwater: Distributions, controls and implications for water supply

Data for Mo in British surface water and groundwater collated from a number of databases show that concentrations are in most cases low, of the order of 2 μg/L or less. However, variability is large and sporadic high values are found in both streamwaters and groundwaters. Data for some 11,600 British streamwater samples indicate a 10–90th percentile range of 0.08–2.44 μg/L with a median of 0.57 μg/L and maximum observation of 230 μg/L. High values tend to be from streams on clay-rich formations and sulphide-mineralised bedrock and streams affected by localised urban and industrial contamination. Monitored lowland river waters also typically have median concentrations <1 μg/L although higher values are present in a number of urban/industrialised English rivers. Highest observed concentrations (median 20 μg/L), occur under low-flow conditions in a river system from an urban, industrial area of NE England and are likely the products of industrial contamination and mine drainage. Concentrations in 96 upland lakewater samples from NW England are universally 0.1 μg/L or less. Data for 1735 groundwater samples from across Britain have a 10–90th percentile range for Mo of 0.035–1.80 μg/L with a median of 0.20 μg/L and maximum observation of 89 μg/L. Relatively high values derive from some Lower Cretaceous greensand, Carboniferous limestone and mudstone (Coal Measures) aquifers, particularly under anaerobic conditions. Release from Fe oxides and possibly Mn oxides by reductive dissolution is a likely mechanism for the concentrations. Under more rarely observed sulphate-reducing conditions in British aquifers, concentrations of Mo diminish, likely due to sequestration by precipitating sulphide minerals. The observed ranges in aqueous samples indicate that most water sources in Britain have Mo concentrations more than an order of magnitude below the WHO health-based value for Mo in drinking water (70 μg/L). Such sources are unlikely to pose a risk to drinking water from the current WHO guidance, provided those impacted locally by mining or other industrial contamination are avoided.