Olowoyo et al. 2026 — Global patterns of heavy metals in drinking water (systematic review)
This systematic review synthesizes findings from studies published 2015–2024 examining heavy metal concentrations in municipal tap water, groundwater, surface water, and bottled/sachet water across geographical regions. The review documents persistent contamination across all four water types, with groundwater generally showing the highest concentrations of several metals due to geological and anthropogenic inputs. The finding that bottled and sachet water — commonly perceived as safer — also showed Pb, Cd, and Cr contamination is particularly relevant from a food-safety perspective given that water is a direct dietary exposure route and an ingredient in processed foods.
Key numbers
All values reported as ranges synthesised from the reviewed literature, not from a single study.
Lead (Pb):
- Tap water: ND to 260.0 µg/L (ND = non-detect; WHO guideline: 10 µg/L)
- Surface water: ND to 0.259 mg/L (259 µg/L)
- Groundwater: ND to 0.791 mg/L (791 µg/L); highest values from Ghana
- Bottled/sachet water: ND to 123.15 µg/L
Arsenic (As):
- Range across all types: ND to 692 µg/L; highest values from groundwater
- WHO guideline: 10 µg/L; US FDA standard: 10 µg/L; some jurisdictions use 50 µg/L
- Exceeds guidelines in multiple studies, especially groundwater in Asia and Africa
Cadmium (Cd):
- WHO limit: 3 µg/L
- Detected range: 0.001 to 2,230 µg/L; highest in groundwater from Ghana (2.23 mg/L)
- Bottled water: 0.15 to 5.51 µg/L in some studies (slightly above WHO guideline in some instances)
Nickel (Ni):
- WHO guideline: 70 µg/L
- Tap water maximum: 1.021 ± 0.1 mg/L (Nigeria)
- Groundwater maximum: 0.953 ± 0.07 mg/L (Nigeria)
- Surface water: 2.33 µg/L (median); up to WHO-exceeding values in some regions
Iron (Fe):
- Tap water: up to 1,346.91 µg/L; some studies above 300 µg/L limit
- Groundwater: up to 20,843.2 µg/L (South Africa)
Zinc (Zn):
- Wide range; groundwater South Africa: up to 19,964.5 µg/L
Mercury (Hg):
- Groundwater: ND to ~0.026 mg/L tap water; ND to ~126.0 µg/L surface water
Copper (Cu):
- Groundwater: up to 996.6 µg/L
Methods (brief)
Systematic review; search strategy and inclusion/exclusion criteria not fully described in the text extraction. Studies from 2015–2024 included. Data synthesised by water type (tap, surface, groundwater, bottled/sachet) and geographic region. No pooled meta-analysis; narrative synthesis with tabulated ranges. Review limitations: wide heterogeneity across studies, varying analytical methods, different WHO and national guideline reference points applied by original studies.
Implications
Certification: Drinking water is both a direct exposure route and an ingredient input for processed foods (reconstituted beverages, infant formula made with tap water, canned goods). The wide ranges for Pb (up to 260 µg/L in tap water) and As (up to 692 µg/L in groundwater) confirm that water source quality is a meaningful variable in cumulative exposure assessment. HMT&C product standards calibrated to food matrices alone will underestimate total exposure if the water used in preparation is contaminated.
Courses: Provides a comprehensive global picture of water-borne heavy metal exposure that complements food-matrix occurrence data; useful for total dietary exposure calculations and for explaining why the wiki cannot treat food as the only exposure pathway.
App: The app’s exposure modelling should flag water as a concurrent exposure pathway, especially for infant formula (which is reconstituted with water). The review’s finding that bottled water is not reliably clean is a useful caveat for “safer water” recommendations.