How Safe is the Water Consumed in Different Parts of Nairobi, Kenya?

Overview

This 2021 study from the University of Nairobi Department of Chemistry analysed seven anonymised brands (A–G) of commercial PET-bottled drinking water purchased from Nairobi supermarkets for seven heavy metals (Sb, Cd, Cr, Cu, Mn, Pb, Zn) by atomic absorption spectrometry and three physico-chemical parameters (pH, electrical conductivity, total dissolved solids). Sb, Cd, and Zn were below detection limit in all seven brands. Cu, Pb, Cr, and Mn were detected in subsets of brands at levels the authors compared to Kenya Bureau of Standards (KEBS), National Environment Management Authority (NEMA), and WHO drinking water limits, with Pb exceeding cited KEBS/WHO limits in two brands and Mn exceeding cited limits in one brand. The paper reports concentrations in units of “mg/g” throughout, which is physically implausible for water samples; method-section calibration standards and limits of detection are reported in mg/L–equivalent units (µg/mL), so the reported concentrations are almost certainly mg/L mis-labelled. The wiki reports the source’s verbatim numbers with this caveat documented in ## Verification notes.

Key numbers

Sample-by-sample heavy metal concentrations from Table 2 (paper labels units “mg/g”; see verification notes for the unit-discrepancy assessment):

Antimony (Sb): BDL in all seven samples (A–G).
Cadmium (Cd): BDL in all seven samples (A–G).
Zinc (Zn): BDL in all seven samples (A–G).
Copper (Cu): A 0.05 ± 0.01; B 0.02 ± 0.03; C BDL; D 0.03 ± 0.03; E BDL; F BDL; G BDL.
Lead (Pb): A 0.003 ± 0.01; B BDL; C BDL; D BDL; E BDL; F 0.32 ± 0.19; G 0.14 ± 0.07.
Chromium (Cr, total): A BDL; B BDL; C BDL; D BDL; E 0.66 ± 0.03; F BDL; G BDL.
Manganese (Mn): A BDL; B BDL; C BDL; D BDL; E BDL; F BDL; G 0.93 ± 0.53.

If the reported units are actually mg/L (the most defensible reading; see verification notes), the highest detected values correspond to:

Pb sample F: 320 ppb (± 190 ppb).
Pb sample G: 140 ppb (± 70 ppb).
Cr sample E: 660 ppb (± 30 ppb).
Mn sample G: 930 ppb (± 530 ppb).
Cu sample A: 50 ppb (± 10 ppb).

AAS method detection limits (Table 1, in µg/mL ≡ mg/L): Sb 0.0016; Cu 0.027; Cd 0.008; Cr 0.030; Zn 0.020; Pb 0.017; Mn 0.050. Calibration correlation coefficients ≥ 0.9990 for all seven analytes.

Physico-chemical parameters (Table 4, four of seven samples analysed):

pH: A 6.72 ± 0.15; B 6.63 ± 0.10; C 7.03 ± 0.23; D 7.32 ± 0.01. (Note: paper text describes sample D as below the NEMA 6.5–8.5 minimum but the tabulated value 7.32 is in range; an internal inconsistency in the discussion of pH outliers — see verification notes.)
Total dissolved solids (TDS, mg/L): A 26.80 ± 0.50; B 63.10 ± 0.33; C 100.7 ± 0.08; D 98.40 ± 0.19. All within NEMA/KEBS/WHO 1500 mg/L ceiling.
Electrical conductivity (EC, µS/cm): A 74.50 ± 0.05; B 125.70 ± 0.70; C 89.40 ± 0.93; D 197.70 ± 0.02. All within NEMA 25–2500, KEBS 30–2500, WHO 26–2500 µS/cm ranges cited by the authors.

Regulatory limits cited by the authors in Table 3 (paper units “mg/g”; the wiki does not propagate these as factual regulatory values — see verification notes for the discrepancy against actual WHO/KEBS drinking-water guideline values):

Cr — KEBS 0.05; WHO 0.06.
Mn — KEBS 0.1; WHO 0.2.
Pb — KEBS 0.05; WHO 0.04.
Cd — KEBS 0.005; WHO 0.004.
Zn — KEBS 5.0; WHO 6.0.
Sb — KEBS — ; WHO 0.006.
Cu — KEBS 0.1; WHO 2.00.

Methods (brief)

Sampling: seven brands of PET-bottled drinking water purchased from major Nairobi supermarkets in triplicate during 2019–2020, transported to University of Nairobi laboratories, digested and refrigerated at 4 °C pending analysis.

Digestion: 100 mL water samples digested with 10 mL aqua regia (HCl:HNO₃ 3:1, analar grade, Sigma-Aldrich) plus 1 mL perchloric acid (Fisher Scientific) in conical flasks on a hot plate at 65 °C for 2.5 h. Cooled, filtered through Whatman No. 1 papers into 50 mL volumetric flasks, diluted with double-distilled water, transferred to nitric-rinsed specimen bottles, refrigerated until AAS analysis.

Instrument: the paper names two distinct AAS instruments in different sections — the abstract identifies “Atomic Absorption Spectrometer (AAS) Spectra AA-10” while the Results section names “computerized AA-7000 Shimadzu Atomic Absorption spectrophotometer”. The wiki cannot resolve which instrument actually produced the reported data; flagged in verification notes. AAS hollow-cathode lamps and operating wavelengths reported as Sb 228.8 nm, Cd 228.8 (paper text actually lists 228.8 for Sb and 357.9 for the next position which corresponds to neither Cd nor a standard wavelength — see verification notes for the wavelength-attribution problem), Cr 324.7, Cu 279.5, Pb 218 nm. Single 100 mg/L standard stock diluted into calibration series; 1 mL concentrated HNO₃ matrix-matched to each standard.

Physico-chemical parameters: pH by glass-electrode pH-meter (50 mL aliquot, 25 °C water bath, two-point calibration with pH 4.0 and pH 10 buffers) — only four samples analysed for pH/EC/TDS, not all seven. EC measured on 50 mL aliquots after calibration against 0.005 M KCl (654 µmho cm⁻¹) at 25 °C. TDS computed from EC by multiplying by a conversion factor of 0.7.

Speciation: Cr reported as total chromium (no Cr(VI)/Cr(III) separation). No arsenic measured. No mercury measured.

Limitations the source itself does not acknowledge but should: (a) reporting units stated as “mg/g” for aqueous samples are physically implausible at the reported magnitudes (0.93 mg/g manganese in water would be ~0.1% solids); (b) pH/EC/TDS subset (n=4) is not aligned with the heavy-metal subset (n=7) and the four samples are not identified by the same letters in both tables (Table 4 labels A–D, Table 2 labels A–G); (c) only single-time-point composites per brand are reported, with no within-brand temporal or batch variability assessment; (d) no procedural blank or certified reference material results are reported.

Implications

Certification: The verbatim values reported by this source would, if accepted as mg/L, place lead in two of seven Nairobi-supermarket PET-bottled brands at 140–320 ppb — well above any drinking-water regulatory ceiling cited in the paper. The wiki carries the values as the source reports them and flags the multiple data-integrity concerns in verification notes.

Courses: Useful as a teaching case for evaluating peer-reviewed drinking-water occurrence papers — the layered data-integrity concerns (unit mis-labelling, dual instrument identification, regulatory-limit transcription errors) illustrate the kind of close reading that distinguishes A-tier from B-tier reporting even within the peer-reviewed corpus.

App: Not directly applicable; bottled drinking water as a category is downstream of ingredient-level contamination modelling.

Microbiome: Not addressed.

Update history

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