Rainwater harvested from rooftops in two Nigerian communities affected by oil industry activity carries metals at levels that pose only low non-carcinogenic health risks, according to a new study published in BMC Chemistry. Researchers led by Chideraa Courage Offor and John Kanayochukwu Nduka examined rooftop-harvested rainwater from Chokocho and Diobu in Rivers State, tracking how metal concentrations changed with the rainfall season and after six months of storage, and applying statistical techniques to trace where the metals came from.
Rainwater harvesting is widely practiced in sub-Saharan African communities as an alternative water supply, but the chemical quality of stored rainwater can be shaped by atmospheric deposition, the roofing materials that catch the water, and the conditions under which it is kept. To capture these influences, the team collected rainwater from five different catchment surfaces: an asbestos sheet roof, an aluminium roof, a zinc roof, a stone-coated roof, and an ambient collection point. Sampling took place during early, mid, and late rainfall periods, allowing the researchers to observe how concentrations shifted as each rainy season progressed.
Fresh samples were analyzed immediately after collection, while a second portion of each sample was stored under mesophilic conditions for six months before being re-analyzed. The researchers designed this storage step to depict the condition of usage by the locals, since harvested rainwater in these communities is typically kept in containers for extended periods before drinking or other household use.
The clearest pattern emerged in Chokocho, where the study documented strong first-flush effects. Early rain samples showed elevated concentrations of sodium at 5.64 milligrams per liter and zinc at 1.10 milligrams per liter, but these values declined to less than 0.2 milligrams per liter during the mid- and late-rain periods. First flush refers to the initial runoff of a rainfall event, which tends to wash accumulated dust, debris, and dissolved materials off roof surfaces into collection vessels, producing a pulse of contaminants that diminishes as the rain continues.
Storage also changed the metal profile of the water. Stored samples from Chokocho showed lower concentrations of iron, potassium, sodium, zinc, and copper than their corresponding fresh samples, with the differences most pronounced during the early and mid-rain periods. This suggests that some metals may settle out, adsorb to container surfaces, or otherwise become less detectable in solution over the months that water sits in storage. Acidity varied across sites and conditions: fresh and stored pH ranged from 5.1 to 7.9 in Chokocho and from 6.0 to 7.8 in Diobu, spanning slightly acidic to near-neutral values.
To identify the origins of the metals, the researchers applied chemometric techniques, statistical and mathematical tools used to disentangle environmental sources from measured concentration patterns. Principal component analysis, one of these techniques, explained 86.1 percent of the variance in the dataset. Positive matrix factorization, a receptor model that apportions measured concentrations among contributing sources, identified three distinct origins for the metals: atmospheric deposition combined with first-flush wash-off, crustal dust, and weathering of the roofing materials themselves.
These three sources map directly onto the practical concerns of rainwater harvesting. Atmospheric deposition reflects what rainfall scavenges from the air as it falls, which in an oil-impacted environment may include combustion-related and industrial particulates. Crustal dust points to soil-derived particles resuspended by wind and deposited on roofs between rain events. Roofing material weathering indicates that the catchment surface itself, whether asbestos sheet, aluminium, zinc, or stone-coated metal, contributes metals through corrosion and degradation over time.
On the public health side, the team assessed non-carcinogenic risks through two exposure pathways: ingestion of the water and dermal contact during everyday use. Hazard index values, which aggregate risk estimates across metals and pathways, were all below 1 for both ingestion and dermal exposure. A hazard index below 1 is generally interpreted as indicating that anticipated exposure doses fall below thresholds at which non-carcinogenic effects would be expected, so the study concluded that overall, harvested rainwater posed low non-carcinogenic risk in these communities.
The authors emphasize that the favorable risk results do not eliminate the need for careful practice. Because the first flush of each rainfall event carries the highest metal loads, particularly early in the season, they recommend first-flush diversion, a simple arrangement that discards or bypasses the initial runoff before storage begins. Proper storage is also recommended to improve water quality, reflecting the finding that storage conditions themselves influence which metals remain in solution over time.
The study offers a detailed picture of how season, catchment surface, and storage interact to determine the chemistry of harvested rainwater in the Niger Delta. By combining direct measurement of fresh and stored samples with source apportionment through principal component analysis and positive matrix factorization, the work links specific metal sources to specific management actions. For communities relying on rooftop rainwater as a primary water supply, the findings suggest that diverting the first flush and maintaining sound storage practices can meaningfully reduce metal exposure, even in an environment shaped by oil industry activity.
Subject of Research: Metal contamination, seasonal variation, and health risks of rooftop-harvested rainwater in the oil-impacted Niger Delta, Nigeria
Article Title: Source patterns, storage-associated changes, and public health implications of metals in harvested rainwater from an oil-impacted environment, Niger Delta
Article References: Offor, C. C., & Nduka, J. K. (2026). Source patterns, storage-associated changes, and public health implications of metals in harvested rainwater from an oil-impacted environment, Niger Delta. BMC Chemistry. https://doi.org/10.1186/s13065-026-01957-0
Image Credits: AI Generated
DOI: 10.1186/s13065-026-01957-0
Keywords: rainwater harvesting, Niger Delta, heavy metals, first flush, water quality, health risk assessment, Nigeria, roofing materials, storage, chemometric analysis, Rivers State, Source
News Source: Bethany Barker. (November 4, 2026). Harvested rainwater in Nigeria’s oil-impacted Niger Delta shows low metal health risks. Scienmag.
