Spatiotemporal analysis and risk assessment of toxic metal(loid)s in harvested rainwater: Insight into storage chemistry as passive treatment processes in oil-impacted Eastern Nigeria

Rainwater is widely harvested, consumed, and stored among rural communities in sub-Saharan Africa, highlighting the need to evaluate storage systems as passive treatment units and assess the dynamics and health risks of toxic metal(loid)s (TMs) in harvested rainwater (HRW) from oil-impacted areas of Rivers State, Nigeria. A total of 112 rainwater samples were collected from different roofing materials across early, mid, and late rainfall periods between 2019 and 2020. Samples were subdivided for initial determination of Pb, Cr, As, and Cd using Atomic Absorption Spectrophotometry, and for microbial enumeration ( Staphylococcus epidermidis, Bacillus subtilis, and Pseudomonas putida ) via ten-fold serial dilution. Remaining portions were stored under controlled conditions for six months, after which repeat analyses of TMs, pH, and microbial populations were conducted to assess temporal changes. Cadmium exhibited highly variable removal, with strong attenuation (95–99%) during early rains in Eleme (2020). Chromium had high and stable removal efficiencies (∼55–100%) in 2019, but became more variable in 2020, ranging from substantial removal (∼61–100%) to notable remobilization (down to −592% in Eleme and −187% in Rumuodomaya). A significant relationship between Cr and Bacillus subtilis was observed in Eleme (F = 5.776, p = 0.002). Total cancer risk for Cr, Cd, and As in Rumuodomaya (fresh and stored HRW) was ≥ 1.0 × 10⁻⁴, indicating potential health concerns. These findings demonstrate that stored HRW can act as a low-cost passive treatment system, but also pose significant health risks, emphasizing the need for appropriate treatment before consumption in oil-impacted communities.