Evaluation of environmental pollution effects on domestic roof-harvested rainwater in Southern part of Nigeria using impact indices

The effects of environmental factors, roof materials and age of roof on some metals usually found in Domestic Roof-Harvested Rainwater (DRHRW) from southern Nigeria were investigated. Samples were collected in Ibadan (residential), Lagos (industrial) and Port-Harcourt (gas-flaring). Four roof types (corrugated-iron-sheet, long-span-aluminium, asbestos and step-tiles) and three ages of roof (<5, 5–10 and >10 years in service conditions) were considered. Sample preparations, handling and analysis were carried out using standard procedures. Enrichment-Factor, Transfer-Factor and Pollution-Load-Index were estimated to identify roof polluting severity, rate of transfer of each metal detected into the DRHRW as well as atmospheric contribution to the overall pollution effects observed. Metal enrichment was generally low indicating low metal transfer into DRHRW from the sites. Occasional moderate enrichment was recorded for Fe, Cu, Zn, Ca and Mg suggesting their natural presence in the environment. Extremely high enrichment observed for Pb and Cd in industrial and gas-flaring rainwater samples as compared with residential indicated that metal contaminations in these locations are higher compared to residential samples which serve as background. Rate of metal transfer from roof material to rainwater was generally low. Thus, the possible source of the metal contaminants found in DRHRW in the areas investigated was the atmosphere. Mean Pollution-Load-Index of 0.95, 0.99 and 1.06 for rainwater samples in Ibadan, Lagos and Port-Harcourt areas respectively suggested higher pollution load in gas-flaring than residential and industrial regions. Pollution-Load-Index applied to age and roof-type indicated that long-span-aluminium and asbestos roofs that are less than 5 years could contaminate rainwater.