Rainwater chemistry of a developing urban-industrial metropolis in Southeast Nigeria

Abstract

Rainwater (RW) over the Calabar metropolis was analyzed to establish its chemistry, influences, and agricultural suitability. Sampling was done in the rainy and dry seasonal cycles for three years (2018–2020). pH, electrical conductivity (EC), and major ions (Ca2+, Na+, Mg2+, K+, HCO3-, Cl-, and SO42-) were registered and the principal component analysis (PCA) and ionic ratios were used to establish relationships between ionic species and project their sources. RW suitability for agriculture was determined by calculating the sodium and magnesium absorption ratios. Results indicated a relative abundance trend of cations towards Ca2+ > Mg2+ > K+ > Na+ while HCO3- > SO42- > Cl- was the trend for anions. pH varied from 6.1 to 7.8 (mean = 6.60). This and the EC reflected influences from atmospheric gases and in-cloud dissolved solids. The volume-weighted mean (VWM) of ions (Ca2+, Mg2+, K+, Na+, HCO3-, SO42-, and Cl-) was 246 eq/l in the rainy season and 198 eq/l in the dry season indicating low-moderate atmospheric pollution. Wet deposition (WD) fluxes for total ionic contents in RW were higher in the rainy season indicating the impact of rainfall. The PCA and the ionic ratios showed that the ionic concentrations were of crustal and marine origins predominantly. RW chemical characteristics in the study area compared with the concentration ranges of other local and global locations with similar geologic settings and low to moderate pollution indices showed close similarity. The RW assessment for agricultural use based on sodium and magnesium absorption ratios indicated good levels of suitability.