Origin of major ions in monthly rainfall events at the Bamenda Highlands, North West Cameroon

摘要

Rainwater characteristics can reveal emissions from various anthropogenic and natural sources into the atmosphere. The physico-chemical characteristics of 44 monthly rainfall events(collected between January and December 2012) from 4 weather stations(Bamenda, Ndop plain, Ndawara and Kumbo) in the Bamenda Highlands(BH) were investigated. The purpose was to determine the sources of chemical species, their seasonal inputs and suitability of the rainwater for drinking. The mean pH of 5 indicated the slightly acidic nature of the rainwater. Average total dissolved solids(TDS) were low(6.7 mg/L), characteristic of unpolluted atmospheric moisture/air. Major ion concentrations(mg/L) were low and in the order K+> Ca2+> Mg2+> Na+for cations and NO-3≥ HCO-3> SO2-4> Cl-> PO3-4> F-for anions. The average rainwater in the area was mixed Ca-Mg-SO4-Cl water type. The Cl-/Na+ratio(1.04) was comparable to that of seawater(1.16), an indication that Na+and Cl-originated mainly from marine(Atlantic Ocean) aerosols. High enrichments of Ca2+, Mg2+and SO2-4to Na+ratios relative to seawater ratios(constituting 44% of the total ions) demonstrated their terrigenous origin, mainly from Saharan and Sahelian arid dusts. The K+/Na+ratio(2.24), which was similar to tropical vegetation ash(2.38), and NO-3was essentially from biomass burning. Light( 100 mm) monsoon rains, indicating a high contribution of major ions during the low convective showers. Despite the acidic nature, the TDS and major ion concentrations classified the rainwater as potable based on the WHO guidelines.