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

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 NO₃^- >>HCO₃^- > SO₄^2- > Cl^- > PO₄^3- > 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 SO₄^2- to 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 NO₃^- was essentially from biomass burning. Light (< 100 mm) pre-monsoon and post-monsoon convective rains were enriched in major ions than the heavy (> 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.