Risk assessment and source apportionment of wet bulk deposition in three typical sites of Gampaha District, Sri Lanka

Scavenging of pollutants emitted to the atmosphere from natural resources and anthropogenic activities changes the chemical composition of rainwater. However, determination of the extent of atmospheric pollution and identification of pollution sources have not been regularly carried out in Sri Lanka. The objective of this research is therefore to analyze bulk deposition samples collected during a 7-month period from 10 June 2017 to 6 January 2018 in three different locations in Sri Lanka, Katunayake, Gampaha and Lunugama, followed by analysis of rainwater quality parameters including anions and cations using standard methods. These locations differ from each other in geography, industrialization and urbanization. Acid rain was not encountered in any of the three locations during the sampling period according to the pH measurements. The volume-weighted mean ion concentration in the three locations determined by ion chromatography varied as K+ < NH4+ < Mg2+ < Ca2+ < Na+. The dominant cations were Ca2+ and Na+, while Cl− and SO42− were identified as the dominant anions in all locations. The ratios between Na+ and the ions Cl−, SO42−, Ca2+, K+ and Mg2+, calculated using the enrichment factor, indicate that some ions from marine and anthropogenic activities contribute to the rainwater. The Katunayake sampling location showed the highest Pearson correlation between Na+ and Cl− because of the sea breeze, while Ca2+ and Mg2+ showed the highest Pearson correlation in the Gampaha and Lunugama locations because of soil dust. Although rainwater is not acidic according to pH measurements, it can be neutralized by calcium salts. This masking effect should thus be considered in risk assessment strategies. Continuous monitoring of rainwater quality is necessary to reach sound conclusions and make predictions about atmospheric quality.