Chemical composition of major ions in rainwater

Abstract

The present study investigated the chemical composition of rainwater at Kabir nagar, Nari, Nagpur, Maharashtra, India. The rainwater samples were collected on event basis during June-July-August-2006 and were analyzed for pH, major anions Cl, NO(3), SO(4)) and cations (Ca, Mg, Na, K, NH4). The pH value varied from 6.0 to 7.3 (avg. 6.3 +/- 0.3) indicating alkaline nature of rainwater. The pH of the rainwater was found well above the reference pH (5.6), showing alkalinity during the monsoon season. The average and standard deviation of ionic composition was found to be 98.1 +/- 10.6 micro eql(-1). The total anions contribute 45.1% and cations 54.9%, respectively to rainwater. Neutralization factors (NF) followed a sequence of NF(Ca) > NF(Mg) > NF(NH4) with factors of 1.1, 0.38 and 0.15 indicating the crustal components are responsible for neutralization of anions. The average ratio of (NO(3) + Cl)/SO(4) observed as 1.1 indicates that nitric and hydrochloric acid influences the acidity of rainwater. The ratio of NH(4)/NO(3) and NH(4)/SO(4) was observed as 0.68 and 0.34 indicate that the possible compounds which may predominate in the atmosphere are NH(4)NO(3) and (NH(4))(2)SO(4). Ionic correlation was established to identify sources of origin. A good correlation was seen between Ca and Mg (r = 0.95); suggesting the common occurrence of these ions from crustal origin. Similarly, the acidic ions SO(4) and NO(3) correlated well (r = 0.60) indicating their origin from similar sources. Other relatively significant correlations were observed between Ca and SO4 (r = 0.92), Mg and SO(4) (r = 0.83), Ca and NO(3) (r = 0.09), Ca and Cl (r = 0.34) and Mg and Cl (r = 0.31), and Mg and NO(3) (r = 0.71). The observed rainwater ratio of Cl/Na (1.1) is closer to that of seawater ratio (1.16) indicates fractionation of sea-salt and modifications by non-marine constituents as the site is 834 km away from the sea coast. The nss-Ca contribution was observed as 95.7% suggesting their crustal origin whereas nss-Mg and nss-K shows their contribution as 87.9% and 83.2% indicating influence of soil sources. The nss-SO(4) contributed as 87.4% shows anthropogenic origin.