Rainwater chemistry at a regional representative urban site: influence of terrestrial sources on ionic composition

In the present investigation, the rainwater chemistry is studied over a one year period at a regional representative urban site, Tirupati. A continuous record of 67 events were included that ranged from 0.5 to 74.6 mm (annual total 850.2 mm), from which 35 events (52%) were chemically analyzed during the period for major inorganic ion composition viz. F, Cl, NO3, SO4, Na, K, Ca, and Mg as well as NH4. The volume-weighted mean (VWM) of the measured ionic sum is 501.28 μeq l−1 and the crustal ion contribution dominating over the acidic ions with a ratio of total cation to that of anions (Σcations/Σanions) is 1.4. Only the Ca and SO4 together made up more than 55% of the total ion, whereas Ca alone contributed about 51% of the total cation. All the rain samples analyzed showed alkaline pH always, to that of reference level 5.6, with VWM of 6.78. On the basis of good correlation between the crustal ions (Ca, Mg, K and NH4) and SO4(r=0.95) and NO3(r=0.91) as well as order of neutralization factors Ca (0.88)>Mg (0.33)>NH4 (0.11), it has been determined that the acid neutralization is brought about by crustal ions, particularly calcium, in the region. The data were subjected for correlation analysis. A good correlation observed between Ca and SO4 and NO3 (r=0.92 and 0.90) and Mg and SO4 and NO3 (r=9.83 and 0.77), respectively, reveals that the ionic composition of rainwater in the region is strongly influenced by terrestrial sources rather than anthropogenic and marine sources.