Composition of rain water at a semi-arid rural site in India

Rain water samples were collected at Gopalpura during the monsoon of 1996. Gopalpura situated 52 km SE of Agra city, is exclusively a rural area with no industries. There are no major cities or towns within a radial distance of 35 km. The cations (Ca, Mg, Na, K and NH4) and anions (F, Cl, NO3, SO4, acetate and formate) along with pH and conductance were measured. The percentage contribution of soil components (Ca, Mg, Na and K) and NH4 are observed to be higher than the acidic substances. In the rainwater samples sea salt fraction was found to be small, indicating the incorporation of non-marine constituents. The ratios of Mg/Ca, NO3/Ca and F/Ca in precipitation indicates that 50–75 percentile of rain water correspond to respective ratios in the local soil. Significant correlation of NO3 with NH4, Ca and Mg (0.56, 0.52 and 0.62) and of SO4 with Ca and Mg (0.65 and 0.66) and neutralization factor for NH4, Ca and Mg suggests that these are the neutralising constituents. Hence, it has been hypothesized that neutralization by NH4 is an incloud process, while that by Ca and Mg is a below cloud process. This is confirmed by results obtained from samples collected sequentially during a rain event. The contribution of terrestrial sources are larger, while the anthropogenic and sea salt fractions are negligible. Hence, the ionic concentration may be used as a reference level to compare data from other industrial and urban areas.