Spatio-temporal variations of quality of rainwater and stormwater and treatment of stormwater runoff using sand–gravel filters: case study of Delhi, India

Abstract

The present study was undertaken to characterise rainwater and stormwater runoff, along with reusability potential of treated stormwater runoff in Delhi, India. The analysis of rainwater and stormwater runoff samples revealed that the acidic ions (SO42− and NO3−) in rainfall are being neutralised by alkaline species resulting in zero event of acidic rain (pH < 5.6) over Delhi. The higher level of alkaline species (Ca2+, NH4+, HCO3−, etc.) in stormwater runoff may be attributed to the alluvial nature of soil and suspended dust which is chiefly being of crustal origin in Delhi. Non-sea-salt fraction (NSSF) indicated no marine contribution towards rainwater quality. Neutralization factor (NF) validated the dominance of alkaline species (NH4+, HCO3−, K+, Ca2+, and Mg2+) over the acidic species (SO42−, NO3−, Cl− etc.), thereby resulting in near neutral pH of rainwater. Physico-chemical analysis of stormwater runoff from different locations in Delhi revealed relatively higher concentration of all the dissolved species (Ca2+, Mg2+, HCO3−, PO43−, NO3−, Cl−) along with higher level of suspended impurities and the spatial variations were observed to be regulated by land use landcover pattern. The treatment of stormwater runoff by sand–gravel filter resulted in complete removal of suspended impurities and partial removal of hardness. This study proposes additional use of a constructed wetland to remove the organic impurities, PO43−, and NO3−. Furthermore, the study concludes that the stormwater runoff in Delhi can easily be treated to the desired level of drinking water supply with additional provision of disinfection to narrow down the demand–supply gap for drinking water.