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Rainwater is no more ‘pure’ or ‘safe’

Nandkumar M Kamat

Decades ago, chemical oceanographers at National Institute of Oceanography (NIO) had analysed the rainwater falling in Goa for various dissolved salts, heavy metals, and had detected traces of sea salt like sodium, potassium, calcium and magnesium and elements like lead. A 2017 paper by Ramaswamy and colleagues had reported deposition of various ions from rainwater. They found per every square metre of surface per year we get eight grams of sodium, half gram ammonium, a gram of magnesium, five grams of calcium, 15 grams of chloride ions, two grams of nitrates, five grams of sulphates from the sea and three grams of sulphate ions from other sources.

At Goa University we had sampled rainwater falling directly from clouds for several years in clean, sterile plastic containers and our results had clearly indicated that it cannot be considered pure anymore. Not only is it mildly to moderately acidic but its contents represent everything that floats in the stratosphere and troposphere.

Since biometeorology is not a much developed field in India no attention has been paid to the biological and microbiological dimension of the rainwater. But it is becoming clear now that the growth of various types of algal biofilms, rock loving and tree loving lichens etc may depend on dispersal of their propagules through rainfall. Dimmick et al (1979), Fuzzi et al (1997) and Sattler et al (2001) suggested that cloud droplets may provide a medium in which airborne cells can divide. Because bacteria require water for their metabolism, they are supposed to be good cloud condensation nuclei. In a changing climate, one can furthermore suppose that even more bacteria find viable conditions in the atmosphere and, thus, may become more abundant in clouds and may be transported more widely. According to Brent (2012) microbes and their metabolic activities could affect meteorological processes in the atmosphere both by changing cloud chemistry.

Chemically speaking rainwater falling in Goa doesn’t only contain various sea salts but it is teeming with carbon soot particles, volcanic ejecta, sulphide particles and absolutely pristine aeolian microcrystalline silica particles. The silica particles may be mixed in the rainclouds by powerful winds. It is known that during the summer the dust storms launch millions of tons of sand from west Asian deserts and dry regions towards Arabian Sea and India’s west coast. It is possible that these particles still floating in the turbulent layer of the troposphere come down with the rainfall. The real microbiological dimension of clouds bringing south west monsoon rainfall is not possible without precision sampling and metagenomic analysis. But our preliminary results indicate that rainwater falling in Goa has plenty of BINs (Biological Ice Nuclei). Biometeorologists believe that BINs help in formation of clouds by acting as nucleating centres. If a drop of rainwater directly and freshly collected is observed under the microscope we are actually supposed to find a clear field under high magnification. But our experience showed that fresh samples collected from different locations in Goa show a definite presence of microbial cells comprising – bacteria, cyanobacteria, algae, yeast, fungal hyphae and spores. The most interesting forms are oval coccoid bacteria and rod shaped motile bacteria. We were surprised to find scattered yeast cells in rainwater samples. Some of these could grow in pure culture and showed some unusual properties. One of these was identified at a molecular level and found to belong to Candida species. These results were presented at an international conference at Benares Hindu University in February 2019 by my Phd research student Sheela Pal in the paper ‘Yeast in Southwest monsoon rainwater: Some ecological and biometeorological implications’.

Detection of pathogenic Candida yeast in rainwater in Goa indicates that there is hitherto ill understood dimension of threats to public health from the rainfall. We are still completely ignorant about the diversity of viruses suspended in the rainwater but only meticulous and ultrasensitive analysis would be able to show the presence of viral nucleic acids.

It is common ethnic knowledge that getting soaked in rainwater is not good for health as it causes various respiratory illnesses. There is traditional wisdom of taking a warm water bath after one gets soaked in the rain because at higher temperature the microbes in rainwater are deactivated. It is difficult to estimate the nature, diversity and composition of chemicals from natural and anthropogenic sources dissolved in rainwater and falling all over the state. It is impossible to estimate the diversity and total biomass of the microbes coming down with the rains. Finally, the dissolved chemicals and the microorganisms enter the terrestrial ecosystems and end up in the local habitats. A major contribution of rainwater in Goa is rapid colonisation of the exposed surfaces of various man-made structures by cyanobacterial films and various types of fungi. These are major agents of biodeterioration in monsoon-washed tropical countries.

In terms of public health now it must be clear that people must protect themselves from getting soaked in the harmful rainwater and avoid getting it in their mouth and nostrils. The results of metagenomic analysis of rainwater would show us the magnitude of threat which we face from exposure to south west monsoon rainfall. We need long time series analytical data from different locations to build a complete picture of the chemical and microbiological cocktail falling in Goa during the monsoon.

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