This bird’s eye view shows the severely contaminated Adyar river flowing into the Bay of Bengal after cutting through Chennai’s Marina beach. Less than half a kilometre on either side, the river mouth is flanked by urban settlements.
| Photo Credit: Google Earth

Through history, human civilisations almost always erupted near bountiful rivers. The waters enriched the soil, hosted lush ecosystems, and fed and washed the people. This affinity for rivers is less true today but the relationship has also become twisted. In many places around the world, including in India, cities and factories have zombified rivers into sewers.

Now, scientists in California have found that the pollutants rivers drain into seas can in fact spray into the air, become aerosolised, and drift back overland, where people can inhale them. Their findings were published in Science Advances on May 28.

In their study, the scientists tracked the water in the Tijuana river flowing into the Pacific Ocean near the U.S.-Mexico border. From January to March 2020, they sampled five places along 35 km of the coast: every day they scooped seawater and, for almost 24 hours, drew air through quartz-fibre filters to catch aerosol particles. They also collected more samples during rain events that sent extra filth down the river.

Then the team used high-resolution liquid-chromatography mass-spectrometry to look for 12 human-made compounds that commonly appear in wastewater. The list covered a sunscreen ingredient called octinoxate, a tire rubber additive called dibenzylamine, prescription drugs, agricultural biocides, and several illicit drugs including methamphetamine. As a proxy for untreated sewage, they tracked benzoylecgonine, a cocaine metabolite that passes unchanged through the human body and persists in seawater.

They found 10 of the 12 compounds were more concentrated in the river water than in non-river water, while aerosols collected at the two spots closest to the river were the dirtiest. The octinoxate, methamphetamine, and dibenzylamine concentrations were closely correlated with that of benzoylecgonine, meaning they were likely from the same source.

Using the measured seawater values and a standard model of water spray, the researchers estimated that 1 km of beach coastline could release 1 kg of octinoxate, up to 100 g of methamphetamine, and several grams of tire additives into the on-shore breeze. Worldwide, they estimated polluted coasts could release roughly 40,000 tonnes of octinoxate and 50 tonnes of dibenzylamine overland air every year.

While a person may inhale less-than-morbid amounts of these substances every hour near the river mouth, the chronic effects of breathing a mix of sunscreen, stimulants, and pesticides — as would be the case for the fishers and marginalised communities living along the coasts — are unknown.