In our previous post, we explored the anthropogenic causes of water pollution at the 2016 Rio Olympic Games and highlighted the pressing need to control these point sources. In this post, however, we will analyse Rio de Janeiro’s water pollution crisis using a different lens — residence time. Through investigating the persistence of pollutants in Rio de Janeiro’s Guanabara Bay, we can gain insight on the physical factors that contributed to the crisis.
You might recall from our second post that residence time refers to the duration that pollutants stay in a medium before being removed. Indeed, in the context of water pollution, residence time is calculated by dividing the amount of pollutant in the water body by the inflow or outflow rate of the pollutant. This means that we not only have to consider the total amount of pollutants, but also the duration needed to flush out pollutants by renewing water bodies.
We can thus deduce that pollutants found in Rio de Janeiro’s untreated waste have a long residence time, as waste is continuously accumulated in water bodies while water renewal rates remain low. As outlined by Fistarol et al. (2015), it takes approximately 11 days to renew 50% of water in Guanabara Bay, meaning that it requires at least 22 days for pollutants to be completely flushed out. While this residence time appears short considering that a massive 25 cubic metres of waste is discharged every second (Cotovicz Jr et al., 2016), it remains problematic as pollutants cannot be removed in time for consecutively-held Olympic water sports events. This is especially so for sailing and open-water swimming events, which often take place in Guanabara Bay and require athletes to cover long distances in the water (Keith, 2016). Furthermore, water renewal rates vary across Guanabara Bay, with these rates being the lowest in inner regions where tidal circulation is low (Fistarol et al., 2015). This suggests that pollutant residence time is particularly long in these regions, as pollutants are seldom flushed out.
Given long residence time of pollutants at Guanabara Bay, Olympic sailors and open-water swimmers are highly susceptible to gastrointestinal illnesses (Wharton, 2014)
In analysing residence time, it is also crucial to consider the breakdown of pollutants, as different pollutants have different residence times. One of the most persistent pollutants in Guanabara Bay is methane, which is released from untreated sewage and degrades water quality by causing acidification (Cotovicz Jr et al., 2016). According to Junge (1974), methane has a residence time of 4 years, suggesting that even after pollutants are flushed out through water renewal every 22 days, traces of methane still exist. This is supported by Cotovicz Jr et al. (2016)’s findings that methane concentrations in central Guanabara Bay are perpetually high, not only because of low tidal circulation but also the degasification of dissolved methane into the atmosphere. Thus, this reinforces that pollutant types also influence the severity of Rio’s water pollution crisis, as they affect residence time and the extent of water quality degradation.
The release of long-lasting pollutants from untreated waste, such as methane, has exacerbated water pollution in Rio de Janeiro (Davis and Manfred, 2015)
In hindsight, while Rio’s water pollution crisis stemmed from poor waste management policy, it was made worse by long pollutant residence times. While there is little that water governing bodies can do to reduce pollutant residence times, which depend on uncontrollable factors like tidal circulation, this awareness can hopefully galvanise them into ramping up waste management efforts. Only then can the extent of long-term waste accumulation be minimised, and Olympic athletes safely compete in the waters.
References
Cotovicz Jr, L. C., Knoppers, B. A., Brandini, N., Poirier, D., Costa Santos, S. J., & Abril, G. (2016). Spatio‐temporal variability of methane (CH4) concentrations and diffusive fluxes from a tropical coastal embayment surrounded by a large urban area (Guanabara Bay, Rio de Janeiro, Brazil). Limnology and Oceanography, 61(S1), S238-S252. https://doi.org/10.1002/lno.10298
Davis, S. & Manfred, T. (2015). A 2016 Rio Olympics waterway has levels of viruses akin to raw sewage — here’s what it looks like [Online image]. Business Insider. https://www.businessinsider.com/rio-olympics-water-pollution-sewage-photos-2015-7
Fistarol, G. O., Coutinho, F. H., Moreira, A. P. B., Venas, T., Cánovas, A., de Paula Jr, S. E., … & Thompson, F. L. (2015). Environmental and sanitary conditions of Guanabara Bay, Rio de Janeiro. Frontiers in microbiology, 6, 1232. https://doi.org/10.3389/fmicb.2015.01232
Junge, C. E. (1974). Residence time and variability of tropospheric trace gases. Tellus, 26(4), 477-488. https://doi.org/10.1111/j.2153-3490.1974.tb01625.x
Keith, E. F. (2016). Treating Rio’s Wastewater Beyond the Olympics. Natural Resources & Environment, 31(4), 48-50. http://www.jstor.org/stable/44213918
Wharton, D. (2014). Sailors test polluted waters at 2016 Rio de Janeiro Olympic site [Online image]. Los Angeles Times. https://www.latimes.com/sports/sportsnow/la-sp-sn-sailors-2016-rio-de-janeiro-olympics-20140811-story.html