In 2019, Bhutan generated more than 172 metric tonnes of solid waste per day, based on a national solid waste inventory survey conducted by the National Statistics Bureau (NSB). Thimphu alone generated 51 metric tons of waste every day, including both dry and wet waste. Currently, municipal solid waste (MSW) across Bhutan is generally dumped into landfills, which are literally open dumpsites with limited engineering designs to mitigate against environmental consequences. As per the National Environment Commission (NEC) records in 2019, Bhutan has about 25 open dumpsites and a majority of them lack a leachate management system. Thus, new landfill development needs to consider the incorporation of leachate collection and subsequent treatment systems, while new landfill sites could be better designed to cope with such challenges.
Landfill leachate is generated as a result of rainwater percolation through solid waste disposed of in a landfill, as well as the moisture present in the waste and the degradation products of residues. The subsurface water flows could also contribute to leachate production when waste directly comes in contact with the land surface. Landfill leachates are highly polluted liquids containing high amounts of toxic and carcinogenic compounds. The flow of the toxic liquid into the nearest water bodies and soil is known to pose adverse effects on human health and the environment. Research conducted else in 2022 also identified landfill leachates as a significant source of emerging contaminants in Bhutan for which there are no data. Emerging contaminants are chemicals of natural or synthetic origin that have been recently discovered and are not commonly monitored, therefore the risk from our landfill leachates cannot be undercut.
Despite those significant risks, Bhutan currently has minimal field monitoring data to evaluate the seepage of the toxic leachate from the existing landfills. For example, leachate from Memelakha landfill in Thimphu is likely to discharge into the nearby Olarongchu stream based on hydrogeological mapping indicating fractures and schist layers dipping 31° toward the northeast. However, it is unclear how much leachate is present and at what rate it moves. Limited studies on the estimation of leachate generation from landfills across the country limit our understanding of better designing our landfills and their management systems. The leachate composition is also significantly influenced by the waste composition and local environmental conditions. Thus, this article suggests the first critical steps needed to be undertaken for managing landfill leachates in dumpsites across the country.
The site selection for new dumpsites needs to incorporate more scientific hydro-geological baseline studies with due consideration to landfill leachate, gas emissions, slope stability, and odour control. Many dumpsites across the country are often located on hilltops which further increases the risks of pollution as leachate flows downhill. Research conducted by Choden et al. (2020) estimated that by the end of the year 2050, the Memelakha landfill in Thimphu would accumulate more than 27.8 million metric tons of MSW, thus accumulating tons of MSW on a hillside could leach significant amounts of toxic and carcinogenic compounds with both short-term and long-term risk to public health. Many of those toxic compounds are known to cause cancer if consumed by drinking polluted water or through products grown in polluted soil (Parvin & Tareq, 2021). Locating the landfill sites upstream of a stream that flows through the city as in the case of Olarongchu should be avoided in case of construction of new landfills. Currently, there are limited investigations into potential threats to downstream water users along Olarongchu or any other water sources that could be polluted by the landfill leachate generated by Memelakha landfill.
Next, comprehensive baseline data on the chemical composition and quantity of landfill leachates needs to be established across all landfill sites. Few landfill sites which have been under operation for a long time could be targeted to start with. This is a piece of critical information for the next phase of leachate treatment systems. A strategy to manage the risk from leachate could be designed using baseline information such as the chemical contents of the leachate generated from landfills at different locations.
Lastly, the plans for the development of the leachate treatment systems are urgently needed, because the leachate once collected at the landfill sites needs to be treated accordingly before releasing into the natural environment. The treatment systems shall be determined by the various toxic elements measured in the leachates as mentioned above. However, leachate treatment also needs to consider other factors such as maximum concentration of pollutants, field capacity, moisture content of the land field, evaporation, and runoff. These are some of the critical parameters identified by Choden et al. (2020), where a team of engineers developed a mathematical model to quantify the leachate generation based on the case study at the Perkarshing landfill site (Phuntsholing). Their model was developed with the aim to contextualize the general theory of landfill design fit for the Bhutanese context.
The article is published based on personal experiences and observations by a group of water researchers from Bhutan. The group can be contacted at firstname.lastname@example.org