Extreme weather events affecting drinking water supply

On July 17, 2021, heavy rainfall devastated Zhengzhou city, China, affecting 14.5 million people, resulting in over 300 deaths, and having a direct economic loss of 114 billion Chinese Yuan. During the following four days, six of the nine total water treatment plants were interrupted, and the water supply to 1864 residential areas covering 1114 km2 in the city was shut down. On July 23, 2021, only three interrupted water treatment plants resumed operation (Case 1 in Table 1). More recently, due to climate change, extreme weather events like this are happening globally with increasing frequency. Table 1 provides some examples of different extreme weather events, including extreme temperatures, droughts, wildfires, and storms, and summarizes their significant impact on an area’s water supply. Unfortunately, these events only account for a small fraction, as more than 9135 extreme weather events have been documented worldwide over the last three decades (Fig. 1).

Table 1 Examples of extreme weather events’ impact on water supply worldwide.
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Fig. 1: Countries/regions affected by extreme weather events, mainly storms and floods, droughts, extreme temperatures, and wildfires, over the last three decades (1990–2021).
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Data source: The International Disaster Database(https://emdat.be/).

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In most cases, extreme weather events affect the drinking water supply by damaging water treatment infrastructure and distribution systems. For example, the recent cold wave in Texas interrupted water supplies for more than 120,000 people, mainly due to intake ice blockages, pipe ruptures, and distribution system failures (Case 2 in Table 1). In Puerto Rico, the island’s entire power grid was inoperable after Hurricane Maria, resulting in drinking water distribution losses across most of the island (Case 3 in Table 1)1. However, it should also be noted that in these extreme situations, even when water is delivered as usual, the quality may be impaired. For example, fecal and opportunistic pathogens were detected in the treated drinking water after being distributed to households during the recovery of Puerto Rico from Hurricane Maria. This occurred from treatment failures such as intermittent chlorination which led to low chlorine concentrations in the water distribution system2. In other cases, when water conservation measures are implemented, time in which water will remain in the distribution system increases, resulting in potential loss of residual disinfectants, increased concentration of disinfection by-products, or the development of nitrification in chloraminated systems3. As a result, there has been growing hesitation on the reliability of grid water supply and growing concerns of it as a significant source of chemical and biological contamination.

Varying with the type of extreme weather events, water quality is also affected at the water source in different ways. In the following section, we mainly discussed the impacts of events, including storms and floods, extreme heat, extreme colds, wildfires, and droughts on the water supply, covering the climate events relevant to extreme impacts as observed by Intergovernmental Panel on Climate Change (IPCC)4.

Storms and floods

Storms, including tornados, sand/dust storms, blizzards, hurricanes, heavy rainfall, and subsequent floods, mudslides, and landslides, can undermine water supplies through a variety of ways (Cases 1–7 in Table 1). Winds and heavy rainfall, which can carry vast pollutants including pathogens, particulate matter, and soluble substances, are able to increase dissolved organic carbon (DOC) and decrease dissolved oxygen (DO)5. Several studies have linked floods to groundwater contamination and enteric diseases, and floods were also reported to increase pathogenic contamination of private wells6,7,8. Moreover, heavy rainfall in urbanized catchments can cause sewer overflow events, which can undermine the quality of receiving surface waters9. Lastly, dust storms can cause an increase in the water turbidity10.

Extreme heats

Extreme heats, especially continuously elevated temperatures, can change the physicochemical properties of water, and thus affect the water quality. For example, high water temperatures caused by extreme heat can promote the decomposition of organic matter, the decrease of DO, the increase of inorganic salt concentrations, and accelerate the release of nutrients from sludge11,12. Meanwhile, changes in water temperature can also alter the abiotic components of water and the species composition of aquatic communities13. Specifically, high temperatures may increase the amount of dissolved organic matter (DOM) in soil, which may further be released into water through rainfall14. Also of concern, heat-tolerant species like cyanobacteria will prevail in sustained high temperatures. These changes elevate the risks of eutrophication and algal blooms15. For example, an increased water temperature was reported to be positively correlated with cyanobacterial abundance in Taihu Lake, China16. This lake is an important water resource for three neighboring provinces, but suffers from algal blooms (Case 8 in Table 1)17. In addition, warm conditions are more suitable for the growth of some enteric pathogens in aquatic environments, such as Vibrio cholerae O1 and acanthamoeba species18,19.

Extreme colds

When temperatures decrease suddenly, the stratification of lakes and reservoirs breaks down and causes the mixing of surface water with deeper water20. This destratification can interfere with the cycling of iron, manganese, and other nutrients, and once led to a cyanobacterial bloom in a major water reservoir in Sydney, Australia (Case 9 in Table 1)21. It should also be noted that rivers and reservoirs that have frozen over increase the chances of flooding once temperatures return to normal. In addition, deicing agents, primarily composed of sodium chloride, that melt ice and snow, can increase the salinity of both surface water and groundwater22. Once a saline water layer is formed at the bottom of a reservoir, it will affect the vertical mixing of water and thus the water quality and ecology23.

Wildfires

Wildfires can generate ash consisting of minerals and oxidized organic substances, which can vary depending on the burnt material, and increase the total nitrogen, phosphorus, organic carbon, and sediment content in the water. For example, wildfires in Australia were reported to increase catchment nutrient exports by 5-6 times (Case 10 in Table 1)24. On the other hand, since forest biomass can serve as a sink for regional urban pollutants, forest fires may in turn release arsenic, chromium, lead, chloride, and sulfate into sediment and local streamflows25. In some other cases, plastic pyrolysis generates hazardous volatile organic compounds (VOCs), which can contaminate the drinking water network, including groundwater sources (Case 11 in Table 1)26. Moreover, fire suppressant and retardant chemicals, such as Foxepan S (a mixture of hydrocarbon surfactants, glycol solvents, and water), and Silv-ex (a mixture of sodium and ammonium salts of fatty alcohol ether sulfates, higher alcohols, and water)27, can cause elevated concentrations of ammonia, phosphorus, and cyanide in recently-burnt catchments28.

Droughts

Droughts will directly cause water shortages (Cases 12 and 13 in Table 1). In addition, due to the lack of rainfall and runoff, the quality of raw water is changed because of the accumulation of sediments and the decline in self-purification capacity (dilution capacity). Rainfall following drought can also often result in sudden influx of nutrients from accumulated sediment and pollutants from other point sources29. The severe droughts in Southern California, USA have reminded local populations of the profound effects of extreme weather events on the availability of water resources, as lawn watering hours were restricted, water rates were increased, new indoor water use standards were signed, and rebates for water-efficient toilets and landscapes were provided30. In other cases, massive exploitation of groundwater because of water shortage can cause seawater intrusion and thus impact the availability of freshwater31.

In addition, climate change has been projected to alter precipitation patterns, possibly resulting in some areas becoming drier and having less water for consumers32. Overall, extreme weather events of every category have been widely affecting the quality and quantity of drinking water from the source to the tap.

Adjustments in drinking water supply to extreme weather events

Trucked or bottled water is commonly used in emergency situations. However, transportation of trucked and bottled water may be affected in extreme weather events. For example, the event - “Windy Thursday” on 18 January 2007 in the United Kingdom, caused almost 50 transport vehicles to overturn33. Moreover, large consumption of bottled water may also result in widespread plastic pollution34. Additional treatment processes, such as increasing the dosage of disinfectants are mostly implemented to address the decreased quality of source water. However, the increased dosage of chemicals may result in higher risks of byproduct formation and cannot guarantee the quality of water at POU35. Diversifying water source options, including long-distance water transfer, wastewater reclamation, groundwater extraction, and desalination, are usually used to address the scarcity of source water. These options have been effective but are potentially more vulnerable to the changing conditions during extreme weather events. For example, Melbourne, Australia responded to the severe droughts (Case 14 in Table 1) by investing in a long-distance conveyance infrastructure for imported water and a desalination plant. However, due to the Millennium Drought ending and the public’s concern for the cost and ecological effects, the system was not operated to provide any water to Melbourne after the completion of the two projects36.

Furthermore, as we depend more on centralized water supply systems that rely heavily on electricity, there is increasing concern that if these were to fail during an emergency case, it may increase the possibility of cascading failures37. For example, during the Zhengzhou heavy rainstorm, four drinking water treatment plants were shut down because of power outages (Case 1 in Table 1). In modern cities, the number of people affected by these failures can be as high millions (Cases 1–7 in Table 1). These centralized approaches could also be vulnerable to damages in the infrastructure and water distribution systems as well. Still in Zhengzhou’s case, one water treatment plant was shut down because of a sudden leak in an outlet pipe caused by a road collapse. Because of these sudden challenges faced by existing centralized water supply systems, adaption to more flexible methods to mitigate the short- or long-term impacts during extreme weather events is warranted.

Compared to centralized ones, decentralized systems that supply drinking water to small communities, households, and even individuals, are able to reduce some uncertainties, both technically and financially38,39. This is largely because decentralized systems supply water to fewer populations, and can be less dependent on major power grids and expertized labor for monitoring and maintenance. With appropriate design, decentralized systems can also be less vulnerable to extreme weather events as they tend to be closer to the end users than centralized systems. Even under circumstances of malfunction or shut down, decentralized systems will impact fewer people for shorter durations. In addition to the above-mentioned benefits, decentralized water supply systems have also shown potential advantages in terms of higher water use efficiency and water quality38. Point-of-entry (POE) and POU treatment systems are two common decentralized systems40. POE systems at large building or small community scale can ensure the quality of water supply from the distribution network, while POU systems at household or personal scale are designed to ensure the water quality at the consumption point. However, since POE systems are mostly fed by the centralized water supply, their stability is highly dependent on centralized systems.

Different from the POE systems, the POU systems that supply drinking water for individuals or families, have more diversified water sources, including not only the water from centralized systems, but also pristine water sources such as lake water, river water, rainwater, etc. In fact, POU systems have already been widely used to tackle global water supply challenges associated with extreme weather events41. Issuing boil water advisories are the most common strategy to tackle decreased disinfection efficiency of centralized water treatment systems subject to extreme weather events. Sodium hypochlorite-based disinfectants have been widely used to treat drinking water in Puerto Rico after Hurricane Maria (2017) (Case 3 in Table 1), Indonesia after tsunami (2004)42, and Bangladesh after floods (1998) (Case 4 in Table 1), when centralized systems were heavily damaged and people were more readily exposed to waterborne diseases. In regions suffering from droughts, household-water storage infrastructures accompanied by facile treatment steps are an appealing choice43. Straw-like filtration devices are also available for individuals in the market to tackle emergent conditions44.

Nevertheless, current POU systems still face several challenges. For example, the December 2004 tsunami in Indonesia destroyed water supply infrastructures and placed 500,000 people at an increased risk of contracting waterborne illnesses. A survey and water sample tests after the tsunami across 21 communities suggested that boiling water was not associated with improved water quality, even among those who described correct practice45, although the reason was not explained. Chlorine tablets quickly went out of stock during the recovery of Puerto Rico after Hurricane Maria landed2. Water storage tanks were found to accumulate various opportunistic pathogens in their sediments. Sachets combining flocculant and slow-release disinfectants are effective in reducing pollutant concentrations and controlling pathogens in emergencies41, but suffer from having a complex treatment protocol and an unsatisfactory taste. Filtration-based devices may not meet the demand when a large volume of water is needed in a short period, and have further fouling problems if the water is very dirty. Meanwhile, different from back-up power generators, which only provide solutions to address emergent power outages, POU drinking water treatment is expected to be able to provide solutions for some long-term extreme weather events as well. Due to the unreliability of current methods previously listed, the adaption of POU methods that help better tackle extreme weather events should be comprehensively considered.

How to improve POU to mitigate the impacts of extreme weather events on reduced drinking water supply

Because of the increasing concerns on water quality, the global market for POU systems is expanding rapidly and estimated to be valued at 34 billion US dollars by 202535. Most POU systems (over 80% containing reverse osmosis cartridges) are purchased to improve water quality at the tap35 under normal circumstances, while products prepared for extreme weather events are probably marginal. This is largely because of a societal conception that extreme weather event-relevant water supply incidents will only affect a few of us for a short period of time. However, as can be seen in Table 1, extreme weather events caused severe impacts on water uses, and over 6 billion victims of extreme weather events have been documented in the past 30 years (Fig. 1), although the number that was affected by associated drinking water supply disruption was not reported. Therefore, greater recognition from society on this issue is warranted.

For individuals and households, regular preparation and maintenance of chemical supplies, such as chlorine tablets and flocculant containing sachets, and simple POU equipment, such as filters, are the most straightforward methods to prepare for the disruption of water supply caused by extreme weather events. Fostering an environment that encourages people to do so is critical. Policymakers should help improve public awareness on this matter. Policy guidance documents will direct consumers and the water industry to pay more attention to potential water supply incidents that may result from extreme weather events. For public facilities (e.g., universities and large/complex buildings) in regions with more likelihood for extreme weather events, mandatory emergency POU water treatment kits/equipment may be considered, similar to the cases of fire extinguishers and first-aid kits. Moreover, financial incentives such as subsidies may further reduce the cost of POU products and stimulate technological innovation.

Technological innovation is also needed to stimulate user demand. Similar to POU systems designed for normal situations, costs, treatment capacity, and ease of operation, and maintenance are all desired features for extreme conditions as well46. In addition, application scenarios must also be considered for these POU systems as different extreme weather events bring different challenges.

For events such as heavy rains, wildfires, extreme heats, and storms, water may still be supplied afterwards, but possibly with undermined quality. As a result, removal efficiencies of pathogens, contaminants, and particulate matter should be reliable even under off-power grid situations. Instead of being dependent on the power grid, POU systems should also be able to operate with connections to back-up power generators, energy storage devices, solar panels, and even hand pumps. These possibilities have been demonstrated in novel in-line chlorine dosers, which do not require electricity, and photovoltaic powered nanofiltration/reverse osmosis systems47.

For other extreme weather events, which may damage water supply facilities, it is possible that no water is supplied from the tap for a period of time. However, plenty of untreated water is available in such scenarios and can be collected from rainwater, surface water, and groundwater. Besides filtration, some prototype POU systems with low or even zero energy consumption have been demonstrated to successful in these situations. For example, a simple water extraction apparatus with a pollutant shielding function has been devised to extract clean water from polluted bulk water sources, although its efficiency of water collection needs to be further improved48. Another example is teabags repacked with adsorbents, coagulants, and disinfectants, separately or in combination, which enables facile POU treatment of pristine water46. Similarly, anti-bacterial hydrogels with catechol-enabled molecular-level hydrogen peroxide generators and quinone-anchored activated carbon particles are designed for effective water treatment without energy input49. However, the performance of these novel materials under real situations and the feasiblity of scable production need to be further evaluated.

Under other situations, water may be unavailable and must be obtained by transport over long distances. Some emerging technologies have also been developed to address this challenge. For example, it has been demonstrated that sorbent-based devices can collect water from air using primarily solar energy: extract water at night when relative humidity is higher and condense during the day50. It has been estimated that atmospheric water harvesting can provide daily drinking water for roughly a billion people51, although some engineering challenges remain to be addressed. For events such as droughts, storage and treatment of used water are two features required for POU. For the water storage devices, the prevention of pathogen accumulation should be seriously considered. For the treatment of used water, as water quality determines its use, POU systems should be able to produce clean water with different qualities.

Overall, extreme weather events are able to affect the water supply by undermining the quality and accelerating the scarcity of source water, which cannot be addressed effectively by centralized water treatment systems alone. Although various strategies have been proposed to help centralized water treatment systems, their inherent limitations call for back-up solutions. Within this context, decentralized systems have shown advantages, particularly in decreasing the financial and technical uncertainties. On a personal or household level, POU has been widely used to mitigate the effects of extreme weather events and has shown several advantages. Herein, we urge greater recognition and public awareness from society and more efforts on technological innovation for POU to tackle the challenges faced with water supply and quality during extreme weather events.