Access to clean water is fundamental for supporting livelihoods, recognised by the United Nations as a basic human right. Safe use of water – whether for direct consumption as drinking water, irrigating croplands, or for fulfilling our sanitation and hygiene needs – is dependent upon both water availability and quality. Similarly, aquatic ecosystems depend upon clean water resources and can be sensitive to changing environmental conditions.

When our water use is constrained by availability, the implications typically manifest in obvious ways: governments may introduce water rationing, shipping activities get disrupted or become unfeasible and the lack of water for irrigation can place immense pressure on food production. Thermo-electric power production can be hampered by a combination of both low water levels and high water temperatures, leading to reduced electricity output.

Comparatively, the problems of using water of unsuitable quality are more inconspicuous. Yet, the implications can be severe. An estimated 829,000 deaths are attributed to diarrhoea caused by the use of pathogen-contaminated water for drinking or sanitation purposes. Poor sanitation and inadequate water quality are the second and third leading risk factors for childhood stunting. The health and functioning of aquatic ecosystems are particularly sensitive to oxygen concentrations, which are strongly linked to water temperature and the amount of organic matter. Nutrient and pharmaceutical pollution can also pose serious risks. Being under-monitored, difficult to detect and often imperceptible to the human eye, the World Bank has branded water quality issues an “invisible crisis”.

Making the “invisible” more visible

In-stream water quality sampling and analysis can be both time-consuming and expensive. Thus, observations from water quality monitoring stations are highly limited in space and fragmented across time.

Water quality models are useful tools for supplementing our knowledge of water quality dynamics, which has predominantly been gained from in-stream sampling and analysis. Models offer possibilities to overcome issues related to geographical and temporal coverage, without the requirement for “boots on the ground” to conduct the laborious tasks of collecting and analysing water samples. Models can also be applied for both historic and future conditions, whereas (unfortunately) it is not possible to travel backwards or forwards in time to collect a water sample.

This was the focus of our study, published in Nature Communications Earth & Environment. Here, we developed a new global water quality model for simulating the concentrations of multiple pollutants at high temporal and spatial resolution. After developing the model framework, we applied the water quality model to a historic time period (1980 – 2015) and compared our modelled results to the observed data collected at water quality monitoring stations.

We also wanted to consider how water quality might change in the coming decade. How we manage our wastewater – water that returns to the environment after we have used it for purposes including flushing toilets, using washing machines or the manufacturing of goods - is a key factor influencing the pollution levels in streams. When our wastewater is collected and undergoes treatment, the impact of releasing this water back to the environment can be limited. Conversely, dumping raw sewage into waterways can cause large water quality issues that threaten both safe human use and may cause disruption to ecosystems. To this end, we investigated the impact of two different future scenarios for wastewater treatment practices up to 2030. In the first scenario, we assumed that there would be no expansions in wastewater treatment from 2016 – 2030. In our other scenario, we assumed that the proportion of untreated wastewater entering the environment in 2030 would be halved. This is in accordance with the wastewater treatment target included in the United Nations Sustainable Development Agenda for 2030, as stipulated by Sustainable Development Goal (SDG) target 6.3.

Cleaner water, but issues remain

In our paper, we link the simulated concentrations to safe use thresholds associated with human activities. We find that exceedances of water quality thresholds have occurred across all world regions for at least part of the year. Salinity pollution is a particular problem in areas with large-scale irrigation systems (e.g. North India) and in heavily industrialised regions (e.g. East China). Exceedances in organic and pathogen pollution thresholds are typically found in areas where wastewater treatment rates are low, and particularly downstream of large urbanised areas. The timing and degree of water quality pollution is also heavily influenced by the dilution capacity – i.e. the amount of water in the stream at a given moment.

Our modelling results suggest that achieving SDG6.3 leads to substantial reductions in water pollution (see map for organic pollution). Increases in wastewater treatment capacities not only reduce point source pollution locally - the benefits to surface water quality also propagate downstream. Where wastewater treatment rates are already quite high (e.g. Europe, North America), reductions in pollution concentrations are widely distributed throughout the region. Conversely, water quality improvements can be very high at the local scale in Africa and South America – but less ubiquitous in space.

However, percentage reductions in pollutant concentrations are only part of the story. If existing pollution issues are severe, even strong percentage reductions may not be enough to improve water quality to safe levels for human uses and ecosystem health. Unfortunately, our modelling results suggest that this is a common pattern across some world regions - particularly in the developing world. Linking our modelled concentrations directly to surface water abstractions for human uses, we find relatively modest reductions in the proportion of abstractions that exceed safe use quality thresholds in regions such as Sub-Saharan Africa, Latin America and Southern Asia.

With our results in mind, and combined with the fact we have now entered the “Decade of Action (2021-2030)” for achieving the Sustainable Development Agenda, we conclude it is time to renew global efforts to go above and beyond SDG 6.3 for global water quality improvements. Furthermore, pollutant emissions will also need to be reduced at source to achieve the overarching goal of SDG6 – clean water and sanitation for all.