While droughts and heatwaves result in lower dissolved oxygen and increased river temperature, rainstorms and floods increase the mobilisation of plastics and other pollutants in rivers, finds a new study
Climate change and extreme weather events such as droughts, heatwaves, rainstorms and floods are impacting all corners of the world. Rising frequency and intensity of such events pose serious challenges for water resources availability and water quality.
A recent study published in Nature has thrown light on the responses and mechanisms of river water quality under more frequent and intense hydroclimatic extremes.
The study compared 965 case studies and found that river water quality generally deteriorates under droughts and heatwaves (68% of compiled cases), rainstorms and floods (51%) and under long-term climate change (56%). However, improvements or mixed responses were also observed owing to counteracting mechanisms, like increased pollutant mobilisation versus dilution during flood events.
The study found that droughts and heatwaves result in lower dissolved oxygen and increased river temperature, algae, salinity and concentrations of pollutants from point sources owing to lower dilution. Contrastingly, low flow during these events leads to reduced pollutant transport from agricultural and urban surface runoff, leading to lower concentrations.
Rainstorms and floods generally increase the mobilisation of plastics, suspended solids, absorbed metals, nutrients and other pollutants from agricultural and urban runoff, although high flow can dilute concentrations for salinity and other dissolved pollutants. The sequence of extreme events, such as droughts followed by floods, also impacts the magnitude and drivers of river water quality responses.
According to the study, river water quality responses under multidecadal climate change are driven by hydrological alterations, rises in water and soil temperatures and interactions among hydroclimatic, land use and human drivers. These complex interactions synergistically influence the sources, transport and transformation of all water quality constituents.
Hydroclimatic drivers (precipitation, evaporation and runoff), geographic factors (land use, geology and soil characteristics) and human activities (sectoral water use, untreated wastewater and fertiliser use)all impact river water quality. These drivers can be interrelated. For instance, warmer, drier climate conditions impact land use and can increase irrigation water use, which in turn might contribute to increased salinisation in several river basins worldwide.
Hydrological events, the report added, can also enhance contaminant pulses —large changes in concentrations over a short period— from land to rivers and impact the potential for water uses downstream. Hence, there are complex water quantity and quality responses to climate change and weather extremes.
The study also noted that interactions exist between different water quality constituents, further adding to the complexity. For instance, water temperature affects other water quality constituents by impacting the rate of biochemical processes, algae growth, dissolved oxygen saturation and decay of chemical substances and microorganisms in rivers. Algal blooms are also strongly driven by water temperature, the study said.
In addition, higher temperatures and associated evaporation contribute to freshwater salinisation, whereas salinity changes influence the growth of microorganisms and metal contaminant mobilisation from soil and sediment.
The study stressed the need to improve understanding of the complex hydroclimatic–geographic–human driver feedbacks, water quality constituent fate, transport, interactions and thresholds, and to develop technologies and water quality frameworks that support the design of robust water quality management strategies under increasing hydroclimatic extremes.
In a world that is facing more frequent and severe hydroclimatic extremes, the report recommended research in tools, techniques and models that support the design of robust water quality management strategies.