Cities today are mostly covered in impermeable surfaces – asphalt roads, concrete walkways and roads, and even soils become extremely compact due to the movement of people and machines over them. In my previous blog posts on water pollution, I mentioned how concrete surfaces may introduce excess nutrients into waterbodies through urban runoff that may cause eutrophication in these waterbodies. In this blog post, I will be sharing another, less-known driver of water pollution –
Thermal Pollution
Wang et al. (2023) ran a lab experiment to find out how the permeability of pavement cities in cities affects the potential of thermal pollution from stormwater runoff. To do so, they compared permeable brick pavements (PBPs) with impermeable asphalt pavements (IAPs) and measured the temperature of the stormwater runoff from the moment of contact with the pavements onwards.
Figure 1: Temperature of stormwater runoff over IAP (top) versus PBP (bottom) (Wang et al., 2023)
Their findings reveal that the permeability of surfaces significantly impacts stormwater runoff temperature where the temperature of the stormwater runoff over the PBP is consistently lower than that of IAP (Figure 1). Furthermore, the higher the initial temperature of the pavement at the moment of contact between precipitation and the pavement (T0), the higher the temperature of the subsequent runoff.
Figure 2: Temperature of stormwater runoff over PBP with different rainfall return periods (assuming T0 = 35⁰C)
The researchers also investigated the influence of rainfall intensity. The greater the intensity of the rainfall as measured by the rainfall’s return period (P), assuming T0 of 35⁰C, the higher the temperature of the stormwater runoff (flowing over PBP) (Figure 2).
The results of Wang et al. (2023) study thus reveal the impact of the built environment on the temperature of stormwater runoff, and how this impact might be more intense with more extreme precipitation events in the future, which is extremely likely for most parts of the world (Tabari, 2020). The temperature of stormwater runoff is crucial as it contributes to thermal pollution in waterbodies.
Thermal pollution occurs when there are heat imbalances in the environment, both in waterbodies and in the surrounding atmosphere (Vallero, 2019). In addition to the higher temperatures of concrete surfaces, thermal pollution is also mostly discussed in the context of cities where factories and power plants (1) generate heat in the atmosphere through their activities such as in the burning of fossil fuels and (2) draw large amounts of water from a water source which flows through cooling systems and the remaining heated water directly released back into the waterbody (Kennedy, 2004). Once-through cooling systems (where water passes through coolant systems once) can potentially result in return water 10⁰C warmer than its initial temperature (Rabinowitz, 2021). While many power plants and factories may not be directly located within cities, the high energy needs of cities undoubtedly contribute to the environmental degradation caused by thermal pollution wherever its energy source is located.
The impacts of thermal pollution are both direct and indirect (Vallero, 2019). In aquatic environments, increasing the temperature, no matter how little, can have great consequences. A slight increase in water temperature could directly be lethal for some aquatic organisms, known as a thermal shock (Speight, 2020)
Figure 3: Morality curves of the Neomysis americana shrimp acclimated to different temperatures (Vellero, 2019)
As seen from the experiment on the Neomysis americana shrimp which typically is acclimated to temperatures ranging from 1⁰C in winter to 30⁰C in summer, when the temperature of water suddenly changes within a short period of 1 to 3 days, morality rises from near 0% to 100% over a 5⁰C increase in temperature (Figure 3).
The mortality rates of aquatic organisms under high water temperatures are also affected by other factors such as lower oxygen levels as the solubility of gas decreases, lower photosynthesis rates due to the inhibition of enzymes, and eutrophication (Speight, 2020).
Additionally, thermal pollution does go both ways – together with hot-water pollution exists cold-water pollution where cooler water is released into a waterbody with higher average water, abruptly lowering the temperature of the water. The effects of cold-water pollution are similar to hot-water pollution (Vallero, 2019).
Where aquatic organisms and ecosystems suffer, impacts will eventually be felt on a larger scale – affecting biodiversity, food security and water supplies. Thermal pollution goes unnoticed where water is uncontrollably used and returned into the ecosystem as part and parcel of factory and power plant activities. This should not be the norm.
With many other pollutants existing in waters at the same time, it is especially difficult to differentiate harm caused by thermal pollution as compared to other pollutants, but there are still ways we can do better. Having stricter regulations to control the temperature of return water and replacing impermeable surfaces with permeable surfaces or increasing the amount of vegetation area on roofs and on land are various easy ways to combat the problem of thermal pollution.
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