The poison in the pipes: How urban runoff affects natural waterways

Despite being residents of the world’s driest inhabited continent, we Australians love our water. We’re currently battling some of the worst droughts that we’ve had in one hundred years. Water is scarce, so we need to protect what we’ve got.

Unfortunately, our urban streams are often very polluted and sick. Litter, pesticides, fertilisers, and other pollutants all contribute to the poor health of our waterways. But it seems that there’s more to stream pollution than just these obvious culprits. Recent research suggests that there is a surprising pollutant impacting our streams – concrete. So how is one of the most common materials in our cities changing our waterways?

From rain to stream

When it rains, rainwater makes its way to the lowest points on the landscape – our streams and rivers. In less urbanised areas, the water travels over bushland, grass, dirt, and other natural surfaces, which are like sponges. They soak up rainwater and release it slowly into nearby streams.

However, in towns, cities and other urban areas, most of these natural surfaces are replaced by man-made structures like roads, driveways, roofs, paths, and drains. Rather than having spongy, green spaces where rainwater is slowly absorbed and channelled to rivers, there’s lots of hard surfaces that are more like waterslides – all the rainwater hits these surfaces and rushes straight into our streams.

A contrast: how rainfall flows into streams from urban vs. green areas.

Not only does this mean that more water is more quickly reaching our waterways, it also means this “urban runoff” carries pollutants built up on these hard surfaces, such as rubbish, grease, fertiliser, and chemicals. These pollutants flow into our rivers and can have a wide range of adverse affects, including algal blooms and death of aquatic organisms.

Pollutants carried from urban surfaces are the obvious problem. But it seems that there is an underlying problem… literally. Our urban surfaces themselves are not only the perfect channels to transfer pollutants – it seems the surfaces themselves affect stream chemistry.

It’s in the pipes

Concrete is the foundation of our buildings, the cement that holds our cities together, the material that makes up our gutters, paths, driveways, and drains. It’s versatile, convenient, and durable, making it common in urban development. However, it’s also extremely reactive. As water flows over concrete surfaces, it slowly dissolves the surface, and concrete components leach into the water.

Concrete makes up almost all of our drainage systems and urban surfaces. Photo: Darkday. Full link:

Recent research suggests that concrete significantly changes water chemistry. Water that has been running over concrete tends to have a higher pH, higher electrical conductivity, higher salinity, and higher concentrations of certain elements such as calcium and sodium, an effect which has been seen in urban streams all over the world. Concrete drains, channels and pipes don’t just carry water to our streams, they also change it and the stream it flows into.

Water is life

Water is important – we all know that. So why does changing the chemistry of our urban streams matter?

Our urban streams feed into rivers, which eventually enter into our drinking catchments. Changing our urban stream chemistry could impact on our drinking water quality. Not only that, but changing stream chemistry essentially means that we are changing the habitats of both land-dwelling and aquatic creatures. Current research suggests that areas around urban streams often have a higher soil pH and higher concentrations of phosphorous and nitrogen. This promotes the growth of weed species. This means that rivers around urban areas can become less attractive and less enjoyable for recreational use. Vegetation changes could also mean a food shortage for animals that feed on native plants. Animals that rely on urban waterways for drinking water may also be negatively affected by these changes, and sensitive aquatic species may be unable to survive in changed conditions.  

Not only that, but small changes in stream chemistry can have disastrous consequences for sensitive habitats. For example, Blue Mountains Upland Swamp (BMUS) communities are unique swamp areas that are home to endangered species and play a vital role in filtering and maintaining water flow to streams. BMUS are under increasing pressure from nearby development and urban BMUS show chemical changes characteristic of concrete exposure, which could be playing a key role in their degradation.

A weed infested, chemically changed swamp vs. pristine BMUS. Photo: Katie Purdy, 2018.

Not set in concrete: what we are yet to find out

We have only recently realised that our urban surfaces themselves play a role in water pollution, so this branch of research is relatively new. There is strong evidence to indicate that concrete in particular affects stream chemistry, but we need to investigate this further to understand exactly what this means. Current research involves confirming the chemical effects that we’ve seen concrete have in previous studies. Researchers are also interested in looking at how different types of concrete and different concrete exposure times affect water chemistry.

Other research involves looking at the bigger biological and ecological effects these chemical changes might have. Researchers at WSU plan to investigate how these changes will impact stream ecosystems, including plants and animals. While differences have been noticed in urban soil chemistry and plant communities, we don’t know how concrete-exposed water will affect plant growth and aquatic species such as insects and snails.

Our urban streams are polluted and sick and, in a country where water is almost always scarce, we need to act. With a deeper understanding of how concrete affects streams and the wider ecological consequences, we will hopefully be able to make decisions that will better protect and preserve our waterways.

3 Replies to “The poison in the pipes: How urban runoff affects natural waterways”

  1. Good science Katie; this is one of the reasons I want to see much more openness and awareness of the construction “skeleton” of our towns and cities..revealing the details of drain topography is just as critical as the geology and topography of our regions; it is intrinsic to know the inputs for our wetlands for instance.

    1. Thanks so much for your comment, Robin! I completely agree – it’s an important but often overlooked topic.

  2. Knowledge is power and the better informed we can become, the better opportunity of finding strategies to correct ineffective management protocols in the past.

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