With all the heavy rains we are experiencing lately, it seems like the dry season has been cancelled. With all of this rain most of us might think that the only thing that could happen is flooding and damage to property.

But the water that runs off from our homes, driveways and pavements can be like poison to the environment. In this article I would like to emphasise how the water that flows down the drains at the side of the road can affect our environment.

First of all, the rain water that runs off any surface in urban areas, for example, roofs, driveways, pavements, roads, lawns and parking lots are referred to as urban runoff.

So what are some of the things you would expect to find in the water running down the pavement after a downpour? Well, first of all rain water is quite pure, however, in areas where there is much air pollution one could be faced with acid rain. But generally as water runs off these impermeable surfaces, it picks up pollutants along the way and the type of pollutant is carries depends on what type of surface it passes over.

For instance, water running off from lawns, gardens, parks or golf courses can carry with it pesticides and fertilisers. The fertiliser causes what is known as eutrophication. Here, the extra nutrients cause an accelerated growth of organisms like algae which use up much of the oxygen in the surrounding water. When the algae die their decaying bodies provide organic material also releases nutrients causing further eutrophication. However, it should be noted that eutrophication occurs in relatively still waters like in ponds and lagoons.

Whereas, water which runs off from roads and parking lots carry with it, heavy metals, such as, cadmium, lead, nickel, zinc and copper.

Also found in this water are polycyclic aromatic hydrocarbons (PAHs). These chemicals are the by products formed by the combustion of gasoline and other fossil fuels. While the run off from roofs carry with it high levels of synthetic organic compounds and zinc from galvanised roofing materials.

One of the most obvious problems caused by urban runoff is that less water is absorbed by the soil, leading to more water on the surface, thus causing flooding.

Urban runoff also lowers the water table, since instead of the water passing over a permeable or porous surface like soil and percolating down to replenish the water table, it is rushed off into a storm-drain and then to a river.

Although these drains facilitate keeping our streets clear of water (when they are not filled with garbage), what also happens is that untreated water is discharged into our rivers and streams.

The water that runs from urban surfaces also carry with it heat. This heat is transferred to rivers and streams. The sudden change in temperature in these watercourses causes marine animals like fish to die from shock.

Urban run off can change the course of waterways; most of the water will contain little sediment. This coupled with the speed at which it flows can erode bends in the river changing its course.

Some natural habitats also contain riverbeds that only get filled up with water during the rainy season. However, when the surrounding area is urbanised, there is a year-round supply of water in these waterways. This may sound like a good thing, but the organisms around this site can be affected. For instance, under normal conditions during part of the year when the river is dry individuals of the same species can travel back and forth so that they have a large territory full of food, mates and suitable habitat. However, the constant presence of the river now presents a barrier and thus fragments the habitat. This could eventually lead to the local extinction of the species as they cannot access resources they need to survive.

So how can we control urban run off? The main method is to reduce the speed and flow of the water. Certain infrastructure can be put in place to lessen the effects of urban run off. Examples of this include a percolation trench where as storm water runs off it passes through permeable soils thereby getting filtered before entering underground aquifers.

Installation and integration of storm water filtration systems which filter urban run off before it enters waterways and underground water sources. Legislation should be put in place to cater for urban run off when an area is going to be developed. Finally, one should also monitor the chemicals used on urban surfaces and in turn minimise the amount of pollutants entering our environment via urban run off.

The bottom line is that everything in our environment is in delicate balance with each other. Although urbanisation is inevitable in some cases, it should be regulated and carried out with our environment in mind.

So one cannot blame the rain, when it come to pollution caused by urban runoff and storm water, but the pollutants that we put on these surfaces is what the water carries with it.