What direct and indirect effects does Canterbury, as a built-up area, have upon the Great Stour river?

The Canterbury district (the City and immediate surrounding urban areas) has a total population approaching 120,000. Population growth rate in this part of East Kent is running at 0.5% per year. This process of urbanisation places increasing stress on the Great Stour in terms of both increased water demand and more effluent disposal. Indirectly, the presence of Canterbury contributes to river pollution through sewage discharge, light and heavy industrial effluent and solid waste disposal. See separate items for details on these. Directly, however, the presence of an urban area threatens a river in other perhaps less obvious ways.

What pollution threats do built up areas have upon rivers?

A common source of pollution is from road accidents involving vehicles e.g. petrol and oil from overturned cars, trucks and tractors. Tankers carrying hazardous substances may be involved; these do not necessarily have to be chemically toxic. If two road tankers were involved in a collision, one carrying diesel fuel, the other milk, spillage from the milk tanker would in fact pose the greatest threat to the ecology of a nearby river. Milk has an extremely high BOD loading (see under arable farming), and will quickly go into suspension in the water, killing all wildlife in its path downstream. Diesel fuel and oil, though unsightly, is more easily contained by floating booms (inflatable plastic tubing) and can then be removed by pumping or soaked up by absorbent materials.

If there is a fire in a road traffic accident, water used to extinguish it will drain away, carrying with it petrol, toxic chemicals and other road surface materials. Road highway drainage can pose other problems especially after a period of dry weather. Gullies and drains quickly fill up with accumulated rubbish. With heavy rain, oil, rubber, metals, salt, grit, bacteria, etc. can all be washed off the road surface. All these may impose high BOD. This is an example of a diffuse pollution source; no particular individuals can be held responsible. Leakage from petrol service stations is an increasing threat to the purity of water supplies.

Under serious storm conditions, the river can be expected to receive 'shock load', as storm drains fill up to capacity and untreated crude sewage from sewage treatment works can overspill into the river. The greater river discharge however will disperse the effluent quickly. Discarded litter is also a constant hazard in the urban environment; objects discarded in rivers range from shopping trolleys and car tyres to fishing hooks. Larger objects may block the watercourse, preventing the natural flow, particularly if the river is contained in an underground culvert, running under the built-up area. Fly-tipping (illegal dumping of trade waste) is also on the increase. This is not merely unsightly, but dangerous and the solid matter can silt up the channel or clog up sluice gates.

What is being done to limit the amount of pollution from built-up areas?

The Environment Agency and the Fire Brigade work closely together to control the amount of runoff in the event of extinguishing road traffic accident fires, or indeed any fires near watercourses or groundwater. Modern petrol stations are designed with multi-layered tanks (double-skinned within a concrete vault, and lined with synthetic membranes and compacted clay); this ensures against leakage of contaminating fuel from storage tanks or from the station forecourt.

Local authorities need to ensure frequent and regular road sweeping and drain clearance, to prevent pollution problems with highway drainage. Modern road design needs to include 'lagoons' to trap roadside spillage; new road surfaces (e.g. supermarket car parks) can be fitted with porous paving to reduce flash flooding during storms. Up to 80% reduction in sediment, phosphorous and nitrate discharge is reported from such surfaces. To prevent dumping and fly-tipping, heavy fines can be imposed by local authorities; trash screens (metal grids) are sometimes placed across culverts to stop large discarded objects floating down the river and causing a blockage; they need to be regularly cleared and cleaned.