Dorset Wildlife Trust is calling on parliamentary candidates to commit to measures to protect and restore the natural environment, including our rivers and coastal seas. The primary causes of this pollution are excess nutrients from sewage discharges, agricultural run-off and soil erosion.
Most of Dorset’s rivers suffer from high levels of both nitrate and phosphate pollution. Nutrients sounds like a good thing, but in excess they cause major problems. Once in the water, they contribute to eutrophication—a process that causes rapid growth of algae which smother the watercourse and depletes oxygen levels which create dead zones where aquatic life cannot survive. This process can occur in both freshwater rivers and streams, as well as in coastal environments such as Poole Harbour.
Nutrient and Soil Run-off
The nutrients in Dorset's rivers come from a number of sources including both treated and untreated sewage, as well as agriculture from poorly managed soils, animals’ waste and fertiliser. Nutrients, in the form of chemical fertilisers or slurry, are applied to increase crop yields. Sadly, these chemicals don’t always stay in the soil, and that which is not taken up by the crop can leach into nearby rivers.
We are increasingly seeing high rainfall levels with extreme weather events, which can lead to the soil itself ending up washed off fields and into watercourses. Soil erosion is a significant issue in Dorset, often but not solely linked to maize cultivation. Eroded soil particles, along with associated nutrients and pesticides, are washed into rivers, causing siltation and further nutrient loading. The build of silt along the riverbed can smother aquatic habitats and introduce additional pollutants, further degrading water quality and detrimentally affecting wildlife.
Sewage Discharge
Sewage discharge is another significant source of pollution which has grabbed the headlines. As well as the nutrients from treated discharges, outdated infrastructure and regular system overflows in stormy weather can result in untreated or partially treated sewage entering rivers. This sewage contains harmful pathogens and other contaminants that degrade water quality. In Poole Harbour, this can lead to shellfish contamination as well as direct health risks, putting restrictions on the local fishing industry.
An increasing additional problem is the level of pharmaceuticals which remain in treated discharges. Numerous studies have found significant levels of drugs which have passed through humans in both river and sea water, including anti-depressants, anti-inflammatories, hormones and anti-diabetes drugs. Waste water treatment sites are ill-equipped to deal with these drugs and their effects on wildlife can be significant, altering reproduction, behaviour and development of invertebrates, amphibians and fish.
