The Great Stour Project - River Pollution

Farmland (arable)

Intensive cereal and field vegetable cropping occurs on the Great Stour valley sides, using the more fertile and better-drained loam soils of the Chalk and river terraces. Grazing marshes occur on the wetter clay and peat soils of the floodplain. Land use quality is generally very good to moderate.

Agricultural pollution (general)

This is potentially very harmful. A wide range of contaminants can reach the river either via groundwater or through drainage ditches, including artificial fertilizer residues, insecticides, herbicides, pesticides and farmyard waste. Accidental milk spillage from dairies is a serious contaminant. Undiluted animal manure (slurry) is one hundred times more concentrated than domestic sewage, and can carry a parasite, Cryptosporidium, which is difficult to detect. Silage liquor (from fermented wet grass) is even stronger than slurry, with a low pH and very high BOD. With a low pH, silage liquor can be highly corrosive; it can attack synthetic materials, causing damage to storage equipment, and leading to accidental spillage.

Milk spillage, silage liquor, cattle and pig slurry; these are all examples of point source pollution. Diffuse source pollution from agricultural fertilizers is more difficult to trace, monitor and control. High nitrate concentrations are found in groundwater and may reach 50mg/litre (the EU Directive limit). In ditches and river courses, nutrient pollution from fertilizers causes eutrophication. This is worse in winter, after autumn ploughing has released a surge of nitrates; winter rainfall is heavier increasing runoff and leaching, and there is lower plant uptake. Phytoplankton and algae thrive in the nutrient-rich water. Normally, blue-green algae are very important in the river ecosystem, photosynthesising sunlight energy, and liberating oxygen into the water. In large numbers, however, algae can become excessive, discolouring the water, giving an unpleasant smell and robbing the water of valuable oxygen as bacteria work overtime feeding on dead algae remains. Blue-green algae can also produce toxins, which kill wildlife, cause skin rashes, and cause pains and stomach upsets.

Eutrophication is thus depriving the river of oxygen (called "oxygen debt"). As algae dominate and turn the water green, the growth of other water plants is suppressed; these die first, disrupting the food chain. Death of invertebrates and fish follow on, and their dead remains in turn lead to excess bacterial activity during decomposition, reducing oxygen levels still further. BOD (Biological Oxygen Demand) is a measure of how quickly organic matter like this can de-oxygenate the water. Water with high BOD figures is thus badly polluted. Lower figures are better.

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Agricultural pollution (local situation and prevention)

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