Water quality within a catchment reflects the activities that take place within the catchment. Human activities have altered the natural flow conditions in catchments across the country and have affected the natural background quality of the water. Natural groundwater and surface water quality reflects the vegetation at the surface and the chemical composition of the rocks that form the aquifers. The development and cultivation of land has affected water quality in rivers, lakes, wetlands and in groundwater. Activities that impact on water quality include:
• mining;
• landfilling;
• pumping of water from the ground which may induce seawater, or deep saline waters to be brought higher in the aquifer;
• spillage, or gradual seepage of man-made chemicals and oils into groundwater and watercourses;
• leaking sewers and waste water;
• discharge of sewerage and industrial effluents directly into watercourses; and
• cultivation of land.
Examples of threats to groundwater are illustrated in the diagram below.
The final point, cultivation of land affects water quality as it affects the natural nutrient cycling, which include nitrogen and phosphorus. Both of these nutrients are naturally occurring and would normally be present in water but human activities have meant that elevated levels are now present. Ploughing grassland for the first time to allow cultivation results in a burst of nutrient availability as the nutrients in the soil are converted into a form that can be leached into water. This slowly declines over time after the grassland is first ploughed. Whitmore et al. (1992) showed that the intensive ploughing of grassland during the 1940's and 50's, when production was key following the second world war, is a probable cause of the increases in nitrate found in aquifers in the 1970's onwards.
In addition to this initial release, the regular growth and harvest of crops depletes soil of nutrients. Crops have nutrient requirements which must be met to ensure they are healthy, vigorous and produce an economically sustainable yield for the farmer. Therefore farmers apply manures and slurries or inorganic fertilisers to land to meet these nutrient requirements. The use of manure and slurries is a very effective way of using material that could otherwise be considered as waste. The application of the necessary amount of any type of fertiliser, be it organic or inorganic, depends highly on the nutrient content of the fertiliser, the soil, the prevailing weather conditions and the crop to be grown. The way in which the nutrient is applied and timing is also key to how much could be lost to the atmosphere, how much is available to the crop and how much could be leached from the soil. The way in which nutrients are stored are key in preventing excessive nutrients leaching and entering watercourses.
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