Pollution by phosphorus and nitrogen in water streams feeding the Zelivka drinking water reservoir

This case study refers to long term monitoring of Martinicky and Sedlický Brooks, which represent general water-quality characteristics well related to the watershed of the whole Zelivka drinking water reservoir. For a period of more than ten years water-quality indices were systematically monitored at each profile together with actual discharges recorded at selected profiles. It is seen that the only critical pollutants are nutrients, namely nitrate nitrogen and total phosphorus, originating from both urban and agricultural sources, while industrial pollution is relatively marginal in the watershed. Phosphorus concentrations do not display any simple time regularity. Persistent all-year background phosphorus load gives evidence of the urban origin of pollution. Total input of phosphorus into the reservoir came mainly from surface inflows, out of rainfall and out of bottom sediments. Phosphorus was found to be the limiting nutrient for phytoplankton growth and was therefore recognised as the principal element responsible for reservoir eutrophication rates, which could probably be effectively controlled by concerted abatement actions. Most of the watershed bom nitrogen pollution has a distinctly non-point (diffuse) character. Typical yearly pattern of nitrate concentrations means distinct and regular increase during end-of-winter and spring period due to processes of mineralization, typical for productive arable land on soils with good drainage properties. The process of state economy transition, which was initiated in 1989, has led to dramatic decrease of fertiliser inputs per hectare of land. Reduced fertilisation of arable land has not yet been reflected in improved quality of surface waters. The proposed strategy for prevention and abatement in the Zelivka reservoir eutrophication should be based on win-win principles applied throughout the entire watershed and, at present, preferentially focused on: • phosphorus cycle control and improvement within both the watershed and reservoir water (diminished use of chemicals producing reactive phosphorus compounds, improved phosphorus removal from urban waste water, to prevent phosphorus release from bottom sediments) • general soil erosion control and prevention - grassland cultivation on vulnerable areas and buffer zones and subsidy for cultivating plants on arable land even between growing periods • proper maintenance and improvement of treatment plant technology to maintain actual standards of produced drink