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 Sedlicky B rooks, which represent general water-quality characteristics well related t o 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 profi les. It is seen that the only critical pollutants are nutrients, namely nit rate nitrogen and total phosphorus, originating from both urban and agricul tural sources, while industrial pollution is relatively marginal in the wat ershed. Phosphorus concentrations do not display any simple time regularity . Persistent all-year background phosphorus load gives evidence of the urba n origin of pollution. Total input of phosphorus into the reservoir came ma inly from surface inflows, out of rainfall and out of bottom sediments. Pho sphorus 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 co ncerted abatement actions. Most of the watershed born nitrogen pollution ha s a distinctly non-point (diffuse) character. Typical yearly pattern of nit rate concentrations means distinct and regular increase during end-of-winte r and spring period due to processes of mineralization, typical for product ive arable land on soils with good drainage properties. The process of stat e economy transition, which was initiated in 1989, has led to dramatic decr ease of fertiliser inputs per hectare of land. Reduced fertilisation of ara ble 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 th e entire watershed and, at present, preferentially focused on: phosphorus cycle control and improvement within both the watershed and rese rvoir water (diminished use of chemicals producing reactive phosphorus comp ounds, improved phosphorus removal from urban waste water, to prevent phosp horus release from bottom sediments) general soil erosion control and prevention - grassland cultivation on vuln erable 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 maintai n actual standards of produced drinking water. (C) 1999 Published by Elsevi er Science Ltd on behalf of the IAWQ. All rights reserved.