Waterquality in Vennesla. There has been a positive developmement of the water quality in the Otra during the1990s.The consentration of phosphor, nitrogen and organic matter is considerably reduced since 1980. The acidity has been reduced on all the actual stations. The main reason for the positive development with regard to phosphor is due to considerable steps taken by the local authorities to improve the environment. The decrease of the total consentration of nitrogen is in all probability due to the fact that the level in the 1980s was artificially high because of construction work in connection with water- power development in the upper of the Otra. The consentration of nitogen has however shown a tendency to increase the last four years. This is possibly due to an increase leak of atmospheric added nitogen. The outletreduction of the wood- processing industry and the establishment of the Otra pipes is the main reason for the reduced organic strain in the lower part of the Otra. The positive pH trend is due to a combination of the pipe measure and the fact that the downfall of sulphur in the area has been reduced the last ten years.In 97 a higher average pH value was registered at Skråstad than at the station upstream Hunsfoss.The acid reduction and deminishing consentration of poisonous, unstable aluminium has led to improved life conditions for water - living organisms in the Otra. It is however doubtfull if the level of aluminium consentration is sufficiently low to avoid harm to organic life, especially the vulnerable baby salmon.
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