Critical loads for nitrogen in fjords; evaluation of effects of niotrogen leaching from explosives used for E18 road construction i Aust-Agder

Abstract

The new road, E18, between Grimstad and Kristiansand in Aust-Agder county, will be constructed by blasting of about 15.9 mill. tons (7.7 mill m3) of rock. This will be carried out by use of 0.65 or 2.5 kg explosives per m3 rock for earth works and tunnel works, respectively, and potentially (if 100 % leaching) result in runoff of 900 tons of nitrogen. Potentially about 500 tons of N will enter the more or less landlocked fjords Kaldvellfjorden, Vallesverdfjorden and Isefjærfjorden. We have compared this loading with background and other man-made N-loads by use of the river transport model TEOTIL. Further, we calibrated and run the NIVA fjord model to estimate the change in concentrations of nitrogen, particulate matter and the oxygen regime in these three fjords. Nitrate concentrations are estimated to potentially increase by a factor of 2 for 2 years at most. The results are discussed on basis of relative increase in nitrate and the impact on algae growth and composition. Due to the dilution by sea water, the general phoshorus limitation in the fjords and grazing by zooplankton, biological effects and effects on the oxygen regime in the fjords are estimated to be minor. If we also assume that the actual N-leaching will be far less than the potential, we are relatively certain that the effects will be insignificant.