Nitrogen stream water: stacionary and non-stacionary models

Abstract

The paper analyzes the possibilities to model the concentration of inorganic nitrogen – a harmful pollutant from the eutrophication point of view – contained in the stream and self-purification processes of the stream. During the studies the data of a small right affluent of the river Mūša (administrative district of Biržai) was used. The mentioned data comprised results of natural observations about the changes in nitrogen concentrations during the vegetation period and cold period of the year. The model of pollution dispersion through the stream is a parabolic differential equation with partial derivatives, initial as well as boundary conditions. Constant nitrogen pollution in a stream stretch containing constant hydrological parameters and adequate environment conditions is proportional to the concentration of pollutants inflow. Further from the pollution source the constant pollution is exponentially decreasing. In the stationary model in respect of time t case at the distance expressed by x(m) from the initial pollution concentration c0 measuring place, nitrogen concentration is expressed by, C(x)=[...][-0.000176; -0.000130] in the cold season of the year and [...][-0.000321; -0.000252] during the vegetation period. Thus it may be stated that the stream polluted in the result of agricultural activity, further flowing through forest-covered area is significantly purified already at a 1.5 km stretch. On the average, nitrogen concentration decreases about 3.0 times a day during the vegetation period and about 2.6 times a day during cold season of the year. In in the case of non-stacionary model the concentation of inorganic nitrogen C(x, t) is proportional to the term , i.e. it depends on time t and distance x exponentially.