THERMODYNAMIC MODELING OF RIVER WATER AS A TOOL FOR ENVIRONMENTAL IMPACT EVALUATION

Abstract

The possibility of applying a thermodynamic approach to calculate the elements’chemical species and thus to more effectively assess the water quality is demonstrated. Iskar river and its three tributaries Lesnovska, Kakach and Blato rivers within Sofia Municipality were chosen as a case study. Experimental field and laboratory studies were done to create an input database for thermodynamic calculations. The obtained analytical data revealed high concentrations of PO43-, NH4+ and NO2- in all studied water samples, which is an indicator for serious domestic pollution. The water was pure in terms of Fe, Co, Ni, Cu, Zn and Cd. Al, Pb and Mn exceeded the maximum allowed concentrations in the water of Iskar river after Sofia city, and along the entire flow of Lesnovska river. The thermodynamically calculated chemical species showed that Na, K, Mg, Ca, NO2-, NO3- and NH4+ exist mainly as free ions. The presence of dissolved ammonia was calculated for all sample stations, which is a serious problem for aquatic animals, especially for fish. PO43- showed a great variety of forms depending on pH. Among the trace metals (Al, Pb and Mn), Pb is potentially the riskiest element for plants, as its dominant organometalic species are easily accumulated by them. Free ions of Mn dominate, which is dangerous for aquatic fauna as Mn2+ ions easily interact with ligands of organic compounds found in the bloodstream and/or within organs. Regarding Al, the dominant hydroxy species are a prerequisite for the precipitation of Al(OH)3 and thus, for the self-purification of waters.