Weathering and mass balance in small drainage basins : environmental applications in the Bohemian Massif (Central Europe). Altération et bilan de masse dans de petits bassins de drainage : applications au Massif bohémien (Europe centrale)

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Abstract
Data on inputs and outputs of chemical elements in small representative basins are used to demonstrate the application of mass balance equation to : (1) Evaluation of the rates of weathering and erosion, (2) Proton balance in acidified forested and agricultural basins, (3) Estimation of dry deposition of S0₂ and acumulation of sulphur in soil, (4) Derivation of rate constants of dissolution of feldspars. Specific mass balance equations are derived for all the cases. Calculations are performed step by step to serve as a guide for similar calculations by interested reader. The mathematical models are tested using field data from small catchments in the Bohemian Massif and the methods of gathering the data are described. The bedrock of the tested basins is biotitic gneiss. It is covered by regolith with well developed typical and acid brown earths. The basins represent forested and arable land which receives various doses of dry S0₂ emitted to atmosphere by industry. The mass balance calculations indicate that the rate of weathering of bedrock has been increased twice by modern agriculture and acid industrial depositions. Only 9 to 21 % of weathered rock are removed by chemical erosion in true solution while the rest is fixed in regolith as secondary and residual solids and minerals. The particles are removed by mechanical erosion. The environmental acidification is caused mainly by dry depositions of S02 and in lesser degree by acid rain and deposition of nitrogen oxides. The dissolution rate of oligoclase in bedrock derived from the measured fluxes of sodium is about two orders of magnitude lower then the rate determined by laboratory experiments with freshly ground feldspars. The processes and mass balance of weathering, erosion and environmental acidification strongly depend on the properties of the interface between minerals and natural aqueous solutions. A further research should be directed towards better understanding of the surface chemistry of the interface under field conditions.

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