GEOCHEMICAL MASS BALANCE OF SOME TOXIC HEAVY METALS IN SMALL ECOSYSTEM, EGYPT. AHMED A. MELEGY Geological and Geophysical Deprt. National Research Centre, Dokki, Cairo, Egypt The geochemical mass balance in Bahtim catchment, Shubra EI-Khima, Egypt, is defined by elemental input and output fluxes of some toxic heavy metals such As, Cd, Pb and Zn. The input-output mass balance, normalized to mass fluxes per unit area of the catchment per year, is the net difference between the input fluxes from weathering, dry atmospheric fallout (particulates), irrigation water, fertilization and output fluxes from biomass uptake and drain water. The atmospheric fallout represents the most toxic heavy metals 96% of their total inputs. In addition, the increase of the application of phosphoric fertilizers is mostly evident for Zn and Cd where their inputs are 58.9 and 5.39 g.ha-1.yr -1, respectively. Geochemical mass balance of the studied toxic heavy metals indicates that the highest accumulation is due to Zn, Pb and Cd, respectively, while the depletion appears only for As.