Abstract

 Zimapan reservoir is one of the deepest reservoirs of Mexico (203m) which receives waste water and industrial sewage from Mexico, Querétaro and Hidalgo states. The goal of this study was to assess the settling rate of Zn in sediments and its impact on native microorganisms. Hydrodynamic model of S. E. Jørgensen was applied for assessing the settling rate of Zn (Y). This included sub models of diffusion and settling process as well as toxicity biometrics in microcosmos using natural assembly of dissimilatory sulfate-reducing bacteria as test organisms. Settling results indicated an average coefficient (Ks) of 0.706-m/24 h and in water column this process decreased with the depth. Exponential model of River Tula sediments showed an average Y of 8.06 x 10-13 g Zn/m2 24 h where as for River Moctezuma sediments the Y was 4.99 x 10-12 g Zn/m2 24 h. Statistically significant correlations were observed with several physicochemical parameters such as dissolved solids and zinc concentrations in the tributaries. However, no correlations were observed with discharges of tributaries, pH and organic dissolved and particulate carbon. On an average the eddy diffusion coefficient (D) was 10.53 m2/seg. The biometrics models showed that the toxic impact on micro-biota due to the settling rate was less than 1.0 % of inhibition probit units. These models predict that Zn might contribute from 12 to 16 % of chronic value in five years. Previously it was believed that the settling rate would not be able to produce toxic effects.