Three shallow lakes from the southeastern pampas, which are different in their physicochemical characteristics, were studied with the aim of comparing live/dead fidelity and preservation of mollusk shells. The studied lakes were Las Mostazas, Los Carpinchos and Nahuel Rucá, which proved different in terms of their specific richness, abundance, compositional fidelity and taphonomic alteration. The lake that presented the higher values of conductivity (3.34±0.72), hardness (286±83 mgl^-1) and carbonates saturation index (SI_calcite;= 1.95±0.21) was the one for which lower specific richness in living assemblages was recorded and differences in richness and diversity between the living and the dead assemblages became apparent. It was also the lake where the number of species was higher in death assemblages (R= 5) than in living communities (R= 2). Such characteristics suggest a higher residence time in the taphonomically active zone. On the other hand, the lakes featuring lower values of the same characteristics (conductivity= 2.2±1.2; 0.8±0.5; hardness= 157±55; 250±127 mgl^-1; saturation index of calcite= 0.35±0.15; 0.43 0.36, for LC and NR respectively) presented a higher species richness, lower numbers of species in death assemblages (R= 8) than in living communities (R= 10) and taphonomic attributes that suggested recently dead and deposited individuals. All these results indicate that water conditions in these lakes result in the decrease of the preservation of carbonate remains in comparison with the previously mentioned lake. Even though the lakes are located close to each other and are of similar shape and depth, such differences in water characteristics may be affecting shell preservation.