tions in the surface water and in the water column
are low, except for the copper in surface water of
Site 5 on 24 May, and in the water column of Site
6 on 5 October. In the case of zinc Fig. 2., the
highest metal concentration level is located at
Site 1, close to the Lerma river inlet; and the
minimum concentration close to the dam. Copper
behaviour is not so obvious Fig. 2., and the lack
of data for nickel Fig. 3. does not allow us to
define with precision its behaviour in the water.
Iron Fig. 3., chromium Fig. 4., lead Fig. 4. and
mercury Fig. 5. concentrations in the surface
water and in the water column, exceed in some
sites the Mexican recommended limit for the
living aquatic resource protection and irrigation
water SEDUE, 1989. Table 3.. The concentration
of iron, chromium and lead in water, similar
to that of zinc, are higher at the entrance of the
Lerma river to the reservoir, reducing their values
as the sampling points get closer to the dam,
where the minimum concentration value is systematically
observed for most of the heavy metals.
These results suggest that a sedimentary process
is taking place in the reservoir as part of selfcleaning
net effect in the reservoir.
The heavy metal concentration determined in
water is composed by both the dissolved and the
suspended metals. Part of the suspended metals
come from the sediments, producing a correlation
between sedimentary and suspended metal concentrations.