Sediments are major repositories for heavy metals.
Within any given compartment of the hydrologic cycle for example, far
less than 1% account for the dissolved heavy metal load (Salomons,
1995). While bulk of the heavy metals is stored in sediments (and
soils), the dissolved phases are more mobile; henceforth, easily
bioavalable.
Present studies dealing with particulate heavy metals
in aquatic systems concern total metal concentrations in the carrier
media (suspended or bottom inorganic/organic particulates). The use of
total metal concentration as a sole criterion to assess the potential
adverse effects of sediment contamination implies that all species of a
given metal have an equal impact on the environment, which is not the
case.
To evaluate the possible risk for metal mobility and
consequent bioavailability, a five-step selective heavy metal extraction
procedure, modified from Calmano (1983) and Calmano and Förstner (1983)
was performed in the pelitic fraction of the Pasig River sediments.
Results show that the moderately reducible phase (iron and manganese
oxides) is the dominant controlling matrix for heavy metal partitioning.
In decreasing order, the general occurrence of metal species trends as
follows: residual > moderately reducible > exchangeable > organic
fraction and sulfides > carbonates and easily reducible. The residual
fraction comprises lattice-bound metals and is not a significant matrix
of heavy metal pollutants.