The concentration of Pb in water during the rainy season was
higher than the concentration of Pb in water during the dry season.
The data highlight the importance of continental urban surface
runoff as a source of pollutants; rain apparently increased the discharge
of Pb. According to Morrison et al. (1984), two important
sources of heavy metals are the atmosphere and vehicles.
Atmospheric deposition can occur during dry weather or after
precipitation because water has the ability to absorb and dissolve
pollutants generated by other sources. Note that the majority of
studies related to the urbanization impact were conducted in cities
with temperate climates in the northern hemisphere. Based on the
information provided by Morrison et al. (1984) for the southern
hemisphere, this source of heavy metals may not be as critical in
cities with tropical climates, which experience higher amounts of
rain and higher temperatures than cities in the northern hemisphere;
however, the high urban concentration in Rio de Janeiro
justifies this hypothesis.