Question

what is heavy metal contamination

Answer 1

The term "heavy metal", despite being widely used among professionals and scientists, has no strict scientific basis or a chemical definition. Although many of the items listed in the term "heavy metal" have a specific gravity greater than five, there are several exceptions to this rule.
Strictly, and from the chemical viewpoint, heavy metals consist of transition elements and post transition including some metalloids such as arsenic and selenium. These elements have a specific gravity significantly greater than that of sodium, calcium, and other light metals. Furthermore, these elements are shown in different oxidation state in water, air and soil and have varying degrees of reactivity, ionic charge and solubility.
An optional way to name this group is as "toxic elements", which, according to the list of priority pollutants of the Environmental Protection Agency of the United States (USEPA), include the following elements: arsenic, chromium, cobalt, nickel, copper, zinc, silver, cadmium, mercury, titanium, selenium and lead.