What are Heavy Metals?
The US EPA's Terms of Environment defines heavy metals as "Metallic elements with high atomic weights; (e.g. mercury, chromium, cadmium, arsenic, and lead); [that] can damage living things at low concentrations and tend to accumulate in the food chain." The term is a bit confusing as some authorities apply it to metals that are not particularly heavy, and sometimes to elements that are only somewhat metallic. See Heavy Metals: A Meaningless Term? for more on that.
In the world of environmental remediation, the term usually refers to one or more of about two dozen elements that may be present at a toxic waste site. Though it is difficult to generalize, there are a half dozen or so metals that often pose the greatest risks at toxic waste sites, and "drive" cleanups at those sites. This may be due to their relatively high toxicity, or it may be because they are often present at high concentrations. My candidates for the top half dozen are:
Arsenic (a metalloid*)
Beryllium (a very light metal)
Chromium (particularly the hexavalent form)
Other possibilities include:
Antimony (a metalloid*)
Lithium (another very light metal)
Selenium (a metalloid*)
*a metalloid is an element possessing both metallic and non-metallic properties.