Primary source: from contact with human excreta or animal waste and mining chemicals for ore abstractions
Most likely: in groundwater when borehole is dug near or in human settlements; downstream rivers (when stream or lake has significant inflow from urban runoff or untreated sewage); mining areas
Effect: the accumulating heavy metals can cause severe (even lethal) health risks through the toxicity of various organs and the nervous system. Children are more susceptible to health risks: heavy metal contaminations contribute to restricted bodily development and growth.
Examples: Mercury, Lead, Copper, Zinc, Cadmium, Chromium.