Concept:Hazardous wastes can leak harmful substances into the environment, especially water sources.Explanation:When hazardous wastes are dumped or stored improperly, toxic chemicals can seep into the soil and reach underground water supplies.This contaminates groundwater, which is a major source of drinking water for many communities.Groundwater contamination is considered the most serious environmental effect because it is difficult to detect, clean up, and can affect human health over a wide area.Answer:A. contamination of groundwater