That is, each is at risk from contamination from different sources, each reacts differently to that contamination, and each poses a different problem for the home buyer. Introduction takes the form of spills or leaks from stationary containers or nearby processes, or from transient sources like trucks or ships. It is extremely rare today when an accidental spill or leak goes unnoticed. Waterfront property or property near rail lines or truck routes is obviously more vulnerable than other places. It is the risk we take for living in an industrial society.
Presence is when contamination exists in a medium due to some historical use.
If a subdivision is built over or near an old landfill or industrial site, for example, hazardous substances could surface because of the disturbance caused by housing construction. The presence of existing hazardous contaminants is difficult to detect. Some states require environmental surveys of land before development proceeds. Naturally occurring substances exist in nature without any assistance from us. We simply choose to build homes where these substances exist.