Probability is the extent to which something is probable; the likelihood of something happening or being the case. (Google)

Probability is a scale of certainty:

How certain is it that an undeveloped piece of land will be contaminated from spills of industrial hazardous substances?

A possible event is able to be done; it is within the power or capacity of someone or something, it can happen, can be done.

How possible is it for the soil in the above site to be contaminated from spills of industrial hazardous substances?

Throw leaking drums on the land and watch!

While it is possible for a piece of land to be contaminated, the question remains as to how probable it is.

In the Phase I environmental site assessment we deal with probabilities.

We research records, inspect the site, and arrive at an estimation of the probability for environmental infliction to have occurred.