Inverse square doesn't work well for most ground contaminants. Rainfall tends to collect the contaminant back into streams and rivers then transport it downstream where concentration is based on different math. Initial concentration * e ^ (-1 * constant * time) where the constant varies per material is how it's typically simplified for point discharge. There are a variety of factors in a material that was concentrated, released in various forms, re-concentrated as runoff, then transported downstream as it settles.

My understanding is that decent modeling software exists but it takes time and money to get decent environmental engineers to do an analysis.