more questions pesticide variance

So why are pesticides found in wells?
This begs more questions on variance.
Save for what is put into a septic system or buried in a "disposal pit" or land fill or injected into 5 foot holes around for termite control, how does something get into ground water and then into a well?
We know CT has a decent frost line in winter, 1 to 3 feet, 3 being less likely now a days.
We know ground water in CT generally flows North to south because of the glaiers and the lay of the land.
If pesticide contamination reached a fairly deep level of porous rock, it should maintain a fairly steady level, decreasing over time, if there is a single source.
If it comes from more surface sources, it wll be affected by frozen ground and rainfall.
If it comes from multiple sources, then there sould be a variance over time with overlapping "plumes".

What is it, is the question.