A coliform test can tell if your well is impacted by septic leakage or manure runoff, but it won’t tell you if residues from agricultural chemicals or spilled gas or oil are getting in your water.
Those are very different kinds of tests.
Across the northeastern US in general, most aquifers are shallow; on average less than 80 feet below the surface.
A drilled well is more secure, but regardless how deep it is, it’s still vulnerable to surface contamination near the wellhead.
Many older pesticides contained high levels of lead, arsenic and copper, heavy metals which do not break down, and some farms still have high levels of these metals in the soil.
How do organic pollutants get into our water?
It’s shockingly easy to pollute groundwater here in the northeast where it rains a lot and the distance to groundwater is relatively small.
(Fortunately, the odor threshold for benzene is 50-100 ppb, so you’d never be drinking benzene at that level).
Testing for organics is complicated: for example, checking for gas and solvents, pesticides, and antifreeze all require different tests.
Most contaminants can be removed with the right kind of filtration system, but systems can be quite expensive to maintain.