They accumulate in various organs, but mainly the liver. Those who handle organic solvents in their work are often at greater risk of liver disease, especially if the exposure was years ago before modern workplace safety standards.

(this is also true of animals, some authorities advise against eating the liver of livestock raised in non optimum conditions)

In Europe and presumably elsewhere there is considerable concern about "volatile organic compounds" entering the environment and various restrictions exist on uses.
Levels of VOCs in paints and varnishes for example have been reduced, and it has been suggested that water based rather than solvent based products be used where possible.

Perchloroethylene (perc) is a solvent used for dry cleaning and many other purposes. Unless inhaled or ingested in large quantities it was considered low risk, but is now considered dangerous in the long term.
Most organic solvents are not readily mixed with water (try mixing gasoline and water !)They do however dissolve in water in minute quantities.
An activated carbon filter will remove such contamination, although a typical filter can only remove minute quantities, this is fine in practice as only minute quantities CAN be dissolved in water,
In the example given of a car crashing into a reservoir, almost all of the gasoline would float to the surface and evaporate. Only a minute trace would dissolve in the water.