What does ethylene glycol decompose to when exposed to air for an extended period of time?
This image highlights the oxidation process of ethylene glycol fairly well




( Taken from Ethylene glycol: properties, synthesis, and applications )


Since it has little tendency to evaporate, unless it is kept in direct sunlight, it would probably oxidize to a mixture of the above liquid compounds (glyoxal, glyoxalic acid) and not directly to water. If it is kept in sunlight, the reaction would be photochemically catalysed and a it may degrade to carbon dioxide faster, but according to theoretical principles you would not get much water.
Ethylene glycol is relatively hydrophilic and also has a low tendency to separate from water, so the only major degradation process available to it in soil would be biodegradation by either aerobic or anaerobic process, which is NOT the same as atmospheric oxidation1
Not sure about reaction energetics regarding the same


Most of the danger regarding glycol spills (and other chemicals in the environment) comes from absorption by other organisms; since glycol is comparatively less volatile and more hydrophilic than, say ethanol, it has a greater tendency to be absorbed by organisms.

1 Taken from William R. Roy, in Handbook of Antistatics (Second Edition), 2016