Does aniline breakdown in groundwater produce benzene
Aniline is not easily converted to benzene in aqueous conditions. A google search turns up several papers describing the breakdown products of aniline in groundwater but benzene is not one of them. Methanogenic and denitrifying bacteria seem to play a key part in aniline breakdown, releasing methane and carbon dioxide as byproducts. The other main products seem to be polymerisation products resulting from condensation and oxidation reactions.

In answer to your question about detection; no you would not expect to see benzene as a breakdown product in a sample of groundwater that has been contaminated by aniline.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1519315/