Mark Buckley
Petroleum Hydrocarbons
The four major hydrocarbons are BTEX -benzene, toluene, ethylbenzene, and xylene.
The major sources are oil spills and leaky pipelines. Petroleum Hydrocarbons are also leaked from underground storage tanks, gas stations, salvage yards, sites of petroleum exploration, and domestic pollution.
In the U.S., there are on average 83 major crude oil spills per year, which average 50,000 barrels total per year.
http://toxics.usgs.gov/pubs/wri99-4018/Volume3/SectionB/intro.html
Useful EPA PDF file general groundwater contamination sources, by chemical
http://www.epa.gov/seahome/groundwater/src/quality.html
http://www.epa.gov/OGWDW/Pubs