Volatile liquid hydrocarbons in waters of the Gulf of Mexico and Caribbean Sea1

Concentrations of volatile liquid hydrocarbons (VLH), C 6 −C 14 hydrocarbons, were determined in 1977 in coastal, shelf, and open‐ocean surface waters of the Gulf of Mexico and Caribbean Sea. In open‐ocean, nonpetroleum‐polluted surface water, VLH concentrations were 60 ng·liter −1 while in heavily polluted Louisiana shelf and coastal water values reached ≈500 ng·liter −1 . Caribbean surface samples had very low concentrations, ≈30 ng·liter −1 . The relationship between anthropogenic gaseous hydrocarbons and VLH was approximately linear. Aromatic VLH accounted for 60–85% of the total VLH in surface waters. Cycloalkane concentrations were <1.0 ng·liter −1 in open ocean water, 60–100 ng·liter −1 in polluted water (20% of total VLH). Alkanes were ≈15 ng·liter −1 in open ocean water, 40 ng·liter −1 in polluted water. The concentrations of five major VLH compounds (aromatics) in water samples—benzenc, toluene, ethylbenzene, m ‐, p ‐xylenes, and o ‐xylene (called BTX)— were sufficient to predict the total VLH. The empirically determined relationship is VLH(ng·liter −1 ) = 1.42 BTX (ng·liter −1 ); r = 0.96. Subsurface VLH concentrations in samples of polluted waters collected from depths of 50 in were only 35–40 ng·liter −1 below surface concentrations. Open ocean subsurface samples had concentrations of only ≈30 ng·liter −1 at 30–50‐m depths, comparable to those of Caribbean surface water.