Petroleum hydrocarbons along the Arabian Gulf coast of United Arab Emirates

In addition to 20 sediment samples, surface and bottom seawater samples were collected monthly during 1993-1994 at 24 sampling stations. Oil concentrations in seawater were below 13 mu g l(-1) and ranged from 0.19 to 12.95 mu g l(-1) Comparatively higher concentration levels (5.97 mu g l(-1)) were recorded at off-shore stations which are located close to oil tankers' routes in the Arabian Gulf. The other stations, on the other hand, which represent coastal waters of the United Arab Emirates showed lower concentration levels (4.18 mu g l(-1)). Oil pollution levels were also higher at surface layers (4.91 mu g l(-1)) as compared with bottom layers (3.70 mu g l(-1)) reflecting higher rate of oil dissolution at surface layers. The presence of oil pollution, on the other hand, at bottom layers was probably due to: 1) the role of bottom sediments on the enrichment of overlying water layers by hydrocarbons, 2) the role of precipitation of thick residual oils after evaporation of lower-boiling-point components of the oil and the turbulence of water column. The present study revealed comparatively lower levels (3.93 mu g l(-1)) and lower vertical gradients (1.17 mu g l(-1)) for the whole area in the summer season compared to the winter season (4.18 and 1.64 mu g l(-1), respectively) as a result of a maximum rate of evaporation of lower boiling-point components of the oil and sinking of thick residual oils during summer months in addition to the intensity of tanker traffic at the time of sampling. Sediment analysis for petroleum hydrocarbons suggested the presence of appreciable quantities of petroleum hydrocarbons in sediments, particularly those from off-shore areas (7.0 mu g g(-1)). However, the levels reported here and in particular those for coastal areas (6.5 mu g g(-1)) were below 10 mu g g(-1) of dry weight which correspond to hydrocarbon concentrations in coastal areas considered unpolluted.