Field and laboratory investigation of tarmat deposits found on Ras Rakan Island and northern beaches of Qatar

Abstract

Beaches of Ras Rakan Island, located off the northern tip of Qatar, are extensively contaminated by highly weathered tarmat deposits. The focus of this study is to determine the possible source of the contamination and complete a preliminary assessment of its potential environmental impacts. The field data collected at this site indicated that the tarmat residues contained highly weathered, black, asphalt-like material and the contamination problem was widespread. Based on these field observations, the following two hypotheses were formulated: (1) the tarmats must have formed from the residual oil deposited by a relatively large, regional-scale oil spill event, and (2) the oil spill must be relatively old. As part of this study, we collected tarmat residues from several beaches located along the northern region of Qatar Peninsula. We found the hopane fingerprints of these tarmat samples were identical to the fingerprints of the samples collected from Ras Rakan Island. These results together with our physical field observational data validated our hypothesis that the oil spill should have been a regional-scale event. Furthermore, we compared the measured hopane fingerprints of our field samples with fingerprints of reference crude oils from Qatar, Saudi Arabia, and Basrah (located close to Kuwait border), and with the literature-derived hopane fingerprints of Kuwaiti and Iranian crude oils. This analysis indicated that the hopane fingerprints of the tarmat samples closely matched the Kuwaiti and Basrah crude oil fingerprints. Since there were no known oil spills of Basrah crude in this region, the highly weathered, asphalt-looking tarmats should have most likely formed from the 1991 Gulf War oil spill, an old oil spill. The concentrations of parent and alkylated PAHs in the tarmat samples were also quantified to provide a preliminary assessment of potential environmental risks posed by these tarmats to Qatar's coastal ecosystem.