Changes in dissolved hydrocarbon content of shallow waters during and shortly after the Deepwater Horizon incident

摘要

The Deepwater Horizon incident began on April 20th 2010 and oil was released into the Gulf of Mexico until July 15th 2010. Over the 1,500 m ascent to the surface, mixing between the rising hydroccarbons and seawater, as well as mixing between the surface slick and seawater resulted in dissolution of components susceptible to partitioning into the aqueous phase. Such compounds include benzene, toluene, ethylbenzene, xylenes, naphthalenes and polyaromatic hydrocarbons. This paper focuses on the dissolved hydrocarbon compounds and oil droplets detected and measured in the surface waters of the Gulf of Mexico during and shortly after the release. Between the 5th of June and 14th of September 2010 the abundance and distribution of these compounds using a prototype hydrocarbon sensor array tuned to detect the presence of dissolved petrooleum hydrocarbons in waters 1-2 meters below the sea surface. The aim of the survey was to perform continuous monitoring of the surface coastal waters in transects parallel to the coastlines of Mississippi, Alabama and Northwestern Florida to determine and map hydrocarbon levels associated with the Block MC252 oil spill The prototype hydrocarbon sensor array used to understand the dissolved hydrocarbon content in shallow waters of the Gulf of Mexico is shown in Figure 1. The system comprised an underwater pump which was towed between 1-2m below the surface. Water was pumped through a food grade tube into a stainless steel tank located on the vessel’s deck. Into this tank, four commercially available hydrocarbon sensors were mounted, past which the water continuously flowed. The Chelsea Aquatracker (S1), Contros (S2) and Trios (S3) fluorometer sensors have excitation and emission wavelengths specifically chosen to detect polyaromatic hydrocarbons (PAH’s, Table 1). The fourth sensor (S4) is a metal oxide (MOS) sensoor which has a partitioning membrane in front it to allow volatile hydrocarbons, but not water, to pass into the measurement chamber. These sensors were chosen for their range in detection (Table 1) and to provide redundancy in measurement, and were operated continuously while the survey vessel was underway. The sensoor signals were logged every 2 seconds via a logging station, with a GPS location appended to each record. During normal operation, the water pumped through the tank passed directly back into the ocean via an outlet hose. However, periodically or when there are changes in the sensor readings a valve is opened at the rear of the tank allowing the collection of a water sample. These 1 litre samples were then subjected to liquid-liquid extraction using dichloromethane and the extract obtained was analysed using gas chromatography mass spectroscopy (GCMS) instrumentation on board.