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Historical contamination of Mississippi River Delta, Tampa Bay, and Galveston Bay sediments
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1998
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[PDF-7.19 MB]
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Description:In order to obtain sediment which has accumulated over the past 100 years or so, 50-80 cm long sediment cores were collected from the submarine Mississippi River Delta, Galveston Bay, Texas and Tampa Bay, Florida. The cores were extruded and sliced into 1 cm thick sections which were then radiometrically age dated and analyzed for those organic compounds and trace metals suspected of being contaminants in the sampling areas.
The Mississippi Delta core was collected about 24 Krn due west of the mouth of Southwest Pass. Radiometric dating showed the sediment accumulated at a relatively constant 0.8 c d y r with no evidence of physical or biological disturbance. Concentrations of total polycyclic aromatic hydrocarbons (PAH's) in these sediments increased sharply from about 200 nglg in the early 1940's to near 800 nglg in the 1970's. The relative importance of individual PAH's to the total PAH concentration varied through time and suggested both petroleum and combustion sources. The depth distribution of total polychlorinated biphenyl (PCB) concentrations was similar to that of the PAH's, with a peak of 20 nglg (10 times background) in sediment deposited in the 1970's. Organochlorine pesticides were low throughout this core but increased near the core top.
Several metals showed concentration gradients in the Mississippi core similar to those of PAH's and PCB's, i.e., peaks in sediment deposited in the 1970's. For Pb, the peak concentration of about 35 uglg is about 40% above background and corresponds to a peak in leaded gasoline use. Silver and cadmium concentrations are more than twice background in the 1970's sediment, and like Pb have decreased in recent times.
The Galveston Bay core sediment had low concentrations of all organic contaminants. Total PCB and DDT concentrations ranged from near zero to 13.8 nglg and 0.44 nglg respectively, while total PAH's ranged from 89 to 479 nglg . The highest concentrations were found at 10- 12 cm, corresponding to about 1970. Most metal concentrations in this core were near background and showed little variation with depth (time). Silver and cadmium, however, showed a small increase in recent times and barium showed a 5-fold increase, starting in the 1940's when barium sulfate-rich oil well drilling muds began to be used in this area.
The Tampa Bay core used for this study was from very shallow water and was coarse grained and carbonate rich. The A1 and Fe concentration varied with depth, which is an indication of variation in grain size and mineralogy. Such variations affected all metals and thus metals ratios were examined for evidence of human influence. Little clear evidence of metal contamination was found. Organic contaminants, however, were in higher concentration in the Tampa core than in either the Galveston or the Mississippi core. Organic contaminants were higher at the core top but showed peaks at various depths complicating interpretation.
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