The Georges Bank monitoring program, 1984: Analysis of trace metals in bottom sediments during the second year of monitoring

Abstract

Of the 12 elements analyzed in bulk (undifferentiated) sediments collected adjacent to drilling rigs on Georges Bank, only barium was found to increase in concentration during the drilling period (July 1981 until September 1982). The maximum postdrilling concentration of barium (a major element in drilling mud) reached 172 ppm in bulk sediments near the drill site in block 410. This concentration is a factor of 5.9 times higher than the predrilling concentration at that location. This maximum postdrilling barium concentration is within the range of predrilling concentrations {28-300 ppm} measured in various sediment types from the regional stations of this program. No drilling-related changes in the concentrations of chromium or other metals have been observed in bulk sediments at any of the locations sampled in this program to date. We estimate that between 21 percent and 31 percent of the barite (principal barium-bearing mineral} discharged at block 312 was present in the sediments within 6 km of the rig, 4 weeks after drilling was completed. The barite deposited near this well was found to decrease in concentration with a half-life of 0.4year. At this rate, the average barium concentration in sediments within 6 km of the drilling rig in block 312 is expected to be only 10 percent higher than the predrilling concentration within approximately 1.5 years. Although the inventory of the barite discharged on Georges Bank is based on only a few data points, most (approximately 69 percent) of the barite discharged by the eight exploratory wells apparently can be found in sediments west of the drilling locations. The increase in barium concentration above background can be measured in the fine fraction of sediment at a distance of 65 km to the west of block 312. Analysis of sediment-trap samples collected 25 m above the bottom in block 312 indicates that the dispersion of barmm-rich fine sediment is enhanced by resuspension from the sea floor and transport to the west with the mean current flow. Evidence exists of small accumulations of barium near the heads of Lydonia and Oceanographer Canyons. However, the increased concentrations can be defined only by analyzing the fine fraction of sediment.