The Georges Bank monitoring program 1985: Analysis of trace metals in bottom sediments during the third 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 period when eight exploratory wells were drilled (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 higher than the predrilling concentration at this location by a factor of 5.9. This maximum 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 the 11 other metals have been observed in bulk sediments at any of the locations sampled in this program. We estimate that about 25 percent of the barite discharged at block 312 was present in the sediments within 6 km of the rig, 4 weeks after drilling was completed at this location (drilling period was December 8, 1981-June 27, 1982). For almost a year following completion of this well, the inventory of barite decreased rapidly, with a half-life of 0.34 year. During the next year, the inventory decreased at a slower rate (half-life of 3.4 years). The faster rate probably reflects resuspension and sediment transport of bariterich material residing at the sediment surface. Elevated barium concentrations in post-drilling sediment-trap samples from block 312 indicate that such resuspension can occur up to at least 25 m above the sea floor. As the remaining barite particles are reworked deeper into the sediments by currents and bioturbation, removal by sediment-transport processes is slower. The barite discharged during the exploratory phase of drilling is associated with the fine fraction of sediment and is widely distributed around the bank. We found evidence for barium transport to Great South Channel, 115 km west of the drilling, and to stations 2 and 3, 35 km east of the easternmost drilling site. Small increases in barium concentrations, present in the fine fraction of sediment only, were measured also at the heads of both Lydonia and Oceanographer Canyons, located 8 and 39 km, respectively, seaward of the nearest exploratory well.