Inpacts of exploratory drilling for oil and gas on the benthic environment on Georges Bank

Abstract

A 3-year monitoring program was performed to assess the impacts of exploratory drilling for oil and gas on the benthic environment of GeorgesBank, an important commercial fishery region in the North Atlantic east of Massachusetts, USA. Surficial sediments were sampled for chemical and benthic infaunal analysis and bottom still photographs were taken to document bottom microtopography and epifauna at 46 stations during 12field surveys. The surveys were performed quarterly from just before drilling began, during drilling, and for nearly 2 years after completion of drilling. Two of the eight drilling sites were selected for monitoring. Twenty-nine stations were positioned in a tight radial array around a drilling site in 80 m of water. A second group of three stations was positioned near another drilling site in 140 m of water. The remaining stations covered a broad expanse of the Bank and adjacent suspected sites of deposition of fine-grained sediments.

Of the 12 elements analyzed in bulk sediments, only barium increased in concentration during the period when drilling was taking place (July 1981 to September 1982). The concentration of barium in surficial sediment increased 4.7-fold from 28 ppm before drilling to 131.7 ppm after drilling at the station closest to the drilling site in 80 m of water and 5-9-fold from 32 ppm before drilling to 172 ppm after drilling at the station closest to the drilling site in 140 m of water. The concentrations of both barium and chromium increased in the fine (silt/clay)fraction (usually less than 5% by weight of sediment frommost stations) of sediments from several stations around one or both rig sites weight of sediment from most stations) of sediments from several stations around one or both rig sites monitored during the period of drilling. Elevated concentrations of chromium (about two fold) occurred only in sediments near the drilling site in 140 m of water. Statistically significant increases in the concentration of barium in the    fine fraction of sediment were detected approximately 65 km west (downcurrent) and 35 km east of the drilling site in 80 m of water after drilling was completed.

The benthic fauna were abundant and diverse throughout the study area. At most stations, the dominant species rernained nearly the same frorn one season to another over the 3 years of sampling. Polychaetes were the most abundant, followed by crustaceans. The number of some species, partieularly the amphipods Erichthonius fasciatus and Unciola inermis, showed large seasonal variations.