Determination of Sediment Impacts Due to Exploratory Oil Operations Using Likelihood Based Statistics.

Understanding the impacts of human-mediated activity on the benthic environment and the ability to predict its response is increasingly important as the ocean bottom is utilized for resources. We performed an analysis of several benthic metrics measured before, during and after exploratory oil drilling operations in the Chukchi Sea. The Chukchi Sea Environmental Studies Program (CSESP) was an extensive multidisciplinary effort conducted by a coalition of participants from 2008 - 2014. CSESP provided the background data to characterize pre-existing sediment conditions at the drill site (Phase I). Near-term post-drilling (Phase III) and longer-term post-drilling surveys (Phase IV) were also conducted. Sediment profile imaging (SPI) surveys were conducted for all three phases while infaunal grabs for benthic community analysis and sediment chemistry were obtained during Phases I and IV. The Phase I data were used to represent unimpacted conditions while Phase III represented the maximally impacted state. The apparent color redox potential discontinuity was determined from the SPI images and a multinomial likelihood ratio (LR) was calculated to provide a quantitative statistical measure as to whether Phase IV conditions resembled pre- versus post-drilling conditions. Several benthic community metrics (total abundance, Pielou’s Eveness, and total taxa) were analyzed and the LR was calculated. Results showed 3 of the 15 locations sampled remained impacted. Two of these locations were areas where the sediment sample contained drill cuttings, and one displayed elevated levels of barium, indicating a potential physical deposition effect caused by drilling activities. Based on the data from Phases I through IV, the findings related to authorized discharges in 2015 indicate a small vertical and horizontal benthic footprint (<100 m from drill site) indicative of drilling activities but no larger benthic changes relative to the background variability observed during the CSEP studies.