Metal Partitioning in Ilmenite- and Barite-Based Drill Cuttings on Seabed Sections in a Mesocosm Laboratory

Abstract

Summary Ilmenite and barite minerals are alternative weight materials frequently used in drilling muds and are discharged to the marine environment. These minerals contain heavy metals that may become remobilized and available for uptake in biota after deposition in sediment. Several oil companies have jointly developed a model to predict the environmental risks associated with discharges of drill cuttings and mud to sea (DREAM). This model applies partition coefficients (Kd) for estimating heavy-metal concentrations in sediment and interstitial waters from which toxic impacts are derived. Kd depend on properties of both the particulate phase and the solute and consequently on local environmental conditions such as redox potential and pH. The literature displays Kd values ranging over several orders of magnitude, and it is a challenge and a major environmental issue to determine the most representative Kd values for drilling discharges in marine offshore areas. An experiment was performed in box-core samples transferred from a fjord location to a marine soft-bottom mesocosm. Weight minerals and cuttings sampled from offshore drilling operations were added in thin layers on top of the fjord sediment in each box. During the next 3 months, the added layers were reworked by the naturally occurring benthic fauna. Environmental parameters such as dissolved oxygen, hydrogen sulfide, and redox potentials were determined as well as metals in added solids, sediments, interstitial water, and fluxes across the sediment/ water interface. Kd values were obtained for seven metals. To the extent that the experimental environment was able to mimic the offshore environment with respect to bioturbation and physicochemical gradients within the sediment surface, the Kd values obtained are considered relevant for modeling interstitial-water concentrations and toxicity in offshore cuttings deposits.