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

Abstract

AbstractWeight minerals used in drilling muds contain traces of heavy metal contaminants (Neff, 2005). After discharge from drilling operations, drill cuttings with associated mud and weight materials end up on the seabed at which some of these metals may become remobilised to pore water and pose a risk to organisms living in or close to the sediment surface (Ruus et al., 2005). Reported effects of metal toxicity from weight materials are scarce (Olsgard and Grey, 1995; Grant and Briggs, 2002). Nevertheless metal toxicity has been addressed in a model developed by several oil companies to predict the environmental risks associated with discharges of drill cuttings (Singsaas et al., 2008, Smit et al., 2008). In this model (DREAM), partition coefficients (Kd = Csed/Cpw) are used to estimate metal concentrations in sediments (Csed) and pore waters (Cpw). Often, the concentration of metals in the discharged cuttings is known or easily determined. The concentration of the corresponding metals in the pore water can then be calculated (Cpw= Csed/Kd) and the toxicity assessed from the ratio between the Predicted Ecosystem Concentration (PEC) and the Potential No Effect Concentration (PNEC) below which there is no risk for toxic effects. This implies that correct values for Kd are crucial to the validity of model predictions.However, the literature displays Kd-values ranging over several orders of magnitude (Neff, 2008). Some of the reasons may be that the pore water concentration will depend not only on the concentration of the trace metal contaminant in the weight mineral, but also on the the type and quality of the minerals involved and the environment in which they are exposed. Traditionally, Kd values have been determined in laboratory experiments conducted by mixing a given solid phase phase with water and allowing the mixture to approach equilibrium before phase separation and determination of concentrations in the solid and dissolved phase (Reuber et al., 1987). For a given weight mineral, a high number of experimental runs is required to obtain the relevant range of environmental conditions in the sample (Crecelius et al., 2007). However, any well mixed sample would fail to mimic the environment at the interface between a seabed deposit and the overlying water. The top few cm of the deposit is the habitat of most of the organisms exposed to drill cuttings. It is characterised by steep gradients with regard to dissolved oxygen, redox potentials and concentrations of ions available for metal complexation and precipitation. Even the pH may vary by 0.5-1 pH units, which is a large variation compared to the range of pH in sea water habitats. In addition lithogenous bacteria may be present to utilise redox sensitive metal species and sediment dwelling animals will be present to create sediment mixing and extend the sediment-water interface to a variety of coves and irrigated burrows (Aller, 1988). Therefore, as a supplement to standard experiments, studies of metal mobility in an environment as similar as possible (or feasible) to a seabed deposit might provide Kd values which are more representative for the target environment in models such as DREAM or the risk assessment tool developed by the Norwegian Pollution Control Authority (SFT, 2007).Therefore, an experiment was conducted in which metal partitioning from ilmenite (a titanium oxide) and barite (mostly barium sulphate) was investigated in box core samples transferred from a fjord location to a benthic mesocosm. In these experiments, collection and transport of box core samples is performed with minimum disturbance of sediment structure and biological niches. Previous studies (Schaanning et al., 2008b, Tranum et al., 2009) have shown that control samples can be maintained in the mesocosm for several months without severe changes of biogeochemical stratification or macrobenthic diversity. In the mesocosm, triplicate seabed samples were treated by addition of thin layers of weight materials and water based cuttings sampled from offshore drilling operations. During the following 3 months, metal partitioning and environmental parameters were measured in the boxes.