Estimation of Bioavailability of Metals from Drilling Mud Barite

EDITOR'S NOTE: This is 1 of 5 papers reporting on the results of a 4-year project to develop an environmental risk-based decision support tool, to assist the oil industry in establishing cost-effective measures for reducing risk to the marine environment from drilling discharges. Drilling mud and associated drill cuttings are the largest volume wastes associated with drilling of oil and gas wells and often are discharged to the ocean from offshore drilling platforms. Barite (BaSO4) often is added as a weighting agent to drilling muds to counteract pressure in the geologic formations being drilled, preventing a blowout. Some commercial drilling mud barites contain elevated (compared to marine sediments) concentrations of several metals. The metals, if bioavailable, may harm the local marine ecosystem. The bioavailable fraction of metals is the fraction that dissolves from the nearly insoluble, solid barite into seawater or sediment porewater. Barite–seawater and barite–porewater distribution coefficients (Kd) were calculated for determining the predicted environmental concentration (PEC; the bioavailable fraction) of metals from drilling mud barite in the water column and sediments, respectively. Values for Kdbarite–seawater and Kdbarite–porewater were calculated for barium, cadmium, chromium, copper, mercury, lead, and zinc in different grades of barite. Log Kdbarite–seawater values were higher (solubility was lower) for metals in the produced water plume than log Kdbarite–porewater values for metals in sediments. The most soluble metals were cadmium and zinc and the least soluble were mercury and copper. Log Kd values can be used with data on concentrations of metals in barite and of barite in the drilling mud–cuttings plume and in bottom sediments to calculate PECseawater and PECsediment.