干旱区生态环境知识资源中心

Burial of spent drilling fluids used in petroleum and natural gas exploration was evaluated for reducing soil contamination caused by conventional, surface disposal of these wastes on arid and semiarid rangelands. Simulated reserve pits at two locations provided burial depths of 30, 90 (with and without a 30-cm capillary barrier of coarse limestone), and 150 cm below the surface, with sequential replacement of stockpiled subsoil and topsoil. The drilling fluids contained extremely high concentrations of soluble salts [electrical conductivities (EC) = 155-185 dS m-1], with Na and Cl being the dominant soluble ions. Upward migration of contaminants was evaluated over a 20-month period. Soluble salts migrated upward 15 to 30 cm into the overlying soil, and salt movement appeared to be governed to a greater extent by diffusive rather than convective flow mechanisms. Capillary barriers of coarse limestone effectively reduced salt movement at one of the two sites. Sodium, Ca, and Cl were the dominant mobile ions. Exchangeable Na percentages did not increase in soil increments > 15 cm above buried drilling wastes. Barium, Cr, Cu, Ni, and Zn in drilling fluids did not migrate into overlying soil. Movement of contaminants was similar where fourwing saltbush [Atriplex canescens (Pursh) Nutt.], a deep-rooted shrub, and buffalograss [Buchloe dactyloides (Nutt.) Engelm], a shallow-rooted grass, were used for revegetation. Selective-placement burial of drilling fluids offers an effective and environmentally sound alternative to conventional surface disposal. The probability of upward movement of excessive soluble salts into the plant root zone appears greater for 30-cm burial, compared to 90- or 150-cm burial, because of greater soil water contents at the soil/waste interface and greater upward water potential gradients in the surface 30 cm.