Contaminant leaching from drill cuttings piles of the northern and central North Sea: a review

Abstract

From the Executive Summary: “This review concentrates on contaminant leaching from drill cuttings piles that have accumulated on the seabed of the northern and central North Sea (56°N-62°N) since th first exploratory wells were drilled in 1961. Drill cuttings and drill cuttings piles refer to the accumulated mixture of drilling mud, fluids and solids, rock fragments, sediment and speciality chemicals resulting from the drilling of exploration, appraisal and production wells. Higher concentrations of certain metals (Cr, Cu, Ni, Pb, Zn and especially Ba) and hydrocarbons are observed in association with the cuttings piles above that seen in natural North Sea sediments.

Once on the seafloor microbial reactions begin to consume available oxidants and the resulting diagenesis may result in the flux of metals and contaminants into and out of (leaching) the cuttings piles. The fate of metal contaminants within the cuttings piles is influenced by: particle size, organic matter content, the type of benthic fauna and the local sedimentation rate along with biogeochemical pathways such as adsorption and desorption from oxyhydroxides of iron and manganese, adsorption into organic matter or the assimilation into the gut of benthic infauna). Hydrocarbons within the cuttings piles remain relatively unchanged with time as a result of the piles depleted oxygen, low ambient temperature, the type of drilling fluids used and the lack of significant bioturbation.

The noticeable effect of drill cuttings piles on nearby benthic communities (~1-2 km) has been well reported. The influence ranges from physical smothering, organic enrichment and chemical contamination (by hydrocarbons, heavy metals, speciality chemicals and sulphides).

Generally drill cuttings piles contain significant amounts of contaminants, the levels of which decrease rapidly from source. No drill cuttings pile is the same, each represents a unique combination of sediment signature, contaminants and benthic community and each is affected by the local hydrodynamic regime.”