Mesocosm study with thermally treated (TCC) and water-based drill cuttings (WBM)

Mesocosm and bottle slurry experiments were conducted to assess and compare the effects of thermally treated cuttings (TCC) versus cuttings with water-based mud (WBM) on benthic communities. While WBM has been discharged for a long time, discharges of TCC have not yet been practised routinely. In a mesocosm experiment, cuttings were added in a layer thickness of 6.3 mm in box-core samples from the Oslofjord, and effects measured on benthic community structure, microprofiles of O2 and biogeochemical fluxes. In addition a bottle incubation experiment was performed on the same mud materials. Results from both experimental approaches showed significantly increased biodegradation measured as consumption of O2 and nitrate+nitrite in WBM and TCC treatments compared to controls. The biodegradation product ΣCO2 was released from WBM, but surprisingly consumed in TCC. This was presumably caused by precipitation of CaCO3(s) triggered by the mud ingredient Ca(OH)2(s) present in TCC. There was a significantly different impact on the benthic communities with mass mortality and reduction in macrofaunal biomass in TCC treatments, but unaltered faunal response in WBM exposure. The documented adverse effect of TCC cuttings was possibly due to intolerable alkaline conditions induced by the calcium oxide.