Studies on the impact of a water-based drilling mud weighting agent (Barite) on some Benthic invertebrates

Abstract

A comparative study was carried out to observe effects of standard grade and fine grade barite on the filtration rates of four suspension feeding bivalves, Modiolus modiolus, Dosinia exoleta, Venerupis senegalensis and Chlamys varia. Standard grade barite, the most commonly used weighting agent in water-based drilling mud, was responsible for altering the filtration rates of the four bivalve species and damaging the gill structure. The four bivalves were exposed to 0.5mm, 1.0mm and 2.0mm daily depth equivalents of standard grade barite, which permanently remained in suspension. All three barite levels altered the filtration rates leading to 100% mortality. The horse mussel, Modiolus modiolus was the most tolerant to standard barite with the scallop, Chlamys varia the least tolerant. Fine grade barite, at a 2mm daily depth equivalent, also altered the filtration rates of the four bivalve species, but only affected mortality of Venerupis senegalensis, with 60% survival at 28 days. In-vivo studies showed damage to the gills, ranging from displaced inter-lamellar junctions to the deletion of large parts of demibranch. Post-mortem microscopy studies showed damage to individual filaments with a marked reduction in the active surface area of the gill. Field studies have shown that the presence of standard grade barite is not acutely toxic to seabed fauna but does alter benthic community structure when it is persistent.