Chemical and bacterial changes during laboratory conditioning of formulated and natural sediments

Within the framework of toxicity testing using formulated sediment, a conditioning treatment prior to toxic contamination has been examined. This preliminary step enables the bacterial colonisation of the sediment, the initiation of organic matter degradation, and the establishment of stable biological and physico-chemical conditions. The treatment involved in keeping the formulated sediment under water in conditions similar to that chosen for toxicity tests. The behaviour of a formulated sediment was compared with a natural sediment. The monitoring of physico-chemical and biological parameters of sediment and water column was carried out over a 30-day incubation in two laboratories. The parameters of pH and redox, dissolved organic carbon (DOC), NH4 and NO2, total organic carbon (TOC) were measured. The bacterial community was characterised by the determination of bacterial density, in total bacteria number or colony forming units (CFU), several exoenzymatic activities (P-glucosidase, xylosidase, leucine-amino-peptidase phosphatase and sulfatase activities), and three gas productions (CO2, N2O and CH4). The same experiment was carried out with a natural sediment. A 10- to 15-day conditioning allowed a physico-chemical stabilisation and corresponded to kinetic changes in hydrolysis activities. As compared to data of the natural sediment, the biological activity of the formulated sediment showed a different dynamic with lower activity levels. For both sediments, an important decrease of activities levels was observed after 15 days because of a substrate limitation. The work showed that a preliminary conditioning treatment of a formulated sediment provides the stabilisation of parameters that can affect toxicant bioavailability. Additional research is needed to determine the real influence of conditioning on the bioavailability of contaminants. The possible advisability of organic matter input, to maintain the sediment bacterial activity, has to be studied.