Laboratory and field studies of the aeration of mangrove sediments as a bioremediation method

Six blocks of mangrove sediments, measuring 30 cm x 30 cm x 15 cm, complete with crab burrows and root fragments were collected one metre within the edge of a Rhizophora stylosa forest adjacent to Cocoa Creek, Cape Cleveland, North Queensland. Sediment blocks were placed in individual plastic bins to form temporary mesocosms. Seawater was added to the mesocosms at 15 and 23 hours daily and siphoned off after one hour to simulate tidal cycles. Light conditions were kept low and shade cloth covers were used to inhibit growth of an algal mat which might alter oxygen conditions in the sediments.Magnesium peroxide powder, an oxygen release compound, (3.14 g dry weight) was evenly distributed along a 13 cm long and 4 cm deep cut into three of the mesocosms. After 40 hours, vertical oxygen profiles were measured down to the aerobic-anaerobic zone interface in 100 µm increments at three sites along the centre line of the buried compound in each mesocosm.In each of the three remaining mesocosms three pre-disturbance vertical oxygen profiles were measured along a 13 cm long and 4 cm deep cut in the sediment. An airstone was then placed into the 4 cm deep cut, with its flat upper surface approximately 2 cm below the surface and completely buried. Air was administered to the sediments via the airstone at a rate of 1L/min at 400 kPa. Following aeration treatments for 40 hours, the aerobic zone was described in reference to the airstone. Oxygen concentration of sediments was measured for several profiles taken vertically from the surface. The first profile was taken near the centre, immediately adjacent to the airstone, and 3.5 cm from the input end. Each successive profile was taken 5 mm further away from the previous, perpendicular to the airstone, until they resembled pre-disturbance profiles. Air flow continued whilst measurements were taken. To establish whether burial of the airstone might influence the oxygen profiles, an additional three profiles were measured after burial of the airstone and before aeration in one of the mesocosms.Field trials of forced aeration were conducted in a Rhizophora stylosa mangrove forest at Fishermans Landing, near Gladstone. The procedures, described earlier, for positioning the airstone in sediments and the depth of burial and measuring oxygen saturation, were repeated in this trial. The delivery of air was pulsed in field trials since this was considered sufficient to aerate sediments based on observations made during mesocosm trials. The pulse cycle used was 60 minutes airflow and 60 minutes off, repeated for around 40 hours prior to measurements being taken.
 As the majority of hydrocarbons introduced to sediment in an oil spill initially reside on the surface, availability of oxygen in this layer is critical for biodegradation by the more active aerobic microbes. The objective of this series of experiments was to determine whether forced aeration and/or magnesium peroxide (an oxygen release compound) might provide significant increases in molecular oxygen saturation in the surface aerobic layer of sediments under common tropical mangrove trees.

Organization: Australian Ocean Data Network