The Gladstone field experiment, Queensland: Weathering and degradation of hydrocarbons in oiled mangrove and salt marsh sediments: Testing the effects of biomediation strategies

Brief description

Experimental plots were established at three sites within mature stands of Rhizophora stylosa (FLNS1, FLNX8, FLNY9), in an area approved for reclamation by the Gladstone Port Authority. The sites were matched visually for tidal elevation, mangrove tree composition and condition, sediment composition and benthic invertebrate populations. Five plots ranging in area from 26 m² to 53 m² were constructed at each site, so that each plot enclosed at least 10 mangrove trees. Around each plot, prop roots were cut in a path about 0.5 m wide to allow installation of plastic retaining walls. The retaining walls were dug into the mud to a depth of 20 cm and supported to a height of 1 m. A gate was installed to allow tidal waters to move in and out while retaining the oil. Plots at three sites, located outside the reclamation area (FLNN, FLSS, CISS), which were established earlier for dispersant experiments were also used as undisturbed control plots for the mangrove bioremediation experiments. Five replicate plots were established at each of four sites (ABS-1, ABS-2, ABS-3 and ABS-4) in a Halosarcia sp. dominated salt marsh area, approved for reclamation by the Gladstone Port Authority. Enclosures consisted of circular walls of light weight plastic sheeting (Corflute), approximately 1.25 m in diameter, buried 7 cm into the sediment, and standing 33 cm above it. Regular tidal flushing in each plot, occurred through two plastic U tubes, each ~12 mm in diameter, and buried with their outlets on either side of the sheeting wall, within and outside the plot. The salt marsh plots did not have an undisturbed control plot.Before oiling, each mangrove plot was sampled to determine sediment grain size, hydrocarbon content and total organic carbon content. Replicate 7 cm diameter cores were collected from each plot, sliced and the 0-2, 10-12 and 20-22 cm sections from each replicate were pooled.One of five treatments was applied to each plot: Gippsland crude oil only; Gippsland crude oil and bioremediation; Bunker C fuel oil only; Bunker C fuel oil and bioremediation; and controls with no oil. The oils were preweathered prior to application and both oil types were applied at the rate of 5L/m² for for mangrove plots and 2L/m² for salt marsh plots. The average rate of dosing in each plot was calculated from four pooled surface sediment (0-2 cm) samples collected from each plot 40 hours after oiling. Bioremediation treatments were applied 40 hours after oiling. The bioremediation strategy used for the mangrove plots involved forced aeration of mangrove sediments and the addition of nutrients. For the salt marsh plots, only nutrients were applied. At 1, 2, 5 or 6 and 12 or 13 months, 4 replicate 7 cm diameter cores were collected from each mangrove plot, sliced and the 0-2, 10-12 and 20-22 cm sections from each replicate were pooled. For the salt marsh plots, cores were collected with 2 cm diameter plastic tubes after 1, 3 and 9 months. Four replicate 0-1 cm and the 9-10 cm core sections from each plot were combined for analysis. Observations, including the presence of roots, animals, burrows and oil were made before the samples were frozen for later analysis.The following analyses were conducted on the sediment samples: Water content of sediments; Total hydrocarbons determined by gas chromatography with flame ionisation detection (GC-FID); The percentage of unresolved hydrocarbons; Concentrations of individual isoprenoid and n-alkanes (µg/g dry weight); Ratios of isoprenoid to n-alkenes as biodegradation indices.At the end of the experiment (August, 1998), mangrove sediments were sampled for a range of nutrients, including total nitrogen, phosphorus and carbon. Sediment samples were taken from 0-5 cm deep cores using a plastic open mouth syringe, approximately 2 cm in diameter, placed in plastic screw top containers and kept on ice until returned to the AIMS laboratory at Townsville for analysis.
Field trials were carried out to assess strategies developed to enhance in situ bioremediation of oil-contaminated sediments in mangrove and salt marsh habitats.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Statement: Pre-weathering of oils:Oils were pre-weathered in outdoor ponds for 24 hrs to simulate the type of oil that might wash ashore from a coastal ocean spill. The oils were then pumped back into steel drums and delivered to the experimental plots via helicopter. Bioremediation strategies: The oils were pumped into the mangrove plots at a nominal dosing rate of 5 L/m². The oil was added at high tide and the pumps were used to distribute the oil over the water surface and produce an even coating of oil on roots and sediments as possible as the tide fell. The dose was 2 L/m² for salt marsh plots. The bioremediation strategies differed for mangrove and salt marsh studies to some extent matching the very different structural characteristics of the two habitats. Salt marshes throughout northern Australia are characterised by their relatively dry condition in relatively arid areas. Their compacted sediments usually consist of very fine, silty clays which usually restrict the penetration of liquids like oil. Salt marsh sediments also have relatively small numbers of smaller crab burrows than those found in mangrove habitats. For these reasons, spilled oil would be expected to remain at the surface and not to penetrate much into the layers below. The oil would also be expected to deposit on the leaves of salt marsh plants since they are usually immersed during monthly spring tides. If an oil spill were to occur during this time, the plants would be completely oiled and they would almost certainly die. Since our objective was to simulate oil arriving from a large oil spill offshore, and depositing on intertidal habitat as the tide dropped, oil was applied to the plots during the spring tide period in November 1997.Because oil was expected to concentrate on the surface, the bioremediation strategy consisted of sprinkling nutrients over the oiled surface within 40 hours of oiling. Approximately, 5.4 kg of nutrients per plot were added as a slow release fertilizer, Osmocote Tropical, with a prescribed composition of: 19.0% nitrogen (9% nitrate; 10% ammonical), 2.5% phosphorus (1.9% water soluble), 10.0% potassium (water soluble, chloride free), 4.8% sulphur (sulphates) and 0.8% calcium. The application rate was ~0.15 kg/m². The fertilizer was sprinkled evenly over the surface of each bioremediation plot. No additional applications of fertilizer were made in salt marsh plots.In the mangrove study, the emphasis of the strategy differed in that spilled oil was expected to penetrate deeply into sediments via the network of burrows and dead roots. The inactive burrows often closed over once the occupants had been killed by oil. In this way, oil would be sealed beneath the surface where it causes serious long term impacts on tree growth and survival. For these reasons, the bioremediation strategy for oiled mangroves involved pumping air beneath sediments and amongst below-ground tree roots. Nutrients were added to both feed the microbial fauna as well as trees. This dual approach was considered both to assist in keeping trees alive, and to promote microbial degradation of oil in contaminated sediments. It was also considered likely that oil might be flushed out of the sediment by the upward flow of air from buried air stones. Nutrients were added as Osmocote Tropical fertilizer, the same as that described in the salt marsh study. Aeration commenced at the same time as fertilizer was added, around 40 hours after oiling. Additional fertilizer was applied in the same dose to mangrove plots in February 1998.The forced aeration strategy for mangrove plots was based on findings and observations from the literature review, flask experiments and mescosm trials. The strategy included forced aeration and addition of nutrients. Forced aeration was delivered to each plot via 39 aquarium airstones (10 cm long each) buried in sediments around the roots of trees. Airstones were mostly distributed around the shoot tree (the tree where shoot observations were made each month) since there were many more roots than airstones in each plot. Aeration commenced approximately 40 hours after addition of oil, and pumping continued for 111 days for Gippsland oil and 140 for Bunker oil. The aeration system was supplied by three 12 volt diaphragm air compressors with a combined capacity, delivering more than 100 litres per minute. The compressors, on a pulsed cycle with 60 mins on and 60 mins off, were powered by deep cycle batteries which were recharged every 24 hours. Compressed air was piped in a common supply line to the three experimental sites and six plots via 19 mm plastic irrigation tubing over distances up to 380 m from the common compressor station. At each site, these lines fed directly to the two bioremediation plots. Each plot had 13 feeder lines of 4 mm aquarium tubing each in turn.Sediment analysis:Sub-samples of the sediments collected prior to oiling were analyzed for grain size by shaking dried sediments on a rotary shaker in a set of stainless steel geological sieves and weighing the sediment retained on each sieve. Sieves had 63 µm, 125 µm and 250 µm mesh sizes. Other sub-samples were analyzed for percent total organic carbon (%TOC) content by a high temperature combustion process corrected for inorganic carbonate (Sandstrom et al., 1986). These samples were also analyzed at AIMS by gas chromatography with flame ionization detection (GC-FID) to determine if any pre-existing petroleum was present in the surface sediments.Sandstrom MW, Tirendi F and Nott ALJ (1986) Direct determination of organic carbon in modern reef sediments and calcareous organisms after dissolution of carbonate. Marine Geology 70: 321-329.Sediments were defrosted, homogenized and sub-sampled for wet/dry weight determination. Other sub-samples were refrozen and sent to a commercial laboratory (Envirotest, Brisbane) for analysis of total hydrocarbons and individual alkanes by gas chromatography with flame ionization detection (GC-FID). The laboratory reported that approximately 2-5 g wet sediment was weighed and mixed with 2 to 3 times its weight with sodium sulfate (Na2SO4) to bind water. This mixture was extracted with dichloromethane (CH2Cl2) by sonication according to protocol 3550A of the US EPA. Extracts were reduced in volume using rotary evaporation and concentrated to small volumes with a gentle stream of dry nitrogen gas. When reduced to a few ml, the extracts were filtered through a Pasteur pipette containing glass wool and one gram of Na2SO4 into glass vials. The oven temperature program for the GC-FID was initially 60°C hold for 4 min, then 60°C to 290°C at 6°C/min, with a final hold for 25 min. The BP1 fused silica column was 0.22 mm i.d, 25 m long, with 0.25 µm film thickness. Calibration and quantification was done against an external standard mixture of n-alkanes in the C10 to C36 elution range plus pristane and phytane.Nutrient analysis:Mangrove sediments were sampled for a range of nutrients, including total nitrogen, phosphorus and carbon, at the end of the experiments in August 1998. Sediment samples were taken from 0-5 cm deep cores using a plastic open mouth syringe, approximately 2 cm in diameter. Samples were placed in plastic screw top containers and kept cool on ice until returned to the AIMS laboratory at Townsville. Samples were dried and ground in preparation for analysis by the Analytical Services Section at AIMS. Total phosphorus analysis was by strong acid digestion of samples. Total phosphorus from digested samples were analysed on a Varian Liberty 220 Inductively Coupled Plasma Atomic Emission Spectrometer (Thompson and Walsh 1993). Total nitrogen and carbon were determined using a Perkin Elmer CHNS2400 analyser by high temperature flash combustion gas chromatography with thermal conductivity detection.

Notes

Credit
Burns, Kathryn A, Dr (Co Investigator)