At the request of Prime Minister and Cabinet (PM&C), Geoscience Australia (GA) prepared this report for the purposes of informing a National Security paper that highlights potential national security issues associated with climate change.

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Objectives 1. To determine the horizontal and vertical extent of hydrogen sulphide (H2S) in Lake Wollumboola sediments. 2. To examine controls on H2S gas production in Lake Wollumboola sediments. Activities 1. During a visit to Lake Wollumboola in November 2001, Geoscience Australia collected sediment samples, from sediment cores to depths of generally 180 mm, and occasionally to 600 mm. 2. The 12 sample sites chosen incorporate the three different sediment types of Lake Wollumboola; marine sands on the eastern side of the lake, central basin muds in the relatively deeper central part of the lake, and fluvial sands and muds on the western side of the lake where the creeks are depositing sediment from the catchment. 3. We measured H2S in sediment porewaters, immediately after sample collection. Porewater sulphate and chloride were measured in the laboratory. 4. Total sulphur, total iron, and total organic carbon were measured in the laboratories at Geoscience Australia, after the survey. Background Bacteria, which occur naturally in Lake Wollumboola's sediments, produce H2S when they breakdown organic matter. The bacteria, which are called sulphate reducing bacteria, utilise sulfate from the water to breakdown the organic matter, and can only operate under oxygen free (anoxic) conditions. Key Findings 1. H2S production in Lake Wollumboola is extensive. The average H2S concentration in the central basin muds and fluvial sands and muds is ~3000 M. At one site in the central basin muds, H2S concentration is greater than 10 000 M. In contrast, the concentration of H2S in the sandy marine sediments on the eastern side of the lake is low in comparison to the central basin muds and fluvial delta sands and muds. The average H2S concentration in the marine sands is 158 M. 2 Measurements of total organic carbon show that the amount of organic matter is higher in the central basin muds and fluvial delta sands and muds (~3.5 wt%) than in the marine sands (~0.8 wt%). Organic matter is the fuel for H2S production. High H2S concentrations in the central basin muds and fluvial delta sands and muds are probably a result of their high organic matter contents. 3. Depth profiles of H2S concentration and sulphate depletion in the central basin muds and fluvial sands and muds show that H2S production is occurring right at the sediment surface and down to depths of 80-100 mm. This implies that H2S could escape directly into the atmosphere, when the central basin muds and fluvial sands and muds are exposed during times of low lake levels. It also suggests that H2S could build up in the bottom layer of water directly above these sediments if the water remains anoxic for periods of time. 4. Total sulphide measurements show that H2S is reacting with iron in the sediments, forming iron sulphide minerals. Iron is an important trapping mechanism for H2S, preventing its escape to the atmosphere. Most sites, however, do not have enough `reactive iron' available and H2S concentrations are able to build-up in the porewaters of the sediment.

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