Bruce G Blunden

Evaluation of surface and groundwater management strategies for drained sulfidic soil using numerical simulation models

The effective management of acid sulfate soils is a major issue for many coastal regions in Australia. Simulations were conducted to evaluate 4 different water management strategies that could be applied to agricultural land on the south coast of New South Wales, Australia, to minimise acid generation from acid sulfate soils. The water management strategies are compared with the existing extensively drained situation which generates and discharges large quantities of acidic pyrite oxidation products. The 4 water management strategies include elevated drain water levels using a weir, 25 mm irrigation on a 7- or 14-day cycle, and elevated drain water levels with irrigation. All of these strategies were designed to minimise the generation of acid by reducing the transport of oxygen to the sulfidic soil. Simulations were conducted for weather and site conditions experienced during a 12-month period starting in July 1997. Model simulations showed that maintenance of elevated drain water levels using a weir in the drain significantly reduced the amount of acid generated by 75% and 57%, at 10 and 90 m distance from the drain, respectively, by comparison with the existing drained state. The addition of 25 mm irrigation on a 14-day cycle to the weir simulation reduced the oxidation of pyrite by a further 1–2%. Application of irrigation only on a 7-day cycle also reduced the acid generated by 89% and 94% at 10 and 90 m distance from the drain, respectively, by comparison with the existing drained state. Irrigation on a 14-day cycle was not as successful in reducing pyrite oxidation as either the 7-day irrigation or weir strategies. Evaluation of the 4 water management options showed that significant improvements can be made with respect to the amount of acid generated by relatively simple and cost-effective land management practices.

Acid sulphate soil remediation techniques on the Shoalhaven River Floodplain, Australia

A commonly used flood mitigation technique in coastal areas of Australia during the late 1960s was the installation of one-way floodgates on flood mitigation drains. In regions affected by acid sulphate soils (the oxidation of pyrite in the soil forms sulphuric acid), the floodgates prevent tidal carbonate/bicarbonate buffering of the drains and thereby create reservoirs of acidic water (pH < 4.5) that discharge during low tide. Several acid sulphate soil remediation techniques have been used in coastal lowland in southeastern NSW, Australia. Following extensive monitoring and finite element modelling of groundwater conditions and quality, fixed level V-notch weirs were installed at three elevations to maintain elevated groundwater levels. The weirs successfully maintained the groundwater level above the acid sulphate soils, preventing additional pyrite oxidation, and reduced the rate of discharge of acid to the drain. Following further monitoring, investigation into anaerobic acid sources, and finite element-based geochemical modelling, modified two-way floodgates that allow tidal ingress were installed. The modified floodgates were successful in buffering the drain water pH before discharging the drain water into adjacent waterways. Numerical analysis based on finite element modelling was extended to illustrate that saline intrusion into the surrounding soil (as a result of tidal ingress and acid buffering in the drains) was not a major concern for the pastureland or other agricultural activities.

Reducing the impact of acid sulphate soils at a site in Shoalhaven Floodplain of New South Wales, Australia

Oxidation of sulphide minerals in acid sulphate soils has acidified a substantial part of the low-lying coastal land in Australia. Such sulphides, including pyrite (FeS2) formed thousands of years ago during saline inundation of soils rich in iron. They do not pose a serious concern when submerged by the water table, because this prevents atmospheric oxygen reacting with the pyritic layer. However, flood protection of low-lying coastal land through the installation of deep surface drains has caused a general lowering of the water table elevation, that in turn has exposed the pyritic layers to atmospheric oxygen. High rainfall following droughts causes acid pollution of the surrounding flood mitigation drains, creeks and river systems. Large ground areas are affected by the transport of acid constituents during seepage. One way of controlling new acid production is through the installation of weirs in the flood mitigation drains in order to raise the water table elevation. In this paper, the acid pollution in New South Wales is reviewed, and the effect of groundwater elevation is examined. Relationship between acid production and groundwater table is examined. Mathematical models are developed to simulate acid production and transport, and two groundwater management regimes are examined. Weir based control of the groundwater table is shown to be successful in controlling acid production.