Year

2000

Degree Name

Doctor of Philosophy

Department

Faculty of Engineering

Abstract

The effectiveness of manipulating drain water levels using weirs in flood mitigation drains to improve the groundwater and drain water quality was investigated for agricultural land affected by previously oxidised acid sulfate soil near Berry on the South Coast of New South Wales. Groundwater elevation data measured prior to the installation of the weirs showed that significant groundwater drawdown was caused by the low water level in the drains. Drawdown from the drains, in conjunction with high rates of evapotranspiration, caused the groundwater elevation to fall into the pyritic soil causing the oxidation of pyrite and the generation of acidic oxidation products. The high hydraulic gradient caused by the low water level in the drains facilitated the rapid transport of these acidic oxidation products into the drains for discharge into a nearby waterway.

Installation of the weirs promoted higher groundwater elevations by reducing the influence of groundwater drawdown from the drain. The lower hydraulic gradients established under the influence of the higher drain water level maintained by the weir reduced the rate of discharge of acidic oxidation products from the groundwater to the drain. In addition to the assessment of installing weirs in drains to manipulate the groundwater table, numerical simulations combining groundwater flow and pyrite oxidation models were used to predict the magnitude and distribution of pyrite oxidation for various boundary conditions that simulate potential groundwater management strategies. Application of these simulation models shows that substantial reductions in the volume of pyritic soil exposed to oxidising conditions can be achieved by maintaining a higher water level in the drains and/or applying regular irrigation. The rate of acid generation was also investigated in the laboratory by maintaining samples at a constant suction for a specific length of time. Simple multiple regression equations can predict the magnitude and rate of acid generation accurately using these easy to measure independent variables.

Higher groundwater levels did not substantially improve the groundwater quality. High concentrations of stored acidity in the form of acidic cations on the cation exchange sites of the soil as well as the presence of aluminium and iron sulfate minerals formed under acidic conditions ensures that the groundwater has low p H (3.5-4) and high concentrations of dissolved aluminium (30-100 mg/L). The hydrolysis of jarosite and the oxidation of pyrite by Fe3+ under acidic, reducing conditions m a y also give rise to low p H in the groundwater. The maintenance of poor groundwater quality was further investigated by using geochemical models to simulate the effect of the dissolution of acidic minerals and the importance of acidic cation exchange sites. Similarly, acid management strategies including controlled saline intrusion into drains and the injection of dilute lime slurry into the soil were investigated.

Implementation of weirs in flood mitigation drains was shown to be beneficial in terms of reducing the generation of 'new' acid from the oxidation of pyrite in the sulfidic soil as well as facilitation the slow leakage of acid products into waterways rather than the discharge of low pH/high aluminium slugs. However, the management of groundwater elevation alone will not substantially improve groundwater quality without due attention to the 'stored' acid in the soil profile.

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