Some soil-related issues in the disposal of effluent on land

There is increasing environmental pressure to dispose of waste effluents on land, rather than discharging them directly into surface waterways. In addition to reducing impacts on water quality, land disposal of effluents offers the possibility of beneficial recycling of water and essential plant nutrients. Frequently, however, waste water contains high concentrations of dissolved salts, particularly sodium, that can affect the levels of salinity and sodicity in soils. This, in turn, can diminish the utility of soils for productive agriculture. Astute soil management can often ameliorate these adverse affects but this may require land-use practices that reduce the commercial return from farming activities. It is therefore important to identify the main purpose of effluent disposal areas and to compensate land users appropriately when the operation of the effluent disposal scheme affects farm profitability. If farmers are forced by commercial pressures to adopt inappropriate land use practices on areas used for effluent disposal, the operation of the scheme will be compromised, and there may even be the risk of complete failure. The results of a limited number of experiments investigating the application of effluents to land in northern Victoria are used to demonstrate these risks. The effects on soil chemical properties of applying saline–sodic waste water to land are now well understood. Considerably less certain are the effects of these changes on soil chemical properties, on the physical properties of soils and the way soils behave under various land uses — such as cultivation or grazing by heavy animals. Various approaches to modelling and predicting the impacts on soil of irrigation with saline–sodic waste water are discussed. A number of simple, classification models, based on practical experience and empirical equations, currently provide valuable assistance for land managers. More complex, mechanistic models have been developed that describe successfully the movement of water and salts through soils, given appropriate soil measurements as input parameters. The major limitations now are the availability of soil data at an appropriate scale and an understanding of the ways changes in soil chemical properties affect soil physical properties and subsequent soil behaviour. In particular there is a need to investigate those critical zones at the soil surface or the top of the B horizon where water movement is impeded in sodic soils.