1. Introduction

Groundwater is water that occurs beneath the surface of the earth. It is available over most of Australia and in many parts of the country, especially the arid and semi-arid inland, it is a resource of critical importance (Jacobson et al. 1983).

Groundwater is both a valuable resource and a contributing agent to the process of salinisation and land degradation in the Murray-Darling Basin. In areas of useable supply, our reliance on groundwater in the Murray-Darling Basin continues to grow. Research has shown that groundwater levels have declined significantly over the 10 year period between 1990 and 2000 in most of the major resource subsystems. This has been attributed to a combination of lower rainfall recharge because of drought conditions and higher use of groundwater. Greater regulation of surface water resources, increasing opportunities and prolonged droughts have all contributed to our increased use of groundwater as an additional water resource.

At the same time, in other areas of the Basin where groundwater quality is poor, there has been increased groundwater recharge as a result of land clearing and changed management practices. Subsequent rises in groundwater levels in these areas may have contributed to dryland and river salinity.

The impacts of overuse of useable groundwater and rising levels of unpotable groundwater have been:

• rising shallow groundwater levels and associated salinisation of land;
• displacement of salt loads to the river systems,
• reduced availability of groundwater for agricultural use, and
• reduced river flows where groundwater and surface water are connected.

These in turn will have impact upon the usefulness of water resources in the Basin in terms of the provision of water of a suitable quality for irrigation and potable use as well as impact upon aquatic, riparian and groundwater-dependant ecosystems.