The Importance of Hydrochemical Investigations in Sustainable Groundwater Resource Management in Irrigated Catchments

The continuing demand for water to support agricultural practices in irrigated catchments has placed significant pressures on groundwater systems and raised questions pertaining , to the long-term sustainability of these resources. The stress placed on groundwater systems threatens not only the sustainability of the agriculture itself, but also the continual access to potable water, and environmental flow needs. The impacts of irrigated agriculture affect not only the hydrodynamics of groundwater systems but the chemistry and quality of the groundwater and these impacts have been witnessed in many groundwater systems in both Australia and internationally. It is only in recent years that resource managers and farmers alike have recognised the need for hydrochemical investigations of groundwater systems to assess the impacts of irrigation on water quality and the threat to groundwater sustainability. In the Lower Namoi River catchment, northern New South Wales, Australia, overexploitation of groundwater resources to support irrigated agriculture over the last 35 years, has not only caused groundwater level declines of up to 35m, but has significantly altered the hydrochemistry and quality of groundwater in the multilayered alluvial aquifer system. Pumping from productive aquifers for irrigation has led to the reversal of natural vertical hydraulic gradients and has allowed the mixing of groundwater of different chemical composition and quality. In the southeast of the catchment, pumping activities have enhanced the upward leakage of Na-HC03-rich groundwater from the Great Artesian Basin into the overlying alluvial aquifers and degraded the quality of the groundwater for irrigation. This paper focuses on the hydrochemical investigation undertaken by the University of New South Wales to delineate the impact of overexploitation on groundwater chemistry. It will discuss the methodology of the investigation, the interpretation and conclusions drawn from analytical data and how the information gained from this investigation can be utilised by resource managers.