The Importance of Groundwater Recharge Assessment for Sustainable Groundwater Management

The Lower Namoi River catchment is one of the most valuable groundwater resources in New South Wales, supplying irrigation water to support the state's cotton industry. It is also one of the state's most threatened groundwater resources, with unmonitored groundwater abstraction prior to 1983 and over-allocation of the resource since this time. Investigation of groundwater level and potential head data taken from 600 monitoring bores in the catchment over the last 30 - 35 years has lead to questions concerning the future viability of intensive groundwater abstraction from the Lower Namoi River alluvial aquifers. In the most heavily exploited parts of the catchment long-term groundwater declines of 35 m have been observed. Seasonal drawdowns of 50 - 70 m are also observed in these areas. Two consequences of groundwater drawdown are the development of reversed lateral and vertical hydraulic gradients and land subsidence. The former situation is of great concern because this has allowed the movement of saline groundwater from aquifers and aquitards into the productive aquifers and subsequent contamination of fresh groundwater resources. Land subsidence of up to 70 mm has been measured in the catchment due to aquifer compaction and has the potential to reduce the groundwater storage capacity of aquifers. A consequence of groundwater mining in the catchment has been the recognition by resource managers and scientists alike of the need to quantify groundwater recharge in order to maintain the sustainability of this resource. One of the aims of this study is to identify recharge sources, examine recharge processes, quantify recharge and determine the age of groundwater resources. This paper shows how geochemical and isotopic (δ18O, δ 2H, δ 13C and a14C) groundwater data can be used to assess and quantify recharge which can then be used to provide a scientific basis to sustainable yield calculations.