The effects of irrigation waste-water disposal in a former discharge zone of the Murray Basin, Australia

Abstract

In the Murray Basin in southeastern Australia, saline waste irrigation waters are often discharged to natural depressions and saline lakes as a salinity and land management strategy. At the Noora disposal basin in South Australia the waste irrigation water ( EC = 17–19 dS m −1 ) has formed a lens in the top of the highly saline (50–80 dS m −1) regional groundwater (Parilla Sands) aquifer. Using salinity and environmental isotopes of water (deuterium and oxygen-18) the lens has been shown to extend about 500 m in a northwesterly direction from the disposal pond. The major effects of this lens have been: (1) to cause upwards displacement of the regional ground water over an area of about 285 km 2, implying increased evaporation from areas surrounding the lens; (2) to reduce evaporation of regional ground water from the central low-lying area. Electromagnetic induction techniques for detecting preferred flowpaths away from the basin were rendered ineffective in this environment because of lithologic variations within the dune system. However, examination of bore-logs and groundwater gradients indicated that there was little evidence of stratigraphic control of mound development. Salinity in the Parilla Sands aquifer was closely related to the depth of the water table from the soil surface. Shallow (2–4 m) water tables were affected by recharge and evaporation to a much greater extent than ground water located below the higher dunes. There was, however, an almost instantaneous pressure response throughout the whole groundwater system to changes induced in the low-lying areas. Analyses of piezometric data showed that there was a seasonal variation imposed on the groundwater mound development. Corrected mean annual water-table increments and estimates of the mound volume and area were derived from a Theis response curve of the water table rise associated with the mound alone. Calculations using fitted parameters from the Theis analyses also suggested high transmissivity values, but are subject to uncertainties in limited data on specific yield. Although comparison of the mound volume and the disposed volume indicates extensive losses, isotopic and salinity data do not support substantial evaporation of the disposal water. However, there is evidence that the already more saline regional waters are subject to increased evaporation in topographic lows which come within the influence of the elevated water table. Hence the problem to be faced in the future is the contamination of the River Murray system by Parilla Sands water rather than from waste water leaking laterally from the disposal basin. results from this study show that the effect of disposal of the waste water is dominated by the density of the water relative to the regional waters. The assesment of the environmental impact of water disposal at other sites should, therefore, give careful consideration to this aspect, which is not adequately incorporated into groundwater models in current use.