The fate of water and solutes following irrigation with retention pond water at Ranger Uranium Mine
Research Report 10
Willett IR, Bond WJ, Akber RA, Lynch DJ & Campbell GD
About the report
This report describes the results of experiments aimed to determine the fate of water and solutes from the application of Ranger Uranium Mine's retention pond number 2 (RP2) water to soils of a land application site near the mine. Laboratory batch sorption studies with the important solutes in the water, column experiments, and a field experiment to simulate land application are reported.
The major soluble ions in RP2 water are sodium, magnesium and sulfate, although there are also measurable amounts of the other common solutes- calcium, potassium and chloride. The major trace constituents are Mn, 238U, and 226Ra, but 21OPb is also of concern. The reaction of trace metals with soils is typically quite different from that of the common soluble ions, and the two are treated separately in this report.
The soils of the land application site are generally highly weathered, shallow, sandy and gravelly. Three horizons of each of the three major soils of the land application area-yellow earths, red earths and siliceous sands (Oxisols) - were used in the batch studies. Measurements of the cation and anion exchange (CEC, AEC) capacities of the soils, which indicate their reactivity with solutes, showed that all soils had very low CECs (< 10 cmol [+]/kg) and no detectable AEC. CEC increased with soil pH, but increases in CEC that may occur from the application of the alkaline RP2 water were small in comparison with the solute loads.
The field experiment was conducted on the most predominant soil type in the land application area (yellow earth). During a 16 week period in one Dry season, 989 min of water with major solute concentrations similar to RP2 water was applied. For experimental purposes, a further 185 min of water without added solutes was applied during the ensuing Wet season. Estimation of the drainage flux using a soil water hydrology model, which had been validated at the site using tritium tracer measurements, showed that irrigation caused an extra 684 mm (above that caused by the natural rainfall) of water to drain beyond 50 cm in the soil profile.
From the CEC and AEC measurements it was concluded that all soils in the land application area have very little capacity to retain the major soluble ions. Results of the field experiment confirmed this for the yellow earth. Calculation of the fluxes for transport of each ion showed that all the Na+, Mg2+ and C1-, and most of the S042- was lost from the soil profile. Some of the K+ and Ca2+ was retained in the soil or taken up by roots but major losses of these ions in the drainage water also occurred. The alteration of the exchange complex of the soil by retention of K+ and Ca2+ would not be expected to be repeated in subsequent irrigation seasons, so that after the first year most of the K+ and Ca2+ will also be lost from the soil profile.
Batch sorption experiments showed that all soils had capacity to retain Mn, 238U and 226Ra, although the capacity of the siliceous sand was much less than that of the other soils. The results for Mn differed from those for 238U and 226Ra because of Mn-bearing minerals in the soils and the ability of U to displace Mn from the soil. From the batch studies it was concluded that 238U and 226Ra are not expected to be mobile in the soil. Mn is expected to be more mobile than the radionuclides.
The field and column experiments demonstrated that the radionuclides were retained in the surface 6 cm layer of soil. Fractionation studies showed that 238U and 21OPb were retained in relatively stable forms, whereas a significant fraction of 226Ra remained in exchangeable form. 226Ra did not move but may remain in a form available for uptake by roots. In general, plant uptake accounted for <2% of the applied radionuclides. This was attributed to foliar uptake because radionuclide concentrations of the foliage decreased rapidly when irrigation stopped. Mn was retained near the surface in the predominant soil type but may be transported down the profile of the sandiest soil.
It was concluded that irrigation with RP2 water would cause significant fluxes of water and the common solutes to the groundwater. The radionuclides, however, would be retained in the surface soil for many years of irrigation, although they may move to greater depths in the siliceous sand than in the other soils.
The retention of the radionuclides in the surface soil renders them susceptible to transport in particulate forms in runoff water or as airborne dust. 'Mere was some indication of a loss of uranium from the surface of the site during the Wet season. This may have been due to mobilisation of surface particulates by rain. However, the errors involved in the measurements on which this conclusion was based were large and direct measurements of uranium in runoff are required to resolve the issue.