Technical Memorandum 34
Chartres CJ, Walker PH, Willett IR, East TJ, Cull RF, Talsma T and Bond WJ
Supervising Scientist, 1991
ISBN 0 644 22040 6
- Soils and hydrology of Ranger Uranium Mine sites in relation to application of retention pond water (PDF 900 KB)
About the report
In 1986 surveys commenced of the soils and hydrology of the Ranger irrigation area and a nearby area which may be suitable for irrigation. The soil survey indicated three soil mapping units differentiated on the basis of soil colour, depth, drainage status and the presence or absence of ferricrete. The soils of all three units have high gravel fractions of quartz and ferruginous material, usually comprising between 20-50% of the soil mass. The quartz gravel is chemically inert and plays little or no part in immobilisation reactions, thereby reducing the effectiveness of the bulk soil as a contaminant repository. The soils are low in clay (mostly <20%) and deficient in organic matter (mostly <1%). Although clay contents increase slightly with depth, the clays are low-activity clay minerals (predominantly kaolinite). Consequently, the cation exchange capacities of the soils are extremely low (typically <0.01 mol (p+)/kg of the bulk soil) indicating a very limited potential for the assimilation of major cations.
Using conservative estimates of soil cation exchange capacity and total cation concentration of RP2 water, the time until the cation exchange complex of the upper 50 cm of soil comes into equilibrium with the irrigation water is calculated to be approximately 5 years. After this time the exchange-complex will be 100% saturated and any additional load of cations will pass through the soil profile (0-50 cm). The soils are also characterised by low soluble salt contents, acidic pH and low concentrations of secondary iron and manganese oxides. Field measurements of hydraulic conductivity indicate that the soils are highly permeable and that lateral flow of groundwater from the irrigation area to Magela Creek could take as little as 9 months. The survey of the alternative potential irrigation site indicated four mapping units with generally similar properties to the soils of the current irrigation area.
The capacity of these soils to assimilate the cations tested does not necessarily apply to other cations such as uranium and radium, present in minor or trace concentrations. In their cases, more specific adsorption reactions are likely, some of which may be effectively non-reversible. There is insufficient information available to enable any quantitative assessment of the likely significance of these mechanisms; however, the ferruginous gravels may play some part in specific adsorption reactions.
Laboratory water saturation experiments with soils from the irrigation area show that, with the exception of the 0-2 cm layer, there is negligible reductive dissolution of secondary iron and manganese oxides. It is therefore considered unlikely that mobilisation or immobilisation of pollutants (such as heavy metals) will be significantly influenced by soil oxidation-reduction cycles.
These soil assimilation results take no account of biological sinks or pathways, which may lead to temporary or longer term storage of applied ions within, or to removal of ions from, the irrigated area. Such biological factors will prolong the period estimated for saturation of the exchange complex of the soil, although their importance is diminished in the longer term. No data are available regarding sorption/desorption reactions that may take place in the weathered rock below the top 50 cm of soil.