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From soil to groundwater: Assessing the leaching potential of cadmium across gradients of soil types and land-uses

The problem: The productivity of agriculture, the mainstay of New Zealand’ economy, relies on millions tons of fertiliser to maintain soil fertility. These fertilisers contain contaminants, including cadmium (Cd) which pose risks to the environment and human health. The assessment of the bioavailability, mobility, and fate of fertiliser-derived contaminants is therefore important in protecting the environment and human health. In order to manage fertiliser inputs accurately, authorities and protection agencies have struggled with tracing the Cd fluxes between fertilised soil, shallow groundwater and the biomass of plants. A number of analytical and experimental challenges hinder the application of conventional methods in these systems. Proposed research: A realistic assessment of the environmental impact of fertiliser-derived contaminants requires non-perturbing and sensitive sampling techniques for detecting contaminants of interest. Diffusive gradients in thin films (DGT) is an established in-situ technique that can provide information on the concentrations and bioavailability of diverse contaminants in soil, sediment and groundwater. The DGT method has rarely been applied to relatively slowly moving groundwater systems, where the increased length of the diffusive boundary layer (DBL) needs to be taken into account. Three different DGT approaches were tested to overcome the limitations associated with the direct application of DGTs to groundwater systems a static DGT suspension, a DGT shaker, and a pumped flow-cell unit coined the “universal DGT monitoring system” (UDMS). The analysis of stable isotope ratios was used to investigate the Cd cycle in agricultural systems. The isotope data enabled us to trace fertiliser-derived Cd in soil−pasture−groundwater systems across gradients of soil types and land-uses. Additionally, the effects of farm management strategies (urine and farm dairy effluent) on the transport of metals and nutrients from soil to groundwater were determined through testing archived samples from the Lincoln University Dairy Farm (LUDF) lysimeter trial. The hydrogeochemical properties of soils and different treatments may enhance metal leaching and transport. Through a statistical analysis of the data, this study aimed to test the hypotheses that the leaching potential of metals and nutrients in soils varies depending on the type of soil and farm management strategies. Research outcome: The Universal DGT Monitoring System (UDMS)-a pumped flow-cell setup was developed to overcome stagnant conditions and reducing diffusive boundary layer thicknesses by fourfold. The UDMS method is particularly suitable for monitoring low-level contaminants like Cd and conditions with frequent fluctuations in shallow aquifer concentrations. Coupling UDMS and stable isotope ratio methods demonstrated that Cd isotopes are significantly and systematically fractionated in groundwater, soil, and pasture systems. Fractionation patterns in these systems provide insight into hydrogeochemical processes controlling the mobilisation of contaminants in the subsurface. According to the results of the study, soil types and hydraulic properties of soils are probably more important factors than farm treatments and strategies for the leaching of metals and nutrients. The intrinsic degree of leaching, however, could not be determined without control treatments. To understand the driving factors of high mobilisation events, further study is necessary to determine how amended versus unamended soils differ.
Type of thesis
The University of Waikato
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