The fate and impact of fertiliser derived contaminants in New Zealand soils – development of a priority assessment model
The main objective of the present research work was a systematic evaluation of the range and quantity of elemental contaminants applied to land in fertilisers and their wider environmental impacts. The retrospective assessment of a wide range of anthropogenic fertiliser associated contaminants across a representative range of natural and productive land uses, and soil orders in New Zealand (NZ) receiving real-world inputs of fertiliser, and their transfer across environmental compartments was carried out. A systematic and transparent prioritisation methodology to model the consequences to the environment and to human wellbeing from the application of trace elements was presented to help identify where issues may arise in the future and what issues were of greatest precedence. Trace elements applied to soil in organic fertilisers as well as those found in inorganic mineral P fertilisers were assessed. The modelled results were validated against a synthesis of a large soil monitoring dataset and smaller groundwater, river and stream, lake and estuarine sediment datasets. Most of the data were from the Waikato region. Such resulting rankings and the underpinning information can be used by policy makers, resource managers and researchers to guide decision making and allocation of priorities. The main findings were: No single fertiliser group was free of all contaminants. Ag, B, Cd, F, REE, Sr, and U in mineral fertilisers and Cu and Zn in fertilisers made from manures and other organic materials were the elements most elevated compared to concentrations found in background soils. Testing the results of the model against a large soil monitoring dataset confirmed the model provided accurate assessments of the environmental risks of elemental contaminants. Elements predicted to accumulate from additions of inorganic P fertiliser and alternative organic fertilisers (e.g. Cd, P, U, F, La, Zn) were elevated in fertilised soils compared to background soils. Elements predicted to be highly mobile generally did not show accumulation in soil, despite considerable amounts being applied in fertiliser, e.g. boron (B). Many of the elements in water samples appeared associated with land use, e.g. low concentrations of B, Ca, and K a were lowest in stream and river water samples from native forest land use catchments and higher in land used for forestry and pasture. Monitoring of groundwater bores showed Cd was detected in just 15% of the samples, but these samples tend to be from high intensity farming areas. However, the maximum level was still only ½ the drinking water standard for NZ and well below the trigger value for freshwater protection for 95% of species. Data for three lakes affected by algal blooms showed water pH increased to >8.5 which may be causing dramatic changes in solution chemistry leading to the release of certain elements into the water column, e.g. Mo, U, V. These occurrences require further research. No relationship was found between fertiliser use and F or U in fresh water lake sediments and F and U added to soil in fertiliser appeared to be fully retained by the soil. On the other hand, P, Cd, Cu, Ag and Zn were elevated in some of the fresh water lake sediments, especially those surrounded by intensively farmed land uses, consistent with fertiliser being a source. This study identified specific impacts from the different trace elements in the NZ environment. These were: F accumulated in the soil with soil ingestion the most likely mechanism for ingestion of F by animals. There was a lack of risk-based guidelines to direct research on the management of the impacts of F to animals and ecosystem receptors. U accumulated in soil or was effectively trapped in the lake sediments. Like for F, there was a lack of risk-based guidelines to direct research on the management of the impacts of U to animals and ecosystem receptors. Not only do fertiliser additions apply U to soil, but they also increase the availability of native, recalcitrant U through enhanced weathering of soil minerals. The mechanisms of this enhanced weathering phenomenon should be further studied. Cd had significantly accumulated in NZ soils under all the fertilised land uses assessed in this study. A proportion of the added Cd was not held on the soil and may be taken up by plants or leached to water and to sediments. Ag levels in some fertilisers were higher than expected and Ag may be an emerging contaminant in fertilisers. Results suggested little Ag has been applied to NZ soils thus far, while the enhanced levels of Ag in estuarine sediments suggest Ag may transfer from soil to estuaries. Therefore, a watching brief should be kept on this element by researchers to better identify transfer and accumulation mechanisms, as well as any environmental risks. B was highly mobile in soils, which were receiving considerable B additions from fertilisers and other products but accumulated in saline estuarine sediments. This process and it implications for life within the brackish environment appears poorly understood and should be further studied. No long-term fertiliser trial sites receiving alternatives to traditional mineral fertilisers were identified in NZ. Consideration should be given to the development of such long-term field trials to compare with conventional fertiliser trials. It is suggested that research should be conducted into expanding the Fertmark Quality Assurance Scheme, analogous to limit values for Cd, to include maximum limits for other contaminants in both mineral and organic fertilisers. Upon author request this thesis is available as printed version only.
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