Lay, Mark C.Bertram, Danielle ValerieGlasgow, Graeme D.E.Majeed, Muhammad Adnan2026-05-112026-05-112026https://hdl.handle.net/10289/18270This doctoral research developed and applied two modelling approaches to simulate and assess estrogen (estrone E1, and 17β-estradiol E2) transport from an intensive dairy farm system located in the Waikato region, New Zealand. The first approach was a numerical unsaturated zone model implemented in MATLAB, coupling Richards’ equation with a Green-Ampt infiltration framework and advection, dispersion, and sorption processes to track estrogen movement through soil. The second approach employed a GIS-based ArcSWAT catchment model to simulate surface runoff, sediment yield, and estrogen transport at the farm watershed scale, including the evaluation of best management practices (BMPs). The novelty of this research lies in the dual-scale modelling of estrogen transport through both soil and surface runoff pathways and the spatially explicit, quantitative assessment of existing best management practices specifically for estrogen mitigation rather than nutrient reduction alone. Soil Model results indicate that estrogens are largely retained and attenuated in the upper soil layers, reducing the risk of leaching to groundwater. The model showed that both E1 and E2 undergo strong sorption near the surface and substantial microbial degradation, with >90% of the applied mass removed within the top ~1 m over a 90-day simulation. Estradiol was more mobile than estrone, with its concentration front advancing deeper (~0.5–0.6 m). Concentrations declined steeply with depth, and values below ~1.5 m were negligible under steady infiltration. Infiltration-driven pulses produced by surface boundary input led to episodic colloid mobilization, which enhanced estrogen transport to mid-depths. Free and attached colloid time series showed synchronous peaks, indicating that mobile colloids can act as vectors for hormone movement. These findings suggest that under normal conditions, leaching risk is low, but during rapid infiltration following effluent application, colloid-facilitated transport may temporarily extend hormone movement deeper into the profile. Overall, the results align with experimental column studies, confirming that estrogen is primarily confined to shallow soil and largely degraded in situ, while emphasizing the importance of managing infiltration timing and flow dynamics. For the dairy farm studied, the ArcSWAT model provided a spatially explicit assessment of estrogen transport via runoff and erosion. Critical source areas (hotspots) for estrogen and sediment loss were identified at the subbasin level. Under baseline (pre-BMP) conditions, eight subbasins were predicted to generate the highest estrogen concentrations in runoff (often >15 ng/L) and severe sediment yields. One subbasin emerged as the most critical, with annual runoff volumes exceeding ~1500 mm and sediment losses on the order of ~3.9 t/ha, concomitant with elevated estrogen loads. Several other subbasins showed overlapping high runoff and erosion, indicating that both dissolved and sediment-bound estrogen transport mechanisms are at play in these areas. The model indicated that surface water contamination is a primary concern, as these high-runoff zones efficiently deliver estrogens to streams (e.g. the Maungatea Stream on the farm’s boundary). To mitigate these exports, the effectiveness of BMP scenarios was evaluated in ArcSWAT, including constructed wetlands, riparian buffer strips, grazing management, and effluent application timing management. All BMPs reduced estrogen and sediment delivery to some degree, but their performance varied. Constructed wetlands placed at critical drainage points showed the greatest overall impact, trapping 50-90% of sediment from upstream areas and removing an estimated 30-70% of estrogen loads via sedimentation, sorption, and microbial degradation in wetland ponds. Riparian buffers were similarly effective: vegetated buffer strips along stream channels filtered runoff, resulting in sediment transport reductions of about 16-80% (average ~42%) in high-erosion subbasins and estrogen concentration reductions on the order of 30-85% in runoff, through enhanced infiltration and filtering of hormone-laden sediment. Improved grazing management (e.g. rotational grazing and reduced stocking rates in wet periods) yielded moderate benefits, with an average ~17.5% decrease in sediment loss (due to better soil cover and less compaction) and commensurate declines in runoff (~12%) and estrogen exports. Effluent management (avoiding manure irrigation during wet weather and optimizing application rates) provided 5-25% lower sediment losses and up to 10-30% reductions in runoff, thereby modestly cutting estrogen runoff concentrations (5-50%). Notably, after implementing an effective riparian buffer scenario, peak estrogen levels in runoff from the worst areas fell from ~19.1 ng/L (baseline) to below 7 ng/L, and peak sediment yields dropped from ~3900 kg/ha to under 1000 kg/ha. These quantitative improvements underscore that targeted BMP adoption can substantially reduce estrogen loading to surface waters. The catchment modelling highlighted where and which interventions yield the greatest water-quality benefits: for instance, combining wetlands and buffers in the most critical subbasins would address both high runoff and erosion, greatly reducing the transport of both dissolved and particle-bound estrogens to streams.enAll items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.Thesis