Arsenic is a naturally occurring element with potential toxicity to humans depending on concentration and chemical state. Contamination of freshwater environments by arsenic has been seen globally, occurring through both natural and anthropogenic sources. This thesis explores the presence, speciation, and movement of arsenic within Lake Tarawera, including the related physical and biogeochemical processes that control the movement and state of arsenic within the water body and from inlet sources. Lake Tarawera, in the Bay of Plenty Region, North Island, New Zealand is a large deep-water lake in an active volcanic region. Field measurements taken from Lake Tarawera were targeted towards a known localised geothermal area to explore spatial heterogeneity. The results show how the monomictic cycling regime of Lake Tarawera contributes to the retention of arsenic in the water column through increasing concentrations of benthic arsenic during the period of stratification across five open-water sites (196% difference between surface and benthic water As concentrations in March 2020), and homogenous distribution through winter overturning (10% maximum difference in As concentrations, statistically insignificant). The physical and chemical conditions of the lake (temperature and pH) and high solubility of arsenic retain dissolved species in the water column. Oxygen depletion is observed in the deepest part of the lake (2.3 mg L⁻¹ at 79 m depth) which corresponds with increased benthic arsenic concentrations (126 µg L⁻¹). Inlets in the southern arm of the lake (Wairua Arm) are higher sources of arsenic into the water body than inlets in the main body of the lake, with arsenic concentrations from all sampled Wairua Arm inlets exceeding the World Heath Organisation 10 µg L⁻¹ drinking water standard; dissolved concentrations ranging from 41.0-947 µg L⁻¹. One high volume waterfall (Rotomahana WateArsenic is a naturally occurring element with potential toxicity to humans depending on concentration and chemical state. Contamination of freshwater environments by arsenic has been seen globally, occurring through both natural and anthropogenic sources. This thesis explores the presence, speciation, and movement of arsenic within Lake Tarawera, including the related physical and biogeochemical processes that control the movement and state of arsenic within the water body and from inlet sources. Lake Tarawera, in the Bay of Plenty Region, North Island, New Zealand is a large deep-water lake in an active volcanic region. Field measurements taken from Lake Tarawera were targeted towards a known localised geothermal area to explore spatial heterogeneity. The results show how the monomictic cycling regime of Lake Tarawera contributes to the retention of arsenic in the water column through increasing concentrations of benthic arsenic during the period of stratification across five open-water sites (196% difference between surface and benthic water As concentrations in March 2020), and homogenous distribution through winter overturning (10% maximum difference in As concentrations, statistically insignificant). The physical and chemical conditions of the lake (temperature and pH) and high solubility of arsenic retain dissolved species in the water column. Oxygen depletion is observed in the deepest part of the lake (2.3 mg L⁻¹ at 79 m depth) which corresponds with increased benthic arsenic concentrations (126 µg L⁻¹). Inlets in the southern arm of the lake (Wairua Arm) are higher sources of arsenic into the water body than inlets in the main body of the lake, with arsenic concentrations from all sampled Wairua Arm inlets exceeding the World Heath Organisation 10 µg L⁻¹ drinking water standard; dissolved concentrations ranging from 41.0-947 µg L⁻¹. One high volume waterfall (Rotomahana Waterfall) provides a per second arsenic loading of 16,400 ± 2,660 µg As s⁻¹. The more toxic form of arsenic (AsIII) was only detected in one instance, from a warm water stream inlet. Iron and manganese concentrations and the partitioning of these transition metals indicates the ratio of arsenic to these binding elements is not of the right order of magnitude to remove substantial arsenic from the water column, as dissolved arsenic concentrations exceed that of both iron and manganese. Water isotope ratios and dissolved arsenic concentrations from Rotomahana Waterfall show a complicated mixing regime, including influences such as wind, rainfall, other inlets, and opposing water flows coming from the main water and the high flow waterfall. High temperature and low flow waters from Hot Water Beach demonstrates the influence of inlet water temperature because they alter the movement and concentrations of arsenic heading out towards the open-water inlets. The high temperature of Hot Water Beach creates a localised area of thermal stratification, which causes surface mixing and dilution of the inlet waters with the nearest open-water site. Mixing then occurs with depth closer to the open-water site, passed the point of the hot water induced thermocline. These targeted samples highlight how well arsenic is diluted and buffered by such a large volume of water, and demonstrate the importance of stratification, temperature, and flow rate on the movement of As. Laboratory and field studies attempted to minimise oxygen introduction to pore water samples using rhizons (pore water filters) and vacutainers. Arsenic speciation was retained in field samples in the vacutainers for up to three months, with concentrations of up to 2,230 µg L-1 of AsIII measured; this result which led to laboratory experiments on the long-term storage and retention of AsIII. However, the data from these laboratory studies were inconclusive for over-time analyses of two experiments; days 2, 26, and 55 after sample spiking for the first experiment, and days 2 and 29 for the second experiment. Vacutainers and rhizons extracted pore waters in-situ in attempt to minimise oxygen introduction to low dissolved oxygen samples; however, the methodology for long-term storage requires further research. Speciation of arsenic occurs in the sediments due to the highly anoxic environment, with pore water AsIII concentrations ranging exceeding 2,000 µg L⁻¹ in the surface sediments and decreasing with depth. Sediment concentrations show distinct accumulation boundaries which are indicative of redox boundaries over time. rfall) provides a per second arsenic loading of 16,400 ± 2,660 µg As s⁻¹. The more toxic form of arsenic (AsIII) was only detected in one instance, from a warm water stream inlet. Iron and manganese concentrations and the partitioning of these transition metals indicates the ratio of arsenic to these binding elements is not of the right order of magnitude to remove substantial arsenic from the water column, as dissolved arsenic concentrations exceed that of both iron and manganese. Water isotope ratios and dissolved arsenic concentrations from Rotomahana Waterfall show a complicated mixing regime, including influences such as wind, rainfall, other inlets, and opposing water flows coming from the main water and the high flow waterfall. High temperature and low flow waters from Hot Water Beach demonstrates the influence of inlet water temperature because they alter the movement and concentrations of arsenic heading out towards the open-water inlets. The high temperature of Hot Water Beach creates a localised area of thermal stratification, which causes surface mixing and dilution of the inlet waters with the nearest open-water site. Mixing then occurs with depth closer to the open-water site, passed the point of the hot water induced thermocline. These targeted samples highlight how well arsenic is diluted and buffered by such a large volume of water, and demonstrate the importance of stratification, temperature, and flow rate on the movement of As. Laboratory and field studies attempted to minimise oxygen introduction to pore water samples using rhizons (pore water filters) and vacutainers. Arsenic speciation was retained in field samples in the vacutainers for up to three months, with concentrations of up to 2,230 µg L⁻¹ of AsIII measured; this result which led to laboratory experiments on the long-term storage and retention of AsIII. However, the data from these laboratory studies were inconclusive for over-time analyses of two experiments; days 2, 26, and 55 after sample spiking for the first experiment, and days 2 and 29 for the second experiment. Vacutainers and rhizons extracted pore waters in-situ in attempt to minimise oxygen introduction to low dissolved oxygen samples; however, the methodology for long-term storage requires further research. Speciation of arsenic occurs in the sediments due to the highly anoxic environment, with pore water AsIII concentrations ranging exceeding 2,000 µg L⁻¹ in the surface sediments and decreasing with depth. Sediment concentrations show distinct accumulation boundaries which are indicative of redox boundaries over time.