Adam Hartland

Association of Arsenic and Phosphorus with Iron Nanoparticles between Streams and Aquifers: Implications for Arsenic Mobility.

The microbial oxidation of organic matter coupled to reductive iron oxide dissolution is widely recognized as the dominant mechanism driving elevated arsenic (As) concentrations in aquifers. This paper considers the potential of nanoparticles to increase the mobility of As in aquifers, thereby accounting for discrepancies between predicted and observed As transport reported elsewhere. Arsenic, phosphorus, and iron size distributions and natural organic matter association were examined along a flow path from surface water via the hyporheic zone to shallow groundwater. Our analysis demonstrates that the colloidal Fe concentration (>1 kDa) correlates with both colloidal P and colloidal As concentrations. Importantly, increases in the concentration of colloidal P (>1 kDa) were positively correlated with increases in the concentration of nominally dissolved As (<1 kDa), but no correlation was observed between colloidal As and nominally dissolved P. This suggests that P actively competes for adsorption sites on Fe nanoparticles, displacing adsorbed As, thus mirroring their interaction with Fe oxides in the aquifer matrix. Dynamic redox fronts at the interface between streams and aquifers may therefore provide globally widespread conditions for the generation of Fe nanoparticles, a mobile phase for As adsorption currently not a part of reactive transport models.

Synchrotron X-ray distinction of seasonal hydrological and temperature patterns in speleothem carbonate

Environmental context Speleothem chemical records are used to reconstruct environmental change on a broad range of timescales. However, one of the biggest challenges is to link the records contained within speleothems at the sub-annual timescale to changing meteorological conditions. Seasonal infiltration patterns and cave ventilation dynamics are reconstructed through high resolution analysis of speleothem trace element content by synchrotron radiation, building towards proxy records of hydrological variability and winter duration as indices of recent climatic change beyond the instrumental period. Abstract Synchrotron micro-X-ray fluorescence (µXRF) spectrometry is used to reveal trace element patterns within speleothem calcite at the sub-annual scale and provide one of the first calibrations to prevailing meteorological conditions. Mapping of Zn and SO42– within speleothem calcite was performed at the European Synchrotron Radiation Facility over three annual cycles (1977–1979). Peaks in µXRF Zn concentrations occur on an annual basis, although banding of lower XRF intensity reveals multiple events at the sub-annual scale. The delivery of Zn to the speleothem was found to be dependent upon the presence of a water excess, the condition of any overlying snowpack and the pH of the soil solution as controlled by microbial activity. This generated a pattern of Zn event laminae that documented increasing concentrations from winter through to the following autumn and complies with existing models inferring surface-active trace metals are delivered to the point of speleothem growth in association with natural organic matter (referred to as NOM–metal complexes). Minimum and maximum concentrations of speleothem SO42– coincide with winter and summer respectively, in contrast to the near constant SO42– concentrations of the drip water. Fluctuations in speleothem SO42– levels closely follow changes in cave external temperatures, thereby validating existing models of sulfate incorporation into carbonate minerals thought to be driven by cave ventilation dynamics and internal cave atmospheric pCO2 (partial pressure). At the current resolution of analysis, this represents some of the first evidence linking event-based meteorological (temperature and precipitation) records to the trace element content of speleothem calcite, building towards reconstruction of indices of climatic change beyond the instrumental period.

Isotope Tracing of Long-Term Cadmium Fluxes in an Agricultural Soil

Globally widespread phosphate fertilizer applications have resulted in long-term increases in the concentration of cadmium (Cd) in soils. The accumulation of this biotoxic, and bioaccumulative metal presents problems for the management of soil-plant-animal systems, because the magnitude and direction of removal fluxes (e.g., crop uptake, leaching) have been difficult to estimate. Here, Cd isotopic compositions (δ114/110Cd) of archived fertilizer and soil samples from a 66 year-long agricultural field trial in Winchmore, New Zealand, were used to constrain the Cd soil mass balance between 1959 and 2015 AD, informing future soil Cd accumulation trajectories. The isotopic partitioning of soil Cd sources in this system was aided by a change in phosphate source rocks in 1998 AD, and a corresponding shift in fertilizer isotope composition. The dominant influence of mixing between isotopically distinct Cd end-members was confirmed by a Bayesian modeling approach. Furthermore, isotope mass balance modeling revealed that Cd removal processes most likely increased in magnitude substantially between 2000 and 2015 AD, implying an increase in Cd bioaccumulation and/or leaching over that interval. Natural-abundance stable isotopes are introduced here as a powerful tool for tracing the fate of Cd in agricultural soils, and potentially the wider environment.

Phosphorus and arsenic distributions in a seasonally-stratified, iron- and manganese-rich lake: microbiological and geochemical controls

Environmental context Despite being present at trace concentrations, arsenic and phosphorus are among the most important of freshwater contaminants. This research highlights the biogeochemical coupling of both elements in a New Zealand lake. We find that the mineralisation of organic residues coupled to the dissolution of colloidal iron and manganese hydroxides may be an important driver of the bioavailability of phosphorus and arsenic. Abstract Seasonal stratification in temperate lakes greater than a few metres deep provides conditions amenable to pronounced vertical zonation of redox chemistry. Such changes are particularly evident in eutrophic systems where high phytoplankton biomass often leads to seasonally established anaerobic hypolimnia and profound changes in geochemical conditions. In this study, we investigated the behaviour of trace elements in the water column of a seasonally stratified, eutrophic lake. Two consecutive years of data from Lake Ngapouri, North Island, New Zealand, demonstrate the occurrence of highly correlated profiles of phosphorus, arsenic, iron and manganese, all of which increased in concentration by 1–2 orders of magnitude within the anaerobic hypolimnion. Stoichiometric and mass-balance considerations demonstrate that increases in alkalinity in hypolimnetic waters were consistent with observed changes in sulfate, Fe and Mn concentrations with depth, corresponding to dissimilatory reduction of sulfate, FeIII and MnIV hydroxides. Thermodynamic constraints on Fe, Mn and Al solubility indicate that amorphous FeIII, MnIV hydroxides most probably controlled Fe and Mn in the surface mixed layer (~0 to 8m) whereas AlIII hydroxides were supersaturated throughout the entire system. Surface complexation modelling indicated that iron hydroxides (HFO) potentially dominated As speciation in the lake. It is likely that other colloidal phases such as allophanic clays also limited HPO42– activity, reducing competition for HAsO42– adsorption to iron hydroxides. This research highlights the coupling of P, As, Fe and Mn in Lake Ngapouri, and the apparent role of multiple colloidal phases in affecting P and As activity within overarching microbiological and geochemical processes.

Investigation of Vial Position Controls for the Development of a Cave Auto-Sampler

Researchers at the University of Waikato visit caves on a periodic basis to collect samples of drip water, which over geological timeframes form stalagmites and other formations. However, most caves are remotely located, which makes it an arduous task to reach the study site and is economically unfeasible for frequent visits. This paper presents the position control aspect of the cave auto-sampler, which is a crucial operation to store water samples in an airtight container. The primary challenge was to position airtight sample vials under the needles for sample collection, within 3.5mm from the centre of the silicone septa. Various methods and makes of stepper motors were tested to find the balance between precision and power saving. A high gear ratio stepper motor was chosen as the primary drives due to its high precision and its ability to hold position when power is turned off between operations. Testing in lab conditions showed satisfactory consistency regarding position control and is integrated into the auto-sampler.