Perran L. M. Cook

Speciation of dissolved iodine in the waters of a humic-rich estuary

Abstract This paper reports the findings of a study into iodine speciation in the humic-rich waters of the Huon Estuary, Tasmania, Australia. Water samples were taken from the estuary at various locations during both summer and winter. The samples were analysed for a range of parameters including iodate, iodide, total iodine, nutrients, chlorophyll a , dissolved organic carbon (DOC) and salinity. Total iodine behaved conservatively within the estuary irrespective of season. Iodate concentrations varied linearly with salinity but became undetectable in the low salinity end of the estuary. Iodide concentrations showed no correlation with salinity, but showed a positive correlation with total dissolved phosphorus during summer suggesting iodide concentrations within the estuary are controlled by biological activity during, or immediately after, periods of high productivity. “Organic” iodine is produced within the estuary and the fraction of iodine present in this form increases in the low salinity waters where DOC concentrations are highest.

Unravelling the origin and fate of nitrate in an agricultural–urban coastal aquifer

Nitrate (NO3−) contamination in groundwater is a worldwide phenomenon and a pervasive environmental problem, particularly when NO3−-enriched groundwater discharges into a nitrogen-limited estuarine environment through submarine groundwater discharge (SGD). SGD is often associated with eutrophication which ultimately alters the coastal ecology of the receiving surface water. Identifying the sources and transformation processes of NO3− in and within the groundwater discharged to the estuary provides baseline information underpinning effective management of the coastal environments. The aims of this study were to: (1) understand the linkages between aquifers at different depths and in different parts of a catchment in the south east of Australia which underlies a eutrophic estuary (the Werribee River estuary); (2) identify and apportion the NO3− source(s) to the aquifers; and (3) identify the major transformation processes of NO3− in the aquifer along the groundwater flowpath. The average δ15N–NO3− values of the deep groundwater (+21xa0‰) at the SGD hotspot lies between the enriched δ15N–NO3− (~+33xa0‰) at the western side of the estuary and relatively depleted δ15N–NO3− (~+14xa0‰) at the eastern side; indicating that the aquifers from both sides of the estuary are connected at the SGD hotspot. The isotopic composition of NO3−, together with the concentrations of excess nitrogen (N2) gas also revealed that SGD-derived NO3− originated predominantly from agricultural activity. Denitrification was not the primary NO3− removal process in the oxic groundwater. Instead, mixing between sewage (69xa0%) and fertiliser-derived (31xa0%) NO3− appeared to be the main control over the observed NO3− concentrations (~1,000xa0µmolxa0L−1) and the relatively enriched δ15N–NO3− values (+20 to +23xa0‰) in the deeper sand and gravel aquifer at the groundwater discharge zone. These results suggest that groundwater is a critical source of NO3− to the receiving surface water and as such should always be included not only in the regional but also the global N budget. The outcome of the study is a key to sustainable management of coastal aquatic ecosystems.

Development of a gas diffusion probe for the rapid measurement of pCO2 in aquatic samples

A probe for the direct measurement of the partial pressure of carbon dioxide (pCO(2)) in aqueous samples is described. It consists of a gas permeable membrane tube containing a flowing acceptor stream of bromothymol blue indicator solution. Carbon dioxide diffuses across the membrane causing a pH change in the acceptor. This pH change decreases the absorbance of the acid-base indicator which is detected photometrically, with high sensitivity using a multi-reflection photometric detector with an LED light source. Unlike many other common methods used to measure pCO(2), this probe has the advantage of not requiring sampling to perform measurements, and avoids potential losses and contamination. This probe has the potential to perform experiments requiring in situ measurements of pCO(2), allowing regular measurements of closed system experiments, without removing any of the water column. Compared to indirect methods used to measure pCO(2), this probe has the potential to provide more portable and faster measurements. The sensitivity, sampling rate and linear range of the probe can be tuned depending on the required sensitivity and range of measurements, and a measurement rate of at least 36 h(-1) can be achieved. An application of this probe in real-time analysis of pCO(2) flux in a sediment core during a large deposition of organic matter has been described. As a comparison, the measurements of the probe have been plotted against pCO(2) calculated from alkalinity using a Gran titration. It is envisaged that the probe could be used for experiments in the laboratory requiring real time in situ measurements, or incorporated into a portable instrument so that field measurements can be easily performed. Although the linear range and sensitivity of this probe can be tuned, the configuration described gave a linear response over the calibration range of 0-5800 μatm pCO(2), with a detection limit of 144 μatm. The precision was 1.2% RSD (n=13) at 430 μatm.

Fe and S K-edge XAS determination of iron-sulfur species present in a range of acid sulfate soils: Effects of particle size and concentration on quantitative XANES determinations

Acid sulfate soils (ASS) are soils and soft sediments in which sulfuric acid may be produced from iron sulfides or have been produced leaving iron oxyhydroxysulfates in amounts that have a long lasting effect on soil characteristics. If soil material is exposed to rotting vegetation or other reducing material, the Fe-oxyhydroxysulfates can be bacterially reduced to sulfides including disulfides (pyrite and marcasite), and Monosulfidic Black Ooze (MBO) a poorly characterised material known to be a mixture of iron sulfides (especially mackinawite) and organic matter. The chemistry of these environments is strongly affected by Fe and S cycling processes and herein we have sought to identify key differences in environments that occur as a function of Fe and S concentration. In addition to our chemical results, we have found that the effects of particle size on self absorption in natural sediments play an important role in the spectroscopic identification of the relative proportions of different species present.