Over a hundred million people across the globe, and particularly in Asia, are chronicallyexposed to high concentrations (>10 ppb) of geogenic arsenic (As) in shallow reducinggroundwaters utilised as drinking water. It is widely accepted that As mobilisation fromsediments into these groundwater requires active metal-reducing microbes and electrondonors such as organic matter (OM). Although OM have been characterised in fewaquifers, there is a dearth of data on other As rich aquifers.In this work, we further investigate the correlation between As release from aquifers andthe OM/microbial communities present. The key findings of this work are:i. OM in Vietnamese and Taiwanese As hotspot areas originates from multiple sources.This is comparable to previous studies in Cambodia and West Bengal, indicating thatOM originates from similar sources in all the areas studied up to date, regardless ofthe age (Holocene/Pleistocene) and the sediment source (Himalayas/non-Himalayas).This suggests that similar electron donors, such as petroleum derived HMW nalkanes,are present in all these groundwater aquifer sediments.ii. No noticeable differences in OM biomarker distribution patterns were observedbetween sediments from two contrasting groundwater As concentration sites (e.g. <10 ppb; Pleistocene and up to 600 ppb; Holocene) in Vietnam, suggesting thatarsenic mobilisation is not associated with a specific OM source at these sites. Inaddition, no microbial activity was determined in these two sites suggesting thatother abiotic factors could lead to As release.iii. Microbiological process of dissimilatory As(V) reduction is active in microcosmexperiments using non-Himalayan Taiwanese sediments, despite low groundwaterAs concentrations, causing the release of As into the groundwater. However, Asrelease in these sediment slurries is not controlled by a specific source of the lipidderived OM, suggesting that other electron donors, not analysed in present studycould be contributing/controlling the rate of As release; and/or that multiple fractionsof the lipid derived OM are used as electron donors in this process.iv. Artificial maturation experiments indicated that a fully 13C-lablled kerogen analoguecan be obtained by using 13C-labelled cyanobacteria biomass as a starting material.However, this kerogen analogue had some differences when compared to the naturalmaterial, suggesting that an artificial degradation precursor step, prior to the actualmaturation process, might be required to generate analogues that better resemble thenatural kerogen.v. 13C-labelled substrates (hexadecane and kerogen) incubation experiments revealedthat As(III) release in all microcosms was microbially driven. Very low 13C-n-alkaneincorporation was observed in association with As release, suggesting that otherelectron donors could be mediating this process. In contrast, kerogen did not haveany effect on As release. Moreover, As(V) amendments enhanced the degradation ofthe shorter carbon chain length n-alkanes more than the longer ones, suggesting thatthese are more important electron donors in the process of As release than the longerones.

Elevated levels of arsenic (As) in soils and water around the world are both a significant human health and environmental hazard. With increasing global water demands, there is a requirement to further the understanding of the biogeochemical cycling of As from soils and sediments. This thesis focussed on exploring the environmental controls on the occurrence and subsequent mobility of As in a range of natural environments. Arsenic was found to undergo mobilisation from both river sediments and upland peats under changing environmental conditions. The transport of As was found to be correlated with both iron (Fe) and organic carbon (OC), however temporal changes in both sediment/soil composition and movement of water through catchments have a important role in controlling the ultimate transport of As within the environment. A range of investigative methods were employed to study the occurrence and mobility of As within the river sediments of the Allier and Loire Rivers (France), including sequential extraction procedures and batch incubation studies. Arsenic was associated with the reducible phases of sediments, indicating the major role of Fe(oxy)hydroxides in the storage of As in river sediments. In addition to the presence of labile As, the rapid release of As was dependent on the initial sediment composition. Temporal changes in sediment composition may therefore play an important role in controlling the movement of As within fluvial systems. The combination of lead (Pb) and strontium (Sr) isotopic analysis with sequential extraction studies of sediments from the Loire and Allier Rivers was able to determine the relative dominance of granites and basalts within the sediments. This approach provided a first order study on which to better understand the mineral origins of the sediments. The analysis of multiple Pb isotopes was able to eliminate possible anthropogenic contribution to contamination within the sediments, confirming the importance of geogenic cycling of As within the rivers. Information on the origin of mineral formation was obtained through 87Sr/86Sr isotopic analysis, with the formation of Fe-minerals not occurring uniformly along the course of the rivers. While the Sr within the sediment phase targeting well-crystallised Fe(oxy)hydroxides was in equilibrium with the sampled river water, the formation of amorphous Fe minerals was likely occurring in waters upstream of the study sites, within the Massif Central. Total concentration profiles peat from two subcatchments within the Peak District (United Kingdom) provided evidence for both the retention and post depositional movement (PDM) of As within the solid phase, dependent on local conditions. For the first time, the partitioning of As was determined within ombrotrophic peat, and found to be in contrast to Pb, with oxidisible As (likely associated with organic matter) dominating, while Pb was found predominantly within the reducible sediment phase. High temporal resolution monitoring of the organic-rich streamwater draining the peat showed the transport of As was variable, with As found largely in the soluble form despite extensive peat erosion. The evidence for PDM, and the subsequent soluble transport of As demonstrated the importance of biogeochemical processes in releasing As from the solid phase. Once mobilised, both the ratio of Fe:OC and the form of Fe were found to be factors controlling transport of As, with the flushing of stored porewaters an important contribution to As transport from the peat. Despite OC-rich waters, the occurrence of high concentrations of Fe may dominate control of As within the aqueous phase. At relatively high (>0.2) Fe:OC ratios, the particle size distribution of As was closely correlated with that of >1um Fe, although the presence of dissolved and colloidal As was found even within these waters. Given the temporal variability of As transport within the streams, knowledge of the mixing order and ratio between Fe, OC, and As within natural waters may be required for prediction of the mobility and ultimate fate of As.