The role of organics in the mobilization of arsenic in shallow aquifers

• Main Author: Al Lawati, Wafa Mustafa Mohammed
• Format: Dissertation
• Language: English
• Published: University of Manchester 2012
• Subjects: arsenic; organic matter

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.