title: High resolution profile of inorganic aqueous geochemistry and key redox zones in an arsenic bearing aquifer in Cambodia.
abstract: Arsenic contamination of groundwaters in South and Southeast Asia is a major threat to public health. In order to better understand the geochemical controls on the mobility of arsenic in a heavily arsenic-affected aquifer in northern Kandal Province, Cambodia, key changes in inorganic aqueous geochemistry have been monitored at high vertical and lateral resolution along dominant groundwater flow paths along two distinct transects. The two transects are characterized by differing geochemical, hydrological and lithological conditions. Arsenic concentrations in groundwater are highly heterogenous, and are broadly positively associated with iron and negatively associated with sulfate and dissolved oxygen. The observed correlations are generally consistent with arsenic mobilization by reductive-dissolution of iron (hydr)oxides. Key redox zones, as identified using groupings of the PHREEQC model equilibrium electron activity of major redox couples (notably ammonium/nitrite; ammonium/nitrate; nitrite/nitrate; dissolved oxygen/water) have been identified and vary with depth, site and season. Mineral saturation is also characterized. Seasonal changes in groundwater chemistry were observed in areas which were (i) sandy and of high permeability; (ii) in close proximity to rivers; and/or (iii) in close proximity to ponds. Such changes are attributed to monsoonal-driven surface-groundwater interactions and are consistent with the separate provenance of recharge sources as identified using stable isotope mixing models.
authors.affiliation: School of Earth and Environmental Sciences and Williamson Research Centre for Molecular Environmental Science, The University of Manchester, Williamson Building, Oxford Road, Manchester M13 9PL, UK. Electronic address: laura.richards@manchester.ac.uk.
authors.forename: Laura A
authors.initials: LA
authors.lastname: Richards
authors.affiliation: School of Earth and Environmental Sciences and Williamson Research Centre for Molecular Environmental Science, The University of Manchester, Williamson Building, Oxford Road, Manchester M13 9PL, UK.
authors.forename: Daniel
authors.initials: D
authors.lastname: Magnone
authors.affiliation: Department of Environmental Science, Royal University of Phnom Penh, Phnom Penh, Cambodia.
authors.forename: Chansopheaktra
authors.initials: C
authors.lastname: Sovann
authors.affiliation: Faculty of Agricultural Economics and Rural Development, Royal University of Agriculture, Phnom Penh, Cambodia.
authors.forename: Chivuth
authors.initials: C
authors.lastname: Kong
authors.affiliation: British Geological Survey, Environmental Science Centre, Keyworth, Nottingham NG12 5GG, UK; ETH Zurich, Institute of Geophysics, Sonneggstrasse 5, 8092 Zurich, Switzerland.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Uhlemann
authors.affiliation: British Geological Survey, Environmental Science Centre, Keyworth, Nottingham NG12 5GG, UK.
authors.forename: Oliver
authors.initials: O
authors.lastname: Kuras
authors.affiliation: School of Earth and Environmental Sciences and Williamson Research Centre for Molecular Environmental Science, The University of Manchester, Williamson Building, Oxford Road, Manchester M13 9PL, UK.
authors.forename: Bart E
authors.initials: BE
authors.lastname: van Dongen
authors.affiliation: Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Ballentine
authors.affiliation: School of Earth and Environmental Sciences and Williamson Research Centre for Molecular Environmental Science, The University of Manchester, Williamson Building, Oxford Road, Manchester M13 9PL, UK.
authors.forename: David A
authors.initials: DA
authors.lastname: Polya
keywords: Arsenic; Groundwater monitoring; Groundwater quality; PHREEQC; Redox modelling; Redox zones