title: Retardation of arsenic transport through a Pleistocene aquifer.
abstract: Groundwater drawn daily from shallow alluvial sands by millions of wells over large areas of south and southeast Asia exposes an estimated population of over a hundred million people to toxic levels of arsenic. Holocene aquifers are the source of widespread arsenic poisoning across the region. In contrast, Pleistocene sands deposited in this region more than 12,000 years ago mostly do not host groundwater with high levels of arsenic. Pleistocene aquifers are increasingly used as a safe source of drinking water and it is therefore important to understand under what conditions low levels of arsenic can be maintained. Here we reconstruct the initial phase of contamination of a Pleistocene aquifer near Hanoi, Vietnam. We demonstrate that changes in groundwater flow conditions and the redox state of the aquifer sands induced by groundwater pumping caused the lateral intrusion of arsenic contamination more than 120 metres from a Holocene aquifer into a previously uncontaminated Pleistocene aquifer. We also find that arsenic adsorbs onto the aquifer sands and that there is a 16-20-fold retardation in the extent of the contamination relative to the reconstructed lateral movement of groundwater over the same period. Our findings suggest that arsenic contamination of Pleistocene aquifers in south and southeast Asia as a consequence of increasing levels of groundwater pumping may have been delayed by the retardation of arsenic transport.
authors.affiliation: Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA. avangeen@ldeo.columbia.edu
authors.forename: Alexander
authors.initials: A
authors.lastname: van Geen
authors.forename: Benjamín C
authors.initials: BC
authors.lastname: Bostick
authors.forename: Thi Kim Trang
authors.initials: TK
authors.lastname: Pham
authors.forename: Mai Lan
authors.initials: ML
authors.lastname: Vi
authors.forename: Mai
authors.initials: M
authors.lastname: Nguyen-Ngoc
authors.forename: Dao Manh
authors.initials: DM
authors.lastname: Phu
authors.forename: Hung Viet
authors.initials: HV
authors.lastname: Pham
authors.forename: Kathleen
authors.initials: K
authors.lastname: Radloff
authors.forename: Zahid
authors.initials: Z
authors.lastname: Aziz
authors.forename: Jacob L
authors.initials: JL
authors.lastname: Mey
authors.forename: Mason O
authors.initials: MO
authors.lastname: Stahl
authors.forename: Charles F
authors.initials: CF
authors.lastname: Harvey
authors.forename: Peter
authors.initials: P
authors.lastname: Oates
authors.forename: Beth
authors.initials: B
authors.lastname: Weinman
authors.forename: Caroline
authors.initials: C
authors.lastname: Stengel
authors.forename: Felix
authors.initials: F
authors.lastname: Frei
authors.forename: Rolf
authors.initials: R
authors.lastname: Kipfer
authors.forename: Michael
authors.initials: M
authors.lastname: Berg
mesh_terms: D001151:Arsenic / Q000032:analysis*; D020261:Arsenic Poisoning; D002244:Carbon / Q000032:analysis; D060766:Drinking Water / Q000737:chemistry; D005506:Food Contamination / Q000032:analysis; D019015:Geologic Sediments / Q000737:chemistry; D060587:Groundwater / Q000032:analysis / Q000737:chemistry*; D006801:Humans; D010084:Oxidation-Reduction; D045483:Rivers / Q000737:chemistry; D012822:Silicon Dioxide / Q000032:analysis / Q000737:chemistry; D014744:Vietnam; D014872:Water Movements; D060986:Water Wells / Q000737:chemistry
chemical_list: D060766:Drinking Water; D002244:Carbon; D012822:Silicon Dioxide; D001151:Arsenic
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