River bank geomorphology controls groundwater arsenic concentrations in aquifers adjacent to the Red River, Hanoi Vietnam

Many aquifers that are highly contaminated by arsenic in South and Southeast Asia are in the floodplains of large river networks. Under natural conditions, these aquifers would discharge into nearby rivers; however, large‐scale groundwater pumping has reversed the flow in some areas so that rivers n...

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Bibliographic Details
Published in:Water resources research 2016-08, Vol.52 (8), p.6321-6334
Main Authors: Stahl, Mason O., Harvey, Charles F., van Geen, Alexander, Sun, Jing, Thi Kim Trang, Pham, Mai Lan, Vi, Mai Phuong, Thao, Hung Viet, Pham, Bostick, Benjamin C.
Format: Article
Language:English
Subjects:
Aquifer recharge
Aquifers
Arsenic
Contamination
Crystal structure
Discharge
Drawing and ironing
Floodplains
Flow
Freshwater
Geochemistry
Geomorphology
Geomorphology of rivers
Groundwater
Groundwater recharge
Hosts
Iron
Iron oxides
Mathematical analysis
Networks
Oxides
Pleistocene
Pollution
Pore water
Pumping
Recharge
Regions
River banks
River beds
River networks
River sediments
River water
Riverbanks
Riverbeds
Rivers
Sediment
Sediments
Vietnam
Water
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Summary:Many aquifers that are highly contaminated by arsenic in South and Southeast Asia are in the floodplains of large river networks. Under natural conditions, these aquifers would discharge into nearby rivers; however, large‐scale groundwater pumping has reversed the flow in some areas so that rivers now recharge aquifers. At a field site near Hanoi Vietnam, we find river water recharging the aquifer becomes high in arsenic, reaching concentrations above 1000 µg/L, within the upper meter of recently (< ∼10 years) deposited riverbed sediments as it is drawn into a heavily pumped aquifer along the Red River. Groundwater arsenic concentrations in aquifers adjacent to the river are largely controlled by river geomorphology. High (>50 µg/L) aqueous arsenic concentrations are found in aquifer regions adjacent to zones where the river has recently deposited sediment and low arsenic concentrations are found in aquifer regions adjacent to erosional zones. High arsenic concentrations are even found adjacent to a depositional river reach in a Pleistocene aquifer, a type of aquifer sediment which generally hosts low arsenic water. Using geochemical and isotopic data, we estimate the in situ rate of arsenic release from riverbed sediments to be up to 1000 times the rates calculated on inland aquifer sediments in Vietnam. Geochemical data for riverbed porewater conditions indicate that the reduction of reactive, poorly crystalline iron oxides controls arsenic release. We suggest that aquifers in these regions may be susceptible to further arsenic contamination where riverine recharge drawn into aquifers by extensive groundwater pumping flows through recently deposited river sediments before entering the aquifer. Key Points: Rates of arsenic release from young riverbed sediments are up to 1000 times the release rates from aquifer sediment River recharge induced by groundwater pumping may contaminate previously safe aquifers with arsenic Riverbank geomorphology can influence groundwater arsenic concentrations in aquifers adjacent to rivers
ISSN:0043-1397
1944-7973
DOI:10.1002/2016WR018891