TY - JOUR

T1 - Palaeo-hydrogeological control on groundwater As levels in Red River delta, Vietnam

AU - Jessen, Søren

AU - Larsen, Flemming

AU - Postma, Dieke

AU - Viet, Pham Hung

AU - Ha, Nguyen Thi

AU - Nhan, Pham Quy

AU - Nhan, Dang Duc

AU - Duc, Mai Thanh

AU - Hue, Nguyen Thi Minh

AU - Huy, Trieu Duc

AU - Luu, Tran Thi

AU - Ha, Dang Hoang

AU - Jakobsen, Rasmus

PY - 2008

Y1 - 2008

N2 - To study the geological control on groundwater As concentrations in Red River delta, depth-specific groundwater sampling and geophysical logging in 11 monitoring wells was conducted along a 45 km transect across the southern and central part of the delta, and the literature on the Red River delta's Quaternary geological development was reviewed. The water samples (n = 30) were analyzed for As, major ions, Fe2+, H2S, NH4, CH4, δ18O and δD, and the geophysical log suite included natural gamma-ray, formation and fluid electrical conductivity. The SW part of the transect intersects deposits of grey estuarine clays and deltaic sands in a 15-20 km wide and 50-60 m deep Holocene incised valley. The NE part of the transect consists of 60-120 m of Pleistocene yellowish alluvial deposits underneath 10-30 m of estuarine clay overlain by a 10-20 m veneer of Holocene sediments. The distribution of δ18O-values (range -12.2‰ to -6.3‰) and hydraulic head in the sample wells indicate that the estuarine clay units divide the flow system into an upper Holocene aquifer and a lower Pleistocene aquifer. The groundwater samples were all anoxic, and contained Fe2+ (0.03-2.0 mM), Mn (0.7-320 μM), SO4 (<2.1 μM-0.75 mM), H2S (<0.1-7.0 μM), NH4 (0.03-4.4 mM), and CH4 (0.08-14.5 mM). Generally, higher concentrations of NH4 and CH4 and low concentrations of SO4 were found in the SW part of the transect, dominated by Holocene deposits, while the opposite was the case for the NE part of the transect. The distribution of the groundwater As concentration (<0.013-11.7 μM; median 0.12 μM (9 μg/L)) is related to the distribution of NH4, CH4 and SO4. Low concentrations of As (≤0.32 μM) were found in the Pleistocene aquifer, while the highest As concentrations were found in the Holocene aquifer. PHREEQC-2 speciation calculations indicated that Fe2+ and H2S concentrations are controlled by equilibrium for disordered mackinawite and precipitation of siderite. An elevated groundwater salinity (Cl range 0.19-65.1 mM) was observed in both aquifers, and dominated in the deep aquifer. A negative correlation between aqueous As and an estimate of reduced SO4 was observed, indicating that Fe sulphide precipitation poses a secondary control on the groundwater As concentration.

AB - To study the geological control on groundwater As concentrations in Red River delta, depth-specific groundwater sampling and geophysical logging in 11 monitoring wells was conducted along a 45 km transect across the southern and central part of the delta, and the literature on the Red River delta's Quaternary geological development was reviewed. The water samples (n = 30) were analyzed for As, major ions, Fe2+, H2S, NH4, CH4, δ18O and δD, and the geophysical log suite included natural gamma-ray, formation and fluid electrical conductivity. The SW part of the transect intersects deposits of grey estuarine clays and deltaic sands in a 15-20 km wide and 50-60 m deep Holocene incised valley. The NE part of the transect consists of 60-120 m of Pleistocene yellowish alluvial deposits underneath 10-30 m of estuarine clay overlain by a 10-20 m veneer of Holocene sediments. The distribution of δ18O-values (range -12.2‰ to -6.3‰) and hydraulic head in the sample wells indicate that the estuarine clay units divide the flow system into an upper Holocene aquifer and a lower Pleistocene aquifer. The groundwater samples were all anoxic, and contained Fe2+ (0.03-2.0 mM), Mn (0.7-320 μM), SO4 (<2.1 μM-0.75 mM), H2S (<0.1-7.0 μM), NH4 (0.03-4.4 mM), and CH4 (0.08-14.5 mM). Generally, higher concentrations of NH4 and CH4 and low concentrations of SO4 were found in the SW part of the transect, dominated by Holocene deposits, while the opposite was the case for the NE part of the transect. The distribution of the groundwater As concentration (<0.013-11.7 μM; median 0.12 μM (9 μg/L)) is related to the distribution of NH4, CH4 and SO4. Low concentrations of As (≤0.32 μM) were found in the Pleistocene aquifer, while the highest As concentrations were found in the Holocene aquifer. PHREEQC-2 speciation calculations indicated that Fe2+ and H2S concentrations are controlled by equilibrium for disordered mackinawite and precipitation of siderite. An elevated groundwater salinity (Cl range 0.19-65.1 mM) was observed in both aquifers, and dominated in the deep aquifer. A negative correlation between aqueous As and an estimate of reduced SO4 was observed, indicating that Fe sulphide precipitation poses a secondary control on the groundwater As concentration.

U2 - 10.1016/j.apgeochem.2008.06.015

DO - 10.1016/j.apgeochem.2008.06.015

M3 - Journal article

AN - SCOPUS:54449097632

VL - 23

SP - 3116

EP - 3126

JO - Applied Geochemistry

JF - Applied Geochemistry

SN - 0883-2927

IS - 11

ER -