Groundwater arsenic concentrations in Vietnam controlled by sediment age

Transcription

1 Groundwater arsenic concentrations in Vietnam controlled by sediment age 5 6 Dieke Postma, Flemming Larsen, Nguyen Thi Thai, Pham Thi Kim Trang, Rasmus Jakobsen, Pham Quy Nhan, Tran Vu Long, Pham Hung Viet, Andrew S. Murray H/ 3 He dating of groundwater The recharge age of the groundwater was determined using the 3 H/ 3 He method. For helium isotopes and neon, samples were taken in 50 cm long 3/8 (0.95 cm) diameter Cu tubes sealed at either end with pinch off clamps 1. Water for tritium was sampled in 1 L PVC bottles. The helium isotopes and neon measurements were done on a mass spectrometric system (MSS) at the University of Bremen 2. The tritium measurements were done on mass spectrometric system at the Institute for Nuclear Sciences and Technology in Hanoi. The total uncertainty of the recharge ages is estimated to be ±2 year. The results are depicted in Supplementary Fig. S1. NATURE GEOSCIENCE 1

2 Supplementary Fig. S1. Tritium helium dating of groundwater: lines indicate an infiltration rate of 0.5 m/yr except for Van Coc where it is 0.25 m/yr Geology and Groundwater flow model A geological cross section was constructed along a line going from Phu Kim in the southwest to the H transect in the northeast (Fig. 1). The geology was compiled from drillings at the five locations indicated in Supplementary Fig. S2, geophysical borehole logging, and 21 transient electromagnetic soundings. Near the margin of flood plain, the thickness of the Holocene deposits is approximately 15 m, and the total thickness of the Holocene and Pleistocene sediments increases to about 45 m near the Red River. The Quaternary deposits are overlying consolidated Neogene deposits. The Holocene and Pleistocene deposits are dominated by sandy fluvial deposits and three, up to 20 m thick, fine grained deposits of silt and clay, probably deposited in oxbow lake systems. These low permeable units are oriented from northwest to southeast, perpendicular to the shown cross section. An up to six meter thick layer of overbank clay and silt is overlying most of the Holocene aquifer At Phu Kim, only Holocene sand deposits are present with a thickness of 12 m. At Phung Thuong, the Holocene aquifer is 10 m thick and overlying an 18 m thick silt and clay layer, which again is on top of a lower aquifer, probably of Pleistocene age. A shallow aquifer is not found in the central part of the cross section (Thuong Coc), as the geology here is dominated by silt and clay down 2 NATURE GEOSCIENCE

3 SUPPLEMENTARY INFORMATION to 26 m. At Van Coc, a 17 m thick upper Holocene aquifer is found on top of a 16 m thick sequence of silt and clay, which separates the Holocene and Pleistocene aquifers. At the H transect, the Holocene and Pleistocene aquifers are in a direct hydraulic contact Supplementary Fig. S2. A geological cross section along a line from Phu Kim to the H transect in Fig. 1. 2D groundwater flow was modeled using MODFLOW 3. Particle tracking lines show the groundwater flow pattern. Note the importance of the surface water bodies for the flow pattern The groundwater flow in the shallow Holocene sandy aquifer is overall towards the Red River in the northeast. The flow is, however, locally controlled by the presence of the three low permeable layers and the interaction with the many bodies of surface water. The location of these surface water bodies, indicated in Fig. 1 and Supplementary Fig. S2, is yielding a groundwater flow regime with a series of local shallow flow systems. Groundwater flow in the cross section has been simulated numerically in a 2D model using the code MODFLOW 3. The use of a 2D groundwater flow model in this setting is justified because the dominant groundwater flow is perpendicular to subsurface geological structures. The applied boundary conditions in the model are, towards the southwest and NATURE GEOSCIENCE 3

4 northeast observed groundwater heads, an infiltration rate of 160 mm/yr derived from the 3H/3He dating (Supplementary Fig. S1), and a lower no flow boundary given by the Neogene deposits. The groundwater/surface water interaction has been simulated using drains with constant heads Simulated groundwater flow lines, i.e. particle tracking lines from the groundwater table, are displayed in the geological cross section (Supplementary Fig. S2). The estimated residence times, or groundwater ages, are within 5 years of the 3H/3He ages. The flow lines indicate that shallow groundwater flow in the Holocene aquifer is controlled by the local interactions with surface water, and not by regional flow. Phu Kim is located between two surface water bodies (Supplementary Fig. S2), Phung Thuong downstream a secondary water divide and Van Coc just downstream a low permeable silt and clay body. Only the H transect shows a homogeneous flow pattern over a larger area but water infiltrated near Van Coc will pass below the sampled depth at the H transect. Leakage into the underlying Pleistocene aquifer is taking place, and a more regional flow system is thus present in the deeper layers, with simulated residence times up to years when the water reaches the Red River Radiotracer rate measurements. The rate of CO 2 reduction and acetate fermentation and oxidation was measured by a modified version of the method of Crill and Martens 4. A very similar description to what is given here can be found in Hansen et al. 5. The main modifications in the method used in this study compared to Hansen et al. 5 were to accommodate for the difficulties in obtaining in Hanoi the required chemicals, implying that Carbosorb was substituted with a 7:3 mixture of ethanolamine and 2 methoxyethanol (called 7:3 mix in the following) and the scintillation cocktail was a mixture of 2 methoxyethanol, dioxane, toluene, POP and POPOP substituting for the Permaflour E. It is recommended that the chemicals described in Hansen et al. 5 are used for easier processing. 4 NATURE GEOSCIENCE

5 SUPPLEMENTARY INFORMATION CH 4, 14 CO 2 and 14 CH 3 COO were separated by means of a stripping apparatus. A frozen core segment (~11 cm) was placed in a reaction vessel, and 50 ml of 1 N NaOH was added to retain carbonate and acetate; the radiolabeled methane was stripped from the sediment by passing atmospheric air through the reaction vessel. Radiolabeled CO 2 and acetate were almost completely retained in the reaction vessel. The escaping traces of 14 CO 2 were sequestered in two traps containing 10 ml of a 7:3 mixture of ethanolamine and 2 methoxyethanol. The methane stripped off was oxidized by passage through a piece of stainless steel tubing filled with CuO, heated to 800 C in a furnace, and collected in two traps mounted in series containing 10 ml of the 7:3 mixture of ethanolamine and 2 methoxyethanol. The content of the traps was mixed with the scintillation cocktail and the radioactivity counted. The carrier gas was cleaned and partially dehydrated in pretraps containing 10 ml of pure 2 methoxyethanol to remove particles and avoid phase separation when mixing the reacted trap fluid with the scintillation cocktail. In the second step, the sediment sample was acidified by adding 60 ml 4 N HCl. This allowed 14 CO 2 to be stripped from the sediment and captured in two fresh traps each containing 10 ml of the 7:3 mix, after passing through a fresh pre trap. As before, the 7:3 mix was mixed with scintillation cocktail and the 14 CO 2 counted. Acetate is retained in the reaction vessel during HCl treatment. Tests in Hansen et al. 5 showed that only 0.2% of the added 14 CH 3 COO escaped from the reaction vessel during the stripping time (90 min), and most of this was sequestered in the pre trap. 14 CH 3 COO was determined by sampling the acidified sediment slurry, neutralizing it, decanting the fluid, and re suspending it in demineralized water before sampling again, this washing was done twice and all three samples were counted after mixing with scintillation cocktail and the total activity was calculated based on the decanted volumes. All samples were counted on a Packard Tri Carb liquid scintillation counter using blanks prepared from the relevant fluids in the lab and quench correction by sample channels ratio, by means of prepared standards covering the relevant quench range. NATURE GEOSCIENCE 5

6 The rate of CO 2 reduction was calculated from: where CRR is the rate of CO 2 reduction, TIC the concentration of total inorganic carbon calculated from alkalinity and ph, a CH4 and a TIC the radioactivities of CH 4 and TIC after incubation, t the incubation time, and α the fractionation factor of 1.08, which takes into account that 14 CO 2 is reduced at a slower rate than 12 CO 2. In contrast to TIC, a significant amount (2 10%) of the acetate tracer is consumed during the incubation and the concentration of 14 CH 3 COO is accordingly not a constant, but is calculated from a rate expression assuming a first order decrease. The acetate turnover rate (ATR) is then calculated from the relation: Where α is the fractionation factor amounting to 1.08, (CH3COO ) the acetate concentration, t incubation time, and a CH3COO, a CH4, and a TIC the radioactivity in the subscripted fractions. The acetate fermentation rate (AFR) and the acetate oxidation rate (AOR) are then calculated as: NATURE GEOSCIENCE

7 SUPPLEMENTARY INFORMATION 130 and; The concentrations of unlabeled TIC and acetate needed for the rate calculations were interpolated from groundwater concentration profiles at the same site. For acetate, outliers where excluded from these interpolations. Incubation times were hours. Acetate concentrations used in the rate calculations were Phu Kim 1.0 µm, Phuong Thuong 1.3 µm, Van Coc 2.0 µm, and H transect 2.0 µm. TIC values were Phu Kim 15.4 mm, Phuong Thuong 10.4 mm, Van Coc 10.9 mm and H transect 10.8 mm References Stute, M. & Schlosser, P. Atmospheric noble gasses. In: Cook, P.G., Herzeg, A.L. (Eds.) Environmental Tracers in Subsurface Hydrology (Klüwer Academic Publ. 2000) Sültenfuss, J., Roether, W. & Rhein, M. The Bremen mass spectrometric facility for the measurement of helium isotopes, neon and tritrum in water. Isotopes in Environmental and Health Studies, 45, 83 95, (2009) McDonald, M., G. and Harbaugh, A.W. A modular three dimensional finite difference ground water flow model. US. Geol. Survey Open File Report (1988). 152 NATURE GEOSCIENCE 7

8 Crill P. M. & Martens C. S. Methane production from bicarbonate and acetate in an anoxic marine sediment. Geochim. Cosmochim. Acta 50, (1986) Hansen L. K., Jakobsen R. & Postma D. Methanogenesis in a shallow sandy aquifer, Rømø, Denmark. Geochim. Cosmochim. Acta 65, (2001) NATURE GEOSCIENCE