1 Supplementary information SUPPLEMENTARY INFORMATION DOI: 10.1038/NGEO1540 2 Sediment age as overall control on the groundwater arsenic concentration in Vietnam Groundwater arsenic concentrations in Vietnam controlled by sediment age 4 3 5 Dieke Postma, Flemming Larsen, Nguyen Thi Thai, Pham Thi Kim Trang, Rasmus Jakobsen, Pham Quy 6 Nhan, Tran Vu Long, Pham Hung Viet, Andrew S. Murray 7 8 9 10 11 H/3He dating of groundwater 12 The recharge age of the groundwater was determined using the 3H/3He method. For helium isotopes 13 and neon, samples were taken in 50 cm long 3/8” (0.95 cm) diameter Cu tubes sealed at either end 14 with pinch‐off clamps1. Water for tritium was sampled in 1 L PVC bottles. The helium isotopes and 15 neon measurements were done on a mass spectrometric system (MSS) at the University of Bremen2. 16 The tritium measurements were done on mass spectrometric system at the Institute for Nuclear 17 Sciences and Technology in Hanoi. The total uncertainty of the recharge ages is estimated to be ±2 18 year. The results are depicted in Supplementary Fig. S1. 19 1 NATURE GEOSCIENCE | www.nature.com/naturegeoscience © 2012 Macmillan Publishers Limited All rights reserved SUPPLEMENTARY INFORMATION DOI: 10.1038/NGEO1540 20 21 Supplementary Fig. S1. Tritium helium dating of groundwater: lines indicate an infiltration rate of 0.5 22 m/yr except for Van Coc where it is 0.25 m/yr. 23 24 Geology and Groundwater flow model 25 A geological cross–section was constructed along a line going from Phu Kim in the southwest to the 26 H‐transect in the northeast (Fig. 1). The geology was compiled from drillings at the five locations 27 indicated in Supplementary Fig. S2, geophysical borehole logging, and 21 transient electromagnetic 28 soundings. Near the margin of flood plain, the thickness of the Holocene deposits is approximately 15 29 m, and the total thickness of the Holocene and Pleistocene sediments increases to about 45 m near 30 the Red River. The Quaternary deposits are overlying consolidated Neogene deposits. The Holocene 31 and Pleistocene deposits are dominated by sandy fluvial deposits and three, up to 20 m thick, fine 32 grained deposits of silt and clay, probably deposited in oxbow lake systems. These low‐permeable 33 units are oriented from northwest to southeast, perpendicular to the shown cross‐section. An up to 34 six meter thick layer of overbank clay and silt is overlying most of the Holocene aquifer. 35 At Phu Kim, only Holocene sand deposits are present with a thickness of 12 m. At Phung 36 Thuong, the Holocene aquifer is 10 m thick and overlying an 18 m thick silt and clay layer, which 37 again is on top of a lower aquifer, probably of Pleistocene age. A shallow aquifer is not found in the 38 central part of the cross‐section (Thuong Coc), as the geology here is dominated by silt and clay down NATURE GEOSCIENCE | www.nature.com/naturegeoscience 2 © 2012 Macmillan Publishers Limited All rights reserved DOI: 10.1038/NGEO1540 SUPPLEMENTARY INFORMATION 39 to 26 m. At Van Coc, a 17 m thick upper Holocene aquifer is found on top of a 16 m thick sequence of 40 silt and clay, which separates the Holocene and Pleistocene aquifers. At the H‐transect, the Holocene 41 and Pleistocene aquifers are in a direct hydraulic contact. 42 43 44 Supplementary Fig. S2. A geological cross‐section along a line from Phu Kim to the H‐transect in Fig. 45 1. 2D groundwater flow was modeled using MODFLOW3. Particle tracking lines show the 46 groundwater flow pattern. Note the importance of the surface water bodies for the flow pattern. 47 48 The groundwater flow in the shallow Holocene sandy aquifer is overall towards the Red River 49 in the northeast. The flow is, however, locally controlled by the presence of the three low‐permeable 50 layers and the interaction with the many bodies of surface water. The location of these surface water 51 bodies, indicated in Fig. 1 and Supplementary Fig. S2, is yielding a groundwater flow regime with a 52 series of local shallow flow systems. Groundwater flow in the cross‐section has been simulated 53 numerically in a 2D model using the code MODFLOW3. The use of a 2D groundwater flow model in 54 this setting is justified because the dominant groundwater flow is perpendicular to subsurface 55 geological structures. The applied boundary conditions in the model are, towards the southwest and NATURE GEOSCIENCE | www.nature.com/naturegeoscience 3 © 2012 Macmillan Publishers Limited All rights reserved SUPPLEMENTARY INFORMATION DOI: 10.1038/NGEO1540 56 northeast observed groundwater heads, an infiltration rate of 160 mm/yr derived from the 3H/3He 57 dating (Supplementary Fig. S1), and a lower no‐flow boundary given by the Neogene deposits. The 58 groundwater/surface water interaction has been simulated using drains with constant heads. 59 Simulated groundwater flow lines, i.e. particle tracking lines from the groundwater table, are 60 displayed in the geological cross‐section (Supplementary Fig. S2). The estimated residence times, or 61 groundwater ages, are within 5 years of the 3H/3He ages. The flow lines indicate that shallow 62 groundwater flow in the Holocene aquifer is controlled by the local interactions with surface water, 63 and not by regional flow. Phu Kim is located between two surface water bodies (Supplementary Fig. 64 S2), Phung Thuong downstream a secondary water divide and Van Coc just downstream a low 65 permeable silt and clay body. Only the H‐transect shows a homogeneous flow pattern over a larger 66 area but water infiltrated near Van Coc will pass below the sampled depth at the H‐transect. Leakage 67 into the underlying Pleistocene aquifer is taking place, and a more regional flow system is thus 68 present in the deeper layers, with simulated residence times up to 500‐600 years when the water 69 reaches the Red River. 70 71 Radiotracer rate measurements. 72 The rate of CO2 reduction and acetate fermentation and oxidation was measured by a modified 73 version of the method of Crill and Martens4. A very similar description to what is given here can be 74 found in Hansen et al.5. The main modifications in the method used in this study compared to 75 Hansen et al.5 were to accommodate for the difficulties in obtaining in Hanoi the required chemicals, 76 implying that Carbosorb was substituted with a 7:3 mixture of ethanolamine and 2‐methoxyethanol 77 (called 7:3 mix in the following) and the scintillation cocktail was a mixture of 2‐methoxyethanol, 78 dioxane, toluene, POP and POPOP substituting for the Permaflour E. It is recommended that the 79 chemicals described in Hansen et al.5 are used for easier processing. 4 NATURE GEOSCIENCE | www.nature.com/naturegeoscience © 2012 Macmillan Publishers Limited All rights reserved SUPPLEMENTARY INFORMATION DOI: 10.1038/NGEO1540 14 CH4, 14CO2 and 14CH3COO‐ were separated by means of a stripping apparatus. A 80 81 frozen core segment (~11 cm) was placed in a reaction vessel, and 50 mL of 1 N NaOH was added to 82 retain carbonate and acetate; the radiolabeled methane was stripped from the sediment by passing 83 atmospheric air through the reaction vessel. Radiolabeled CO2 and acetate were almost completely 84 retained in the reaction vessel. The escaping traces of 14CO2 were sequestered in two traps 85 containing 10 mL of a 7:3 mixture of ethanolamine and 2‐methoxyethanol. The methane stripped off 86 was oxidized by passage through a piece of stainless steel tubing filled with CuO, heated to 800°C in a 87 furnace, and collected in two traps mounted in series containing 10 mL of the 7:3 mixture of 88 ethanolamine and 2‐methoxyethanol. The content of the traps was mixed with the scintillation 89 cocktail and the radioactivity counted. The carrier gas was cleaned and partially dehydrated in pre‐ 90 traps containing 10 mL of pure 2‐methoxyethanol to remove particles and avoid phase separation 91 when mixing the reacted trap fluid with the scintillation cocktail. 92 In the second step, the sediment sample was acidified by adding 60 mL 4 N HCl. This 93 allowed 14CO2 to be stripped from the sediment and captured in two fresh traps each containing 10 94 mL of the 7:3 mix, after passing through a fresh pre‐trap. As before, the 7:3 mix was mixed with 95 scintillation cocktail and the 14CO2 counted. Acetate is retained in the reaction vessel during HCl 96 treatment. Tests in Hansen et al.5 showed that only 0.2% of the added 14CH3COO‐ escaped from the 97 reaction vessel during the stripping time (90 min), and most of this was sequestered in the pre‐trap. 98 14 99 fluid, and re‐suspending it in demineralized water before sampling again, this washing was done CH3COO‐ was determined by sampling the acidified sediment slurry, neutralizing it, decanting the 100 twice and all three samples were counted after mixing with scintillation cocktail and the total activity 101 was calculated based on the decanted volumes. All samples were counted on a Packard Tri‐Carb 102 liquid scintillation counter using blanks prepared from the relevant fluids in the lab and quench 103 correction by sample channels ratio, by means of prepared standards covering the relevant quench 104 range. NATURE GEOSCIENCE | www.nature.com/naturegeoscience 5 © 2012 Macmillan Publishers Limited All rights reserved SUPPLEMENTARY INFORMATION DOI: 10.1038/NGEO1540 105 106 The rate of CO2 reduction was calculated from: 107 108 109 110 111 where CRR is the rate of CO2 reduction, TIC the concentration of total inorganic carbon calculated 112 from alkalinity and pH, aCH4 and aTIC the radioactivities of CH4 and TIC after incubation, t the 113 incubation time, 114 and α the fractionation factor of 1.08, which takes into account that 14CO2 is reduced at a slower rate 115 than 12CO2. 116 In contrast to TIC, a significant amount (2–10%) of the acetate tracer is consumed 117 during the incubation and the concentration of 14CH3COO‐ is accordingly not a constant, but is 118 calculated from a rate expression assuming a first order decrease. The acetate turnover rate (ATR) is 119 then calculated from the relation: 120 121 122 123 Where α is the fractionation factor amounting to 1.08, (CH3COO‐) the acetate concentration, t 124 incubation time, and aCH3COO‐, aCH4, and aTIC the radioactivity in the subscripted fractions. The acetate 125 fermentation rate (AFR) and the acetate oxidation rate (AOR) are then calculated as: 126 127 128 129 6 NATURE GEOSCIENCE | www.nature.com/naturegeoscience © 2012 Macmillan Publishers Limited All rights reserved DOI: 10.1038/NGEO1540 SUPPLEMENTARY INFORMATION 130 and; 131 132 133 134 The concentrations of unlabeled TIC and acetate needed for the rate calculations were interpolated 135 from groundwater concentration profiles at the same site. For acetate, outliers where excluded from 136 these interpolations. Incubation times were 18‐21 hours. Acetate concentrations used in the rate 137 calculations were Phu Kim 1.0 µM, Phuong Thuong 1.3 µM, Van Coc 2.0 µM, and H‐transect 2.0 µM. 138 TIC values were Phu Kim 15.4 mM, Phuong Thuong 10.4 mM, Van Coc 10.9 mM and H‐transect 10.8 139 mM. 140 141 References 142 143 1. Stute, M. & Schlosser, P. Atmospheric noble gasses. In: Cook, P.G., Herzeg, A.L. (Eds.) 144 Environmental Tracers in Subsurface Hydrology (Klüwer Academic Publ. 2000). 145 146 2. Sültenfuss, J., Roether, W. & Rhein, M. The Bremen mass spectrometric facility for the 147 measurement of helium isotopes, neon and tritrum in water. Isotopes in Environmental and Health 148 Studies, 45, 83‐95, (2009). 149 150 3. McDonald, M., G. and Harbaugh, A.W. A modular three‐dimensional finite‐difference ground‐ 151 water flow model. US. Geol. Survey Open‐File Report 83‐875. (1988). 152 7 NATURE GEOSCIENCE | www.nature.com/naturegeoscience © 2012 Macmillan Publishers Limited All rights reserved SUPPLEMENTARY INFORMATION DOI: 10.1038/NGEO1540 153 4. Crill P. M. & Martens C. S. Methane production from bicarbonate and acetate in an anoxic marine 154 sediment. Geochim. Cosmochim. Acta 50, 2089–2097. (1986) 155 156 5. Hansen L. K., Jakobsen R. & Postma D. Methanogenesis in a shallow sandy aquifer, Rømø, 157 Denmark. Geochim. Cosmochim. Acta 65, 2925–2935 (2001). 158 159 160 161 162 8 NATURE GEOSCIENCE | www.nature.com/naturegeoscience © 2012 Macmillan Publishers Limited All rights reserved ... displayed in the geological cross‐section (Supplementary Fig. S2). The estimated residence times, or 61 groundwater ages, are within 5 years of the 3H/3He ages. The flow lines indicate that shallow 62 groundwater flow in the Holocene aquifer is controlled by the local interactions with surface water, ... The concentrations of unlabeled TIC and acetate needed for the rate calculations were interpolated 135 from groundwater concentration profiles at the same site. For acetate, outliers where excluded from 136 these interpolations. Incubation times were 18‐21 hours. Acetate concentrations used in the rate ...SUPPLEMENTARY INFORMATION DOI: 10.1038/NGEO1540 20 21 Supplementary Fig. S1. Tritium helium dating of groundwater: lines indicate an infiltration rate of 0.5 22 m/yr except for Van Coc where it is 0.25 m/yr.