reactive transport modeling of arsenic mobilization in groundwater of the red river floodplain vietnam

Available online at www.sciencedirect.com ScienceDirect Procedia Earth and Planetary Science 17 (2017) 85 – 87 15th Water-Rock Interaction International Symposium, WRI-15 Reactive transport modeling of arsenic mobilization in groundwater of the Red River floodplain, Vietnam Dieke Postma1,a, Pham Thi Kim Trangb, Helle Ugilt Søa, Vi Mai Lanb, Rasmus Jakobsena b a GEUS, Østervoldgade 10, 1350 Copenhagen Denmark Research Centre for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science (VNU), Hanoi, Vietnam Abstract The arsenic content in groundwater of the Red River floodplain decreases with the burial age of the aquifer sediment over a 6000 year period This decrease is caused by diminishing reactivities of both sedimentary organic carbon, Fe-oxides as well as CaCO3 Here we present a 1-D reactive transport model developed in PHREEQC-3 that quantifies the resulting changes in groundwater bulk chemistry as well as arsenic content over the last six millennia © 2017 2017Published The Authors Published by Elsevier B.V © by Elsevier B.V This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of WRI-15 (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 Keywords: geochemistry, groundwater, arsenic, Red River, Vietnam, reactive transport, modeling Introduction Geogenic arsenic in groundwater probably constitutes the largest global drinking water contamination problem Alone in southern and southeast Asia it has been estimated that more than 100 million people are exposed to drinking water with a arsenic content exceeding the WHO recommended maximum value of 10 µg/L As 1,2 Chronic intake of too much arsenic enhances risks for developing various forms of cancers and heart diseases On the big delta complexes of S and SE Asia, arsenic enters the system in association with Fe-oxides which together with clays and sands have been deposited on the floodplain Once the sediment the sediment becomes part of the saturated groundwater zone anoxic conditions develop because of organic matter degradation Organic matter degradation leads to the reduction of As-containing Fe-oxide, resulting in the release of Fe(II) and As(III) to the groundwater3,4,5 Secondary processes that may affect the groundwater arsenic concentration, comprise the adsorption and desorption of arsenic to the sediment6 * Corresponding author Tel.:+45 30273702; fax:+45 4538142050 E-mail address: diekepostma@gmail.com 1878-5220 © 2017 Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 doi:10.1016/j.proeps.2016.12.003 86 Dieke Postma et al / Procedia Earth and Planetary Science 17 (2017) 85 – 87 We have studied the geochemical processes related to arsenic mobilization in the floodplain of the Red River, Vietnam, at a field site located 30 km NW of Hanoi5,7,8,9 Here local aquifers are found were the sediment has burial ages ranging from 600 to 6000 years We have shown previously that the arsenic content of the groundwater is related to the sediment burial age8 In young sediments the groundwater concentration is up to 500 µg/L As, while in 5900 year old sediments the maximum arsenic concentration only reaches 20 µg/L In addition we have shown that the rates of organic matter degradation strongly decreases with increasing burial age Clearly, the geochemical properties of the sediments change significantly during the 6000 year period, leading to a diminishing mobilization of arsenic over time In this contribution we present a reactive transport model that quantifies the geochemical processes related to arsenic mobilization to the groundwater over the last 6000 years The model has been constructed as a one-dimensional flow tube in PHREEQC-310 considering only the vertical component of groundwater flow This simplification is based on groundwater dating showing a rather homogeneous infiltration rate of 0.5 m/yr at the various sites and the assumption that the infiltration rate has been the same over the last 6000 years The objective is to obtain a quantitative understanding of the geochemical processes controlling the bulk groundwater chemistry as well as the arsenic concentration on a 6000 year time scale Geochemical Processes and Model Developments The measured content of arsenic in the iron oxides of the aquifer sediment corresponds to a As/Fe ratio of about 1.2 mmol/mol M corresponding to a stoichiometric composition of Fe(OH)2.9964(AsO4)0.0012 Both Fe and As(V) can be mobilized during reductive dissolution with organic carbon functioning as the electron donor: CH2O + 4FeOOH + 7H+ → 4Fe2+ + HCO3- + 6H2O (1) In this reaction arsenic has been omitted for simplicity but in the course of the reaction the As(V) in the Fe-oxide will become reduced to As(III) and enter the groundwater No other electron acceptors, such as sulfate, are important in the aquifer but methanogenesis constitutes the second pathway for organic carbon degradation following the overall reaction: 2CH2O → CH4 + CO2 (2) The measured methane concentration of up to 1.5 mM CH indicates methanogenesis to be of importance The overall rate of both reactions (1) and (2) is controlled by the kinetics of organic matter degradation which again depends on the reactivity of the organic carbon present in the aquifer It must be expected that the organic carbon reactivity will diminish over time as the most reactive material is consumed first To describe this process we have developed the following rate equation: -dCC/dt = mo ∙9.3∙10-12∙(mt/mo)2.5 (mol/sec) (3) Here mo = 1.36 mol/L is the initial mass of organic matter based on the measured concentration of 0.27 % C in the youngest sediment recalculated to the concentration per liter of contacting groundwater, mt, the mass of organic carbon at time t and 9.3∙10-12 sec-1 the rate constant The exponent to (mt/mo) reflects the heterogeneity of the organic carbon reactivity, the difference between the most and least reactive organic carbon The parameters in this rate equation were found by trial and error fitting to the groundwater chemistry data The distribution of the electron flow coming from organic matter to either the reduction of Fe-oxides (reaction (1)) or to methanogenesis (reaction (2)) is controlled by the kinetics of reductive dissolution of Fe-oxide The kinetics of Fe-oxide reduction will depend on factors like surface area, crystallinity and microbial catalysis, but is also dependent on the thermodynamic energy release of the Fe-oxide reduction reaction which is a function of the mineralogy of the Fe-oxide as well as the solution composition In the presence of an unstable and poorly crystalline ferrihydrite, iron reduction would be the predominant process while in the presence of well crystalline goethite or hematite, methanogenesis may become most important The rate equation for Fe-oxide reduction is constructed as: -dCFeOOH/dt = mo ∙ 2.54∙10-11∙(mt/mo)1.5 ∙ (1 – SRFeOOH) (mol/s) (4) Dieke Postma et al / Procedia Earth and Planetary Science 17 (2017) 85 – 87 Here mo is 0.394 mol/L corresponding to 65 µmol FeOOH/g obtained by extrapolation from the field data SR FeOOH is the Saturation Ratio (IAP/K) representing the thermodynamic contribution In the model, the solubility of Feoxide (K) is much closer to goethite than to ferrihydrite There is a very strong effect of the pH on the reduction of iron by organic carbon (Reaction (1)) This pH effect on the energy release of iron reduction (1) may result in both positive and negative feed-back mechanisms Iron oxide reduction by itself increases the pH (reaction (1)) and disfavors the process The CO2 produced by methanogenesis (reaction (2)) will dissolve as carbonic acid and lower the pH, thereby stimulating Fe-oxide reduction Additional processes like the dissolution or precipitation of siderite or calcite and the CO pressure of infiltrating water from the soil zone will also affect the pH Together, all these reactions form an interacting reaction network where the relative importance of the different reactions changes over time Model results The results of the PHREEQC model are able to satisfactorily describe the water chemistry in the aquifers with different burial ages, including the decreases in arsenic, alkalinity, Ca and pH over time as well as the increase in dissolved Fe(II) The model results indicate methanogenesis to be the more important pathway for organic carbon degradation as compared to iron reduction Both the rates of organic carbon degradation (3) and the rate of iron oxide reduction (4) decrease over time but the latter decreases faster than the first and as the result the reaction stoichiometry changes over time The rate of arsenic release is directly related to the rate of iron oxide reduction and therefore decreases over time If this was the only process to occur, the groundwater arsenic concentration should initially be highest and thereafter diminish However, the model predicts that the groundwater arsenic content will increase over the first 1200 years because adsorption of As(III) onto the sediment removes much of the arsenic and delays the build-up in the groundwater After the first 1200 years, the decreasing arsenic release rates will, in combination with desorption of arsenic from the sediment, slowly decrease the groundwater arsenic content After 6000 years the arsenic concentration will have decreased to 20 µg/L corresponding to twice the WHO recommended maximum level Almost all of the Fe(II) that is released by Fe-oxide reduction in the model is precipitated in the sediment as siderite Over time the CaCO in the soil and aquifer sediment is leached out, which causes the pH and alkalinity to decrease while enabling more Fe(II) to stay in solution and finally also results in the dissolution of siderite Acknowledgements This research has been funded by the European Research Council under the ERC Advanced Grant ERG-2013ADG Grant Agreement Number 338972 References Ravenscroft P, Brammer H, Richards K Arsenic Pollution: A Global Synthesis, Wiley-Blackwell; 2009 Fendorf S, Michael HA, van Geen A, Spatial and temporal variations of groundwater arsenic in south and southeast Asia Science 2010; 328: 1123-1127 McArthur JM, Ravenscroft P, Safiulla S, Thirlwall MF, Arsenic in groundwater: testing pollution mechanisms for sedimentary aquifers in Bangladesh Water Resour Res 2001; 37: 109–117 Dowling CB, Poreda RJ, Basu AR, Peters SL, Aggarwal PK, Geochemical study of arsenic release mechanisms in the Bengal Basin groundwater Water Resour Res 2002; 38: 1173, doi:10.1029/2001WR000968 Postma D, Larsen F, Nguyen TMH, Mai TD, Pham HV, Pham QN, Jessen S, Arsenic in groundwater of the Red River floodplain, Vietnam: Controlling geochemical processes and reactive transport modeling Geochim Cosmochim Acta 2007; 71: 5054–5071 Nguyen THM, Postma D, Pham TKT, Jessen S, Pham HV, Larsen F, Adsorption and desorption of arsenic to aquifer sediment on the Red River floodplain at Nam Du, Vietnam Geochim Cosmochim Acta 2014; 142: 587-600 Postma D, Jessen S, Nguyen TMH, Mai TD, Koch CB, Pham HV, Pham QN, Larsen F, Mobilization of arsenic and iron from Red River floodplain sediments, Vietnam Geochim Cosmochim Acta 2010; 74: 3367-338 Postma D, Larsen F, Nguyen TT, Pham TKT, Jakobsen R, Pham QN, Tran VL, Pham HV, Murray AS, Groundwater arsenic concentration in Vietnam controlled by sediment age Nature Geoscience 2012; 5: 656-661 Jessen S, Postma D, Larsen F, Pham QN, Le QH, Pham TKT, Tran VL, Pham HV, Jakobsen R, Surface complexation modeling of groundwater arsenic mobility: Results of a forced gradient experiment in a Red River flood plain aquifer, Vietnam Geochim Cosmochim Acta 2012; 98: 186–201 10 Parkhurst DL, Appelo CAJ, Description of input and examples for PHREEQC version 3—A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations: U.S Geological Survey Techniques and Methods, 2013, book 6, chap A43, 497 p., available only at http://pubs.usgs.gov/tm/06/a43 87 ... build-up in the groundwater After the first 1200 years, the decreasing arsenic release rates will, in combination with desorption of arsenic from the sediment, slowly decrease the groundwater arsenic. .. Mai TD, Pham HV, Pham QN, Jessen S, Arsenic in groundwater of the Red River floodplain, Vietnam: Controlling geochemical processes and reactive transport modeling Geochim Cosmochim Acta 2007; 71:... studied the geochemical processes related to arsenic mobilization in the floodplain of the Red River, Vietnam, at a field site located 30 km NW of Hanoi5,7,8,9 Here local aquifers are found were the