Pham Quy Nhan

Fate of Arsenic during Red River Water Infiltration into Aquifers beneath Hanoi, Vietnam

Recharge of Red River water into arsenic-contaminated aquifers below Hanoi was investigated. The groundwater age at 40 m depth in the aquifer underlying the river was 1.3 ± 0.8 years, determined by tritium–helium dating. This corresponds to a vertical flow rate into the aquifer of 19 m/year. Electrical conductivity and partial pressure of CO2 (PCO2) indicate that water recharged from the river is present in both the sandy Holocene and gravelly Pleistocene aquifers and is also abstracted by the pumping station. Infiltrating river water becomes anoxic in the uppermost aquifer due to the oxidation of dissolved organic carbon. Further downward, sedimentary carbon oxidation causes the reduction of As-containing Fe-oxides. Because the release of arsenic by reduction of Fe-oxides is controlled by the reaction rate, arsenic entering the solution becomes highly diluted in the high water flux and contributes little to the groundwater arsenic concentration. Instead, the As concentration in the groundwater of up to 1 μM is due to equilibrium-controlled desorption of arsenic, adsorbed to the sediment before river water started to infiltrate due to municipal pumping. Calculations indicate that it will take several decades of river water infiltration to leach arsenic from the Holocene aquifer to below the World Health Organization limit of 10 μg/L.

CONTROLLING FACTORS TO THE DISTRIBUTION OF SALINITY OF POREWATER IN QUATERNARY MARINE SEDIMENTS IN NAM DINH AREA

This paper presents the results of studies and analysis, assessment the controlling factors to the distribution of salinity (mainly NaCl) in the Quaternary marine sedimentary in Nam Dinh area by using a combination of different methods such as boreholes logging, undisturbed sediment sampling, pore water squeezing, analysis of chemical and stable isotope compositions of pore water. Based on the correlation between concentration of Cl-, Na+ ions and NaCl versus electrical conductivity of marine sediment at different depth according to geophysical borehole results determine the distribution of concentration of salinity (NaCl) of pore water at different depths. In addition, results from the analysis of stable isotopes δ18O and δ2H of porewater in the marine sedimentary layer show the mixing of marine water and meteoritic water in the sediments. Distribution of NaCl of marine sediments is due to difference in density and molecular diffusion. References Charles D. Shackelford, David E. Daniel, 1990. Diffusion in Saturated soil. Journal of Geotechnical Engineering 117, 467 - 484. Crooks, V. E. and Quigley, R. M., 1984. Saline leachate migration through clay: A comparative laboratory and field investigation. Can. Geotech. J., 21(2), 349 - 362. Đoan Văn Canh, Le Thị Lai, Hoang Văn Hưng, Nguyễn Đức Rỡi, Nguyễn Văn Nghĩa, 2005. Groundwater Resource Of Nam Định Province, J. of Geology, B/25, Ha Nội. Frank Wagner, Dang Tran Trung, Hoang Dai Phuc, Falk Lindenmaier, 2011. Assessment of Groundwater Resources in Nam Dinh Province. Final Technical Report of improvement of Groundwater Protection in Vietnam, Ha Noi. Hoang Văn Hoan, Flemming Larsen, 2007. Phương phap xac định độ lỗ hổng hiệu dụng, hệ số thấm của tầng chứa nước va TDS của nước bằng phương phap địa vật lý lỗ khoan, lấy vi dụ vung đồng bằng song Hồng. TC KHKT Mỏ - Địa chất, 20/10-2007. Ha Nội, 101-107. Hoan V. Hoang, Nhan Q. Pham, Flemming Larsen, Long V. Tran, Frank Wagner And Anders V. Christiansen, 2011. Processes Controlling High Saline Groundwater in the Nam Dinh Province, Vietnam. 2nd Asia-Pacific Coastal Aquifer Management Meeting October 18-21, 2011, Jeju Island, Korea. Hoang Văn Hoan, Phạm Quý Nhân, Flemming Larsen, Trần Vũ Long, Nguyễn Thế Chuyen, Trần Thị Lựu, 2012. Ảnh hưởng của qua trinh khuếch tan tới sự phân bố độ mặn của nước lỗ rỗng trong lớp trầm tich biển tuổi Đệ tứ khu vực Nam Định. Bao cao hội nghị khoa học lần thứ 20, ĐH Mỏ - Địa chất. Ha Nội. Nguyễn Kim Ngọc, Kiều Vân Anh, Nguyễn Thị Hạ, Hoang Văn Hoan, Đỗ Tiến Hung, Hoang Văn Hưng, Nguyễn Văn Lâm, Phạm Quý Nhân, Nguyễn Thị Thanh Thủy, 2005. Thủy Địa Hoa học. Nha xuất bản Giao thong vận tải, Ha Nội, 315tr. Kooi H., Groen J., Leijnse A., 2000. Modes of seawater intrusion during trangressions, Water resources research, Vol. 36, No. 12, phương phap 3581-3589. Rowe, R. K., Caers, C. J., and Barone, F., 1988. Laboratory determination of diffusion and distribution coefficients of contanminants using undisturbed clayey soil. Can. Geotech. J., 25, 108-118. Schincariol RA, Schwartz FW, Mendoza CA., 1997. Instabilities in variable density flows: stability and sensitivity analyses for homogeneous and heterogeneous media. Water Resour Res; 33(1), 31-41. Simmons CT, Fenstemaker TR, Sharp JM., 2001. Variable-density groundwater flow and solute transport in heterogeneous media approaches resolutions and future challenges. J Contam Hydrol; 52(1-4), 245-75. 148. Tanabe, S., Hori, K., Saito, Y., Haruyama, S., Vu, V.P., Kitamura, A., 2003a. Song Hong (Red River) delta evolution related to millen-nium-scale Holocene sea-level changes. Quaternary Science Reviews 22, 2345-2361. Tanabe, S., Hori, K., Saito, Y., Haruyama, S., Doanh, L.Q., Sato, Y., Hiraide, S., 2003b. Sedimentary facies and radiocarbon dates of the Nam Dinh-1 core from the Song Hong (Red River) delta. Viet-nam. Journal of Asian Earth Science 21, 503-513. Tanabe, S., Hori, K., Saito, Quang Lan Vu, Till J. J. Hanebuth, Quang Lan Ngo, Akihisa Kitamura, 2006. Holocene evolution of Song Hong (Red River) delta system, north Vietnam. Sedimentary Geology 187, 29-61. Weixing Guo and Christian D. Langevin, 2002. User’s Guide to SEAWAT: A computer program for simulation of three-dimensional variable-density groundwater flow, USGS, Florida, USA.