HR: 13:40h
AN: H33H-01    [Abstracts]
TI: Declining Temporal Variability of Major Ions Concentrations but not Arsenic Concentration as Function of Groundwater age in the Ganges-Brahmaputra-Meghna Delta of Bangladesh
AU: * Dhar, R K
EM: rdhar@gc.cuny.edu
AF: School of Earth and Environmental Sciences, Queens College of the City University of New York, 65-30 Kissena Blvd, Flushing, NY 11367 United States
AU: Zheng, Y
EM: Yan_Zheng@qc.edu
AF: School of Earth and Environmental Sciences, Queens College of the City University of New York, 65-30 Kissena Blvd, Flushing, NY 11367 United States
AU: Zheng, Y
EM: Yan_Zheng@qc.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Rte 9W, Palisades, NY 10964 United States
AU: Stute, M
EM: martins@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Rte 9W, Palisades, NY 10964 United States
AU: Stute, M
EM: martins@ldeo.columbia.edu
AF: Barnard College of Columbia University, 3009 Broadway, New York, NY 10027 United States
AU: Cheng, Z
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Rte 9W, Palisades, NY 10964 United States
AU: vanGeen, A
EM: avangeen@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Rte 9W, Palisades, NY 10964 United States
AU: Shamsudduha, M
AF: Department of Geology, Dhaka University, Dhaka, Dhaka, 1000 Bangladesh
AU: Hoque, M A
AF: Department of Geology, Dhaka University, Dhaka, Dhaka, 1000 Bangladesh
AU: Shanewaz, M
AF: Department of Geology, Dhaka University, Dhaka, Dhaka, 1000 Bangladesh
AU: Ahmed, K
AF: Department of Geology, Dhaka University, Dhaka, Dhaka, 1000 Bangladesh
AB: Elevated arsenic concentrations in groundwater of the Ganges-Brahmaputra-Meghna (GBM) delta region pose a devastating health threat. One issue of considerable uncertainty is the magnitude of temporal variations in groundwater As concentrations. Despite the importance of the issue, there are very few high quality, long-term, time series data for groundwater As from the region. This presentation addresses the issue with a series of samples from Araihazar, Bangladesh, that were collected biweekly or monthly between 2001 and 2004 and analyzed by high resolution inductively-coupled plasma mass spectrometry. The method is precise (better than 5 percent long-term reproducibility for As and generates data for 32 other elements. Our study site comprises six nests of monitoring wells installed in an area where the hydrology and geochemistry have been carefully studied. Dissolved As concentrations range from $<$ 1 ug/L to 600 ug/L in the shallow Holocene aquifer (upper 30 m) and mostly $<$ 1 ug/L (As $<$ 1-6.6 ug/L) in the deep Pleistocene aquifer ($>$ 30 m depth). Dating of shallow aquifer water resulted in tritium-helium ages ranging from $<$ 1 yr to $>$ 40 yr. Deep Pleistocene aquifer water was 14C-dated and found to be up to thousands of years old. In the shallow aquifers, variations in groundwater As concentrations in all but 2 out of 28 wells did not display any discernable pattern linked to seasonal water table fluctuations. In addition, As concentration fluctuations were limited (0-15 percent) in all 28 wells. The two wells displayed seasonal pattern of As variations (10-15 percent) were particularly shallow (depths 6 m and 9 m). Redox-sensitive indicators such as Fe and Mn in both wells also displayed seasonal pattern. In contrast to As, most of wells with tritium-helium ages $<$21 years showed much larger variations in dissolved Cl- concentrations (10-43 percent) and total dissolved cations (5-31 percent) concentrations. Temporal variability was considerably reduced for groundwater $>$21 years old: 6-7 percent and 10-14 percent, major cations and anions, respectively. This suggests that despite the apparent influence of recharge on major ion concentrations, dissolved As concentrations in most shallow aquifers are relatively well buffered. No notable variability ($<$2 percent) in As concentrations was observed in groundwaters from the Pleistocene aquifer.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting