Hydrology Dissolved Organic Carbon (DOC) (mM) Dissolved Carbon 0 0.4 0.8 1.2 Tr/He ~35yrs 20 Bomb C-14 40 60 Depth (m) Methane 80 DIC 100 DOC 120 140 160 180 0 2 4 6 8 Inorganic (DIC) and Methane (mM) 10 Carbon Isotopes 1000 • Inflow of young carbon Surface Water Carbon (Atmospheric values over last 50 years) 500 • Young carbon drives biochemistry Inorganic Carbon C0/00 0 Methanogenesis Methane 3m 19m 0 700 30m 60m 2000 4000 Organic Carbon -500 CaCO3 -1000 -70 -30 -25 -20 -15 -10 13 C0/00 -5 0 Radiocarbon Age 14 • Mixture of young and old carbon is not the result of pore water mixing, but mobilization of old organic carbon Hydraulic Characteristics at Intensive Site 0 4.7E-05 8.2E-05 Clay May 01 2.5E-04 3.6E-04 20 Jan 02 ? 1.2E-04 1.8E-04 40 2.2E-04 60 3.9E-04 May 01 2.4E-04 80 Depth (m) 100 7.2E-05 -10 120 0 Clay 140 160 3.9E-05 0 2E-04 4E-04 . Hydraulic Conductivity (m/s) -300 -200 Relative Head -100 0 100 -4 -3 (mm difference4 from head at 19m) 18 -2 -1 δ O (per thousand) 0 Depth-wise variation of stable water isotopic values at the filed site Monitoring Rice field Pumping River Exchange Exchange With River Ponds Model Aquifer: S ( ) ( ) dha = h f − ha K f f f + h p − ha K p f p + (hr − ha )K r fr + (hv − ha )K v fv − qI − favα v ET0 dt Village: Field: Pond: dhv Sy = (ha − hv )K v − (1− fav )α v ET0 + R dt Sy dh f dt dhp dt ( ) = ha − h f K f − α f ET0 + R + = (ha − hp )K p − α p ET0 + R qI ff Case-A Case-B clay aquifer Kf (1/d) [conductance for field] 8.9x10-4 8.9x10-4 Kv (1/d) [conductance for village] 6.3x10-6 9.1x10-4 Kp (1/d) [conductance for pond] 9.3x10-3 8.3x10-3 Kr (1/d) [conductance for river] 7.7x10-2 8.7x10-2 Objective Function w/ pumping 5.9x10-1 5.7x10-1 w/ pumping 19 13 w/o pumping 42 22 Village ETtree from Residence Time (yrs) Table 1. The estimated conductance parameter values when the storage coefficients are fixed, the respective objective functions (sum of square errors), and modeled residence times for the aquifer. Case A: Village ET out of Clay Kf Kp Kr Kv CV Kf Kp Kr Kv 1 -0.18 1 -0.28 0.29 1 -0.30 0.01 0.09 1 0.11 0.10 0.11 5.45 Kf Kf Kp Kr Kv Sy S Case B: Village ET out of Aquifer Kf Kp Kr Kv CV Kf Kp Kr Kv 1 -0.21 1 -0.31 0.23 1 -0.11 0.05 0.00 1 0.10 0.10 0.10 0.06 1 -0.17 -0.31 -0.34 -0.26 -0.24 Case A: Village ET out of Clay Kp Kr Kv Sy 1 0.04 0.02 0.02 -0.07 1 -0.22 1 -0.29 0.92 0.81 -0.15 1 -0.19 Case B: Village ET out of Aquifer Kf Kp Kr Kv Sy Kf Kp Kr Kv Sy S 1 -0.11 -0.39 -0.36 -0.36 -0.26 1 -0.06 0.10 0.11 -0.14 1 -0.07 1 -0.10 0.79 0.83 -0.16 1 -0.22 S CV 1 0.15 0.09 0.10 0.77 0.14 0.13 S CV 1 0.18 0.09 0.11 0.08 0.09 0.15 Transient Three-Dimensional Flow Model River cells Pond cells Village areas Irrigated rice areas Irrigation wells Drinking wells Other agriculture area Area of interest 114 row and columns 15 layers Frequency 0.35 Estimated Groundwater Age Distribution at 30Estimated Groundwater Age Distribution m w/ irrigated agriculture w/o irrigated agriculture 0.3 0.25 0.2 Median age of 66 years with irrigated agriculture 0.15 240 years without irrigated agriculture 0.1 0.05 0 40 200 360 520 680 840 1000 Groundwater Age (years) 1160 1320 1480 + Conclusions • Arsenic concentrations are subject to change and irrigation pumping is sufficient to have significantly changed flow paths, drawing young water and chemicals into the aquifer. • Geochemical parameters at our site are consistent with a scenario of concomitant arsenic release and organic carbon oxidation. • Deeper wells have the potential to alleviate the problem, but could also become contaminated. Tremendous disparity with US groundwater contamination problems • In the developed world people don’t drink seriously contaminated groundwater when contamination is known. • Relative to US, efforts to understand the physical and chemical processes are not funded. Need a serious scientific/engineering program People MIT Chris Swartz Nicole Keon Winston Yu Jenny Jay Dan Brabander Peter Oates Harry Hemond U. Cincinatti Shafik Islam BUET Borhan Badruzzaman Ashraf Ali Feroze Ahmed Khandaker Ashfaque Roger Beckie Volker Niedan Future directions • Arsenic in other regions in Asia Does Bangladesh indicate the future? • Arsenic in agriculture and food chain • Combined surface-water groundwater management pathogens vs. arsenic Can these be done without a detailed hydro-bio-geo-chemical model? P 100-m Irrigation Wells P P P P P P P P P P P P P P P P P P P P P P P P Agriculture Areas 40% boro rice 71% irrigated P P P P Field Site P P MIT/BUET/NSF Arsenic Project Small N2 glove bag at night