The impact of low dissolved oxygen in recharge water on arsenic pollution in groundwater of Bangladesh Md. Nazrul Islam Department of Civil Engineering, University of Toronto, Toronto, Canada R.D. Von Bernuth Department of Biosystems Engineering, Michigan State University, East Lansing, Michigan, USA ABSTRACT: This study is based on the concept that lack of dissolved oxygen (DO) at or below the water table and water extraction (Q) through shallow irrigation wells at a rate greater than the aquifer recharge rate are the main causes of arsenic release in the groundwater of Bangladesh. This study identified the hydrogeochemical processes related to shortage of DO that eventually pro- duce high arsenic concentrations and their migration into the groundwater systems. The existing theories of arsenic release by oxidation and reduction in the context of dissolved oxygen shortage in recharging groundwater were studied. Both numerical and thermodynamic analyses were used to demonstrate how oxidation theory of arsenic release is inadequate to explain the release of arsenic into the groundwater of Bangladesh. This study quantified the amount of dissolved oxy- gen level in deeper layers of the aquifer and their relation to the variations in redox potential values and arsenic release processes. It also analyzed groundwater velocity and flow patterns to establish a link between dissolved oxygen shortage and arsenic release into the groundwater. On the basis of the findings, it was concluded that shortage of dissolved oxygen in recharging water is the most likely the root cause of arsenic occurrence in Bangladesh groundwater. 1 INTRODUCTION The arsenic contamination problem in Bangladesh groundwater is most likely to be associated with shortage of dissolved oxygen in recharge water at or below the water table. The presence or absence of dissolved oxygen (DO) in natural surface or groundwater systems has been known as an index of Oxidation Reduction Potential (ORP) or redox potential. The ORP is a measure of ten- dency for donating or accepting electrons during chemical reactions. As long as any measurable amount of dissolved oxygen is present in the groundwater systems, the redox potential (p e ) level is controlled by the dissolved oxygen concentration and the system will remain in oxic conditions. Under higher oxic conditions, ion activities and electron donating tendencies are less (Stumm & Morgan 1996, Khan et al. 2000). On the contrary, under anoxic conditions, the tendency for donat- ing electrons is high. Arsenic concentrations in the groundwater in Bangladesh are a by-product of redox reactions where microbial derived oxidation of organic carbon plays an important role in donating electron, and the terminal electrons are accepted by the hydrated ferric oxides and/or hydroxides present in the groundwater systems. Upon electron acceptance, ferric oxides are reduced from solid particulate form to dissolved ferrous ions and the associated arsenic is released into the groundwater systems (Nickson et al. 2000). This is the most widely accepted reduction theory of arsenic release into the groundwater of Bangladesh. The origin of arsenic in the aquifer sediment is from natural geological processes. There are two theories of arsenic release. One is that over time the microbial degradation of organic carbon has caused arsenic to be released into water by reductive dissolution of iron oxides under anoxic or 173 Natural Arsenic in Groundwater: Occurrence, Remediation and Management – Bundschuh, Bhattacharya and Chandrasekharam (eds) © 2005, Taylor & Francis Group, London, ISBN 04 1536 700 X Copyright © 2005 Taylor & Francis Group plc, London, UK mildly reduced conditions. This is known as reduction theory of arsenic release. However, some experts believe that exposure of arsenic rich pyrites to the atmospheric oxygen due to over extrac- tion of groundwater is the main cause of arsenic release into the groundwater of Bangladesh (Bridge & Hussain 1999). This idea is known as the oxidation theory of arsenic release. Neither of these two theories is unequivocally accepted by all and a strong debate has been continuing on these two current theories of arsenic release. It is not the purpose of this research to engage in this debate. In both cases dissolved oxygen level plays an important role in arsenic contamination problem, and that is the focus of this study. The ORP or redox potential of the sediment water systems mainly controls the arsenic specia- tion in groundwater systems (Bhattacharya et al. 2002a b, Ahmed et al. 2004). The Speciation or different oxidation states of arsenic species in water is highly attributed to the physicochemical (electrostatic force, ionic strength, pH) and molecular interactions (thermodynamic stability, water solubility, hydrogen bonding ability etc.) (Gazsó 2001). The physicochemical and molecu- lar interactions between arsenic species and aquifer sediments are largely influenced by the asso- ciated biogeochemistry of the aquifer systems. The amount of dissolved oxygen present in the systems and the rate of consumption of dissolved oxygen generally play an important role in con- trolling the redox potential. As long as the water table remains close to the ground surface, the dissolved oxygen content in groundwater remains in equilibrium with atmospheric oxygen. Consumption of oxygen by micro- organisms can shift the equilibrium from oxic to suboxic states. The amount of atmospheric oxygen diffusion into the groundwater through the unsaturated porous medium basically depends on the depth to the water table and the oxygen diffusion rate (ODR) (Mukhtar et al. 1996). The ODR into 174 Figure 1. Probability of exceedence of arsenic above threshold level (50 ppb) is higher in the SE (southeast) and SW (southwest) zones of Bangladesh. Copyright © 2005 Taylor & Francis Group plc, London, UK the groundwater varies inversely with the depth to the water surface of the aquifer. If water table moves downward, supply of atmospheric oxygen will be less in the groundwater. The dissolved oxygen level in aquifer recharge water is attributed to the surface runoff and seepage from river bed. Most recently, upstream diversion or withdrawal of river water from the major river systems (Ganges and Jamuna rivers, Fig. 1) and large scale installation of shallow wells in Bangladesh is believed to be responsible for rapid lowering water table. Rapid consump- tion of dissolved oxygen in recharging groundwater due to the lowering of water table and micro- bial use of organic carbon as their energy-supplying-electron donor might have triggered the arsenic contamination problem in the groundwater of Bangladesh. This is the principal working hypothesis of this study. This hypothesis is in complete contrast with the existing oxidation theory of arsenic release but partially supports theory of arsenic release by reduction. The main purpose of this investigation was to examine how the shortage of dissolved oxygen in the recharging water contributed to groundwater arsenic pollution problem in Bangladesh. The second purpose is to understand whether the concept of dissolved oxygen shortage in recharging groundwater does or does not contradict the current theories of arsenic release by oxidation and or reduction. The third purpose of this study is to understand how dissolved oxygen shortage might influence the microbial activities that eventually influence the arsenic speciation processes. Addressing these questions may help in finding a suitable bioremediation solution to groundwater arsenic problem in Bangladesh. 2 METHODOLOGY OF THE STUDY To accomplish the purposes of this study, five analytical approaches were adapted; (1) statistical analyses of spatial distribution patterns of arsenic concentrations, (2) a multiple layer oxygen dif- fusion model analysis to estimate the amount of dissolved oxygen concentration in deeper layers, (3) analyses of hydrological factors such as hydraulic gradient, groundwater flow patterns and their relation to arsenic contamination problem and dissolved oxygen shortage, (4) computation of redox potential values at different depths of the aquifer and their correlation with vertical arsenic concentration distribution, and (5) thermodynamic analyses and explanations of existing theories of arsenic release and the validity of the hypothesis presented in this study. Thermodynamic analy- ses were done to establish a link between dissolved oxygen shortage, lowering of water table and their impact on thermodynamic stability of arsenic species and redox potentials values. 2.1 Statistical analyses of arsenic concentrations distribution patterns The analyses of spatial arsenic distribution patterns were done to understand the impact of dis- solved oxygen shortage on the arsenic release mechanism as a function of the associated hydro- geological conditions. A question is, “Which hydrologic zone in Bangladesh did correlate best to the arsenic concentration and how the dissolved oxygen shortage in that process does increase arsenic concentration?” The arsenic concentration records of about 3500 well water samples were analyzed by the BGS/DPHE. The water quality data were analyzed at different universities in the USA and abroad. The authors computed the probability of arsenic concentrations exceeding the threshold level (50 ppb) by using Gumbel exponential distribution method. Based on the volume of available water resources (rainfall, recharge, and surface water), Bangladesh was divided into six hydrolog- ical planning zones (MPO 1987) such as, northeast (NE), north-center (NC), north-west (NW), southeast (SE), south-center (SC) and southwest (SW) zones (Fig. 1). The coordinates (latitude and longitude) of the arsenic contaminated well records were inserted into the map of Bangladesh. The distribution patterns of arsenic concentration were statistically analyzed within the boundary of each zone and results are presented in Table 1. It was found from the analyses that the prob- ability of arsenic exceeding the threshold level (50 ppb) in the south-east (SE) zone was computed as 71.4% by using Gumbel equation. The probability of exceeding arsenic below threshold levels 175 Copyright © 2005 Taylor & Francis Group plc, London, UK (50 ppb) in the northwest (NW), north center (NC) zones are shown in Figure 1 and the computed values are tabulated in Table 1. 2.2 Multi layer oxygen diffusion model to estimate the dissolved oxygen level In order to determine the impact of shortage of dissolved oxygen in the recharge water at deeper layers of aquifers as a function of lowering the water table, a numerical oxygen diffusion model was built using the Finite Element Analyses technique. The Finite Element technique was used because of its higher accuracy and adaptabilities to numerical solutions for physical processes like convection diffusion, pollution distribution and contaminant transportation. 2.2.1 Conceptual oxygen diffusion model For the sake of simplicity, this oxygen diffusion model considers a constant oxygen consumption rate (␣ϭ0.0021 cm 3 /cm 3 /hr) by organic carbon in both saturated and unsaturated zone of the aquifer. This model was built to validate the theory of arsenic release by oxidation where the arsenic rich aquifer layers are exposed to the atmospheric oxygen. It was hypothetically assumed that the arsenopyrite-rich sediments layers L-6 an L-10 are located respectively at 6 and 10 meter below the ground surface (Fig. 2). A layered 10 m thick aquifer was modeled, representing the layer numbers by L-1 to L-10 (Fig. 2) with each layer being 100 cm thick. WT-1 and WT-2 show 176 Table 1. Probability of exceedence of arsenic concentrations in different hydrologic planning zones of Bangladesh. Hydrologic No. of Average As zone wells conc. (g/L) St. Dev Probability of exceedence (%) 10 ppb 25 ppb 50 ppb 100ppb 250 ppb Northeastern (NE) 1039 34.0 (0.5–572) 68 58.7 48.7 34.0 14.9 Ͻ1 North center (NC) 192 28.6 (0.5–284.0) 51.4 53.9 48.0 38.9 24.4 4.9 Northwest (NW) 1072 12.3 (0.5–708) 47.1 44.9 32.7 18.1 5 0.3 Southeastern (SE) 295 174.1 (0.5–1090) 199. 80.1 76.2 71.4 59.55 29.1 Southwest (SW) 474 84.8 (0.5–1660) 145 66.2 61.4 53.3 38.74 12 South center (SC) 295 38.6 (0.5–862) 113.18 53.9 48.05 38.95 24.42 4.98 Figure 2. Conceptual oxygen diffusion model to estimate oxygen concentrations at deeper layers before and after lowering of water table. Copyright © 2005 Taylor & Francis Group plc, London, UK the water table elevation before and after large-scale well installation and Case-1 and Case-2 reflect hydrologic conditions respectively before and after large-scale well installation. This model estimated the change in amount of dissolved oxygen at the exposed deeper layers (L-6 and L-10, Fig. 2) after lowering water table from WT-1 to WT-2. The oxygen diffusion model was simulated up to 350 hours from beginning with time step ⌬t is equal to 1 and 4 hours. However, results of 1-hour time step are printed every 10 hours (Fig. 3). The change in oxygen exposure took place at layer L-6 and L-10 before and after introduction of large-scale shallow irri- gation wells in Bangladesh was investigated. In the diffusion model the following values were addumed: the diffusion coefficient Dx was 259.2cm 2 /hr (4.166 * 10 Ϫ4 m 2 /s) and the oxygen diffu- sion rate was Ϫ 0.002125cm 3 /cm 3 /h. The results of the transient oxygen diffusion model were analyzed and plotted (Fig. 3). Figure 3 shows that a 4 m lowering of the water table from layer L-2 to L-6 did not result in an increase of the oxygen concentration at L-10 and L-6. The oxygen concentration in Case-1 at layer L-6 was predicted 0.09 atm (3.64 mg/L) after 150 hours. After lowering of the water table, using the same time interval and the same aquifer properties, the oxygen concentration was found to be 0.06atm (2.39mg/L). This result implies that lowering of the water table cannot increase the oxygen supply to the deeper layers of the aquifer. These results are help to establish a link between current theory of arsenic release and role of oxygen shortage and are further addressed in the results and discussion section. 2.3 Hydrological factors and their relation to arsenic contamination problem The relations among arsenic distribution patterns and groundwater velocity and flow directions were analyzed by dividing the whole country into a number of square grids where arsenic concentration records and groundwater flow direction maps were available. The whole country was divided into 16 177 Figure 3. Change in dissolved oxygen concentration in layer L-6 and L-10 after lowering of water table 5m from the position of WT-1 to WT-2. Copyright © 2005 Taylor & Francis Group plc, London, UK 178 Figure 4. The schematic groundwater flow direction is shown towards the main river system and the Bay of Bengal where the difference between the mean sea level (MSL) is about 95 m (from north to south). Arsenic concentration and groundwater flow direction 0 6789 10 200 400 600 800 1000 1200 Location of grid numbers along groundwater flow direction (North to South) Arsenic concentration(ppb) Increasing trend of arsenic contamination Figure 5. Increased trend of arsenic concentration from north to south along the groundwater flow direction. Copyright © 2005 Taylor & Francis Group plc, London, UK grids where arsenic contaminated wells were available. Grids 6, 7, 8, 9 and 10 (Fig. 4) have the same general hydraulic gradient and direction of flow. The arsenic concentration records in those five grids showed a strong correlation between arsenic concentration and groundwater flow direction (Fig. 5). 2.4 Computation of redox potential values and their relation with arsenic concentration distribution The redox potential values at different layers of the aquifer system were computed by measuring the activity of electrons in a solution expressed in units of volts (E h ) or in units of electron activity (p e ). The aquifer sediment contains solid amorphous Fe(OH) 3 and it was assumed that the poten- tial corresponds to an oxidation-reduction potential of the aquatic environment. By computing equilibrium constant value of log K, and the standard state free energy value ⌬G 0 , the equation that expressed the equilibrium of iron oxides with dissolved Fe 2ϩ gave the value of the redox potential in the groundwater systems (Figs. 7 and 8). 3 RESULT AND DISCUSSIONS The results of the analyses are discussed in the light of the hypothesis of this study. The working hypothesis was that layers at or below the water table receive less oxygenated water because of low- ering water table which is due to large scale installation of irrigation wells and upstream withdrawal of river water in Bangladesh. The shortage of dissolved oxygen in the recharge water may also be due to microbial activity. The immediate question one would ask is how a shortage of dissolved oxygen occurred in the recharging groundwater and how it contributes to arsenic mobilization. It should be kept in mind that the cause of arsenic contamination in the groundwater of Bangladesh is still poorly understood and a strongly debatable issue. However, the above questions could be answered in the context of hydrogeochemical issues. It was apparent from Table 1 that the probability of arsenic exceeding threshold level (50 ppb) was highest in the southeast (SE) and 179 Figure 6. Arsenic concentration distribution over aquifer depth. Copyright © 2005 Taylor & Francis Group plc, London, UK southwest (SW) zones of Bangladesh. These two regions were built by the sediment of the Meghna and Ganges River Flood Plain. This delta usually experiences a high rate of sediment flow of about 479 million tons per annum (BWDB 1993). The least arsenic contaminated regions are located in the northwest (NW) and north center (NC) regions where the surface geology is built up by the Old Himalayan Fan, Tista-Jamuna Flood Plains, and Madhupur Tract. The top of the main aquifer systems in the NW zone is closer to the surface than that of the SE and SW regions. The regional groundwater gradient in the NW zone is 2 m/km but the gradient in the SE and zone is 0.1 m/km (MPO, 1987). It is apparent that the groundwater gradient in NW zone is 20 times higher than southern zones(SE & SW) where the arsenic concentration is maximum (Fig. 1). 3.1 Arsenic concentration is greater in the southeast (SE ) and southwest (SW) zones The sedimentology of northwest (NW) zone is different from SE and SW zone. Moreover, the hydraulic gradient in NW zone is twenty times higher than SW zone; therefore, the residence time of groundwater in NW zone would be less than SW and SE zones. On the contrary, the SW and SE zones were mostly built up through the sedimentary deposition, and the hydraulic gradient in the SW and SE zones is less; therefore, the groundwater residence time is expected to be higher than 180 Redox potential, p e (V) Aquifer depth (m) Figure 7. Redox potential values (p e ) decrease with aquifer depth from 10 to 30 m below the surface. Redox potential, p e (V) Aquifer depth (m) Figure 8. The redox potential values (p e ) start increasing with aquifer depth from 150 to 350 m below the surface indicating that the recharge water is rich in dissolved oxygen. Copyright © 2005 Taylor & Francis Group plc, London, UK in the NW zone. Potential recharge water in the shallow aquifer is usually rich in dissolved oxy- gen. Under the influence of the high hydraulic gradient in NW zone, the oxygenated groundwater recharge can quickly and easily replace the old groundwater. After rejuvenating the aquifers by recharge water, the groundwater in NW zone is expected to remain oxic if there is not too much organic carbon leaching into the aquifer from the top of the ground surface. Continuous percolation of organic carbon from the increased agricultural activities can also result in rapid consumption of dissolved oxygen in the groundwater. The groundwater in max- imum arsenic contaminated area (SW and SE zones, Fig. 1) might be continuously lacking in dis- solved oxygen because it can not be easily replaced by the oxygenated recharge water because of low gradient. As a result, the redox potential value always remains low in SW and SE region. Only mixing with oxygenated water or lack of reductant minerals can maintain the desired oxic condi- tion in SW and SE regions, but achieving oxic conditions is difficult in these zones due to rapid consumption of dissolved oxygen. Therefore, arsenic mobilization is the higher in SW and SE region than NW, NE and north center (NC) zone in Bangladesh. 3.1.1 Lack of correlation between arsenic concentrations and depth of water table The investigation report (BGS, 2001) showed that arsenic contamination does not have any rela- tionship with depth to the water table or amount of irrigation withdrawal rate in Bangladesh. Moreover, in the most contaminated zone (SE and SW zone) the water table does not match with the most intensive extraction rate. Because both arsenic release mechanisms (oxidation or reduc- tion) are time dependent processes but concentrations spatially differ, it is inappropriate to explain differences using a space–dependent relationship. Therefore, it might be misleading to establish a direct correlation between arsenic concentra- tions and depth of water table or extraction rate (Q). For example, if the volume of irrigation with- drawal is considered as a function of water table depth, a consistent relationship can be obtained only when the specific yields and effective porosity are the same for every aquifer. Otherwise, the correlation will be influenced by those parameters and may destroy the relationship. The lack of correlation between arsenic concentrations and lowering of water table or abstraction rates (Q) does not necessarily lead to rejecting the idea that over-pumping of irrigation wells may cause arsenic mobilization in the groundwater of Bangladesh. Recently, a group of researchers based in Manchester University found evidence that influxes of organic carbon in groundwater are known to occur when irrigation wells are drawn down. This group also found that introduction of organic carbon by over-pumping of irrigation wells can be a factor in increasing arsenic mobility in shal- low groundwater in Bangladesh (Roach 2004). Organic carbon in the sediment acts as a food source of bacteria. Bacteria would consume dissolved oxygen and eventually could lead to change the biogeochemistry of the arsenic contaminated groundwater. 3.2 Maximum arsenic concentration at the depth 30 to 50 meter below the ground surface The analysis of vertical distribution pattern of arsenic concentrations is shown in Figure 6. That shows that arsenic concentrations above threshold level (Ͼ50 to 1600 ppb) are mostly confined within between 30 and 50 m below the ground level. But, arsenic concentrations were less than 50 ppb between 150 and 350m. The sequence of vertical arsenic distribution can be interpreted in terms of redox potential values as shown in Figures 7 and 8. The increasing trend of arsenic at the depth ranging from 9 to 50 m below the ground (Fig. 6) was found to be associated with decreas- ing trend of redox potential as shown in Figure 7. The decreasing trend of arsenic from 150 to 350 m in Figure 6 is related to the increasing trend in redox values as shown in Figure 8. Although, the two aquifer systems have many differences, the sequence of decreasing and increasing trend of the redox potential values (Figs. 7 and 8) could be associated with the dissolved oxygen content in the groundwater. The recharging groundwater at the depth of 150 to 350m is mostly attributed to the regional thorough-flow which is rich in dissolved oxygen. The presence of electron donors at the deeper aquifer layers is also less. On the other hand, the recharging groundwater at the upper part of the aquifer is also rich in dissolved oxygen since it comes from the surface runoff and river 181 Copyright © 2005 Taylor & Francis Group plc, London, UK bed seepage, but this dissolved oxygen content can be quickly consumed by the electron donors. Therefore, the shortage of dissolved oxygen might play an important role in mobilizing arsenic in the groundwater system because dissolution of iron oxides occurs under anoxic conditions (Lee et al. 2003). 3.2.1 How does the dissolved oxygen level in recharging water decrease over aquifer depth? It is a common misconception that lowering of water table would increase the unsaturated vadose zone and eventually the oxygen concentration level in deeper layers of the aquifer will be increased. The oxygen diffusion model analyses as shown in Figure 2 demonstrated that lowering of water table does not increase the oxygen supply rate into the deeper layers of the aquifer. To prove this, a one dimensional oxygen diffusion model was built, where the water table elevations at WT-1 and WT-2 (Fig. 2) represent the levels before and after installation of irrigation wells respectively. The oxygen diffusion model suggests that lowering the water table about four meters (from WT-1 to WT-2) did not increase the dissolved oxygen concentration at the hypothetical arsenic contaminated layers at L-6 and L-10 (Fig. 2). The oxygen concentration in Case-1 at layer L-6 was estimated as 0.09atm (3.64mg/L) after 150 hours. But in Case-2, after lowering of the water table, using the same interval of time and same aquifer properties, the oxygen concentration at L-6 was found as 0.06atm (2.39mg/L). Therefore, lowering water table decreased the dissolved oxygen concentrations in the same aquifer. Consumption or reduction of dissolved oxygen supply helps develop reducing conditions. 3.2.2 Shortage of dissolved oxygen decreases the redox potential values The vertical arsenic distribution patterns as shown in Figure 6 indicate that the wells contaminated with arsenic in concentrations greater than 50 ppb are mostly located at the depth ranging from 10 to 35 m below the ground. The associated redox potential values (p e ) were computed using the dis- solved iron concentration records and are depicted in the Figures 7 and 8. It is apparent in Figure 7 that the redox potential values start decreasing (0 to Ϫ0.5) from the surface of the water table and continue up to 35 m below the ground level where the p e values fall below zero (Ϫ1.5) (Fig. 7). Certainly, the main reason for the decreasing redox value is the consumption of dissolved oxygen by electron donors. But the redox potential values at the top most surface of the water table are higher than the deeper part of the aquifer because of its close proximity to the ground surface. At the top layer of the aquifer, the dissolved oxygen level is detectable and much higher than deeper layers. The redox potential values again start increasing from 150 to 350 m below the ground. This is also associated with the dissolved oxygen concentration in the recharging groundwater. This can be reasoned that recharging groundwater at the depth below 150 m is known as the regional through- flow. The regional through-flow is usually rich in dissolved oxygen level, and in the deeper layers the oxygen consumption rate is negligible since organic carbon can not reach that point. 3.3 Validity of the existing theories of arsenic release from the context of dissolved oxygen shortage There have been divergences of views among the researchers and considerable difficulties in explaining the cause of arsenic mobilization in Bangladesh groundwater. While it was not the pur- pose of this analysis to focus on this debate, it was necessary to evaluate both oxidation and reduc- tion theory of arsenic release in the context of shortage of dissolved oxygen in order to support the working hypothesis. Since, the working hypothesis of this study directly opposes the oxidation theory of arsenic release; inadequacies in the oxidation theory will be discussed. The proponents of the reduction theory have rejected the explanations of oxidation theory of arsenic release. They argue that if pyrite oxidation were the real cause of arsenic release, arsenic concentrations must have been positively correlated with the amount of sulfate in the arsenic con- taminated groundwater. Arsenic contaminated groundwater in Bangladesh does not provide any evidence of correlation between arsenic and sulfate in the field. However, Bridge & Hussain (2000) mentioned that the lack of correlation between arsenic and sulfate does not indicate the 182 Copyright © 2005 Taylor & Francis Group plc, London, UK [...]... Burgess, W.G & Ahmed K.M 200 0 Mechanism of arsenic release to groundwater of Bangladesh and West Bengal Appl Geochem 15: 403–413 Roach, J 200 4 Arsenic in Asian Drinking Water Linked to Microbes Published in National Geographic News URL: http://news.nationalgeographic.com/news /200 4/06/0630_040630 _arsenic. html#main Ravenscroft, P., McArthur, J.M & Hoque, B.A 200 1 Geochemical and palaeohydrological controls... cause of arsenic release, why was arsenic contamination not found in the earlier days, and why is there not any prior report of arsenic toxicity in Bangladesh (Adel 200 0, Bridge & Hussein 1999) 3.4 Role of dissolved oxygen to influence the microbial activities in arsenic immobilization process Immobilization of toxic metals and radio nuclides are usually brought about by precipitation, biosorption and bioaccumulation... cell surface-associated or extra cellular polysaccharides and proteins (Gazso 200 1).Therefore, the dissolved oxygen content in recharging groundwater may influence the biosorption process of arsenic removal 3.4.1 Groundwater flow direction and its relation to arsenic mobilization The groundwater flow direction maps (Figs 4 and 5) demonstrate a general trend that arsenic concentrations increased as... lowering the water table and extraction rate may influence the arsenic mobilization but that it is further influenced by the dissolved oxygen content Recent studies have demonstrated that lowering the water table and/ or the extraction rate influence the amount of organic carbon available in the deeper layers and that is believed to be responsible for arsenic mobilization In addition, the pumping rate... Taylor & Francis Group plc, London, UK Bridge, T.E & Hussain, M.T 1999 The increased draw down and recharge in groundwater aquifer and their relationships to the arsenic problem in Bangladesh URL: http://www.dainichi-consul.co.jp./english/ arsenic/ arsarticles.htm Gazsó, Lajos, G 200 1 The key microbial processes in the removal of toxic metals and radionuclides from the environment CEJOEM 7 (3–4): 178–185... maximum arsenic concentration should be located at the topmost layer of the aquifer (0 to 15 m) because dissolved oxygen is the highest in the topmost layer Since it takes time for arsenic contaminated water to reach the screen of the pumping wells a concentration gradient is expected to occur during pumping But arsenic contaminated wells do not show any concentration gradient during pumping in Bangladesh... Alauddin, M Newaz, S S & A Hussam 200 0 Appraisal of a simple arsenic removal method for groundwater of Bangladesh J Environ Sci Health A35 (7): 1021–1041 Lee,-Y., Um,-I-H & Yoon, J 200 3 Arsenic (III) oxidation by iron (VI) (ferrate) and subsequent removal of arsenic (V) by iron (III) coagulation Environ Sci Technol 37(24): 5750–5756 McArthur, J.M., Ravenscroft, P., Safiullah, S & Thirlwall, M.F 200 1 Arsenic. .. recharging (NW zone) to the discharging southeast (SE) zone The groundwater residence time increases with decreasing hydraulic gradient in the southeast zone of Bangladesh Over time, the dissolved oxygen is consumed up by the electron donors and helps develop a mild reducing condition Usually, the tendency to donate electrons is high in reducing conditions Metal-reducing bacteria “breathe” by passing... during pumping in Bangladesh In addition, the oxidation theory of arsenic release can not explain why arsenic concentration is maximum at 30 m below the ground level and why contamination is not maximum just at the top of the water table Therefore, oxidation theory is inadequate to explain the arsenic mobilization in Bangladesh groundwater The iron oxides reduction model of arsenic release is now widely... the groundwater and would contribute to the arsenic mobilization in Bangladesh groundwater Since arsenic mobilization is not solely due to chemical changes; but rather it is a resultant of complex multidimensional biogeochemical and hydro-geological processes Although it is true that arsenic mobilization is poorly understood, but it is most likely that arsenic contamination problem in Bangladesh is . change in oxygen exposure took place at layer L-6 and L-10 before and after introduction of large-scale shallow irri- gation wells in Bangladesh was investigated. In the diffusion model the following. that introduction of organic carbon by over-pumping of irrigation wells can be a factor in increasing arsenic mobility in shal- low groundwater in Bangladesh (Roach 200 4). Organic carbon in the. dur- ing pumping. But arsenic contaminated wells do not show any concentration gradient during pumping in Bangladesh. In addition, the oxidation theory of arsenic release can not explain why arsenic