Impact of irrigating rice paddies with groundwater containing arsenic in Bangladesh

Vetiver grass cleans up arsenic contaminated field for subsequent safe cultivation of rice with low arsenic in grains: A two year field study

The presence of high concentrations of arsenic (As) in agricultural soils and its subsequent accumulation in rice crop is a serious issue threatening sustainability of agriculture and human health. In the present work, remediation of As contaminated field in Nadia, West Bengal, India was done through the cultivation of Vetiver (Vetiveria zizanoides L. Nash) and the same field was subsequently used for rice (Oryza sativa L.) cultivation. The results showed that V. zizanoides could reduce As concentrations in the field to bring it lower than the maximum permissible limit (20 mg kg-1) in 11 months' time. The rice plants grown in remediated field showed improvement in growth and photosynthesis parameters as compared to that of contaminated field. Importantly, yield related parameters (filled seed, 1000 grain weight, number of panicles etc.) were also significantly higher in remediated field than that in contaminated field. Arsenic concentration in roots, shoot, husk and grains of rice was found to be significantly lower in remediated field than in contaminated field. Grain As decreased from 0.75 to 0.77 μg g-1 dw in contaminated field to 0.15-0.18 μg g-1 dw. In conclusion, replacing rice for single year with V. zizanoides crop can significantly remediate the field and can be a viable option.

Evaluation of the potentials of rice varieties and water management practices for reducing human health risks associated with polluted river water irrigated rice in Bangladesh

The consumption of arsenic and trace-metal-contaminated rice is a human health concern worldwide, particularly in Bangladesh. In this study, the effects of rice varieties and water management practices on the concentrations of arsenic and trace metals in rice grains were investigated to reduce human health risks related to rice consumption. In addition, the performance of risk reduction using the optimum combination of rice variety and water management practices was quantitatively assessed using Monte Carlo simulation, in which non-carcinogenic and carcinogenic risk distributions under the status quo and the optimum combination were compared. The experimental results revealed that Dular and BRRI dhan45 (rice varieties) cultivated under alternate wetting and drying (AWD) and continuous flooding (CF) conditions showed the lowest hazard quotient (HQ) values for copper, cadmium, and arsenic and the lowest target cancer risk (TR) for arsenic. In Dular and BRRI dhan45 (AWD and CF) varieties, the proportion of the population for which HQs exceeded 1.0 (the reference value) tended to decrease (except for arsenic), compared with populations for which the rice varieties and water management practices were not specified. These results suggest that the use of optimum combinations of rice varieties and water management practices could reduce non-carcinogenic and carcinogenic risks associated with arsenic and trace metals uptake via rice grain consumption by the Bangladeshi people.

Arsenic contamination in groundwater and food chain with mitigation options in Bengal delta with special reference to Bangladesh

Bangladesh, situated in Bengal delta, is one of the worst affected countries by arsenic contamination in groundwater. Most of the people in the country are dependent on groundwater for domestic and irrigation purposes. Currently, 61 districts out of 64 districts of Bangladesh are affected by arsenic contamination. Drinking arsenic contaminated groundwater is the main pathway of arsenic exposure in the population. Additionally, the use of arsenic-contaminated groundwater for irrigation purpose in crop fields in Bangladesh has elevated arsenic concentration in surface soil and in the plants. In many arsenic-affected countries, including Bangladesh, rice is reported to be one of the significant sources of arsenic contamination. This review discussed scenario of groundwater arsenic contamination and transmission of arsenic through food chain in Bangladesh. The study further highlighted the human health perspectives of arsenic exposure in Bangladesh with possible mitigation and remediation options employed in the country.

Silicon-based additive on heavy metal remediation in soils: Toxicological effects, remediation techniques, and perspectives

Chemical fertilizer is gaining increasing attention and has been the center of much research which indicating complex beneficial and harmful effects. Chemical fertilizer might cause some environmental hazards to the biosphere, especially in the agricultural ecosystem. The application of silicon (Si) fertilizer in agriculture has been proved to be able to create good economic and environmental benefits. Si is the second most abundant earth crust element. Si fertilizer improves soil quality and alleviates biotic and abiotic crop stress. It is of great significance to understand the function of Si fertilizer in agricultural utilization and environmental remediation. This paper reviews the Si-based fertilizer in farmland use and summarizes prior research relevant with characterization, soil quality improvement, and pollution remediation effects. Its use in agriculture enhances plant silicon uptake, mediates plant salt and drought stress and remediates heavy metals such as Al, As, Cd, Cu, Zn and Cr. This article also summarizes the detoxification mechanism of silicon and its effects on plant physiological activity such as photosynthesis and transpiration. Fertilizer materials and crop fertilizer management were also considered. Foliar spraying is an effective method to improve crop growth and yield and reduce biotic or abiotic stress. Silicon nanoparticle material provides potential with great potential and prospects. More investigation and research are prospected to better understand how silicon impacts the environment and whether it is a beneficial additive.

Urinary element profiles and associations with cardiometabolic diseases: A cross-sectional study across ten areas in China

BACKGROUND

Existing evidence on the associations of urinary element profiles with related food intake and cardiometabolic diseases has been limited in China.

OBJECTIVES

To examine the associations of urinary toxic metals and other elements with food intakes and with the prevalence of cardiometabolic diseases.

METHODS

Inductively coupled plasma mass spectrometry was used to measure the concentrations of cadmium (Cd), arsenic (As), nickel (Ni), aluminum (Al), copper (Cu), and 16 other elements in spot urine samples collected from 19,380 adults in 10 geographically diverse areas of China during 2013-2014. The levels of creatinine-corrected elements were used to analyze their correlations with self-reported dietary intake and associations with prevalent diabetes (n = 1862), stroke (n = 1322) and ischemic heart disease (IHD) (n = 1690).

RESULTS

Overall, the mean (SD) age was 59.2 (10.1) years with a mean BMI of 24.2 (3.5) kg/m². Of the 21 elements, the median (IQR) concentrations varied from 0.49 (0.31-0.82) μg/g creatinine for vanadium (V) to 1666 (1189-2321) mg/g creatinine for potassium (K). Nine urinary elements [Cd, As, Ni, lead (Pb), boron (B), magnesium (Mg), rubidium (Rb), strontium (Sr), and cesium (Cs); all rs > 0.20, p < 0.001] were positively correlated with staple food intake, five [Cd, As, selenium (Se), Rb, and Cs; all rs > 0.20, p < 0.001] with animal-sourced food group, and one (Cd; r = 0.21, p < 0.05) with pickled vegetable intake. For diabetes, adjusted prevalence ratios (PRs) per SD of specific element levels were 1.10 [95% confidence interval (CI): 1.03-1.18] for Cd, 1.24 (1.18-1.31) for As, 1.33 (1.27-1.39) for Ni, 1.14 (1.09-1.20) for Al, and 1.24 (1.18-1.30) for Cu. Cd was positively associated with stroke (PR per SD = 1.13, 1.04-1.23), while none of the elements were significantly associated with IHD.

CONCLUSION

In China, the urinary levels of several toxic metals were significantly associated with the consumption of specific food groups and the risk of cardiometabolic diseases including diabetes and stroke.

Groundwater quality: Global threats, opportunities and realising the potential of groundwater

Groundwater is a critical resource enabling adaptation due to land use change, population growth, environmental degradation, and climate change. It can be a driver of change and adaptation, as well as effectively mitigate impacts brought about by a range of human activities. Groundwater quality is key to assessing groundwater resources and we need to improve our understanding and coverage of groundwater quality threats if we are to use groundwater sustainably to not further burden future generations by limiting resources and/or increasing treatment or abstraction costs. Good groundwater quality is key to progress on a range of Sustainable Development Goals, but achievement of those goals most affected by groundwater contamination is often hindered by of a lack of resources to enable adaptation. A range of threats to groundwater quality exist, both natural and anthropogenic, which may constrain groundwater use. However, groundwater often provides good quality water for a range of purposes and is the most important water resource in many settings. This special issue explores some of the key groundwater quality challenges we face today as well as the opportunities good groundwater quality and treatment solutions bring to enhance safe groundwater use. Legacy anthropogenic contaminants and geogenic contaminants may be well documented in certain places, such as N America, Europe and parts of Asia. However, there is a real issue of data accessibility in some regions, even for more common contaminants. This paucity of information can restrict our understanding and ability to manage and protect groundwater sources. Compared to surface water quality, large scale assessments for groundwater quality are still scarce and often rely on inadequate data sets. Better access to existing data sets and more research is needed on many groundwater quality threats. Identification and quantification of these threats will support the wise use and protection of this subsurface resource, allow society to adequately address future challenges, and help communities realise the full potential of groundwater.

Evaluation of a field kit for testing arsenic in paddy soil contaminated by irrigation water

Rice is the primary crop in Bangladesh and rice yield is diminished due to the buildup of arsenic (As) in soil from irrigation with high-As groundwater. Soil testing with an inexpensive kit could help farmers target high-As soil for mitigation or decide to switch to a different crop that is less sensitive to As in soil. A total of 3,240 field kit measurements of As in 0.5 g of fresh soil added to 50 mL of water were compared with total soil As concentrations measured on oven-dried homogenized soil by X-ray fluorescence (XRF). For sets of 12 soil samples collected within a series of rice fields, the average of kit As measurements was a linear function of the average of XRF measurements (r2=0.69). Taking into account that the kit overestimates water As concentrations by about a factor of two, the relationship suggests that about a quarter of the As in paddy soil is released in the kit's reaction vessel. Using the relationship and considering XRF measurements as the reference, the 12-sample average determined correctly whether soil As was above or below a 30 mg/kg threshold in 86% of cases where soil As was above the threshold and in 79% of cases where soil As was below the threshold. We also used a Bayesian approach using 12 kit measurements to estimate the probability that soil As was above a given threshold indicated by XRF measurements. The Bayesian approach is theoretically optimal but was only slightly more accurate than the linear regression. These results show that rice farmers can identify high-As portions of their fields for mitigation using a dozen field kit measurements on fresh soil and base their decisions on this information.

Assessment and source identification of As and Cd contamination in soil and plants in the vicinity of the Nui Phao Mine, Vietnam

This study investigated the contamination levels and sources of As and Cd vicinity area from Nui Phao mine that is one of the largest tungsten (W) open pit mines in the world. Soil and plant samples were collected from the study area to identify the concentrations of As and Cd using aqua-regia or HNO₃ digestion. According to the Vietnamese agricultural soil criteria, all soil samples were contaminated with As and Cd. The distribution of As concentration is related to the distance from the Nui Phao mine. The higher As concentrations were measured in the area close to the mine. However, the Cd distribution in the soil showed a different pattern from As. Enrichment factor and Geoaccumulation Index (Igeo) indicated that As in the soil is derived from the mining activities, while Cd could have other geogenic or anthropogenic sources. The ranges of As and Cd concentration in polished rice grains in the Nui Phao mine area exceeded the CODEX criteria (0.2 mg/kg), which indicated extreme contamination. The arsenic concentration between soil and plant samples was determined to be a positive correlation, while the Cd concentration showed a negative correlation, implying that As and Cd have different geochemical behavior based on their sources.

Arsenic in a groundwater environment in Bangladesh: Occurrence and mobilization

Groundwater with an excessive level of Arsenic (As) is a threat to human health. In Bangladesh, out of 64 districts, the groundwater of 50 and 59 districts contains As exceeding the Bangladesh (50 μg/L) and WHO (10 μg/L) standards for potable water. This review focuses on the occurrence, origin, plausible sources, and mobilization mechanisms of As in the groundwater of Bangladesh to better understand its environmental as well as public health consequences. High As concentrations mainly was mainly occur from the natural origin of the Himalayan orogenic tract. Consequently, sedimentary processes transport the As-loaded sediments from the orogenic tract to the marginal foreland of Bangladesh, and under the favorable biogeochemical circumstances, As is discharged from the sediment to the groundwater. Rock weathering, regular floods, volcanic movement, deposition of hydrochemical ore, and leaching of geological formations in the Himalayan range cause As occurrence in the groundwater of Bangladesh. Redox and desorption processes along with microbe-related reduction are the key geochemical processes for As enrichment. Under reducing conditions, both reductive dissolution of Fe-oxides and desorption of As are the root causes of As mobilization. A medium alkaline and reductive environment, resulting from biochemical reactions, is the major factor mobilizing As in groundwater. An elevated pH value along with decoupling of As and HCO₃^- plays a vital role in mobilizing As. The As mobilization process is related to the reductive solution of metal oxides as well as hydroxides that exists in sporadic sediments in Bangladesh. Other mechanisms, such as pyrite oxidation, redox cycling, and competitive ion exchange processes, are also postulated as probable mechanisms of As mobilization. The reductive dissolution of MnOOH adds dissolved As and redox-sensitive components such as SO₄^2- and oxidized pyrite, which act as the major mechanisms to mobilize As. The reductive suspension of Mn(IV)-oxyhydroxides has also accelerated the As mobilization process in the groundwater of Bangladesh. Infiltration from the irrigation return flow and surface-wash water are also potential factors to remobilize As. Over-exploitation of groundwater and the competitive ion exchange process are also responsible for releasing As into the aquifers of Bangladesh.

Soil arsenic but not rice arsenic increasing with arsenic in irrigation water in the Punjab plains of Pakistan

AIM

Irrigating rice with groundwater can lead to As accumulation in soil and rice grains. Matched sets of irrigation water, paddy soil, and rice grains were collected to assess the scale of the problem in the Punjab plains of Pakistan.

METHODS

From a total of 60 sites, irrigation water and rice grains as well as 103 soil samples were collected and analyzed in the laboratory. Irrigation water and 660 soil samples were also analyzed in the field using a field kit.

RESULTS

Concentrations of As in irrigation water (65±32 μg/L) are higher in the floodplain of the Ravi River compared to the Chenab (13±9 μg/L) and Jhelum (4±5 μg/L) rivers, as well as the intervening Rechna (6±6 μg/L) and Chaj doabs (0.8±0.2 μg/L). Area-weighted mean soil As concentrations are 12±3 mg/kg along the Ravi, 8.9±2 and 8.1±2 mg/kg along the Chenab and Jhelum, respectively, and 6.2±0.2 mg/kg within the Rachna and 6.1±0.1 mg/kg in Chaj doabs. The As content of polished grains export-quality basmati rice of 0.09±0.05 mg/kg, however, is low across the entire area.

CONCLUSIONS

Groundwater irrigation leads to elevated As concentrations in paddy soil of some rice-growing regions of Punjab but does not result in increased uptake of As in basmati rice grains.

Variation in arbuscular mycorrhizal fungal communities associated with lowland rice (Oryza sativa) along a gradient of soil salinity and arsenic contamination in Bangladesh

Rice is an essential food crop that nourishes >50% of the world population. In many regions of Bangladesh rice production is constrained by high soil salinity and heavy metal contamination due to irrigation practices. Plants may naturally overcome such stress through mutualistic interactions with arbuscular mycorrhizal fungi (AMF). Yet, little is known regarding the diversity and composition of AMF communities in rice fields with high saline and arsenic concentration. Here we used high throughput Illumina sequencing to characterize AMF communities in rice roots from 45 Bangladeshi rice fields, along a large geographical gradient of soil salinity and arsenic contamination. We obtained 77 operational taxonomic units (OTUs, based on a sequence similarity threshold of 97%) from eight AMF families, and showed that high soil salinity and arsenic concentration are significantly associated with low AMF diversity in rice roots. Soil salinity and arsenic concentration also explained a large part of the variation in AMF community composition, but also soil pH, moisture, organic matter content and plant available soil phosphorus played an important role. Overall, our study showed that even at very high salinity and arsenic levels, some AMF OTUs are present in rice roots. Their potential role in mediating a reduction of rice stress and arsenic uptake remains to be investigated.

Arsenic speciation dynamics in paddy rice soil-water environment: sources, physico-chemical, and biological factors - A review

Rice is the main staple carbohydrate source for billions of people worldwide. Natural geogenic and anthropogenic sources has led to high arsenic (As) concentrations in rice grains. This is because As is highly bioavailable to rice roots under conditions in which rice is cultivated. A multifaceted and interdisciplinary understanding, both of short-term and long-term effects, are required to identify spatial and temporal changes in As contamination levels in paddy soil-water systems. During flooding, soil pore waters are elevated in inorganic As compared to dryland cultivation systems, as anaerobism results in poorly mobile As(V), being reduced to highly mobile As(III). The formation of iron (Fe) plaque on roots, availability of metal (hydro)oxides (Fe and Mn), organic matter, clay mineralogy and competing ions and compounds (PO₄^3- and Si(OH)₄) are all known to influence As(V) and As(III) mobility in paddy soil-water environments. Microorganisms play a key role in As transformation through oxidation/reduction, and methylation/volatilization reactions, but transformation kinetics are poorly understood. Scientific-based optimization of all biogeochemical parameters may help to significantly reduce the bioavailability of inorganic As.

Non-essential element concentrations in brown grain rice: Assessment by advanced data mining techniques

The concentrations of 17 non-essential elements (Al, As, Ba, Be, Cd, Ce, Cr, Hg, La, Li, Pb, Sb, Sn, Sr, Th, Ti, and Tl) were determined in brown grain rice samples of two varieties: Fortuna and Largo Fino. The samples were collected from the four main producing regions of Corrientes province (Argentina). Quantitative determinations were performed by inductively coupled plasma mass spectrometry (ICP-MS), using a validated method. The contents of As, Be, Cd, Ce, Cr, Hg, Pb, Sb, Sn, Th, and Tl were very low or not detected in most samples. The non-essential element levels detected were in line with studies conducted in rice from different parts of the world. In order to characterize the influence of geographical origin in the samples, the following classification methods were carried out: linear discriminant analysis (LDA), k-nearest neighbors (k-NN), partial least squares discriminant analysis (PLS-DA), support vector machine (SVM) and random forests (RF). The best performance was obtained by using RF (96%) and SVM (96%). The results reported here showed the variation in the non-essential element profiles in rice grain depending on the geographical origin.

The evaluation of arsenic contamination potential, speciation and hydrogeochemical behaviour in aquifers of Punjab, Pakistan

In this study, we tested 123 groundwater wells from five different areas of Punjab, Pakistan for arsenic (As) contamination level and species, as well as delineated hydrogeochemical behaviour of As in aquifers. Results revealed that 75% and 41% of the groundwater wells exceeded the safe As limit of World Health Organisation (WHO, 10 μg L^-1) and Pakistan-EPA (50 μg L^-1), respectively. Arsenite (As(III)) and arsenate (As(V)) spanned 0-80% and 20-100% of total As (1.2-206 μg L^-1), respectively. The mean As content (5.2 μg L^-1) of shallow wells at 9-40 m depth did not exceed the WHO safe limit, representing a safe aquifer zone for pumping of groundwater compared to deeper wells at 41-90 m (51 μg L^-1) and >90 m (23 μg L^-1) depths. Piper-plot elucidated that the aqueous chemistry was dominated with Na-SO₄, Na-Ca-SO₄, Na-Mg-SO₄ type saline water. Principal component analysis grouped As concentration with well depth, pH, salinity, Fe and CO₃, exhibiting that these hydrogeochemical factors could have potential role in controlling As release/sequestration into the aquifers of study area. Geochemical modeling showed positive saturation indices only for iron (Fe) oxide-phases, indicating Fe oxides as the major carriers of As. Overall, this study provides insights to tackle emerging As threat to the communities in Punjab, Pakistan, as well as help develop suitable management/mitigation strategies - based on the baseline knowledge of As levels/species and factors governing As contamination in the study area.

Opportunities and challenges in the use of mineral nutrition for minimizing arsenic toxicity and accumulation in rice: A critical review

Growing rice on arsenic (As)-contaminated soil or irrigating with As-contaminated water leads to significant accumulation of As in grains. Moreover, rice accumulates more As into grains than other cereal crops. Thus, rice consumption has been identified as a major route of human exposure to As in many countries. Inorganic As species are carcinogenic and could pose a considerable health risk to humans even at low dietary concentration. Genotypic variation and concentration of nutrients such as iron, manganese, phosphate, sulfur and silicon are the two main factors that affect As accumulation in rice grains. Therefore, in addition to better growth and yield of plants, application of specific nutrients in optimum quantities offers an added benefit of decreasing As content in rice grains. These nutrient elements influence speciation of As in rhizosphere, compete with As for root uptake and interfere with As translocations to the shoot and ultimately accumulation in grains. This papers critically appraises the methods, forms and rate of application, mechanisms and extent of efficiency of different mineral nutrients in decreasing As accumulation in rice grains.

Field Study of Rice Yield Diminished by Soil Arsenic in Bangladesh

Rice was traditionally grown only during the summer (aman) monsoon in Bangladesh but more than half is now grown during the dry winter (boro) season and requires irrigation. A previous field study conducted in a small area irrigated by a single high-arsenic well has shown that the accumulation of arsenic (As) in soil from irrigating with high-As groundwater can reduce rice yield. We investigated the effect of soil As on rice yield under a range of field conditions by exchanging the top 15 cm of soil between 13 high-As and 13 low-As plots managed by 16 different farmers, and we explore the implications for mitigation. Soil As and rice yields were measured for soil replacement plots where the soil was exchanged and adjacent control plots where the soil was not exchanged. Differences in yield (ranging from +2 to -2 t/ha) were negatively correlated to the differences in soil As (ranging from -9 to +19 mg/kg) between adjacent replacement and control plots during two boro seasons. The relationship between soil As and yield suggests a boro rice yield loss over the entire country of 1.4-4.9 million tons annually, or 7-26% of the annual boro harvest, due to the accumulation of As in soil over the past 25 years.

Mitigating arsenic contamination in rice plants with an aquatic fern, Marsilea minuta

Dangers of arsenic contamination are well known in human civilization. The threat increases when arsenic is accumulated in food and livestock through irrigated crops or animal food. Hence, it is important to mitigate the effects of arsenic as much as possible. This paper discusses a process for reducing the level of arsenic in different parts of rice plants with an aquatic fern, Marsilea minuta L. A pot experiment was done to study the possibility of using Marsilea minuta as a phytoremediator of arsenic. Rice and Marsilea minuta were allowed to grow together in soils. As a control, Marsilea minuta was also cultured alone in the presence and absence of arsenic (applied at 1 mg/L as irrigation water). We did not find any significant change in the growth of rice due to the association of Marsilea minuta, though it showed a reduction of approximately 58.64% arsenic accumulation in the roots of rice grown with the association of fern compared to that grown without fern. We measured a bioaccumulation factor (BF) of > 5.34, indicating that Marsilea minuta could be a good phytoremediator of arsenic in rice fields.

Associations between Diet and Toenail Arsenic Concentration among Pregnant Women in Bangladesh: A Prospective Study

This prospective study evaluated the relationship between long-term dietary habits and total arsenic (As) concentration in toenail clippings in a cohort of 1616 pregnant women in the Bangladeshi administrative regions of Sirajdikhan and Pabna Sadar. Diet was assessed at Gestation Week 28 and at Postpartum Month 1, using a locally-validated dish-based semi-quantitative food-frequency questionnaire. Toenail As concentration was analyzed by microwave-assisted acid digestion and inductively coupled plasma mass spectrometry. Associations between natural log-transformed consumption of individual food items and temporally matched natural log-transformed toenail As concentration were quantified using general linear models that accounted for As concentration in the primary drinking water source and other potential confounders. The analysis was stratified by As in drinking water (≤50 μg/L versus >50 μg/L) and the time of dietary assessment (Gestation Week 28 versus Postpartum Week 1). Interestingly, toenail As was not significantly associated with consumption of plain rice as hypothesized. However, toenail As was positively associated with consumption of several vegetable, fish and meat items and was negatively associated with consumption of rice, cereal, fruits, and milk based food items. Further studies in pregnant women are needed to compare As metabolism at different levels of As exposure and the interaction between dietary composition and As absorption.

Arsenic contamination in agricultural soils of Bengal deltaic region of West Bengal and its higher assimilation in monsoon rice

In the Bengal deltaic region, the shallow groundwater laced with arsenic is used for irrigation frequently and has elevated the soil arsenic in agricultural soil. However, the areas with seasonal flooding reduce arsenic in top layers of the soils. Study shows arsenic accumulation in the deeper soil layers with time in the contaminated agricultural soil (19.40±0.38mg/kg in 0-5cm, 27.17±0.44mg/kg in 5-10cm and 41.24±0.48mg/kg in 10-15cm) in 2013 whereas depletion in 2014 and its buildup in different parts of monsoon rice plant in Nadia, India. Principal Component Analysis and Cluster Analysis were performed, and Enrichment Factor was calculated to identify the sources of arsenic in the soil. Potential Ecological Risk was also calculated to estimate the extent of risk posed by arsenic in soil, along with the potential risk of dietary arsenic exposure. Remarkably, the concentration of arsenic detected in the rice grain showed average value of 1.4mg/kg in 2013 which has increased to 1.6 in 2014, both being above the permissible limit (1mg/kg). These results indicate that monsoon flooding enhances the infiltration of arsenic in the deeper soil layer, which lead to further contamination of shallow groundwater.

Redox Zonation and Oscillation in the Hyporheic Zone of the Ganges-Brahmaputra-Meghna Delta: Implications for the Fate of Groundwater Arsenic during Discharge

Riverbank sediment cores and pore waters, shallow well waters, seepage waters and river waters were collected along the Meghna Riverbank in Gazaria Upazila, Bangladesh in Jan. 2006 and Oct.-Nov. 2007 to investigate hydrogeochemical processes controlling the fate of groundwater As during discharge. Redox transition zones from suboxic (0-2 m depth) to reducing (2-5 m depth) then suboxic conditions (5-7 m depth) exist at sites with sandy surficial deposits, as evidenced by depth profiles of pore water (n=7) and sediment (n=11; diffuse reflectance, Fe(III)/Fe ratios and Fe(III) concentrations). The sediment As enrichment zone (up to ~700 mg kg^-1) is associated with the suboxic zones mostly between 0-2 m depth and less frequently between 5-7 m depth. The As enriched zones consist of several 5 to 10 cm-thick dispersed layers and span a length of ~5-15 m horizontally from the river shore. Depth profiles of riverbank pore water deployed along a 32 m transect perpendicular to the river shore show elevated levels of dissolved Fe (11.6±11.7 mg L^-1) and As (118±91 μg L^-1, mostly as arsenite) between 2-5 m depth, but lower concentrations between 0-2 m depth (0.13±0.19 mg L^-1 Fe, 1±1 μg L^-1 As) and between 5-6 m depth (1.14±0.45 mg L^-1 Fe, 28±17 μg L^-1 As). Because it would take more than a few hundred years of steady groundwater discharge (~10 m yr^-1) to accumulate hundreds of mg kg^-1 of As in the riverbank sediment, it is concluded that groundwater As must have been naturally elevated prior to anthropogenic pumping of the aquifer since the 1970s. Not only does this lend unequivocal support to the argument that As occurrence in the Ganges-Brahmaputra-Meghna Delta groundwater is of geogenic origin, it also calls attention to the fate of this As enriched sediment as it may recycle As into the aquifer.

Using DET and DGT probes (ferrihydrite and titanium dioxide) to investigate arsenic concentrations in soil porewater of an arsenic-contaminated paddy field in Bangladesh

Arsenic concentration in the pore water of paddy fields (Csoln) irrigated with arsenic-rich groundwater is a key parameter in arsenic uptake by rice. Pore water extracts from cores and in situ deployment of DET and DGT probes were used to measure the arsenic concentration in the pore water. Ferrihydrite (Fe) and titanium dioxide (Ti) were used as DGT binding agents. Six sampling events during different growing stages of the rice, inducing different biogeochemical conditions, were performed in one rice field. A time series of DGT experiments allow the determination of an in situ arsenic diffusion coefficient in the diffusive gel (3.34×10(-6) cm(2) s(-1)) needed to calculate the so-called CDGT(Fe) and CDGT(Ti) concentrations. Over 3 days of a given sampling event and for cores sampled at intervals smaller than 50 cm, great variability in arsenic Csoln concentrations between vertical profiles was observed, with maxima of concentrations varying from 690 to 2800 μg L(-1). Comparisons between arsenic measured Csol and CDET and calculated CDGT(Fe) and CDGT(Ti) concentrations show either, in a few cases, roughly similar vertical profiles, or in other cases, significantly different profiles. An established iron oxyhydroxide precipitation in the DET gel may explain why measured arsenic CDET concentrations occasionally exceeded Csoln. The large spread in results suggests limitations to the use of DET and type of DGT probes used here for similarly representing the spatio-temporal variations of arsenic content in soil pore water in specific environmental such as paddy soils.

Effects of silicon (Si) on arsenic (As) accumulation and speciation in rice (Oryza sativa L.) genotypes with different radial oxygen loss (ROL)

Arsenic (As) contamination of paddy soils has adversely affected the health of millions of people those consuming rice for staple food. The present study was aimed at investigating the effects of silicon (Si) fertilization on As uptake, speciation in rice plants with different radial oxygen loss (ROL). Six genotypes were planted in pot soils under greenhouse conditions until late tillering state. The results showed that the rates of ROL were higher in hybrid rice genotypes varying from 19.76 to 27 μmol O2 g(-1) root dry weight h(-1) than that in conventional indica rice genotypes varying from 9.55 to 15.41 μmol O2 g(-1) root dry weight h(-1). Si addition significantly increased straw biomass (p<0.005), but with no significant effects on root biomass. Si fertilization significantly reduced shoot and root total As concentrations (p<0.001) in six genotypes grown in 40 mg As/kg soil. Si addition decreased the inorganic As in shoots of 'Xiangfengyou-9' with lower ROL and 'Xiangwanxian-12' with higher ROL by 31% and 25% respectively and had the tendency to increase DMA concentrations. It is potential to reduce As contamination of rice efficiently by combining Si fertilization and selecting genotypes with high radial oxygen loss.

Heavy metal exposure from ingesting rice and its related potential hazardous health risks to humans

Different types of rice grains imported from 11 different countries and available on the Kuwaiti retail market were collected for heavy metal analysis. The surveyed rice samples were grouped according to their country of origin into four different regions namely Asia, Europe, Middle East, and North America. The samples were analyzed for total arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg) using inductively coupled plasma mass spectrometry (ICP-MS). The data showed that the highest geometric means (GMs) of total As, Cd, Pb, and Hg contents were in rice samples imported from Europe, Asia, Europe, and the Middle East, respectively. The total As, Cd, Pb, and Hg concentrations in the analyzed rice samples were compared to the heavy metal content of 5800 rice samples from 25 different countries reported in the literature; furthermore, the heavy metal (total As, Cd, Pb, and Hg) concentrations determined in this study were implemented to calculate the daily dietary intake of toxic metals for the general population in the state of Kuwait, while the collected ones from the literature were implemented to calculate and then compare the daily dietary intake of toxic metals for the general population in 29 different countries around the world.

Arsenic concentrations in paddy soil and rice and health implications for major rice-growing regions of Cambodia

Despite the global importance of As in rice, research has primarily focused on Bangladesh, India, China, and the United States with limited attention given to other countries. Owing to both indigenous As within the soil and the possible increases arising from the onset of irrigation with groundwater, an assessment of As in rice within Cambodia is needed, which offers a "base-case" comparison against sediments of similar origin that comprise rice paddy soils where As-contaminated water is used for irrigation (e.g., Bangladesh). Here, we evaluated the As content of rice from five provinces (Kandal, Prey Veng, Battambang, Banteay Meanchey, and Kampong Thom) in the rice-growing regions of Cambodia and coupled that data to soil-chemical factors based on extractions of paddy soil collected and processed under anoxic conditions. At total soil As concentrations ranging 0.8 to 18 μg g(-1), total grain As concentrations averaged 0.2 μg g(-1) and ranged from 0.1 to 0.37 with Banteay Meanchey rice having significantly higher values than Prey Veng rice. Overall, soil-extractable concentrations of As, Fe, P, and Si and total As were poor predictors of grain As concentrations. While biogeochemical factors leading to reduction of As(V)-bearing Fe(III) oxides are likely most important for predicting plant-available As, husk and straw As concentrations were the most significant predictors of grain-As levels among our measured parameters.

Diet and toenail arsenic concentrations in a New Hampshire population with arsenic-containing water

BACKGROUND

Limited data exist on the contribution of dietary sources of arsenic to an individual's total exposure, particularly in populations with exposure via drinking water. Here, the association between diet and toenail arsenic concentrations (a long-term biomarker of exposure) was evaluated for individuals with measured household tap water arsenic. Foods known to be high in arsenic, including rice and seafood, were of particular interest.

METHODS

Associations between toenail arsenic and consumption of 120 individual diet items were quantified using general linear models that also accounted for household tap water arsenic and potentially confounding factors (e.g., age, caloric intake, sex, smoking) (n = 852). As part of the analysis, we assessed whether associations between log-transformed toenail arsenic and each diet item differed between subjects with household drinking water arsenic concentrations <1 μg/L versus ≥1 μg/L.

RESULTS

As expected, toenail arsenic concentrations increased with household water arsenic concentrations. Among the foods known to be high in arsenic, no clear relationship between toenail arsenic and rice consumption was detected, but there was a positive association with consumption of dark meat fish, a category that includes tuna steaks, mackerel, salmon, sardines, bluefish, and swordfish. Positive associations between toenail arsenic and consumption of white wine, beer, and Brussels sprouts were also observed; these and most other associations were not modified by exposure via water. However, consumption of two foods cooked in water, beans/lentils and cooked oatmeal, was more strongly related to toenail arsenic among those with arsenic-containing drinking water (≥1 μg/L).

CONCLUSIONS

This study suggests that diet can be an important contributor to total arsenic exposure in U.S. populations regardless of arsenic concentrations in drinking water. Thus, dietary exposure to arsenic in the US warrants consideration as a potential health risk.

Silicate mineral impacts on the uptake and storage of arsenic and plant nutrients in rice ( Oryza sativa L.)

Arsenic-contaminated rice grain may threaten human health globally. Since H₃AsO₃⁰ is the predominant As species found in paddy pore-waters, and H₄SiO₄⁰ and H₃AsO₃⁰ share an uptake pathway, silica amendments have been proposed to decrease As uptake and consequent As concentrations in grains. Here, we evaluated the impact of two silicate mineral additions differing in solubility (+Si(L), diatomaceous earth, 0.29 mM Si; +Si(H), Si-gel, 1.1 mM Si) to soils differing in mineralogy on arsenic concentration in rice. The +Si(L) addition either did not change or decreased As concentration in pore-water but did not change or increased grain-As levels relative to the (+As--Si) control. The +Si(H) addition increased As in pore-water, but it significantly decreased grain-As relative to the (+As--Si) control. Only the +Si(H) addition resulted in significant increases in straw- and husk-Si. Total grain- and straw-As was negatively correlated with pore-water Si, and the relationship differed between two soils exhibiting different mineralogy. These differing results are a consequence of competition between H₄SiO₄⁰ and H₃AsO₃⁰ for adsorption sites on soil solids and subsequent plant-uptake, and illustrate the importance of Si mineralogy on arsenic uptake.

Phytoremediation of arsenic contaminated soil by Pteris vittata L. I. Influence of phosphatic fertilizers and repeated harvests

A greenhouse experiment was conducted to evaluate the effectiveness of diammonium phosphate (DAP), single superphosphate (SSP) and two growing cycles on arsenic removal by Chinese Brake Fern (Pteris vittata L.) from an arsenic contaminated Typic Haplustept of the Indian state of West Bengal. After harvest of Pteris vittata the total, Olsen's extractable and other five soil arsenic fractions were determined. The total biomass yield of P. vittata ranged from 10.7 to 16.2 g pot(-1) in first growing cycle and from 7.53 to 11.57 g pot(-1) in second growing cycle. The frond arsenic concentrations ranged from 990 to 1374 mg kg(-1) in first growing cycle and from 875 to 1371 mg kg(-1) in second growing cycle. DAP was most efficient in enhancing biomass yield, frond and root arsenic concentrations and total arsenic removal from soil. After first growing cycle, P. vittata reduced soil arsenic by 10 to 20%, while after two growing cycles Pteris reduced it by 18 to 34%. Among the different arsenic fractions, Fe-bound arsenic dominated over other fractions. Two successive harvests with DAP as the phosphate fertilizer emerged as the promising management strategy for amelioration of arsenic contaminated soil of West Bengal through phyotoextraction by P. vittata.

Phytoremediation of arsenic contaminated soil by Pteris vittata L. II. Effect on arsenic uptake and rice yield

A greenhouse experiment evaluated the effect of phytoextraction of arsenic from a contaminated soil by Chinese Brake Fern (Pteris vittata L.) and its subsequent effects on growth and uptake of arsenic by rice (Oryza sativa L.) crop. Pteris vittata was grown for one or two growing cycles of four months each with two phosphate sources, using single super phosphate (SSP) and di-ammonium phosphate (DAP). Rice was grown on phytoextracted soils followed by measurements of biomass yield (grain, straw, and root), arsenic concentration and, uptake by individual plant parts. The biomass yield (grain, straw and rice) of rice was highest in soil phytoextracted with Pteris vittata grown for two cycles and fertilized with diammonium phosphate (DAP). Total arsenic uptake in contaminated soil ranged from 8.2 to 16.9 mg pot(-1) in first growing cycle and 5.5 to 12.0 mg pot(-1) in second growing cycle of Pteris vittata. There was thus a mean reduction of 52% in arsenic content of rice grain after two growing cycle of Pteris vittata and 29% after the one growing cycle. The phytoextraction of arsenic contaminated soil by Pteris vittata was beneficial for growing rice resulted in decreased arsenic content in rice grain of <1 ppm. There was a mean improvement in rice grain yield 14% after two growing cycle and 8% after the one growing cycle of brake fern.

Do radial oxygen loss and external aeration affect iron plaque formation and arsenic accumulation and speciation in rice?

Hydroponic experiments were conducted to investigate the effect of radial oxygen loss (ROL) and external aeration on iron (Fe) plaque formation, and arsenic (As) accumulation and speciation in rice (Oryza sativa L.). The data showed that there were significant correlations between ROL and Fe concentrations in Fe plaque produced on different genotypes of rice. There were also significant differences in the amounts of Fe plaque formed between different genotypes in different positions of roots and under different aeration conditions (aerated, normal, and stagnant treatments). In aerated treatments, rice tended to have a higher Fe plaque formation than in a stagnant solution, with the greatest formation at the root tip decreasing with increasing distances away, in accordance with a trend of spatial ROL. Genotypes with higher rates of ROL induced higher degrees of Fe plaque formation. Plaques sequestered As on rice roots, with arsenate almost double that with arsenite, leading to decreased As accumulation in both roots and shoots. The major As species detected in roots and shoots was arsenite, ranging from 34 to 78% of the total As in the different treatments and genotypes. These results contribute to our understanding of genotypic differences in As uptake by rice and the mechanisms causing rice genotypes with higher ROL to show lower overall As accumulation.

Partitioning of arsenic in soil-crop systems irrigated using groundwater: a case study of rice paddy soils in southwestern Taiwan

The accumulation of As in rice due to groundwater irrigation in paddy fields represents a serious health hazard in South and Southeast Asia. In Taiwan, the fate of As in long-term irrigated paddy fields is poorly understood. Groundwater, surface soil, and rice samples were collected from a paddy field that was irrigated with As-containing groundwater in southwestern Taiwan. The purpose of this study is to elucidate the source and sink of As in the paddy field by comparing the As fractions in the soils that were obtained by a sequential extraction procedure (SEP) with the As uptake of rice. The risks associated with eating rice from the field can thus be better understood. The concentration of As in groundwater varied with time throughout the growing seasons of rice, but always exceeded the permitted maximum (10 μg L(-1)) for drinking water by the WHO. The As concentration increased with the concentration of Fe in the groundwater, supporting the claim that a large amount of As was concentrated in the Fe flocs collected from the internal wall of the groundwater pump. The results of the SEP revealed that As bound with amorphous and crystalline hydrous oxides exhibited high availability in the soils. The root of rice accumulated the largest amount of As, followed by the straw, husk, and grain. Although the As concentration in the rice grain was less than 1.0 mg kg(-1), the estimated intake level was close to the maximum tolerable daily intake of As, as specified by the WHO.

Vincent AP, Roberts LC, Badruzzaman ABM, Ali MA, Harvey CF. Rice field geochemistry and hydrology: an explanation for why groundwater irrigated fields in Bangladesh are net sinks of arsenic from groundwater

Irrigation of rice fields in Bangladesh with arsenic-contaminated groundwater transfers tens of cubic kilometers of water and thousands of tons of arsenic from aquifers to rice fields each year. Here we combine observations of infiltration patterns with measurements of porewater chemical composition from our field site in Munshiganj Bangladesh to characterize the mobility of arsenic in soils beneath rice fields. We find that very little arsenic delivered by irrigation returns to the aquifer, and that recharging water mobilizes little, if any, arsenic from rice field subsoils. Arsenic from irrigation water is deposited on surface soils and sequestered along flow paths that pass through bunds, the raised soil boundaries around fields. Additionally, timing of flow into bunds limits the transport of biologically available organic carbon from rice fields into the subsurface where it could stimulate reduction processes that mobilize arsenic from soils and sediments. Together, these results explain why groundwater irrigated rice fields act as net sinks of arsenic from groundwater.

Arsenic dynamics in porewater of an intermittently irrigated paddy field in Bangladesh

In Bangladesh, irrigation of dry season rice (boro) with arsenic-contaminated groundwater is leading to increased As levels in soils and rice, and to concerns about As-induced yield reduction. Arsenic concentrations and speciation in soil porewater are strongly influenced by redox conditions, and thus by water management during rice growth. We studied the dynamics of As, Fe, P, Si, and other elements in porewater of a paddy field near Sreenagar (Munshiganj), irrigated according to local practice, in which flooding was intermittent. During early rice growth, As porewater concentrations reached up to 500 μg L(-1) and were dominated by As(III), but As release was constrained to the lower portion of the soil above the plow pan. In the later part of the season, soil conditions were oxic throughout the depth range relevant to rice roots and porewater concentrations only intermittently increased to ∼150 μg L(-1) As(V) following irrigation events. Our findings suggest that intermittent irrigation, currently advocated in Bangladesh for water-saving purposes, may be a promising means of reducing As input to paddy soils and rice plant exposure to As.

Arsenic in soil and irrigation water affects arsenic uptake by rice: complementary insights from field and pot studies

Groundwater rich in arsenic (As) is extensively used for dry season boro rice cultivation in Bangladesh, leading to long-term As accumulation in soils. This may result in increasing levels of As in rice straw and grain, and eventually, in decreasing rice yields due to As phytotoxicity. In this study, we investigated the As contents of rice straw and grain over three consecutive harvest seasons (2005-2007) in a paddy field in Munshiganj, Bangladesh, which exhibits a documented gradient in soil As caused by annual irrigation with As-rich groundwater since the early 1990s. The field data revealed that straw and grain As concentrations were elevated in the field and highest near the irrigation water inlet, where As concentrations in both soil and irrigation water were highest. Additionally, a pot experiment with soils and rice seeds from the field site was carried out in which soil and irrigation water As were varied in a full factorial design. The results suggested that both soil As accumulated in previous years and As freshly introduced with irrigation water influence As uptake during rice growth. At similar soil As contents, plants grown in pots exhibited similar grain and straw As contents as plants grown in the field. This suggested that the results from pot experiments performed at higher soil As levels can be used to assess the effect of continuing soil As accumulation on As content and yield of rice. On the basis of a recently published scenario of long-term As accumulation at the study site, we estimate that, under unchanged irrigation practice, average grain As concentrations will increase from currently ∼0.15 mg As kg(-1) to 0.25-0.58 mg As kg(-1) by the year 2050. This translates to a 1.5-3.8 times higher As intake by the local population via rice, possibly exceeding the provisional tolerable As intake value defined by FAO/WHO.

Comparison of drinking water, raw rice and cooking of rice as arsenic exposure routes in three contrasting areas of West Bengal, India

Remediation aimed at reducing human exposure to groundwater arsenic in West Bengal, one of the regions most impacted by this environmental hazard, are currently largely focussed on reducing arsenic in drinking water. Rice and cooking of rice, however, have also been identified as important or potentially important exposure routes. Quantifying the relative importance of these exposure routes is critically required to inform the prioritisation and selection of remediation strategies. The aim of our study, therefore, was to determine the relative contributions of drinking water, rice and cooking of rice to human exposure in three contrasting areas of West Bengal with different overall levels of exposure to arsenic, viz. high (Bhawangola-I Block, Murshidibad District), moderate (Chakdha Block, Nadia District) and low (Khejuri-I Block, Midnapur District). Arsenic exposure from water was highly variable, median exposures being 0.02 μg/kg/d (Midnapur), 0.77 μg/kg/d (Nadia) and 2.03 μg/kg/d (Murshidabad). In contrast arsenic exposure from cooked rice was relatively uniform, with median exposures being 0.30 μg/kg/d (Midnapur), 0.50 μg/kg/d (Nadia) and 0.84 μg/kg/d (Murshidabad). Cooking rice typically resulted in arsenic exposures of lower magnitude, indeed in Midnapur, median exposure from cooking was slightly negative. Water was the dominant route of exposure in Murshidabad, both water and rice were major exposure routes in Nadia, whereas rice was the dominant exposure route in Midnapur. Notwithstanding the differences in balance of exposure routes, median excess lifetime cancer risk for all the blocks were found to exceed the USEPA regulatory threshold target cancer risk level of 10(-4)-10(-6). The difference in balance of exposure routes indicate a difference in balance of remediation approaches in the three districts.

Arsenic accumulation in a paddy field in Bangladesh: seasonal dynamics and trends over a three-year monitoring period

Shallow groundwater, often rich in arsenic (As), is widely used for irrigation of dry season boro rice in Bangladesh. In the long term, this may lead to increasing As contents in rice paddy soils, which threatens rice yields, food quality, and human health. The objective of this study was to quantify gains and losses of soil As in a rice paddy field during irrigation and monsoon flooding over a three-year period. Samples were collected twice a year on a 3D-sampling grid to account for the spatially heterogeneous As distribution within the soil. Gains and losses of soil As in different depth segments were calculated using a mass-balance approach. Annual As input with irrigation water was estimated as 4.4 +/- 0.4 kg ha(-1) a(-1). Within the top 40 cm of soil, the mean As accumulation over three years amounted to 2.4 +/- 0.4 kg ha(-1) a(-1), implying that on average 2.0 kg ha(-1) a(-1) were lost from the soil. Seasonal changes of soil As showed that 1.05 to 2.1 kg ha(-1) a(-1) were lost during monsoon flooding. The remaining As-loss (up to 0.95 kg ha(-1) a(-1)) was attributed to downward flow with percolating irrigation water. Despite these losses, we estimate that total As within the top 40 cm of soil at our field site would further increase by a factor of 1.5 to 2 by the year 2050 under current cultivation practices.

Arsenic burden from cooked rice in the populations of arsenic affected and nonaffected areas and Kolkata City in West-Bengal, India

Arsenic contamination of rice irrigated with contaminated groundwater contributes to the additional arsenic burden of the population where rice is the staple food. In an arsenic contaminated area, an experimental field-based study done on nine fields elucidated significant positive correlation between arsenic in irrigation water and soil, irrigation water and rice, and also soil and rice both for Boro (groundwater) and Aman (rainwater) rice. Speciation studies showed that for both Boro (cooked) and Aman (raw) rice from contaminated area, 90% of total recovered arsenic was inorganic. In arsenic contaminated, uncontaminated villages, and Kolkata city, daily quantities of arsenic ingested by adult population from cooked rice diet are equivalent to 6.5, 1.8, and 2.3 L respectively, of drinking water containing WHO guideline value. In contaminated area, daily intake only from cooked Boro rice for 34.6% of the samples exceeded the WHO recommended MTDI value (2 microg In-As day(-1) kg(-1) body wt), whereas daily intake from Aman rice was below MTDI value as was rice from uncontaminated areas and Kolkata city. Our study indicated that employing traditional rice cooking method as followed in Bengal delta and using water having arsenic <3 microg L(-1) for cooking, actual exposure to arsenic from rice would be much less.

Enhanced transfer of arsenic to grain for Bangladesh grown rice compared to US and EU

A field survey was conducted in arsenic impacted and non-impacted paddies of Bangladesh to assess how arsenic levels in rice (Oryza sativa L.) grain are related to soil and shoot concentrations. Ten field sites from an arsenic contaminated tubewell irrigation region (Faridpur) were compared to 10 field sites from a non-affected region (Gazipur). Analysis of the overall data set found that both grain and shoot total arsenic concentrations were highly correlated (P<0.001) with soil arsenic. Median arsenic concentrations varied by 14, 10 and 3 fold for soil, shoot and grain respectively comparing the two regions. The reason for the sharp decline in the magnitude of difference between Gazipur and Faridpur for grain arsenic was due to an exponential decline in the grain/shoot arsenic concentration ratio with increasing shoot arsenic concentration. When the Bangladesh data were compared to EU and US soil-shoot-grain transfers, the same generic pattern could be found with the exception that arsenic was more efficiently transferred to grain from soil/shoot in the Bangladesh grown plants. This may reflect climatic or cultivar differences.

Accumulation of iron and arsenic in the Chandina alluvium of the lower delta plain, Southeastern Bangladesh

Accumulations of iron, manganese, and arsenic occur in the Chandina alluvium of southeastern Bangladesh within 2.5 m of the ground surface. These distinctive orange-brown horizons are subhorizontal and consistently occur within 1 m of the contact of the aerated (yellow-brown) and water-saturated (gray) sediment. Ferric oxyhydroxide precipitates that define the horizons form by oxidation of reduced iron in pore waters near the top of the saturated zone when exposed to air in the unsaturated sediment. Hydrous Fe-oxide has a high specific surface area and thus a high adsorption capacity that absorbs the bulk of arsenic also present in the reduced pore water, resulting in accumulations containing as much as 280 ppm arsenic. The steep redox gradient that characterizes the transition of saturated and unsaturated sediment also favors accumulation of manganese oxides in the oxidized sediment. Anomalous concentrations of phosphate and molybdenum also detected in the ferric oxyhydroxide-enriched sediment are attributed to sorption processes.

Baseline soil variation is a major factor in arsenic accumulation in Bengal Delta paddy rice

Factors responsible for paddy soil arsenic accumulation in the tubewell irrigated systems of the Bengal Delta were investigated. Baseline (i.e., nonirrigated) and paddy soils were collected from 30 field systems across Bangladesh. For each field, soil sampled at dry season (Boro) harvest i.e., the crop cycle irrigated with tubewell water, was collected along a 90 m transect away from the tubewell irrigation source. Baseline soil arsenic levels ranged from 0.8 to 21. mg/kg, with lower values found on the Pliestocene Terrace around Gazipur (average, 1.6 +/- 0.2 mg/kg), and higher levels found in Holecene sediment tracts of Jessore and Faridpur (average, 6.6 +/- 1.0 mg/kg). Two independent approaches were used to assess the extent of arsenic build-up in irrigated paddy soils. First, arsenic build-up in paddy soil at the end of dry season production (irrigated - baseline soil arsenic) was regressed against number of years irrigated and tubewell arsenic concentration. Years of irrigation was not significant (P = 0.711), indicating no year-on-year arsenic build-up, whereas tubewell As concentration was significant (P = 0.008). The second approach was analysis of irrigated soils for 20 fields over 2 successive years. For nine of the fields there was a significant (P < 0.05) decrease in soil arsenic from year 1 to 2, one field had a significant increase, whereas there was no change for the remaining 10. Over the dry season irrigation cycle, soil arsenic built-up in soils at a rate dependent on irrigation tubewell water, 35* (tubewell water concentration in mg/kg, triple bond mg/L). Grain arsenic rises steeply at low soil/shoot arsenic levels, plateauing out at concentratations. Baseline soil arsenic at Faridpur sites corresponded to grain arsenic levels at the start of this saturation phase. Therefore, variation in baseline levels of soil arsenic leads to a large range in grain arsenic. Where sites have high baseline soil arsenic, further additional arsenic from irrigation water only leads to a gradual increase in grain arsenic concentration.

Flocculant-disinfectant point-of-use water treatment for reducing arsenic exposure in rural Bangladesh

We introduced flocculant-disinfectant water treatment for 12 weeks in 103 households in Bangladesh to assess if drinking water would be chemically and microbiologically improved and the body burden of arsenic reduced. The median concentration of arsenic in tubewell water decreased by 88% after introduction of the flocculant-disinfectant from 136 microg/l at baseline to 16 (p < 0.001). The median concentration of total urinary arsenic decreased 42% from 385 microg/g creatinine at baseline to 225 microg/g creatinine after intervention (p < 0.001). Among 206 post-intervention drinking water samples that were reportedly treated on the date the sample was collected, 99 (48%) lacked residual free chlorine and 100 (49%) were contaminated with thermotolerant coliforms. The flocculant-disinfectant markedly reduced arsenic in drinking water, but treated drinking water was frequently contaminated with fecal organisms. The lesser reduction in urinary arsenic compared to water arsenic and the health consequences of this reduction require further research.

Spatial variability of arsenic concentration in soils and plants, and its relationship with iron, manganese and phosphorus

Spatial distribution of arsenic (As) concentrations of irrigation water, soil and plant (rice) in a shallow tube-well (STW) command area (8 ha), and their relationship with Fe, Mn and P were studied. Arsenic concentrations of water in the 110 m long irrigation channel clearly decreased with distance from the STW point, the range being 68-136 microg L(-1). Such decreasing trend was also noticed with Fe and P concentrations, but the trend for Mn concentrations was not remarkable. Concerning soil As, the concentration showed a decreasing tendency with distance from the pump. The NH(4)-oxalate extractable As contributed 36% of total As and this amount of As was associated with poorly crystalline Fe-oxides. Furthermore only 22% of total As was phosphate extractable so that most of the As was tightly retained by soil constituents and was not readily exchangeable by phosphate. Soil As (both total and extractable As) was significantly and positively correlated with rice grain As (0.296+/-0.063 microg g(-1), n=56). Next to drinking water, rice could be a potential source of As exposure of the people living in the As affected areas of Bangladesh.

Evaluation of the sustainability of deep groundwater as an arsenic-safe resource in the Bengal Basin

Tens of millions of people in the Bengal Basin region of Bangladesh and India drink groundwater containing unsafe concentrations of arsenic. This high-arsenic groundwater is produced from shallow (<100 m) depths by domestic and irrigation wells in the Bengal Basin aquifer system. The government of Bangladesh has begun to install wells to depths of >150 m where groundwater arsenic concentrations are nearly uniformly low, and many more wells are needed, however, the sustainability of deep, arsenic-safe groundwater has not been previously assessed. Deeper pumping could induce downward migration of dissolved arsenic, permanently destroying the deep resource. Here, it is shown, through quantitative, large-scale hydrogeologic analysis and simulation of the entire basin, that the deeper part of the aquifer system may provide a sustainable source of arsenic-safe water if its utilization is limited to domestic supply. Simulations provide two explanations for this result: deep domestic pumping only slightly perturbs the deep groundwater flow system, and substantial shallow pumping for irrigation forms a hydraulic barrier that protects deeper resources from shallow arsenic sources. Additional analysis indicates that this simple management approach could provide arsenic-safe drinking water to >90% of the arsenic-impacted region over a 1,000-year timescale. This insight may assist water-resources managers in alleviating one of the world's largest groundwater contamination problems.

Degradation rates of CFC-11, CFC-12 and CFC-113 in anoxic shallow aquifers of Araihazar, Bangladesh

Chlorofluorocarbons CFC-11 (CCl(3)F), CFC-12 (CCl(2)F(2)), and CFC-113 (CCl(2)F-CClF(2)) are used in hydrology as transient tracers under the assumption of conservative behavior in the unsaturated and saturated soil zones. However, laboratory and field studies have shown that these compounds are not stable under anaerobic conditions. To determine the degradation rates of CFCs in a tropical environment, atmospheric air, unsaturated zone soil gas, and anoxic groundwater samples were collected in Araihazar upazila, Bangladesh. Observed CFC concentrations in both soil gas and groundwater were significantly below those expected from atmospheric levels. The CFC deficits in the unsaturated zone can be explained by gas exchange with groundwater undersaturated in CFCs. The CFC deficits observed in (3)H/(3)He dated groundwater were used to estimate degradation rates in the saturated zone. The results show that CFCs are degraded to the point where practically no (<5%) CFC-11, CFC-12, or CFC-113 remains in groundwater with (3)H/(3)He ages above 10 yr. In groundwater sampled at our site CFC-11 and CFC-12 appear to degrade at similar rates with estimated degradation rates ranging from approximately 0.25 yr(-1) to approximately 6 yr(-1). Degradation rates increased as a function of reducing conditions. This indicates that CFC dating of groundwater in regions of humid tropical climate has to be carried out with great caution.

Changes in phosphate content and phosphatase activities in rice seedlings exposed to arsenite

The effect of arsenite (As2O3) in situ on the level of the phosphate pool and activities of phosphohydrolytic enzymes was examined in rice (Oryza sativa L.) seedlings grown for 5-20 d in sand cultures. The effects were manifested via a decline in phosphate content and inhibition of the activities of key phosphatases. Application of 50 µM As2O3 in situ resulted in 34 to 77% inhibition of acid phosphatase activity in roots and about 38 to 50% inhibition of activity in shoots of 15-20-d-old seedlings. Similarly, alkaline phosphatase activity was inhibited in shoots under in situ As (III) toxicity. Varietal as well as organ specific differences were observed in the response of inorganic pyrophosphatase activity to in situ As (III) treatment. A moderately toxic in situ As2O3 level of 25 µM as well as a highly toxic level of 50 µM inhibited mitochondrial-ATPase activity whereas 25 µM As (III) stimulated the chloroplastic isoform of ATPase but at a higher level (50 µM) As (III) was inhibitory. The results suggest that exposure of rice plants to arsenite leads to lowering of the phosphate pool and alteration in the activities of key phosphohydrolytic enzymes which might contribute to metabolic perturbations and decreased growth of rice plants in an As (III) polluted environment.