Sealing rice field boundaries in Bangladesh: a pilot study demonstrating reductions in water use, arsenic loading to field soils, and methane emissions from irrigation water.

Irrigation of rice fields in Bangladesh with arsenic-contaminated and methane-rich groundwater loads arsenic into field soils and releases methane into the atmosphere. We tested the water-savings potential of sealing field bunds (raised boundaries around field edges) as a way to mitigate these negative outcomes. We found that, on average, bund sealing reduced seasonal water use by 52 ± 17% and decreased arsenic loading to field soils by 15 ± 4%; greater savings in both water use and arsenic loading were achieved in fields with larger perimeter-to-area ratios (i.e., smaller fields). Our study is the first to quantify emission of methane from irrigation water in Bangladesh, a currently unaccounted-for methane source. Irrigation water applied to unsealed fields at our site emits 18 to 31 g of methane per square-meter of field area per season, potentially doubling the atmospheric input of methane from rice cultivation. Bund sealing reduced the emission of methane from irrigation water by 4 to 19 g/m(2). While the studied outcomes of bund sealing are positive and compelling, widespread implementation of the technique should consider other factors, such as effect on yields, financial costs, and impact on the hydrologic system. We provide an initial and preliminary assessment of these implementation factors.

Arsenic removal from flowing irrigation water in bangladesh: impacts of channel properties.

Across Bangladesh, dry-season irrigation with arsenic-contaminated well water is loading arsenic onto rice paddies, leading to increased arsenic concentrations in plants, diminished crop yields, and increased human health risks. As irrigation water flows through conveyance channels between wells and rice fields, arsenic concentrations change over space and time, indicating that channels may provide a location for removing arsenic from solution. However, few studies have systematically evaluated the processes controlling arsenic concentrations in irrigation channels, limiting the ability to manipulate these systems and enhance arsenic removal from solution. The central goal of this study was to quantify how channel design affected removal of dissolved arsenic from flowing irrigation water. Field experiments were conducted in Bangladesh using a chemically constant source of arsenic-contaminated irrigation water and an array of constructed channels with varying geometries. The resulting hydraulic conditions affected the quantity of arsenic removed from solution within the channels by promoting known hydrogeochemical processes. Channels three times the width of control channels removed ∼3 times the mass of arsenic over 32 min of flowing conditions, whereas negligible arsenic removal was observed in tarp-lined channels, which prevented soil-water contact. Arsenic removal from solution was ∼7 times higher in a winding, 200-m-long channel than in the straight, 45-m-long control channels. Arsenic concentrations were governed by oxidative iron-arsenic coprecipitation within the water column, sorption to soils, and phosphate competition. Collectively, these results suggest that better design and management of irrigation channels may play a part in arsenic mitigation strategies for rice fields in Southern Asia.

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.

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.

Spatial distribution and temporal variability of arsenic in irrigated rice fields in Bangladesh. 1. Irrigation water.

Around 38% of the area of Bangladesh is irrigated with groundwater to grow dry season crops, most importantly boro rice. Due to high As concentrations in many groundwaters, over 1000 tons of As are thus transferred to arable soils each year, creating a potential risk for future food production. We studied the reactions and changing speciation of As, Fe, P, and other elements in initially anoxic water during and after irrigation and the resulting spatial distribution of As input to paddy soils near Sreenagar (Munshiganj), 30 km south of Dhaka, in January and April 2005 and February 2006. The irrigation water had a constant concentration of 397 +/- 7 microg L(-1) As (approximately 84% As(III)), 11 +/- 0.1 mg L(-1) Fe, and 2 +/- 0.1 mg L(-1) P. During the fast flow along the longest irrigation channel (152 m) As, Fe, and P speciation changed, but total concentrations did not decrease significantly, indicating that As input to fields was independent of the length of the irrigation channels. In contrast, during slow water flow across the fields, As, Fe, and P concentrations decreased strongly with increasing distance from the water inlet, due to formation and settling of As- and P-bearing Fe aggregates and by adsorption to soil minerals. Total As concentrations in field water were approximately 3 times higher close to the inlet than in the opposite field corner shortly after irrigation, and decreased to below 35 microg L(-1) over the next 72 h. The laterally heterogeneous transfer of As, Fe, and P from irrigation waterto soil has important consequences for their distribution in irrigated fields and needs to be considered in sampling and in assessing the dynamics and mass balances of As fluxes among irrigation water, soil, and floodwater.

Spatial distribution and temporal variability of arsenic in irrigated rice fields in Bangladesh. 2. Paddy soil.

Arsenic-rich groundwater from shallow tube wells is widely used for the irrigation of boro rice in Bangladesh and West Bengal. In the long term this may lead to the accumulation of As in paddy soils and potentially have adverse effects on rice yield and quality. In the companion article in this issue, we have shown that As input into paddy fields with irrigation water is laterally heterogeneous. To assess the potential for As accumulation in soil, we investigated the lateral and vertical distribution of As in rice field soils near Sreenagar (Munshiganj, Bangladesh) and its changes over a 1 year cycle of irrigation and monsoon flooding. At the study site, 18 paddy fields are irrigated with water from a shallow tube well containing 397 +/- 7 microg L(-1) As. The analysis of soil samples collected before irrigation in December 2004 showed that soil As concentrations in paddy fields did not depend on the length of the irrigation channel between well and field inlet. Within individual fields, however, soil As contents decreased with increasing distance to the water inlet, leading to highly variable topsoil As contents (11-35 mg kg(-1), 0-10 cm). Soil As contents after irrigation (May 2005) showed that most As input occurred close to the water inlet and that most As was retained in the top few centimeters of soil. After monsoon flooding (December 2005), topsoil As contents were again close to levels measured before irrigation. Thus, As input during irrigation was at least partly counteracted by As mobilization during monsoon flooding. However, the persisting lateral As distribution suggests net arsenic accumulation over the past 15 years. More pronounced As accumulation may occur in regions with several rice crops per year, less intense monsoon flooding, or different irrigation schemes. The high lateral and vertical heterogeneity of soil As contents must be taken into account in future studies related to As accumulation in paddy soils and potential As transfer into rice.

Dynamics of arsenic in agricultural soils irrigated with arsenic contaminated groundwater in Bangladesh.

Arsenic (As) concentrations in the soil layers of 12 rice fields located in four As affected areas and two unaffected areas in Bangladesh were monitored during 2003. In the unaffected areas, where irrigation water contained little As (<1 microg/L), As concentrations of rice field soils ranged from 1.5 to 3.0 mg/kg and did not vary significantly with either depth or sampling time throughout the irrigation period. In the As affected areas where the irrigation water contained elevated As (79 to 436 microg/L), As concentrations of rice field soils were much higher compared to those in the unaffected areas and varied significantly with both depth and sampling time. For the top 0 to 150 mm of the soil, the As concentration increased significantly at the end of the irrigation season (May-June 2003). About 71% of the As that is applied to the rice field with irrigation water accumulates in the top 0 to 75 mm soil layer by the end of the irrigation season. After the wet season during which the rice fields were inundated with flood/rain water, the As concentrations in the soil layer decreased significantly and were reduced to levels comparable to those found in soil samples collected at the beginning of the irrigation period. The long-term As accumulation in agricultural soil appears to be counteracted by biogeochemical pathways leading to As removal from soil.

Arsenic mobility and groundwater extraction in Bangladesh.

High levels of arsenic in well water are causing widespread poisoning in Bangladesh. In a typical aquifer in southern Bangladesh, chemical data imply that arsenic mobilization is associated with recent inflow of carbon. High concentrations of radiocarbon-young methane indicate that young carbon has driven recent biogeochemical processes, and irrigation pumping is sufficient to have drawn water to the depth where dissolved arsenic is at a maximum. The results of field injection of molasses, nitrate, and low-arsenic water show that organic carbon or its degradation products may quickly mobilize arsenic, oxidants may lower arsenic concentrations, and sorption of arsenic is limited by saturation of aquifer materials.

Hepatitis C virus detection and genotyping in liver tissues and sera of Saudi patients using PCR and a line probe assay.

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