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Amending Irrigation Channels with Jute-mesh Structures to Decrease Arsenic Loading to Rice Fields in Bangladesh

Matthew L. Polizzotto*
Soil Science, North Carolina State University, Raleigh, NC, USA

François Birgand
Biological and Agricultural Engineering, North Carolina State University, Raleigh, NC, USA

A. Borhan M. Badruzzaman
Civil Engineering, Bangladesh University of Engineering & Technology, Dhaka, Bangladesh

M. Ashraf Ali
Civil Engineering, Bangladesh University of Engineering & Technology, Dhaka, Bangladesh

Abstract/ Executive Summary:

Extensive use of arsenic-contaminated well water for irrigation of rice fields in Bangladesh has led to elevated arsenic concentrations in rice plants, decreased rice yields, and increased human exposure to arsenic. The goal of this study was to investigate whether arsenic removal from irrigation water could be improved within distribution channels by amending them with physical structures that both induce water treatment and maintain water-conveyance capacities. Chemical and hydraulic effects of amending channels with jute-mesh structures were characterized within 27 m-long experimental channels at a Bangladesh field site. Removal of total arsenic, iron and phosphorus from solution was enhanced within amended channels over unamended channels, with 7% of total As removed in amended channels vs. 3% in unamended channels. Increased elemental removal in amended channels was largely due to increases in residence time and particle-trapping efficiency, but removal via oxidative particle formation did not appear to be substantially enhanced. Results suggest that in-channel structures could be a useful tool for decreasing arsenic loading to rice fields, particularly where constrained channel spatial geometries limit the ability to overcome hydrogeochemical thresholds for enhanced arsenic removal. To improve the practical utility of structure-amended channels, future work could optimize structure designs and establish the season-long sustainability of enhanced arsenic-removal strategies.

Keywords: Arsenic, Irrigation, Rice, Bangladesh, Channels, Mitigation

Location: In Bangladesh

Start Date:

End Date:

Program Area: Risk Management in Agriculture

Commodity: Arsenic, Jute, Water Arsenic

Objectives:

We hypothesized that in-channel physical structures, preferably constructed from local materials, could be utilized to improve As removal from flowing irrigation water by (1) increasing As contact with soil and structure substrates; (2) increasing inchannel water paths, local flow velocities around the structures, and reaeration processes to enhance oxidative (co-) precipitation of As and Fe; (3) decreasing local flow velocities within structures to induce settling/trapping of suspended As-bearing particles; and (4) increasing water residence times and dispersion to enhance reaction time. Accordingly, the primary goal of this study was to investigate whether As removal from irrigation water would increase in structure-amended distribution channels.

Methodology:

2.1. Experimental design: Our well-characterized field site is located 30 km south of Dhaka in Bangladesh. Experimental irrigation channels were constructed within a bare ground field in December 2012, using techniques described previously. Channels were located 73 m downstream of the irrigation well, were 27 m in length, and were 1.35 m in width, roughly 3 the width of local channels. Once constructed, one set of channels was left unaltered (“base” channels), whereas a final channel was amended with a series of 39 tent-like jute-mesh structures (“jute” channels), made from locally available and inexpensive materials. Structure frames (averaging 1 m long and 19 cm high) were built from thin (2 cm diameter) bamboo sticks, and 1 cm-mesh jute fabric was draped across the top with ends driven into the channel soils. Structures were installed in a staggered fashion, oriented perpendicular to the primary flow direction. The average distance between jute structures was 66 cm. Channel experiments were run in triplicate, based on an approach described previously. Three floats were used to define sample collection times downstream of the channel inlet so that specific sample times were aligned with bulk flow rates. Filtered (0.2mm) and unfiltered water samples were collected, immediately acidified with concentrated nitric acid, and stored and transported on ice (max. 4 C). Turbidity, conductivity, pH, ORP and dissolved oxygen were measured in situ on 1- and 15 s bases using Hanna and YSI multiparameter probes. Water stages were recorded every minute using custom-built time-lapse cameras aimed at staff gauges installed at the up- and downstream ends of the channels (Supplementary Fig. S4). Average channel effective velocities at steady flow were measured using floats and calculated from the lag between induced turbidity plumes and tracer chemographs. Flow rates were computed from stage and velocity measurements. Arsenic, Fe, and P concentrations from water samples were measured by inductively coupled plasma-mass spectrometry and inductively coupled plasma-optical emission spectroscopy, with methods provided previously.

2.2. Tracer test To quantify differences in residence time and dispersion between channels, a 200 g/L solution of NaCl was added as a tracer with channels running at 10.4 L/s. Tracer was added at constant rates (1943 ml/min and 1630 ml/min for jute and base, respectively) for 4.23 and 4.60 min (jute and base) using a Mariotte jar at the upstream ends of the channels. The arrival and shape of the tracer plumes were measured using conductivity probes at the mid-channel and downstream points.

2.3. Removal-rate calculations Outflow concentrations are plotted as fractions of inflow concentrations. Material removal rates within channels were calculated from differences between steady-state in- and outflow concentrations. Synthetic chemographs were created by linearly interpolating concentrations between consecutive samples, and time-weighted concentrations and removal rates were calculated for the steady-state portions of flow (5 min). Removal is reported as: (1) the average percentage difference between inflow- and outflow-weighted concentrations; and (2) the average normalized mass of material removed per channel bottom surface area per unit time (mg/m² /min), a unit common for reporting contaminant dissipation in streams. Percentage values describe removal for particular channel conditions, whereas normalized rates can theoretically be compared across channels. Significance of differences between values was established for p 9617;0.05 in paired t- tests

Source of Information: Ecological Engineering Volume 74, January 2015, Pages 101-106

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Overall, our results highlight the promise and principles of low-cost additions to irrigation channels that may help mitigate As loading to irrigated rice fields of Bangladesh. Ultimately, the utility of in-channel structures will depend on their effectiveness over time as well as material disposal needs and space availability. Although total As, Fe, and P removal were elevated in the jute-amended channels over the base channels, the fate of these elements requires quantification. Dissolved species and particulate forms may have bound to channel walls (due to enhanced contact through sinuous bulk water flow) or to the jute-mesh material (since particle accumulation on the mesh was visually observed), and additional particles may have settled in low-velocity waters under the structures. Accordingly, to evaluate the sustainability of designed in-channel structures, As-removal processes and locations should be resolved throughout the course of a growing season so that strategic material harvesting and structure maintenance may be used to effectively limit As desorption, resuspension, or adverse accumulation during subsequent irrigation events.

Knowledge Management: Journal