Short communicationAmending irrigation channels with jute-mesh structures to decrease arsenic loading to rice fields in Bangladesh
Introduction
Around the globe, millions of people are exposed to dangerous levels of arsenic (As) through rice consumption, potentially leading to a myriad of health problems (Brammer, 2009, Consumer Reports, 2012, Duxbury et al., 2003, Meharg et al., 2009, Smedley and Kinniburgh, 2002, Stone, 2008). This issue is of particular concern in Southern Asia, where naturally As-contaminated well water has been used for irrigation of dry-season rice for the past 40 years (Brammer, 2009, Brammer and Ravenscroft, 2009, Hossain et al., 2003). Arsenic loading to fields has led to reductions in rice yields and elevated concentrations within rice grains, creating a pathway for As exposure through the staple food crop (Abedin et al., 2002, Liao et al., 2010, Meharg and Rahman, 2003, Meharg et al., 2009, Stroud et al., 2011, Williams et al., 2006). Practical, low-cost methods for large-scale mitigation of As are limited by the volumes of irrigation water required for rice production (i.e., ∼1 m/y of water) and the lack of alternate dry-season water sources (Brammer, 2009, Brammer and Ravenscroft, 2009).
In Southern Asia, As-laden irrigation water is generally pumped from wells, conveyed through soil-based distribution channels, and applied to fields. Dissolved As concentrations have been observed to decrease along the lengths of the channels (Hossain et al., 2008, Lineberger et al., 2013, Roberts et al., 2007), suggesting that despite short residence times (>10 min), high As-removal kinetics enable channels to be a potential location for As mitigation (Brammer, 2009). High As removal (>75%) from surface water has been previously reported in constructed wetlands, but such treatments have relied on hour-to-day residence times (Schwindaman et al., 2014).
The efficiency of contaminant removal from flowing-water systems can be related to a number of major factors, including: (1) kinetics of relevant biogeochemical reactions; (2) contact between water-column contaminants and a reactive soil substrate; (3) trapping of contaminant-bearing particles; (4) water residence times; and (5) hydraulic loads (Birgand et al., 2007). In Southern Asian irrigation water, specific flow properties may cause these factors to oppose one another, limiting As-removal efficiencies within channels. Increased flow velocities and induced turbulence may promote reaeration of water and soil-water contact, thereby favoring oxidative removal of dissolved Fe and As from solution and As sorption to channel soils–the two processes thought to govern in-channel As removal (Lineberger et al., 2013, Nemade et al., 2009). In contrast, increased velocities and hydraulic loads may also decrease As-bearing-particle settling capacities and water residence times, allowing for As transport through the channels.
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 in-channel 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. Here, we compare flow dynamics and transport of As (plus Fe and P, elements known to influence As) in irrigation water within unamended and jute-mesh-structure-amended channels in Bangladesh. Results suggest that in-channel structures could be useful tools for helping to decrease As loading to rice fields, but future work should first optimize removal rates and determine the season-long sustainability of such engineered systems.
Section snippets
Experimental design
Our well-characterized field site is located ∼30 km south of Dhaka in Bangladesh (Supplementary Fig. S1). Experimental irrigation channels were constructed within a bareground field in December 2012, using techniques described previously (Lineberger et al., 2013). 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 (Supplementary Fig. S2). Once constructed, one set of channels was left unaltered
Flow conditions in base and jute-amended channels
Amending the channels with jute structures led to an overall increase in channel residence time, with faster channel flow around the structures and slower flow within and through the structures (Supplementary Fig. S3). Jute increased the theoretical flow path around the structures from 27 to 66 m, a 2.44-fold increase. With the same overall channel slope, the increased flow path induced higher stage at the upstream end of the jute channel from 6 to 16 cm, a 2.67-fold increase. The jute structures
Acknowledgements
We thank Mehedi Hasan Tarek, Anisur Rahman, Sojib Chowdhury, Ethan Lineberger and residents of Bashailbhog village for field assistance, Kim Hutchison for analytical support, and Dean Hesterberg for helpful discussions. This work was supported by North Carolina State University and facilitated by the Bangladesh University of Engineering and Technology.
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