Abstract
It is now well-established that not just drinking water, but irrigation water contaminated with arsenic (As) is an important source of human As exposure through water-soil-rice transfer. While drinking water As has a permissible, or guideline value, quantification of guideline values for soil and irrigation water is limited. Using published data from 26 field studies (not pot-based experiments) from Asia, each of which reported irrigation water, soil and rice grain As concentrations from the same site, this meta-analysis quantitatively evaluated the relationship between soil and irrigation water As concentrations and the As concentration in the rice grain. A generalized linear regression model revealed As in soil to be a stronger predictor of As in rice than As in irrigation water (beta of 16.72 and 0.6, respectively, p < 0.01). Based on the better performing decision tree model, using soil and irrigation water As as independent variables we determined that Asian paddy soil As concentrations greater than 14 mg kg−1 may result in rice grains exceeding the Codex recommended maximum allowable inorganic As (i-As) concentrations of 0.2 mg kg−1 for polished rice and 0.35 mg kg−1 for husked rice. Both logistic regression and decision tree models, identified soil As as the main determining factor and irrigation water to be a non-significant factor, preventing determination of any guideline value for irrigation water. The seemingly non-significant contribution of irrigation water in predicting grain i-As concentrations below or above the Codex recommendation may be due to the complexity in the relationship between irrigation water As and rice grains. Despite modeling limitations and heterogeneity in meta-data, our findings can inform the maximum permissible As concentrations in Asian paddy soil.
Highlights
Arsenic (As) is a toxic, carcinogenic (Cohen et al, 2013) metalloid that occurs naturally in terrestrial and aquatic environments
We systematically reviewed published articles reporting As concentrations in paddy soil, irrigation water, and rice grains cultivated in Asian countries
Studies were only included in subsequent meta-analysis if (1) the research was carried out in the field and not as pot experiments in the laboratory; (2) it was undertaken in Asian countries; (3) the As concentration data presented included total arsenic of soil, rice grain, and irrigation water from the same study location; (4) the analysis of As was carried out using appropriate laboratory instruments rather than Field Testing Kits; and (5) details of the analytical method(s) and quality assurance procedures used for the study were provided
Summary
Arsenic (As) is a toxic, carcinogenic (Cohen et al, 2013) metalloid that occurs naturally in terrestrial and aquatic environments. Contaminated irrigation water contributes to As exposure by enhancing As concentrations in food crops (Mandal et al, 2019; Bhattacharyya et al, 2021), no WHO or international guideline value for irrigation water has been established to date. Rice plants are major accumulators of As compared to other cereal crops (Williams et al, 2007) and irrigation of a paddy field with As contaminated water elevates As concentrations in paddy soil (Meharg and Rahman, 2003), rice straw, and grain (Panaullah et al, 2008). In Asia, rice is the basic staple food for the majority of the population, including the region’s 560 million poor (GRiSP Global Rice Science Partnership, 2013). During 2018–19, rice consumption in China was to the extent of 146.7 million tons, followed by India at 102 million tons (ICAR-NRRI Annual Report 2020). In Bangladesh, 2.4 million out of 4 million hectares of paddy field have been found to be As contaminated (Akinbile and Haque, 2012)
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