ARTICLE pubs.acs.org/est
Testing Tubewell Platform Color as a Rapid Screening Tool for Arsenic and Manganese in Drinking Water Wells Ashis Biswas,*,†,‡ Bibhash Nath,§ Prosun Bhattacharya,† Dipti Halder,†,‡ Amit K. Kundu,‡ Ujjal Mandal,‡ Abhijit Mukherjee,|| Debashis Chatterjee,‡ and Gunnar Jacks† †
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KTH-International Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, Royal Institute of Technology, Teknikringen 76, SE-100 44 Stockholm, Sweden ‡ Department of Chemistry, University of Kalyani, Kalyani 741 235, West Bengal, India § School of Geosciences, The University of Sydney, Sydney, NSW 2006, Australia Department of Geology and Geophysics, Indian Institute of Technology-Kharagpur, Kharagpur-721 302, West Bengal, India
bS Supporting Information ABSTRACT: A low-cost rapid screening tool for arsenic (As) and manganese (Mn) in groundwater is urgently needed to formulate mitigation policies for sustainable drinking water supply. This study attempts to make statistical comparison between tubewell (TW) platform color and the level of As and Mn concentration in groundwater extracted from the respective TW (n = 423), to validate platform color as a screening tool for As and Mn in groundwater. The result shows that a black colored platform with 73% certainty indicates that well water is safe from As, while with 84% certainty a red colored platform indicates that well water is enriched with As, compared to WHO drinking water guideline of 10 μg/L. With this guideline the efficiency, sensitivity, and specificity of the tool are 79%, 77%, and 81%, respectively. However, the certainty values become 93% and 38%, respectively, for black and red colored platforms at 50 μg/L, the drinking water standards for India and Bangladesh. The respective efficiency, sensitivity, and specificity are 65%, 85%, and 59%. Similarly for Mn, black and red colored platform with 78% and 64% certainty, respectively, indicates that well water is either enriched or free from Mn at the Indian national drinking water standard of 300 μg/L. With this guideline the efficiency, sensitivity, and specificity of the tool are 71%, 67%, and 76%, respectively. Thus, this study demonstrates that TW platform color can be potentially used as an initial screening tool for identifying TWs with elevated dissolved As and Mn, to make further rigorous groundwater testing more intensive and implement mitigation options for safe drinking water supplies.
’ INTRODUCTION The natural contamination of groundwater by geogenic arsenic (As) has been recognized as a crucial water quality problem around the globe.1 The problem is most severe in southeast Asia, particularly in West Bengal (Eastern India) and Bangladesh which form the integral part of the Bengal Basin.2,3 Over the past few decades, millions of hand-pumped tubewells (TWs) have been installed as an initiative of both local households and governments to shift drinking water sources from surface water to groundwater to avoid waterborne diseases such as diarrhea and cholera.4 Moreover, due to easy accessibility, the groundwater has since become a preferred drinking water source. Most of these TWs are installed in the shallow aquifers with depth ranging between 10 and 50 m, where the concentrations of dissolve As have been reported to be the highest.5 Consequently about 60 million people, living in the Bengal Basin, are estimated to be exposed with As in drinking water above the World Health Organization (WHO) drinking water guideline of 10 μg/L.6 8 r 2011 American Chemical Society
The distribution of As in groundwater is highly heterogeneous. The concentration varies widely, both vertically and spatially, within a scale of a few meters.9 11 Moreover the concentration of As in groundwater shows temporal variability12,13 and massive groundwater extraction for irrigation and drinking purposes may cross-contaminate a currently safe aquifer.14,15 Thus from a public health perspective, it is not acceptable to assess health risk regionally by sporadic testing of TWs for As. Recently, elevated manganese (Mn) in drinking water has also been identified as an underlying threat to the human population worldwide.16 20 Prolonged consumption of drinking water containing high Mn may cause neurotoxic effects, such as diminished intellectual function in children.21 Though the relative health risk of Mn exposure is comparatively lower than that of As Received: September 1, 2011 Accepted: November 13, 2011 Revised: November 9, 2011 Published: November 14, 2011 434
dx.doi.org/10.1021/es203058a | Environ. Sci. Technol. 2012, 46, 434–440
Environmental Science & Technology
ARTICLE
Figure 1. Map of the study area in and around Chakdaha Block, in Nadia district in West Bengal, India with the location of sampled tubewell sites. Hooghly River and Chakdaha rail station (RS) are marked. The satellite image of the study site was acquired from Google Earth 6.0.2.
of manganese oxides, produce As free water (10 μg/L). The formation of particular colors on the TW platform and its relation to As, Fe, and Mn concentration in well water is governed by interplay of various biogeochemical interactions. In the aquifer, during progressive lowering of redox status associated with microbially mediated oxidation of organic matter, Mn oxyhydroxides (being a stronger terminal electron acceptor) are reduced before Fe oxy-hydroxides,30 releasing both As and Mn in groundwater. However As released from Mn oxy-hydroxides reduction is readily readsorbed onto Fe oxy-hydroxides in the aquifer sediment.31 When the redox status of the aquifer reaches the stage of Fe reduction, As is mobilized into groundwater together with Fe,32 34 which in contact with atmospheric oxygen gives red coloration to the well platform. However in the aquifer, Mn is often precipitated as secondary mineral phases such as rhodochrosite (MnCO3) in presence of high bicarbonate (produced from oxidation of organic matter),35,36 and/or complexed with dissolved phosphate as MnHPO4.18 When lowering of the redox status in the aquifer is limited to the Mn reduction stage only, groundwater becomes enriched with Mn, and this gives black coloration, in contact with atmospheric oxygen to the well platform. Therefore, it is expected that groundwater enriched with high As possibly would be low in Mn and vice versa. A few recent studies have also reported contrasting relation of As and Mn in groundwater from various parts of southeast Asia.16,34,37,38 Thus color of the stain developed on the TW platform may also guide to screen Mn in groundwater.
exposure,22 it is indeed necessary to monitor As and Mn in all the TWs of a region in a regular basis to assess extent and distribution in groundwater and identify the safe aquifer(s) containing low As and Mn water, as part of the sustainable drinking water management.23 Monitoring As and Mn in all the TWs of a region is a challenging task from the point of view of technology, manpower, time, social acceptance, and economy for countries like India and Bangladesh, which are mostly affected. It is almost impossible to test all the TWs using standard analytical methods. Hence, van Geen et al. (ref 24) and Jakariya et al. (ref 25) advocated for the use of field test kits for mass screening of As in TW water. A number of commercially available field test kits, e.g., Merck, Hach EZ, Quick arsenic, Wagtech Digital Arsenator (WFTK), and Chem-In Corp field test kit (CFTK), have been deployed at a mass scale and have proven their worth for As screening.24 27 However most of these field test kits are based on the principle of generating arsine gas, which is very harmful even at a trace level and might cause health hazards to the analyst.28 Moreover, the minimum time required for As measurement in a single sample by these kits is 20 min,27 which is not very fast, and, further, people living in the rural villages have very low access to these test kits. For Mn, a Hach field test kit (model MN-5) is available in the market, but its reliability has not been properly examined yet. Recently, McArthur et al. (ref 29) have proposed that the color of stain developed on the TW platform, where available, may guide to screen for As in TW installed in the shallow aquifer (10 μg/L of As in the well water.
In this study we have assessed the TW platform color as a tool for mass screening of As and Mn in well waters to validate and recommend for its wider application. For As, the tool is validated with respect to both the WHO drinking water guideline of 10 μg/L and the national drinking water standard of India and Bangladesh (50 μg/L), as they comprise the major part of the As affected region. Interestingly, at present, there is no WHO recommended drinking water guideline for Mn.39 Therefore the tool is validated with respect to the Indian national standard (300 μg/L) only.
’ MATERIALS AND METHODS Sampling of groundwater from TWs installed in shallow aquifer (
