Extensive arsenic contamination in high-pH unconfined aquifers in the Indus Valley

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Science Advances  23 Aug 2017:
Vol. 3, no. 8, e1700935
DOI: 10.1126/sciadv.1700935

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  • Response to eLetter by Unaib Rabbani from 28 August 2017
    • Joel Edwin Podgorski, Project Coordinator, EAWAG Swiss Federal Institute of Aquatic Science and Technology
    • Other Contributors:
      • Syed Ali Musstjab Akber Eqani, Comsats University
      • Michael Berg, EAWAG Swiss Federal Institute of Aquatic Science and Technology

    Dear Science Advances editors

    We would like to respond to some of the specific points made in the letter of Unaib Rabbani, Ghulam Murtaza and Zafar Fatmi on 28 August 2017.

    Firstly, please note that our article does not assert that half of the wells in Pakistan contain arsenic concentrations exceeding 10 µg/l. However, the fact that about half of our dataset happens to have such arsenic concentrations is very useful for our logistic regression model, which can be optimally trained with an even mix of high and low values. It is the relationships found between the predictor variables of the model (i.e. fluvisols, Holocene fluvial sediments, soil organic carbon, soil pH and slope) and the measured arsenic concentrations that determine the model of areas prone to containing high arsenic concentrations. Regardless, our hazard and risk maps provide a large-scale approximation to the extent of arsenic contamination and are meant to be used as a guide for further sampling studies.

    Yes, our grid scale is one square kilometer, which is the coarsest resolution of the available predictor datasets. However, these predictor variables are spatially continuous, which allows us to make predictions at a one-kilometer resolution throughout the entire country, even for vast areas where we have no measurements. This is one of the fundamental advantages of our method to that of, for example, interpolation, which is dependent only upon distance from points of known concentra...

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    Competing Interests: None declared.
  • RE: Answer to the comments of E-letter (by Nabeel Niazi)

    Dear Editor,

    I am writing this letter with regards to the concerns by the local researcher (E-letter published in Science Advances on 29 August in response to our report). As a (second) author of the arsenic study recently published in Science Advances, we take strong exception to the many uninformed points and misleading statements made. While going through certain statements in the response to the report, I got the impression that they did not (yet) read the publication in detail or are lacking the updated knowledge about statistical modeling and its application in the field of hydrology. This could be one of the possible reasons why country wide dataset/hazard risk maps of arsenic contamination have not been generated previously. An undeclared conflict of interest and/or jealously with fellow researchers is also very common in developing countries. I have read a few misleading statements by a fellow researcher in the field (who has also already reported the severe arsenic contamination in fewer areas of Punjab). In any case, we would plead that anyone publicly commenting on our study make the effort to first fully understand it, so as to avoid unfounded, and sometimes illogical, criticism as contained in the recently published E-letter.

    Below we briefly respond to some of the incorrect statements made:

    • The study referred to in the E-letter recently sent to the editor of Science Advances, is conducted by a very good Pakistani research group working...

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    Competing Interests: None declared.
  • Response to eLetter by Nabeel Khan Niazi from 29 August 2017
    • Joel Edwin Podgorski, Project Coordinator, EAWAG Swiss Federal Institute of Aquatic Science and Technology
    • Other Contributors:
      • Michael Berg, Head of Department, EAWAG Swiss Federal Institute of Aquatic Science and Technology

    Dear Editor,

    We would like to respond to the eLetter by Nabeel Khan Niazi from 29 August 2017.

    We would first like to emphasize that the point of our modelling approach is to remove the need to densely sample all of a study area in order to obtain an approximation of the overall state of contamination. As opposed to interpolation/extrapolation, such as regression kriging or nearest neighbor spatial analysis that is based only on arsenic measurements, our modelling finds statistical relationships between arsenic concentrations and various geospatial parameters of geology, soil properties, climate and hydrology related to the natural biogeochemical process of arsenic accumulation in groundwater. The resulting arsenic hazard maps show areas where the probability of contamination is above a certain threshold (also see the Supplementary Information). However, these maps are unable to capture the fine heterogeneity of aquifer systems that can result in contaminated wells existing in close proximity to uncontaminated ones.

    The success of modeling is strongly dependent on the variability and resolution of the predictor variables, since a larger number of samples will not necessarily produce a better hazard map. Compared to our group’s previous hazard maps for China (Rodríguez-Lado et al., 2013), Southeast Asia (Winkel et al., 2008), the Red River Delta (Winkel et al., 2011), or even the world (Amini et al., 2008), the number of samples used for the Pakistan mo...

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    Competing Interests: None declared.
  • RE: Comment on the “Extensive arsenic contamination in high-pH unconfined aquifers in the Indus Valley”
    • Nabeel Khan Niazi, Assistant Professor/Research Scientist, University of Agriculture Faisalabad and University of Bremen

    Dear Editor,

    This is an interesting study which indicates about arsenic contamination issue in groundwater wells in different areas of Pakistan. Arsenic contamination of groundwater in Pakistan is an emerging issue and needs an attention to develop suitable management and remediation measures in arsenic-affected areas.

    However, this report, about prediction of arsenic-induced population risk (50 to 60 million people), appears to be too much ‘exaggerated and overestimated’.
    There are some critically-important points that authors should have considered before claiming that ’50 million to 60 million are at high risk of arsenic poisoning in the Indus Valley of Pakistan’.

    1. Firstly, it is not mentioned in paper which areas of Punjab and Sindh provinces in Pakistan were exactly targeted for groundwater sampling. This is crucial to know and indicate arsenic contamination extent and trend in groundwater from geologically different settings, and compare with previous studies in those areas. For example, arsenic in majority of groundwater wells from areas around central Punjab (e.g., Gujranwala, Hafizabad, Faisalabad (our own unpublished data) is well below the WHO safe limit (10 μg/L) of arsenic in drinking water – the problem mainly exists in southern Punjab (e.g., Vehari, Multan, Bahawalpur) (1), and near Lahore (2) with number of safe wells exist in the same area (3).
    2. A total of 1184 groundwater samples were taken in this study, of which 39...

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    Competing Interests: None declared.
  • RE: “Extensive arsenic contamination in high-pH unconfined aquifers in the Indus Valley”
    • Unaib Rabbani, Preventive Supervisor, Ministry of Health, Kingdom of Saudi Arabia
    • Other Contributors:
      • Ghulam Murtaza, Senior Research Officer, Pakistan Council for Research in Water Resources, Karachi, Pakistan
      • Zafar Fatmi, Professor & Head Research Group, Environmental, Occupational Health & Non-communicable Diseases, Department of Community Health Sciences, Aga Khan University, Karachi, Pakistan

    We read with interest the article published (online) on 23rd August, 2017 by Podgorski et al. in Science Advances (1). The author presented the extent of contamination and estimated population at risk due to arsenic contamination of underground water in Pakistan. Of 1184 water samples, 49.8% were ≥10 µg/L and estimated 50-60 million populations at risk of arsenic exposure in Pakistan.

    Arsenic contamination in underground water is a well-known public health risk along river Indus, which this article validates. However, the estimation of population at risk by the authors initiated serious debate. About half the sample found above ≥10 µg/L is a gross overestimate and does not corroborate with any of the small or large scale studies so far conducted in Pakistan. The national survey in 35 of 104 districts found about 9% of samples above 10 µg/L and about 0.7% above 50 µg/L in 2001 (2). Furthermore, the recent Multiple Indicator Cluster Survey of Sindh province in Pakistan, found only 3% of the water samples above 10 µg/L (3). The reason we believe is a sampling bias as most data points seems concentrated along river Indus. Authors described that data was collected from 1 km2 grids, taking one or two sample from each area, and labeled the whole area as contaminated if results were found higher than 10 µg/L. The geographical distribution of arsenic in underground water is very typical, that you find safe water well along contaminated wells in close distances (even with...

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    Competing Interests: None declared.