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Depth stratification leads to distinct zones of manganese and arsenic contaminated groundwater Samantha C. Ying, Michael V. Schaefer, Alicea Cock-Esteb, Jun Li, and Scott Fendorf Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01121 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 12, 2017
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Environmental Science & Technology
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Depth stratification leads to distinct zones of manganese and arsenic contaminated
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groundwater
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Samantha C. Ying*1, Michael V. Schaefer1, Alicea Cock-Esteb2, Jun Li3, and Scott Fendorf2
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CA 92521, USA;
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*Corresponding author:
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Samantha C. Ying
Environmental Sciences Dept., University of California-Riverside, 900 University Ave, Riverside,
Earth System Science Dept. Stanford University, 473 Via Ortega, Stanford, CA 94305, USA;
Statistics Dept., University of California-Riverside, 900 University Ave, Riverside, CA 92521, USA;
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Department of Environmental Science
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900 University Ave, 2460 Geology, Riverside, CA 92521
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951-827-4505 (phone), 951-827-4652 (fax)
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email:
[email protected] 14
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KEYWORDS: Arsenic, manganese, groundwater, redox processes
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Abstract
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Providing access to safe drinking water is a global challenge, for which groundwater is
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increasingly being used throughout the world. However, geogenic contaminants limit the
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suitability of groundwater for domestic purposes over large geographic areas across most
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continents. Geogenic contaminants in groundwater are often evaluated individually, but here
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we demonstrate the need to evaluate multiple contaminants to ensure that groundwater is safe
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for human consumption and agricultural usage. We compiled groundwater chemical data from
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three aquifer regions across the world that have been reported to have widespread As and Mn
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contamination including the Glacial Aquifer in the U.S., the Ganges-Brahmaputra-Mehta Basin
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within Bangladesh, and the Mekong Delta in Cambodia, along with newly sampled wells in the
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Yangtze River Basin of China. The proportion of contaminated wells increase by up to 40% in
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some cases when both As and Mn contaminants are considered. Wilcoxon rank-sum analysis
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indicates that Mn contamination consistently occurs at significantly shallower depths than As
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contaminated wells in all regions. Arsenic concentrations in groundwater are well predicted by
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redox indicators (Eh and dissolved oxygen) whereas Mn shows no significant relationship with
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either parameter. These findings illustrate that the number of safe wells may be drastically over-
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estimated in some regions when Mn contamination is not taken into account and that depth
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may be used as a distinguishing variable in efforts to predict the presence of groundwater
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contaminants regionally.
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Introduction
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The scale and extent of arsenic groundwater contamination has been studied across Asia,
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especially in Bangladesh, West Bengal India, Vietnam, and Cambodia.1,2 Sediments containing
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geogenic arsenic derived from the Himalayas are transported down the major river systems and
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deposited in the low-lying river floodplains.1 Additionally, high levels of manganese have been
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observed in many locations across South and Southeast Asia.3,4 Consumption of arsenic and
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manganese has increased with the rise in the number of tube wells used as a drinking water
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source in an effort to move away from surface waters laden with diarrheal disease causing
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pathogens.5 Long-term use of arsenic-contaminated water for rice irrigation is also of particular
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concern since it is a staple crop in much of Asia; further, rice yields have been shown to
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decrease with increased soil arsenic.6
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Arsenic is a known carcinogen that is a health concern for humans when present in
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drinking water at concentrations greater than the World Health Organization’s (WHO) guideline
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of 10 μg L-1 (ppb).7 Although a health-based suggested limit of 400 μg L-1 Mn previously set by
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the WHO was recently abolished,7 a reevaluation is necessary8,9 as many studies report the
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detrimental health effects measured in children exposed to manganese concentrations as low as
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100 μg L-1. 3,10–12 Higher mean values (up to 2300 μg L-1) have been found in groundwater in the
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United States, Canada, Sweden, Greece, China, Bangladesh, Cambodia, and Vietnam. 3,10,11,13–17
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Manganese metabolizes differently when ingested in water as opposed to food,
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and has detrimental neurotoxic health effects, including intellectual impairments, muscular
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weakness, and delay in reproductive maturation.3,11,15 Children and infants chronically exposed
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to manganese have neurobehavioral problems in addition to scoring lower on mathematical,
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language, and IQ tests.10,11 Considering these effects have been reported in numerous countries
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and at much lower levels than the reported guidelines, more attention needs to be focused on
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the extent and implications of manganese contamination.
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Though manganese contamination of groundwater has been reported in many regions,
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greater emphasis has been placed upon assessing arsenic exposure. The number of wells
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considered contaminated can be greatly underestimated if only arsenic contamination is taken
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into account. In this study, we utilize data from over 16,000 wells within four regions including
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Mekong River basin (Cambodia), glacial aquifer (USA), Ganges-Brahmaputra-Meghna (GBM)
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River basin (Bangladesh), and newly collected data from Yangtze River basin (China) to quantify
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the discrepancy in the total number of wells considered contaminated when only arsenic is
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taken into account as compared to considering the presence of both As and Mn contaminants
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within each region. We hypothesized that redox processes were responsible for the release of
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As and Mn into groundwater and that redox status of a well as indicated by Eh can be used to
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distinguish As and Mn contaminated wells. Although some studies have shown that spatial
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distribution of wells within geographically and geologically distinct areas produce Mn instead of
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As contaminated water,16 other physical variables have not been shown to make such a
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distinction. We reveal that although redox parameters are reliable indicators of As
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concentration, they are poorly correlated with Mn concentrations, contradicting our initial
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hypothesis. Instead, we demonstrate that relative well depth can be an indicator of the
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likelihood of finding Mn versus As contamination within a well in all four regions. The results of
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this study can contribute to improving mitigation strategies and policy development in
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determining best use of groundwater of different depths.
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Materials and Methods
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We compiled chemical and physical data from 16691 wells in four regions, including 271
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newly sampled wells in this study, to examine manganese and arsenic contamination of
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groundwater as a function of depth. Included in the study are data collected by the USGS
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Groundwater Resource Program for the U.S. glacial aquifer system (n = 1567),18 the DWQI data
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set collected by Resource Development International-Cambodia (RDIC) for the Mekong River
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basin of Cambodia (n = 11319),19 the National Hydrochemical Survey dataset for wells within the
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Brahmaputra-Ganges-Meghna (BGM) River basin collected by British Geological Survey/DPHE (n
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= 3534),20 and new chemical and physical data collected in this study from the Yangtze River
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basin, China (n = 271).
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Description of Study Areas. The study regions generally exhibit similar aquifer
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stratigraphy with a sandy layer overlain by a clayey layer, with the exception of the glacial
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aquifer which includes large expanses that have sandy layers at the surface.21 The Yangtze River
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basin is an alluvial plain composing a majority of central and southern Hubei Province, China,
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which features a humid subtropical monsoon climate. The majority of the Jianghan Plain is
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characterized as a low-lying, topographically flat basin formed from transformation of the
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Presinian metamorphosed basement (late Paleozoic era)22 with two major rivers, the Yangtze
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and Hanshui Rivers, traversing the region.23 Approximately 20 m of clay and silt overlays sandy
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deposits interbedded with clay layers.24 The Mekong River basin in Cambodia was formed
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through sediment deposition over the last 7.5 ka after the last Holocene maximum flooding
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event; a clay layer (~3-20 m thick) overlays the reduced sandy aquifer extending from 20 to
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>50m below surface. The Ganges-Brahmaputra-Meghna river basin is >1.7 million km2 and the
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basin stratigraphy is characterized by the following layers from the surface: a thin clayey layer
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(at 0-10 m), a thick Holocene sandy upper aquifer (10-100 m), a second clay layer (100-150 m), a
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sandy Pleistocene lower aquifer (>150 m).25 The glacial aquifer extends across 26 states,
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providing drinking water for 41 million people.21 The glacial aquifer system is composed of
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unconsolidated glacial deposits on bedrock north of the continental glaciation, ranging in
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thickness from
