Depth Stratification Leads to Distinct Zones of Manganese and Arsenic Contaminated Groundwater

Abstract

Providing access to safe drinking water is a global challenge, for which groundwater is increasingly being used throughout the world. However, geogenic contaminants limit the suitability of groundwater for domestic purposes over large geographic areas across most continents. Geogenic contaminants in groundwater are often evaluated individually, but here we demonstrate the need to evaluate multiple contaminants to ensure that groundwater is safe for human consumption and agricultural usage. We compiled groundwater chemical data from three aquifer regions across the world that have been reported to have widespread As and Mn contamination including the Glacial Aquifer in the U.S., the Ganges-Brahmaputra-Mehta Basin within Bangladesh, and the Mekong Delta in Cambodia, along with newly sampled wells in the Yangtze River Basin of China. The proportion of contaminated wells increase by up to 40% in some cases when both As and Mn contaminants are considered. Wilcoxon rank-sum analysis indicates that Mn contamination consistently occurs at significantly shallower depths than As contaminated wells in all regions. Arsenic concentrations in groundwater are well predicted by redox indicators (Eh and dissolved oxygen) whereas Mn shows no significant relationship with either parameter. These findings illustrate that the number of safe wells may be drastically overestimated in some regions when Mn contamination is not taken into account and that depth may be used as a distinguishing variable in efforts to predict the presence of groundwater contaminants regionally.

Predicting geogenic Arsenic in Drinking Water Wells in Glacial Aquifers, North‐Central USA: Accounting for Depth‐Dependent Features

“…Well depth characteristics were also influential predictor variables for our study (CaseDepth, rank = 14; DepthComp, rank = 27, Figure ). Our findings are consistent with and augment those by others that well depth (Ayotte et al, ; Ying et al, ), and depth below the water table (Tesoriero et al, ) are proxies for important As mobilization mechanisms. In our study, deeper depths generally increased the likelihood of predicted elevated As.…”

“…Naturally occurring, geogenic arsenic (As) adversely affects drinking water quality of groundwater in geologically diverse aquifers in Europe, Africa, and North and South America, and particularly Southeast Asia (Amini et al, ; Bonsor et al, ; McArthur et al, ; Smedley & Kinniburgh, ; Wen et al, ). Within North America, naturally occurring As is widespread in groundwater throughout Late Quaternary glacial aquifers in the northern United States (Ayotte et al, ; Erickson, ; Root et al, ; Thomas, , ; Thomas et al, ; Warner, ; Warner & Ayotte, ; Welch et al, ; Ying et al, ). Groundwater supplies 50% of drinking water worldwide (Zekster & Everett, ) and 30% in the United States (U.S.) (U.S. Environmental Protection Agency, ), but compromised water quality from anthropogenic and natural contaminants can limit usage of groundwater as a drinking water source (Foster & Chilton, ; Nordstrom, ).…”

Si and Water Management Drives Changes in Fe and Mn Pools that Affect As Cycling and Uptake in Rice

“…Most at-risk populations are individuals with low body mass, those who consume high quantities of rice, and those who also ingest iAs via drinking water [7]. In an effort to devise approaches to minimize As entry into the human food chain, many studies have been conducted to understand how As is released from soils and sediments to water [8][9][10][11][12][13][14][15][16], where it can be taken up by rice roots. It has become clear that plant-availability of As in rice paddy agroecosystems is intimately tied to biogeochemical Fe and Mn cycling [17,18].…”

Arsenite Depletion by Manganese Oxides: A Case Study on the Limitations of Observed First Order Rate Constants

“…Exposure to As, a known carcinogen, causes pulmonary, epidermal, liver, and kidney problems [19][20][21][22]. The fate of As in the subsurface is controlled, in part, by Mn oxides and aqueous Mn(II), due to the redox chemistry of As [23][24][25]. The mobile As III oxidation state is more toxic than the relatively immobile As V state [26].…”

“…Concentrations of As and Mn in these studies were as high as 1340 µg/L [28] and 4000 µg/L [27], respectively. An aggregation of many studies shows that Mn contamination (>400 µg/L) typically occurs in more shallow groundwater than As contamination (>10 µg/L) [25]. Furthermore, As depletion from solution can be predicted by sediment Mn oxide concentration [33].…”