Publications by authors named "Mason Stahl"

Groundwater hydrographs contain a rich set of information on the dynamics of aquifer systems and the processes and properties that influence them. While the importance of seasonal cycles in hydrologic and environmental state variables is widely recognized there has yet to be a comprehensive analysis of the seasonal dynamics of groundwater across the United States. Here we use time series of groundwater level measurements from 997 wells from the National Groundwater Monitoring Network to identify and describe groundwater seasonal cycles in unconfined aquifers across the United States.

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Chronic exposure to groundwater contaminated with geogenic arsenic (As) poses a significant threat to human health worldwide, especially for those living on floodplains in South and Southeast (S-SE) Asia. In the alluvial and deltaic aquifers of S-SE Asia, aqueous As concentrations vary sharply over small spatial scales (10-100 m), making it challenging to identify where As contamination is present and mitigate exposure. Improved mechanistic understanding of the factors that control groundwater As levels is essential to develop models that accurately predict spatially variable groundwater As concentrations.

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Iron oxides control the mobility of a host of contaminants in aquifer systems, and the microbial reduction of iron oxides in the subsurface is linked to high levels of arsenic in groundwater that affects greater than 150 million people globally. Paired observations of groundwater and solid-phase aquifer composition are critical to understand spatial and temporal trends in contamination and effectively manage changing water resources, yet field-representative mineralogical data are sparse across redox gradients relevant to arsenic contamination. We characterize iron mineralogy using X-ray absorption spectroscopy across a natural gradient of groundwater arsenic contamination in Vietnam.

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The role of man-made ponds on arsenic mobilization was examined in Bangladesh. Here, we describe a field experiment that shows how recharge from a newly constructed pond creates a reactive front that moves downward into the underlying aquifer, but only advances slowly, less than 8 cm/year. We found that pond recharge introduces organic carbon that likely drives the reduction of sulfate and solid-phase iron.

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Widespread contamination of groundwater with geogenic arsenic is attributed to microbial dissolution of arsenic-bearing iron (oxyhydr)oxides minerals coupled to the oxidation of organic carbon. The recharge sources to an aquifer can influence groundwater arsenic concentrations by transport of dissolved arsenic or reactive constituents that affect arsenic mobilization. To understand how different recharge sources affect arsenic contamination-in particular through their influence on organic carbon and sulfate cycling-we delineated and quantified recharge sources in the arsenic affected region around Hanoi, Vietnam.

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Quantifying anthropogenic contributions to elemental cycles provides useful information regarding the flow of elements important to industrial and agricultural development and is key to understanding the environmental impacts of human activity. In particular, when anthropogenic fluxes reach levels large enough to influence an element's overall cycle the risk of adverse environmental impacts rises. While intensive groundwater pumping has been observed to affect a wide-range of environmental processes, the role of intensive groundwater extraction on global anthropogenic element cycles has not yet been characterized.

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Groundwater drawn daily from shallow alluvial sands by millions of wells over large areas of south and southeast Asia exposes an estimated population of over a hundred million people to toxic levels of arsenic. Holocene aquifers are the source of widespread arsenic poisoning across the region. In contrast, Pleistocene sands deposited in this region more than 12,000 years ago mostly do not host groundwater with high levels of arsenic.

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