Quantifying Riverine Recharge Impacts on Redox Conditions and Arsenic Release in Groundwater Aquifers Along the Red River, Vietnam

Abstract

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. We constrained potential end‐member compositions and employed a novel end‐member mixing model using an ensemble approach to apportion recharge sources. Groundwater arsenic and dissolved organic carbon concentrations are controlled by the dominant source of recharge. High arsenic concentrations are prevalent regardless of high dissolved organic carbon or ammonium levels, indicative of organic matter decomposition, where the dominant recharge source is riverine. In contrast, high dissolved organic carbon and significant organic matter decomposition are required to generate elevated groundwater arsenic where recharge is largely nonriverine. These findings suggest that in areas of riverine recharge, arsenic may be efficiently mobilized from reactive surficial environments and carried from river‐aquifer interfaces into groundwater. In groundwaters derived from nonriverine recharge areas, significantly more organic carbon mineralization is required to obtain equivalent levels of arsenic mobilization within inland sediments. This method can be broadly applied to examine the connection between hydrology, geochemistry and groundwater quality.

Mapped Predictions of Manganese and Arsenic in an Alluvial Aquifer Using Boosted Regression Trees

“…Therefore, the high resistivities near streams are interpreted as coarse-grained materials associated with natural levee and point bar deposits. Previous research has found increased As near rivers, driven by variation in sediment texture and organic carbon (Yang et al 2014;Nghiem et al 2019).…”

“…Hydrogeologic characteristics include groundwater age, NGWA.org K.J. Knierim et al Groundwater 60, depth to groundwater, recharge source, and sediment texture (Stute et al 2007;Aziz et al 2008;Sharif et al 2008;Yang et al 2014;McMahon et al 2019;Nghiem et al 2019). The BRT models were developed with direct indicators and surrogates for these characteristics.…”

“…Geochemical conditions affecting Mn and As include redox and pH (Welch et al 2000; Smedley and Kinniburgh 2002; McMahon et al 2019). Hydrogeologic characteristics include groundwater age, depth to groundwater, recharge source, and sediment texture (Stute et al 2007; Aziz et al 2008; Sharif et al 2008; Yang et al 2014; McMahon et al 2019; Nghiem et al 2019). The BRT models were developed with direct indicators and surrogates for these characteristics.…”

Isotopic variation in groundwater across the conterminous United States – Insight into hydrologic processes

“…Groundwater recharge is controlled by a number of factors including precipitation, evapotranspiration, soil type, bedrock geology, climate, vegetation, population among others. Stable isotopes of groundwater provide insight into the seasonal timing and relative efficiency of recharge between seasons (Jasechko et al, 2014(Jasechko et al, , 2017 along with a host of other processes including the degree of evaporation experienced prior to recharge, recharge elevation, surface water-groundwater interaction and connectivity, and paleo-recharge (Siegel and Mandle, 1984;Smith et al, 1992;Squeo et al, 2006;Skrzypek et al, 2015;Follstad Shah et al, 2019;Jasechko, 2019;Nghiem et al, 2019). Thus, understanding spatial patterns in groundwater isotopes greatly aides in characterizing a region's hydrology.…”

Groundwater pollution: Occurrence, detection, and remediation of organic and inorganic pollutants

“…Significant arsenic contamination is also reported in Vietnam. Nghiem et al (2019) reported that 29.2% of randomly sampled household well waters during the regional sampling survey have arsenic concentration below the WHO standards, while the arsenic concentration in 39.3% of the well waters sampled exceeds the Vietnamese standard of 50 µg/L. The authors stated that a high concentration of arsenic was observed in the western (Hanoi) and eastern sides of the Red River.…”