T148 Arsenic, Fluoride, Manganese, and Radiogenic Contaminants in Groundwater Systems—Scientific Knowledge, Public Health Concerns, and Removal Technology II

Wednesday, 28 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

NITROGEN CYCLING IN HIGH ARSENIC GROUNDWATER SYSTEM

Paper number: T148-14
Program: Technical Sessions
Time: 3:30 PM – 3:40 PM

Authors

Abstract

Co-occurrence of elevated NH4+ and arsenic (As) has been widely found in anoxic aquifers, which was naturally sourced from organic matter degradation coupled with Fe(III) oxide reduction. Anaerobic NH4+ oxidation coupled to Fe(III) oxide reduction (Feammox) may also contribute to As and Fe(II) mobility. Knowledge of co-cycling Fe(III)/Fe(II) and nitrogen species is crucial on As enrichment processes, which has been not well documented yet.

In deep groundwater along an approximate flow path from the alluvial fan to the flat plain in the Hetao Basin, China, dissolved NH4+, Fe(II), and As concentrations consistently increased, and ORP values and dissolved NO3- concentrations decreased, accompanied by the increases in δ15N-NO3 and δ18O-NO3 values and decrease in δ15N-NH4 values. Isotopic data suggest that NH4+ were mainly sourced from organic nitrogen mineralization, which was thereafter processed to nitrification pathway in the alluvial fan. Nitrification of NH4+ contributed to groundwater NO3- in the alluvial fan, which was discharged into the transition area. Values of δ15N-NO3 and δ18O-NO3 were positively correlated in deep groundwater from the transition area and the flat plain, both of which were negatively correlated with NO3- concentrations following Rayleigh fractionations, supporting the occurrence of denitrification. In these two zones, δ15N-NH4 values became gradually enriched as NH4+ concentration decreased. However, changes in δ15N-NO3 and δ15N-NH4 values were unassociated, possibly indicating that NH4+ oxidation was coupled to Feammox process, rather than anammox process. Higher δ15N-NH4 values in the transition area than those in the flat plain possibly suggested stronger Feammox rates in the former one, where lower dissolved As concentrations were observed. Higher Feammox rates were typically related to lower organic matter degradation rates. Therefore, the Feammox process may contribute slightly to Fe(II) and As enrichment. Higher As concentrations were related to stronger organic matter degradation and Fe(III) oxide reduction, leading to higher NH4+ and Fe(II) but lower NO3- concentrations, accompanied by lower δ15N-NH4 and higher δ15N-NO3 values. The study provided a clear picture on co-cycling processes of dissolved nitrogen species and As behavior in groundwater systems.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-357406

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