Aquifer Arsenic Cycling Induced by Seasonal Hydrologic Changes within the Yangtze River Basin

Abstract

Consumption of groundwater containing >10 μg L(-1) arsenic (As) adversely impacts more than 100 million people worldwide. Multiyear trends in aquifer As concentrations have been documented, but strong seasonal variations are not commonly observed. Here we report dramatic seasonal changes in As concentrations and aquifer chemistry within the Jianghan Plain of the Yangtze River, China. At some wells, concentrations fluctuate by more than an order of magnitude within a single year (100-1200 μg L(-1)). Groundwater extraction and sustained water levels of surface channels during the dry season induces a strong downward hydraulic gradient, seasonally supplying oxidizing (oxygen, nitrate) water to the otherwise anoxic aquifer. Oxygen and/or nitrate addition promotes a transient drop in As concentrations for 1-3 months. When recharge ceases, reducing, low-arsenic conditions are reestablished by reactive, endogenous organic carbon. Temporal variability in As concentrations is especially problematic because it increases the probability of false-negative well testing during low-arsenic seasons. However, periods of low As may also provide a source of less toxic water for irrigation or other uses. Our results highlight the vulnerability and variability of groundwater resources in the Jianghan Plain and other inland basins within Asia to changing geochemical conditions, both natural and anthropogenic, and reinforce that continued monitoring of wells in high-risk regions is essential.

Identification of Hydrobiogeochemical Processes Controlling Seasonal Variations in Arsenic Concentrations Within a Riverbank Aquifer at Jianghan Plain, China

“…Arsenic concentrations in groundwater ranged from 2 to 1,072 μg/L across the field site during the monitoring period from May 2012 to March 2014. The highest average groundwater arsenic concentration in all the wells was observed at 25 m (mean 127 μg/L) and the lowest at 10 m (mean 24 μg/L) during the monitoring period (Duan et al, ; Schaefer et al, ).…”

“…Jianghan Plain is an alluvial plain surrounded by Han River at the north and Yangtze River at the south, covering an area of >55,000 km 2 (Schaefer et al, ). The area is subtropical with an annual precipitation of 1,300 mm, of which 50% occurs in the rainy season (May to August; Duan et al, ).…”

“…Groundwater was pumped for irrigation and domestic water usage, causing short‐term, frequent variations in groundwater levels between May and August (Duan et al, ). This would enhance the downward hydraulic gradient between the confined aquifer and upper aquitard, potentially inducing leaking recharge of surface water through the upper aquitard (Schaefer et al, ).…”

“…Seasonal fluctuations of surface water and groundwater levels cause seasonal flow direction and redox condition changes, inducing seasonal variation in arsenic concentration in groundwater (Schaefer et al, ). At the site, the groundwater level dynamics are affected by irrigation extraction, hydraulic exchange between the confined aquifer and upper aquitard, and surface water and groundwater exchange.…”

“…Microbial reduction of iron oxides and consequent release of sorbed arsenic from the iron oxide surfaces are generally considered important processes controlling arsenic reactive transport in groundwater (Fendorf, Michael, & van Green, ; Jones et al, ; Polizzotto et al, ; Postma et al, ). Recent studies also implied that the reductive dissolution of iron oxides is the main process controlling arsenic release at Jianghan Plain under anoxic conditions (Duan et al, ; Schaefer et al, ). Microbially mediated reduction of iron oxides may be described as where Sediment‐Fe (OH) 3 ‐As (V) denotes the arsenic sorbed on iron oxide surfaces (here Fe (OH) 3 was used to represent iron oxides) in sediments.…”