Seasonal Arsenic Accumulation in Stream Sediments at a Groundwater Discharge Zone

Abstract

Seasonal changes in arsenic and iron accumulation rates were examined in the sediments of a brook that receives groundwater discharges of arsenic and reduced iron. Clean glass bead columns were deployed in sediments for known periods over the annual hydrologic cycle to monitor changes in arsenic and iron concentrations in bead coatings. The highest accumulation rates occurred during the dry summer period (July-October) when groundwater discharges were likely greatest at the sample locations. The intermediate flow period (October-March), with higher surface water levels, was associated with losses of arsenic and iron from bead column coatings at depths below 2-6 cm. Batch incubations indicated iron releases from solids to be induced by biological reduction of iron (oxy)hydroxide solids. Congruent arsenic releases during incubation were limited by the high arsenic sorption capacity (0.536 mg(As)/mg(Fe)) of unreacted iron oxide solids. The flooded spring (March-June) with high surface water flows showed the lowest arsenic and iron accumulation rates in the sediments. Comparisons of accumulation rates across a shoreline transect were consistent with greater rates at regions exposed above surface water levels for longer times and greater losses at locations submerged below surface water. Iron (oxy)hydroxide solids in the shallowest sediments likely serve as a passive barrier to sorb arsenic released to pore water at depth by biological iron reduction.

Redox zonation and oscillation in the hyporheic zone of the Ganges-Brahmaputra-Meghna Delta: Implications for the fate of groundwater arsenic during discharge

“…Several lines of evidence suggest that temporal redox oscillation is a key feature of the natural reactive barrier consisting of Fe(III) oxyhydroxides at the Meghna riverbank (Jung et al, 2012; Lee et al, 2014; MacKay et al, 2014). The sandy riverbank sediment may undergo temporal redox oscillation due to the ~4-5 m seasonal fluctuation in the groundwater table and river water level (Fig.…”

“…Investigation of the behavior of As in hyporheic zones of Waquoit Bay has shown trapping of As by the “iron curtain” at ~1 m depth in the sediment (Bone et al, 2006; Jung et al, 2009). Recent studies have also reported that groundwater As is naturally immobilized by Fe(III) oxides formed in the hyporheic zone where groundwater interacts with lake water or stream water, and the removal of dissolved Fe and As varies seasonally depending on the hydrologic condition (Lee et al, 2014; MacKay et al, 2014; Baken et al, 2015). However questions remain regarding how aquifer heterogeneity influences the magnitude of the geochemical attenuation during discharge (Burnett et al, 2006).…”

Biogeochemistry of Arsenic Cycling in a Tidally Influenced River-Bank Aquifer in Bangladesh

“…This zone includes shallow surface water, porewater sediments below the sediment-water column interface, and shallow groundwater (MacKay et al, 2014). Intermediate mixing between the groundwater and surface water may promote chemical and biological reactions among species present only within the HZ (MacKay et al, 2014).…”

“…As reduction through bacterially mediated respiratory pathways under appropriate environmental conditions (Kocar and Fendorf, 2009, Borch, 2010, Xiu et al, 2015. In a river that experiences daily and seasonal tidal fluctuations, intermediate mixing between two endmember mixing waters may promote biological reactions that characterize that mixing zone (MacKay et al, 2014).…”

Bioavailability and risk assessment of arsenic in surface sediments of the Yangtze River estuary

“…Conversely, the amount of As bound to organic fractions was lower in wet seasons than in dry seasons. Seasonal variations can affect the oxidant capacity of sediments (MacKay et al, 2013). The As bound to Fe-Mn oxides and organic fractions was susceptible to a change in redox conditions .…”