Heterogeneous oxygenation of wetland soils with increasing inundation: Redox potential, water depth, and preferential flow paths

Abstract

Saturated wetland soils are typically depleted in oxygen due to rapid consumption by biological processes. Fine‐grained sediments limit the vertical flux of oxygenated surface water. In forested wetlands potential exists for advective transport along preferential flow pathways created by decomposing roots or tree remains, resulting in isolated zones of oxygenated pore water. This potential is enhanced in wetlands that are perched above the regional water table, where increases in surface‐water depth create significant downward hydraulic gradients. Along the Mississippi River floodplain, many wetlands are now perched as a result of groundwater mining. This study was conducted at Sky Lake, MS, through a growing season, with high temporal‐resolution monitoring of redox potential as a proxy for the delivery of oxygenated surface water at 30 and 60 cm depths at six locations. Results showed reducing conditions at all locations under moderate inundation, but redox potential rose significantly at a subset of locations when surface‐water exceeded one meter in depth. Rising redox potential is argued to represent localised advective transport of oxygen in excess of the rate of biological consumption. Wetland plants growing in perennially saturated soils may benefit from periodic increases in water depth and subsequent increase in the flux of oxygen along preferential flow paths.

Groundwater recharge from an oxbow lake‐wetland system in the Mississippi Alluvial Plain

“…Recharge to the aquifer is attributed to rising water levels spreading over point bar deposits and into the surrounding forested wetlands where preferential flow pathways are likely to exist due to buried and decomposing tree remains. An earlier study in the wetland demonstrated an increasing redox potential in isolated zones, consistent with the existence of preferential flow pathways through the bottom sediments (Lahiri & Davidson, 2020).…”

“…Bald cypress can grow in perennially saturated soils, though they need aerated conditions to germinate (Keim et al, 2006) and have been observed to experience greater radial growth when aerated or receiving oxygenated water to the root zone (Davidson et al, 2006). Results of this study and that of a companion paper in this issue, Lahiri and Davidson (2020), suggest a possible working solution of holding lake levels higher in the fall and winter and allowing natural outflow during the spring and summer.…”

“…In a forested wetland, there is ample opportunity for these pathways to be formed by decomposing roots, or buried limbs and trunks of fallen trees (Figure ), clear evidence of which was found in the wetland cores. In a companion paper in this issue, Lahiri and Davidson () report the results of changes in redox potential in the wetland soils in the vicinity of SLW‐1. That study was completed prior to the high‐water event reported in the current paper but was carried out during a period when lake levels varied from dry conditions to over 1 m in depth.…”

“…One was positioned in open water at the bottom of the lake. The second was positioned in a stilling well near ground surface in the wetland next to SLW‐1 (location of the companion redox study, Lahiri & Davidson, ). Lake level was recorded every 4 hr.…”

Can Weathering of Banded Iron Formations Generate Natural Hydrogen? Evidence from Australia, Brazil and South Africa

“…Is the circulating water still oxygenated at such depth? Numerous studies show that groundwater usually falls toward reducing conditions, since all conventional oxidants (such as O 2 , NO 3 − or SO 4 2− ) are quickly consumed in the first meters of soil/sediments by organic matter oxidation [51][52][53]. This favors water being the main oxidant, and thus the possibility for BIF-contained iron to be directly oxidized by water molecules is highly possible.…”