Abstract

Forest rings are large, circular features in boreal forests that commonly exceed 500m in diameter and are visible on aerial photographs. A detailed study of redox conditions and spontaneous potential (SP) was carried out over a forest ring that overlies an H2S(aq) accumulation. Studies included drilling, monitoring well installation, and downhole SP using both polarizing and nonpolarizing electrodes. Also measured were redox potential of groundwater and soils, concentrations of sulfur species in groundwater, and headspace concentrations of redox-sensitive gases in monitoring wells. The results show positive SP anomalies in the shallow subsurface and near-horizontal, negative-inward redox gradients in the water-saturated overburden at theedges of the ring. SP anomalies are spatially correlated with redox gradients, suggesting that the two are related. The SP anomalies may be produced in response to redox gradients as redox-active ions and polar molecules spontaneously align with the negative poles toward the oxidizing end of the gradient, i.e., toward their more electronegative neighbors. This orientation of dipoles imparts a macroscopic electrical polarity to the redox gradient and results in the observed positive electrical anomaly inside the forest ring. Ongoing oxidation reactions occurring around the periphery of the forest ring maintain strong HS concentration gradients, which result in an outward steady-state diffusive flux of HS. Electromigration of redox-active ions in the redox-induced electrical field may also contribute to maintenance of the redox gradient.

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