FIELD-MEASURED REDOX POTENTIALS IN SOILS AT THE GROUNDWATER SURFACE-WATER INTERFACE

At the groundwater-surface-water interface the last modification of discharging groundwater takes Place. In this study the redox potential was measured in situ in four groundwater discharge areas with a size of 3-4 m2. Three of them were the upper reaches of streams in coniferous forests and one was an alder fen. Measurements were made at different seasons, with 50 and 100 platinum micro-electrodes respectively, in each area. Readings were taken in the upper 30 cm of the soil, at two depths. A method study showed that the electrodes needed a longer equilibrium time than is usually reported. The main finding of this study was the large spatial variation within soils in the discharge zone; regions as small as 10 cm in diameter could have redox potentials covering several hundred mV. It was not possible to identify a seasonal variation due to the large spatial variation. Differences in the distribution of the redox potential with depth may be explained by the structure of the peat affecting flow patterns and the residence time of water.