STEEL ROD OXIDATION AS A HYDROLOGIC INDICATOR IN WETLAND SOILS

Depth of rusting on steel rods has been proposed as an inexpensive means of determining depth to the water table and the reducing zone in wetland soils. The suitability of steel-rod oxidation as a hydrologic indicator in wetland soils was tested in a series of laboratory and field experiments in both Histosols and a mineral soil (Typic Fluvaquent). In laboratory microcosms, the steel-rod rusting depth exactly matched water-table levels under both permanently flooded and drained conditions. But under a rapidly fluctuating hydrology, once heavy rust formed on the rods it did not dissolve upon partial reflooding. Field experiments showed a good correspondence between steel-rod rusting depth and water-table depth under both relatively constant and seasonally changing hydrology in Histosols (r2 = 0.80, slope = 0.81). In the Typic Fluvaquent, a close correspondence between steel-rod rusting depth and water-table depth occurred under relatively constant hydrological conditions but, with a rapidly dropping water table, rod oxidation showed a lag period in response. We conclude that the steel-rod technique is valuable as a hydrologic indicator of the reducing zone in basic science studies. We recommend frequent sampling (monthly) and the use of other hydrological measures to ensure unambiguous results. The technique is unsuitable for jurisdictional delineation in areas with a fluctuating hydrology due to prolonged lag periods in rod oxidation under some conditions and the inability of previously formed rust to dissolve upon reflooding.