Identification of throughflow using the distribution of secondary iron oxides in soils

Abstract

The hypothesis that the spatial distribution of secondary iron (Fe) oxides and hydroxides in soils can be used an indicator for subsurface flow (throughflow) is tested on a podzolised hillslope at Bicknoller Combe, Somerset, UK. Acid ammonium oxalate solution and citrate-bicarbonate-dithionite solution are used to estimate the amount of Fe oxides (Fe o and Fe d respectively) in the soils collected at five depths from 64 soil profiles across the hillslope. The spatial distribution of Fe oxides matches the pattern of throughflow over the slope which had been previously identified by mapping of soil water potential and soil morphological analysis. The distribution of Fe d on the hillslope correlates well with the relative intensity of lateral throughflow in topsoils; the upper convex shoulder slope shows the highest intensity of throughflow, gradually decreasing towards the base of the slope. On convergent slopes, the intensity of throughflow greatly increases along the line of convergence. The spatial distribution of Fe o and Fe o/Fe d reflects a more dynamic aspect of the removal and deposition processes of Fe associated with podzolisation and gleying processes than that of Fe d on the study slope. Significant amounts of Fe removed from upslope soils are precipitated in oxidised subsurface soil horizons at lower slope positions. In the comparison with the ratio Mn d/Fe d which has been previously proposed as an indicator of lateral throughflow in an acid hillslope, the distribution of Fe d, Fe o, and Fe o/Fe d better reflects spatial patterns of throughflow and its relative intensity.