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

The hypothesis that the spatial distribution of secondary iron (Fe) oxides and hydroxides in soils can be used an indicator for subsurface flow (throu ghflow) is tested on a podzolised hillslope at Bicknoller Combe, Somerset, UK. Acid ammonium oxalate solution and citrate-bicarbonate-dithionite solut ion are used to estimate the amount of Fe oxides (Fe-o and Fe-d respectivel y) in the soils collected at five depths from 64 soil profiles across the h illslope. The spatial distribution of Fe oxides matches the pattern of thro ughflow over the slope which had been previously identified by mapping of s oil 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 slop e. On convergent slopes, the intensity of throughflow greatly increases alo ng 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 F e associated with podzolisation and gleying processes than that of Fe-d on the study slope. Significant amounts of Fe removed from upslope soils are p recipitated in oxidised subsurface soil horizons at lower slope positions. In the comparison with the ratio Mn-d/Fe-d which has been previously propos ed as an indicator of lateral throughflow in an acid hillslope, the distrib ution of Fe-d, Fe-o, and Fe-o/Fe-d better reflects spatial patterns of thro ughflow and its relative intensity. (C) 1999 Elsevier Science B.V. All righ ts reserved.