Extended periods of flooding can impair the quality of surfacewaters by mobilising and transporting heavy metals from land to water bodies. The aim of this study was to test the hypothesis that soil drying followed by extended flooding enhances solubilisation of metals (Fe, Cu, Co, Zn and Ni).

We tested this hypothesis by flooding samples of two contrasting soils: a clay soil and a brown earth, which had each been either dried (40°C for two weeks) or kept at field moisture conditions (36% and 54% of WHC moisture content respectively). The soils were flooded for 31 days with a given amount of water (so that 150 g dry-weight equivalent soil received 400 ml of water). Water sub-samples were collected after day 1, 3, 7, 10, 15, 22 and 31, filtered (0.45µm) and analysed by ICP-MS.

The onset of flooding increased dissolved metal concentrations. The release of metals coincided with a reduction in redox potential, suggesting reductive dissolution of metal oxides and desorption of previously sorbed metals. Soil drying resulted in a significant increase in dissolved metal concentrations relative to the moist counterparts following flooding. This could be due to a combination of soil drying (e.g. microbial death, increased crystallinity of metal oxides) and flooding (reductive dissolution of metal oxides) associated factors. The clayey soil released significantly (p<0.05) greater quantities of metals, perhaps due to its greater organic matter (OM), as soil drying may reduce the stability of OM-metal complexes.

Our results suggest that extended periods of drying-flooding have the potential to promote mobilisation of soil metals and thus have implications for catchment water quality.