Restoration of wetlands: successes and failures on scalds comprising an iron oxide clogged layer with areas of acid sulfate soils

Abstract

This paper reports on successes and failures experienced over nearly two decades while attempting to remediate degraded scalds typified by iron clogged layersinterspersed with patches of acid sulphate soil on the Eastern Dundas Tablelands in Victoria, Australia. In 2004 (Gardner et al. I Overview Plant Soil 267:51–59, 2004a, Plant Soil 267:85–95, b), it was suggested that redox processes similar to acid sulfate soil reactions were acting to exacerbate and amplify salinity effects in the landscape. Soil permeability to 1 m depth was measured and compared to that at 3-5 m depths. Additional analysis of soils in the discharge zone were undertaken (mineralogy, SEM, XRF, XRD) to seek evidence of typical acid sulfate reactions which had not been found in 2004. Water quality (EC, pH) and hydraulic pressures were measured in 2005- 2006, after shallow trenchs were dug across the site, and revegetation attempted with a range of native species. Following observations of improved growth at leaking peizometers, those with above ground water levels were drilled to allow water to escape. Assessment of revegetation outcomes were conducted in 2006 and 2016. Soil permeability was lower in the upper 1 m layer of the soil than at 3-5m depth, Clear evidence of acid sulfate soil mineralogy was found, however small scale variation was the norm. Clogging of soil macropores was observed, which could be manually cleared. Fracturing the soil to increase discharge either with explosives or excavation of both shallow and deep trenchs failed because the increased discharge was insufficient to overcome evaporation and salt concentration, and redox processes continued which would eventually clog the soil again. Success was achieved with leaking piezometers to 4.5 m depth, which allowed the groundwater to reach the surface without reducing ferric oxides. Rushes planted at the discharge point maintained an oxidised environment thereby halting the redox processes. Evaporation effects were prevented because the increased discharge occurred at a point. This combination allowed a range of native species to flourish and after 10 years, to spread beyond what we estimate the leaking piezometers would support. Evaporation causing concentration of salts is very important, but this is linked to the redox processes, which cause soil clogging and reduced permeability, and thus the evaporation and salt issue. Success was achieved when the reduced groundwater could access the surface in an oxidised environment, at a sufficient rate to prevent salt concentration by evaporation. The revegetation has expanded beyond what could be supported by the original discharge, suggesting that the plants are breaking the clogged soil layers and increasing discharge. The successful colonising species typically produce specialised structures such as dauciform and proteoid roots which are able to reduce and chelate iron oxides.