Evaluation of iron-coated tubes to detect reduction in soils for wetland identification in the Kruger National Park
Johnson, Tracey Lee
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The identification of hydric soils is important for wetland delineation and protection. South Africa currently uses the Department of Water Affairs and Forestry (DWAF, 2005) delineation guidelines which can be subjective in certain cases. A robust technical standard that can be legally conclusive is required. The National Technical Committee of Hydric Soils (NTCHS) in the United States of America has accepted the IRIS tube methodology as a technical standard. This method has not yet been tested or accepted in South Africa. IRIS is an acronym for Indicator of Reduction in Soils and the IRIS tubes consist of PVC conduit piping, coated in a synthesised Fe oxide paint comprising of mainly ferrihydrite. These tubes are installed in the soil and if reducing conditions are present, the paint will be removed from the tubes. The following study took place in the Kruger National Park and the IRIS tubes were tested in three different wetland systems, namely Malahlapanga, Nshawu and the Tshuthsi spruit. Four wetness zones were identified, based on vegetation, at each wetland and with three repetitions. Water table monitoring wells were installed, the soils were classified, soil wetness indicators were identified, vegetation was described, soil analyses were undertaken and the pH and Eh of the water table was recorded monthly in order to calculate rH values. The study took place from September 2012-September 2013. The area percentage of paint removed from the top 300 mm of the IRIS tubes was quantified by scanning the tubes and using Adobe Photoshop. The IRIS results were compared to the DWAF indicators and it was found that the methods were in agreement, however, it was found that the conditions at the Tshutshi spruit were not favourable for Fe reduction due to the high pH values recorded. The limitations and advantages of the method are explored. It was found that the wetter summer months were the most favourable months for the installation of the tubes. The success of the DWAF (2005) wetland indicators was evaluated for each wetland’s lithology and when consulting the ancillary data, potential ancillary variables were identified. There were trends in the pH, organic carbon as well as the exchangeable sodium at all of the wetlands (with the exception of the upland zone at the Basaltic wetland). At the Gneiss and Basalt lithologies, there were similarities in the patterns of Fe and Mn distribution along the catena, however, the opposite trend was observed at the Granitic wetland. The IRIS tubes are thought to be a useful tool for wetland delineation in South Africa, however, further research is required over a wider geographical area to determine where they will work and also to test the MIRIS tube methodology (Manganese Indicators of Reduction in Soils) in wetlands which are unfavourable to the reduction of Fe.