Procedure for deriving qualitative contaminant attenuation maps from land type data

Abstract

A procedure is presented for deriving qualitative contaminant attenuation maps from available soils information. Unfortunately, in South Africa, no soil map with national coverage exists at a scale larger than 1:2 500 000. However, 1:250 000 land type maps, which depict areas of relatively uniform soil pattern, are available. The spatial distribution of each soil type within a land type is not mapped and thus the position of each unique soil type is not known. However, the percentage of land covered by each soil type is quantified. The approach adopted here was to use land type data to derive soil characteristic maps per land type. Using Geographic Information Systems (GIS), the soils characteristic maps were then combined with reported contaminant attenuation characteristics from the literature to obtain a qualitative indication of the attenuation potential of a group of contaminants in a given land type. For example, the average clay content map was used to generate the attenuation potential of cation forming contaminants. The higher the clay content, the higher the contaminant attenuation potential. For attenuating, anion forming and contaminants, sesquioxide content plays a primary role. In general, the higher the sesquioxide content, the higher the contaminant attenuation potential. Thus the iron content map was used to depict the attenuation potential for anion forming contaminants. These qualitative contaminant attenuation maps can be used for regional planning purposes. From the maps, areas with soils that have favourable attenuation properties for a given contaminant or group of contaminants can be identified. To pinpoint sites with the required properties, further detailed investigations of target areas would need to be undertaken. It is concluded that land type maps can be used in lieu of soil maps, to identify areas with favourable contaminant attenuation properties for more detailed investigation.