In situ remediation potential for Southern African groundwater resources

摘要

In situ groundwater remediation is practised in Europe, the United States, New Zealand and Canada. Widely accepted treatment methods include, permeable reactive barriers, redox manipulation, the Vyredox® method and biological denitrification. The permeable reactive barrier is widely used for contaminant removal. It consists of a constructed trench filled with a contaminant specific reagent, such as sawdust or wood chips for the promotion of biological denitrification. Other techniques include in situ redox manipulation which requires dithionite injection and in situ biological denitrification, which uses substrates such as ethanol or methanol. The Nitredox® and Vyredox® methods use a sophisticated arrangement of injection and aeration boreholes to manipulate oxidation and reduction to control nitrate, iron and manganese concentrations in the aquifer. Concern about nitrate as a chemical constituent of groundwater is increasing, especially in the arid and semi-arid regions of Southern Africa. Livestock losses, and "blue baby syndrome" in humans, result from high nitrate concentrations in drinking water. Hence, there is a need to remediate groundwater with nitrate concentrations above the required standard. Low cost, robust and simple treatment technologies are needed for rural water supply in Southern Africa. This paper gives an overview of the performance of full scale and pilot scale treatment plants. An estimate was made of the costs of applying selected in situ treatment options for a South African town. The order of difference in cost between in situ and ex situ treatment plants is calculated. The geological and hydrogeological parameters required for successful operation of most in situ treatment systems are described. Advantages and disadvantages of in situ treatment are also mentioned.