298014

Permeable Reactive Barriers (Proceedings of the International Symposium held at Belfast, Northern Ireland, March 2004). IAHS Publ. 298, 2005, 97-104.

Assessment of different carbon sources and delivery techniques to promote an in situ reactive zone for bioprecipitation of metals in groundwater

BRADLEY M. PATTERSON¹, YAMIN MA^1,2, MICHELLE E. GRASSI^1,2,BLAIR S. ROBERTSON¹, GREG B. DAVIS¹, MARK LIPMAN³, STUART RHODES³, ALLAN J. MCKINLEY² & ANDREW W. RATE²

1 CSIRO Land and Water, Private Bag No. 5, Wembley, Western Australia 6913, Australia

bradley.patterson@csiro.au

2 The University of Western Australia, Nedlands, Western Australia 6009, Australia

3 Rio Tinto Technical Services, Milsons Point, New South Wales 2061, Australia

Abstract Large-scale soil column experiments were undertaken using low metal-sorbing soil to assess the efficiency of different carbon sources to promote and maintain a sulphate-reducing in situ reactive zone for bioprecipitation of metals in groundwater. Carbon sources were delivered by either direct injection (molasses), diffusion delivery through polymers (ethanol) or as a non-aqueous phase liquid (emulsified vegetable oil). All three carbon sources promoted denitrification with denitrification half lives between 1.3 and 2.1 days. However, only molasses and ethanol promoted sulphate reduction, with average sulphate-removal efficiencies of 53% and 95%, and sulphate reduction half lives of 8.3 and 1.2 days, respectively. Based on this information, ethanol was a more efficient carbon source in promoting and maintaining sulphate reduction within the column compared to molasses. Under these experimental conditions, non-aqueous phase vegetable oil amendment did not prove suitable as a carbon source to promote sulphate reduction. Substantial removal of zinc and copper (>98%) was observed in groundwater for both the ethanol-amended and molasses-amended columns. Zinc and copper were removed from the groundwater as it flowed past the sulphate-reducing regions of the columns, suggesting that the metals were deposited on the soil, possibly as zinc and copper sulphides. The longevity of sulphate reduction after ethanol and molasses delivery had ceased was in the order of two weeks or less, suggesting that in a field application, relatively continuous carbon addition would be required to maintain sulphate reduction for long-term treatment of metal-contaminated groundwater.

Key words bioprecipitation; groundwater; in situ reactive zone; metals; sulphate reduction