GQ07: Securing Groundwater Quality in Urban and Industrial Environments (Proc. 6th International Groundwater Quality Conference held in Fremantle, Western Australia, 2–7 December 2007). 1 In situ remediation of arsenic at a highly contaminated site in Northern Germany T. KRÜGER 1 , H. M. HOLLÄNDER 1,2 , P.-W. BOOCHS 1 , M. BILLIB 1 , J. STUMMEYER 3 & B. HARAZIM 3 1 Institute of Water Resources Management, Hydrology and Agricultural Hydraulic Engineering, Leibniz University of Hannover, Appelstr 9A, 30167 Hannover, Germany krueger@iww.uni-hannover.de 2 CSIRO Land and Water, Private Bag No. 2, Urrbrae 5064, South Australia, Australia 3 Federal Institute of Geosciences and Natural Resources (BGR), Stilleweg 2, 30655 Hannover, Germany Abstract High arsenic concentrations (up to 8800 μg/L) were detected in groundwater at a military site in Northern Germany. The majority of the arsenic is organically bound (80-95%). Arsenate and arsenite (each <10%) were also detected. In soil column studies it was shown that the arsenic effectively can be immobilized using bivalent iron chloride. This process forms the base of an in situ treatment technique, which allows the immobilization of arsenic within the aquifer. A pilot plant consisting of two wells which alternately can be used for pumping or infiltrating water enriched with reactive chemicals, in this case iron chloride and oxygen was built. Preliminary experiments with NaCl instead of FeCl 2 injection were carried out to find out an optimal operation cycle for the creation of a defined and stable reactive area in the underground around the wells. Key words arsenic; contaminated groundwater; in situ immobilization INTRODUCTION Arsenic in groundwater – a global issue Arsenic contamination of groundwater from anthropogenic (e.g. Holländer et al., 2006; Krüger et al., 2006) or natural sources (Ravencroft et al., 2005; Planer-Friedrich et al., 2001) present a human health risk. The population of developing countries living in areas with high naturally occurring arsenic concentrations often has little choice other than drinking the arsenic polluted water which can lead to serious health issues like cancer (Scott et al., 1995). At industrial sites, the polluted water often threatens the water quality of the downstream aquifers and ecosystems. Arsenic in groundwater – a site specific problem At a military site in Northern Germany groundwater contamination across large areas results from arsenic used in chemical warfare agents during 1917 - 1945. Concentrations up to 9,000 μg As/L have been detected and are mostly organically bound (80-85%). Inorganic species such as arsenite and arsenate were also found (each <10%). Currently the groundwater is remediated using pump-and-treat technology which reduces the average arsenic concentration from 400 μg/L to an effluent of <100 μg/L.