Arsenic Mobilization Near a Closed Landfill, Ayer MA: A Case Study
Christopher Abate, AMEC Earth and Environmental Inc., 2 Robbins Road, Westford, MA 01886, Tel: 978-692-9090 , Fax: 978-692-6633 , christopher.abate@amec.com
Kathleen Sellers, AMEC Earth and Environmental Inc., 2 Robbins Road, Westford, MA 01886, Tel: 978-692-9090 , Fax: 978-692-6633 , kathleen.sellers@amec.com
Michael J. Robinson, AMEC Earth and Environmental Inc., 2 Robbins Road, Westford, MA 01886, Tel: 978-692-9090 , Fax: 978-692-6633 , mike.j.robinson@amec.com
Robert J. Simeone, Department of the Army, Base Realignment and Closure Division, Devens Reserve Forces Training Area, 30 Quebec Street, Unit 100, Devens, MA 01434-4479, Tel: 978-796-2205 , robert.j.simeone@us.army.mil

Elevated concentrations of Arsenic have been detected in groundwater downgradient from the Shepley’s Hill Landfill near Ayer, Massachusetts.  Located in the ‘Arsenic Belt’ of central Massachusetts, this landfill has operated since the 19^th century and was capped and closed in 1993.  Though groundwater levels within the landfill have declined since capping as recharge was diverted, a substantial portion of the waste mass remains saturated.   As a result, microbial degradation of organic waste has generated reducing conditions (ORP < 0) in groundwater, which have in turn resulted in reductive dissolution of iron hydroxides in the fractured rocks and glacial sediments that make up the surrounding aquifer.  In this manner, Arsenic sorbed onto oxide mineral surfaces is mobilized until either: 1) oxidizing conditions are restored (ORP > 0), the oxides reprecipitate, and Arsenic is resorbed or 2) sulfides precipitate under extreme reducing conditions (ORP < -200) which also sorb Arsenic.

Due to persisting Arsenic concentrations in offsite monitoring wells, a 50 gpm pump and treat system was installed at the downgradient end of the landfill to contain the impacted groundwater and eventually restore offsite conditions.  An evaluation of the effectiveness of this system in its first year of operation is underway.  Concurrently, supplemental investigations are presently being conducted to define the extent of offsite impacts and the potential for completed receptor pathways to define the associated risks.  The preliminary results of these studies suggest: 1) the distribution of elevated Arsenic is closely related to redox zonation within the aquifer, 2) Arsenic concentrations systematically decline several orders of magnitude with distance from the presumed source of reducing conditions, and 3) Arsenic in groundwater is apparently attenuated prior to discharge to surface water. 

Microbial Transformation of Arsenic Compounds
D. Freikowsky, J. Winter and C. Gallert, Institut für Ingenieurbiologie und Biotechnologie des Abwassers, Universität Karlsruhe, Email: Claudia.Gallert@iba.uka.de

Arsenic contaminated groundwater is a serious worldwide problem. Arsenic concentrations above 50 µg l-1 could be found in groundwater of Asian countries as well as in the US and South America. In the West Bengal Delta plain, the arsenic concentration in the groundwater could reach more than 400 µ l-1 and in the sediments 2 – 8 mg kg-1. This high arsenic concentrations causes skin deformations and health  problems for several million people.

The role of microorganisms in the mobilisation of arsenate by reduction of As(V) to the mobile As (III) and the role of Fe (III) reducing microorganisms in the sediment and groundwater of the West Bengal Delta plain will be elucidated by population analysis and activity tests of sediments and groundwater samples. Isolations of microbes under oxic and anaerobic conditions, as well as PCR experiments with specific primers for As(III) oxidizers and As(V) reducing bacteria are done to analyse the microbial population in samples from the experimental site. The isolated pure cultures will be characterised by classical microbial physiology as well as by analyses of 16 S rRNA genes.

Column experiments with high and low contaminated sediment samples and addition of selected carbon sources are done to demonstrate the role of micororganisms and electron acceptors in arsenic mobilisation, as compared to a sterile control column, which allows to check for abiotic arsenic transformations. The laboratory experiments are compared with field examinations at the experimental site by analysing arsenic concentrations and dissolved organic carbon concentrations (DOC) in water samples. Also, results of a drilling campagne for getting sediment samples and analysing the geochemical conditions and microbial populations will be reported.