ARSENIC SPECIATION, 18O AND 2H ISOTOPIC COMPOSITION OF GROUND WATER AT A LANDFILL SITE IN CENTRAL MASSACHUSETTS

An arsenic contaminant plume, originating beneath a closed and capped landfill in Central Massachusetts, USA, contains elevated concentrations of arsenic ranging from near normal (5 to 10 ppb) levels to very high levels exceeding in some places 10,000 ppb of As in groundwater. Four different sources of arsenic have been proposed at various times based on specific geochemical, geological, and hydrological indicators without a general consensus if any single source or a combination of sources is the principal source(s) of As. The possible As sources include (1) landfill waste pile; (2) underlying glacial sediments; (3) As mineralized underlying bedrock; and (4) 4 m thick peat layer beneath the waste but overlying the glacial deposits.

Each of the above listed sources of As can be by itself observed as the principal source of arsenic for groundwater contamination at different locations in the vicinity of the studied landfill site. Geochemical indicators at these locations include correlations with other trace metals (Ni, Co, Cu, and others), spatial distribution, concentration levels, redox potential, pH, presence of sulphates, and alkalinity.

Arsenic speciation and variations of stable isotope compositions of groundwater samples (D & 18O) from different sampling points at and near the landfill are addtional geochemical parameters we hope to use to gain a better understanding of arsenic mobilization and arsenic sources. Arsenic species, including inorganic As(III), As(V), plus organic monomethyl arsenic acid (MMA) and dimethyl arsenic acid (DMA) are separated on a HPLC column and analyzed by HG/AFS system. Field samples were collected into 125 mL amber bottles each containing 1.7 mL of 0.1 M EDTA (Samanta and Clifford, 2006) to preserve As speciation from ferrihydroxide precipitation. Preserved and unpreserved samples yielded nearly identical values when analyzed within a few hours after field sampling. After 24 hours the EDTA samples remained unchanged while unpreserved samples lost significant total As as well as As(III). Samples for isotope analysis were collected into separate standard bottles and sent for analysis by a CRDS methodology. Results from both sets of analyses, once completed, will be compared with the existing geochemical parameters.