ROLE OF DISSOLVED INORGANIC CARBON IN ARSENIC MOBILIZATION AT LANDFILL SITES IN CENTRAL MASSACHUSETTS, USA

Central Massachusetts is characterized by a N-S trending region with significantly elevated arsenic levels in the unconsolidated strata overlying rocks of the Merrimack Belt. Arsenic concentrations within the overburden (10 to 1000 mg/kg) can pose a threat to the environment if and when the inert arsenic becomes mobilized by groundwater. Many landfills in this area, now capped and closed, were constructed in the past without a leachate collection system and are presently discharging their leachate into the subsurface system. Data from the long term post-closure monitoring of landfills and our data from five landfill sites show that arsenic is present in high levels, exceeding in some places 1000 micrograms per liter. A comprehensive effort was initiated to obtain a fully characterized set of data on 24 hydrogeochemical parameters of 60 samples collected from upgradient to directly downgradient of the five landfills. The database, augmented by data from a continuing longterm monitoring, indicates that reductive dissolution of hydrous ferric oxide (HFO) veneered surfaces is the most likely mechanism of arsenic release. We present data on dissolved organic carbon, dissolved inorganic carbon, arsenic, oxidation-reduction potential, pH, and specific conductance for 57 samples from five landfill sites to illustrate the role of carbon in the mechanism of reductive dissolution of HFO coatings. Total dissolved carbon varies from 4.9 ppm C to 164.6 ppm C, lower values being about equally divided between dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC). This ratio changes and becomes exclusively DIC at higher total C values. Arsenic above 100 ppb positively correlates with DIC in excess of 10 ppm C and negatively with ORP below 100 mV. Specific conductance rises to near 900 microSiemens and pH increases to neutral values at 7. These data suggests that landfill leachate enriched in organic carbon triggers a microbial activity near the discharge point into the groundwater system (DOC becomes DIC) creating a reducing environment that transforms HFO and other sorbed constituents into their soluble forms (increased specific conductance) which neutralize the pH to near neutral values.