ARSENIC IN GROUNDWATER AND SURFACE WATER AT LANDFILLS IN CENTRAL MASSACHUSETTS

Arsenic at concentrations exceeding 4,000 parts per billion (ppb) have been reported in groundwater and surface water downgradient of many landfills located within central Massachusetts. These landfills are located close to or within a region well documented as having high arsenic concentrations in both bedrock and unconsolidated deposits and where arsenic has been observed in groundwater resources at concentrations greater than 10 ppb. Data obtained from three landfills located within this region show similar hydrochemical profiles to many other landfills previously studied within this area.

At one particular landfill site, arsenic exceeding 4,000 ppb in downgradient groundwater was concurrent with iron concentrations exceeding 100,000 ppb, manganese concentrations greater than 10,000 ppb, chloride concentrations higher 65 parts per million (ppm), and elevated concentrations of dissolved inorganic carbon with respect to upgradient groundwater. Elevated arsenic at this site was also related to oxidation-reduction potential (ORP) values less than -70 millivolts (mV), and dissolved oxygen concentrations less than 0.3 ppm.

Two other sites where arsenic has been observed at concentrations greater than 600 ppb and 4,000 ppb also showed iron at concentrations up to 200,000 ppb, and manganese concentrations up to 8,200 ppb. Surface water with high arsenic concentrations (>20 mg/L) downgradient from one of these sites also showed ORP values of less than -50 mV and chemical oxygen demand greater than 210 ppm. Temporal variations in surface water chemistry at one site may be attributed to hydrologic changes affecting groundwater/surface water interactions.

Chemical signature patterns similar to those observed at these three landfills can also be expected in other landfills located throughout this area of Massachusetts. The similarities between the hydrochemical data from landfills in this region suggest the occurrence high arsenic concentrations in downgradient water systems may be due to the reductive dissolution of naturally occurring arsenic bearing materials.