ARSENIC TRANSPORT ACROSS THE GROUNDWATER - SURFACE WATER INTERFACE AT A SITE IN CENTRAL MASSACHUSETTS

Plow Shop Pond, located in central Massachusetts within the New England ‘arsenic belt,' receives water from a series of interconnected upstream ponds as well as from upward-discharging groundwater. A small, shallow embayment on the southwest side of the pond is known as Red Cove due to abundant orange iron precipitates on the sediment surface. Groundwater in the vicinity of the Cove is characterized by low oxidation-reduction potential and elevated levels of dissolved arsenic (As), iron (Fe), and other elements. Overburden consists of ~10 m of fine to coarse sands and gravel; average hydraulic conductivity is estimated to be 14 m/d. The horizontal gradient approaching the Cove is about 0.008, based on water-level measurements in surrounding monitoring wells. The resulting groundwater flux is 0.11 m/d. Groundwater chemistry was profiled in two direct-push borings, which gave an average arsenic concentration of 0.43 mg/L. The total As mass flux across the ~120 m perimeter of the Cove is thus estimated to be 17 kg/yr. Over a period of 100 years, the order-of-magnitude age of the pond, the accumulated As total is 1700 kg. The average concentration over the total shallow (<0.3 m) sediment mass in the Cove (area ~1900 m²; dry bulk density 1800 kg/m³) is 1700 mg/kg. In comparison, twelve shallow sediment samples in Red Cove reported As concentrations ranging from 310 to 6800 mg/kg, with a geometric mean of 1400 mg/kg.

The association of reducing groundwater with high Fe and high As concentrations is observed throughout the region. Groundwater As and Fe concentrations are positively correlated, suggesting that reductive dissolution of ferric oxyhydroxides and release of sorbed constituents is responsible for the elevated arsenic in groundwater discharging toward the Cove. In the zone surrounding the redox interface, the formation of iron oxides or sulfides results in coprecipitation of As and other dissolved trace metals. This scenario is supported by the observed heterogeneous distribution of As oxidation states in pore waters and sediments, along with high concentrations of Fe and As in Red Cove sediments. The estimated cumulative mass compares well with observed sediment concentrations, not only for arsenic but also for iron and manganese. The order-of-magnitude agreement supports the proposed accumulation mechanism.