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Automated Monitoring for the Zero-Valent Iron Wall Project at the Massachusetts Military Reservation Research Site

by Jennifer Savoie, and Gregory Granato, Hydrologists

The Massachusetts Military Reservation's Installation Restoration Program has installed two permeable zero-valent iron walls, or curtains, in series within the plume downgradient from the CS-10 source area. The zero-valent iron will be installed to enhance abiotic degradation of the hydrocarbons. As the plume moves through the permeable wall, the zero-valent iron will become oxidized and reductively dechlorinate the organic compounds producing harmless compounds, such as water, hydrogen, and chloride, and aliphatic hydrocarbons, such as methane and ethane (which are harmless in low concentrations)(fig. 1). Two walls are to be installed in series to guarantee the degradation of the PCE. The walls are to be fifty feet wide and will intercept the vertical extent of the plume, from about 80 to the 140 ft below ground surface. The water table is located about 80 ft below ground surface.

The presence of the zero-valent iron is expected to create a very reducing environment, altering the existing ground-water chemistry and microbial population. The U.S. Geological Survey (USGS), through it's Toxics Substance Hydrology Program and National Research Program (NRP), with cooperation from the Air Force Center for Environmental and the University of Waterloo, is conducting a study to examine the initial and long-term effects of the zero-valent iron on the ground-water chemistry and microbial population downgradient from the wall. Changes in ground-water chemistry and microbial population will be assessed using a series of manual sampling rounds that will collect samples to be analyzed for cations, anions, metals, colloids, nitrate, ammonia, dissolved inorganic carbon, dissolved organic carbon, total organic carbon, methane, nitrous oxide, bacterial size and abundance, and volatile organic compounds (VOCs). The field properties pH, alkalinity, specific conductance, temperature, turbidity, and dissolved oxygen will also be determined at each site as part of the manual sampling program at the time of sampling. Samples will be collected before and after installation of the reactive wall to assess the nature, extent, and effects of geochemical changes.

The automated ground-water monitoring system was installed in a well downgradient from the walls that is screened from 100 to 102 feet below ground surface. The system utilizes a bladder pump with packer to pump and purge the screened interval of the well until the automated control system determines that the purge criteria have been met (a minimum number of screen volumes and geochemical stability indicated by water-quality probe readings) and records the final measurements for the sampling period (typically 1-2 days). The automated ground-water monitoring system measures the field parameters dissolved oxygen, pH, specific conductance, and temperature. The automated ground-water monitoring system started measuring the field parameters before the zero-valent iron wall was installed. The USGS will use this remote monitoring device as an early-warning system to indicate water-quality changes. The researchers also may use the real time record to help interpret geochemical changes between manual sampling rounds.

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