MONITORING PHOSPHATE REMOVAL BY A POND-BOTTOM ZERO-VALENT IRON REACTIVE BARRIER BY USING HORIZONTAL AND VERTICAL MULTIPORT SAMPLERS, VERTICAL DIFFUSION CHAMBERS, AND SEEPAGE METERS, CAPE COD, MA

The Air Force Center for Environmental Excellence installed a zero-valent-iron reactive barrier on a pond bottom on Cape Cod, MA, in August 2004 to remove phosphate from a ground-water contaminant plume discharging to the pond. The plume resulted from land disposal of treated wastewater from 1936 to 1995 and contains phosphate (PO₄) concentrations as high as 6 mg/L (as P). The reactive barrier, which contains a mixture of native sediment and added iron (about 3% by weight), is 1 m thick, extends from 0-12 m from shore, and is about 90 m wide parallel to shore. To monitor the effectiveness of phosphate removal, various types of custom-designed sampling devices were installed in the barrier during its emplacement because it was uncertain which devices would still be operable after the installation process and subsequently be effective in the substantially altered and amended sediments. Vertical multiport wells with 5 sampling ports spaced 24-37 cm apart were installed at 10 locations. Horizontal multiport samplers with 15 ports and extending to about 12 m from shore were installed in pairs at 15 and 90 cm below the pond bottom at two locations. Diffusion chambers with 13 removable 60-mL sampling bottles spaced 6-12 cm apart vertically were installed at 8 locations. Permanent seepage meters with removable lids were installed in pairs at four locations. All sampling devices were operable after installation, and water samples from the devices have been collected three times (11/04, 4/05, and 7/05) since the barrier installation. Data from the multiport samplers and diffusion chambers show that, as the plume discharges upward through the barrier to the pond, phosphate concentrations decrease from as high as 3 mg/L PO₄ (as P) below the barrier to less than 0.1 mg/L near the pond bottom. Of the various devices, the diffusion chambers provide the most detailed vertical chemical profiles in the barrier, but yield only 60-mL samples, whereas the multiport samplers provide less vertical detail but larger-volume samples for chemical analysis.