REMOVAL OF PHOSPHORUS AND NITROGEN FROM GROUND WATER BY A GEOCHEMICAL BARRIER AT A DISCHARGE POINT TO A KETTLE POND ON CAPE COD, MASSACHUSETTS

Twenty-three months after the installation of a geochemical barrier at the discharge area of a treated-wastewater plume to Ashumet Pond, Cape Cod, MA, results from pond-bottom ground-water sampling indicate a substantial decrease in dissolved phosphorus and nitrogen discharging to the pond. Ashumet Pond is a ground-water fed kettle pond about 500 m downgradient from a former wastewater-disposal site. Previous studies reported elevated concentrations of dissolved phosphorus (up to 3 mg/L) and nitrogen (up to 13 mg/L) in ground water below the pond bottom where the wastewater plume discharges to the pond. Estimated loading rates of phosphorus and nitrogen from the plume to the pond are 110 and 940 kg/yr, respectively.

In August 2004, the Air Force Center for Environmental Excellence installed a geochemical barrier to remove phosphorus from the ground water before it discharged to the pond to reduce eutrophication of the pond. Zero-valent iron was mixed into a volume of pond-bottom sediments (3% by weight) about 0.6 m deep, 12.2 m perpendicular, and 91.4 m parallel to the shore in the area of highest phosphorus concentrations in pond-bottom ground water. In August 2006, water was sampled from 200 drive points installed at depths of 0.15, 0.45, and 0.91 m below the pond bottom to delineate the distributions of phosphorus and nitrogen in ground water at the barrier. The pH and concentrations of dissolved oxygen, phosphorus, and nitrate changed substantially as ground water flowed upward through the barrier. Values of pH increased and dissolved oxygen, phosphorus, and nitrate concentrations decreased at progressively shallower depths within the barrier. These chemical changes were consistent with changes associated with oxidation of the zero-valent iron. Near the barrier, the mean phosphorus concentrations (from GIS interpolation of the point values of concentration) were 1.01, 0.28, and 0.24 mg/L for the 0.91-, 0.45- and 0.15-m sampling depths, respectively. By using model-calculated water fluxes through the barrier and comparing concentrations between the 0.15 m and 0.91 m intervals, estimated mass-flux removal rates are 8.78 x 10-2 kg/d (32.1 kg/yr) for phosphorus, 3.89 x10-2 kg/d (14.2 kg/yr) for nitrate (as N), and 9.58 x 10-4 kg/d (0.35 kg/yr) for ammonium (as N).