PERMEABLE REACTIVE BARRIERS (PRB) FOR GROUNDWATER NITROGEN REMOVAL, WAQUOIT BAY, MA

Waquoit Bay is located in the towns of Falmouth and Mashpee on the south side of Cape Cod. It is part of the Waquoit Bay National Estuarine Research Reserve (WBNERR). Accelerated eutrophication, caused by elevated concentrations of nitrogen, has many negative affects on the aquifer ecology of the Waquoit Bay watershed. As part of a Cooperative Institute for Coastal and Estuarine Environmental Technology (CICEET) funded demonstrated project, the Woods Hole Marine Biological Lab teamed with Lombardo Associates, Inc. (LAI) for the installation and testing of LAI's patented NitrexTM groundwater nitrogen removal technology at two locations in the Waquoit Bay watershed. The NitrexTM filter is a slow carbon feed system, which enables heterotrophic denitrification to occur. Media life is expected to be greater than 20 years.

The two installations are:

· Childs River

· Waquoit Bay Reserve Property – 149 Waquoit Highway, East Falmouth, MA

and they were constructed in summer 2005.

The subsurface materials consist of beach sand. Grain size analyses of sediment core samples retrieved at Childs River and the Waquoit Bay Reserve sites indicate that the sediments consist of medium to coarse, silt-free sand with hydraulic conductivity (K) estimated to be 0.06 cm/s (170 ft/day). The effective grain size, d10, is approximately 0.125 mm and the uniformity coefficient (d60/d10) is approximately 1.3.

Groundwater tracer experiments were conducted at the two sites to determine the groundwater flow patterns and velocities. Mini piezometers were installed at the Childs River and Waquoit Bay Reserve sites. The estimated groundwater velocity at the Childs River site is approximately 50 to 80 cm/day (1.64 to 2.62 ft/day). At the Waquoit Bay Reserve site the estimated groundwater velocity is approximately 16 cm/day (0.52 ft/day) from the tracer set.

The dimensions of the two PRB are as follows: Childs River Site: Barrier length - 40 ft; Barrier width - 6 ft; Barrier depth - 5 ft. Waquoit Bay Reserve Site: Barrier length - 65 ft; Barrier width - 12 ft; Barrier depth - 2.5 ft.

Groundwater samples taken by MBL has indicated that the NitrexTM PRB has eliminated all groundwater nitrate from approximately 2 mg/l to non-detect.

The paper will present the results of this and other NitrexTM PRB installations.