THE MIGRATION OF A SODIUM BROMIDE TRACER IN A LARGE MAINE PEATLAND

A solute transport experiment is in progress within Caribou Bog, a 2200 hectare peatland located in central Maine. Hydraulic head measurements indicate that ground-water flow in this peatland shift seasonly, with similar patterns observed over several years. In June 2001, 40 liters of NaBr solution were injected into the peat through a 7.6 cm diameter pipe slotted from 1.0 to 1.5 meters below ground surface. The saline tracer has been monitored using electrical resistivity measurements and ground-water sampling. The concentration of bromide in the injections well has dropped from about 25.0 mmol/l (June 2001) to about 0.19 mmol/l (May 2002) and resistivity measurements collected in July 2002 indicate that the center of the saline plume has migrated about 1 meter. The leading edge of the plume appears to have reached one cluster of monitoring wells, located 1.8 m from injection well, in May 2002. Bromide remained below the detection limit (0.002 ppm) in 4 other clusters of wells positioned near the injection well. The migration rate is much slower than the ground-water velocities estimated using Darcy's law (3 m/yr) or the point dilution method (18 m/yr), indicating the plume migration is slowed by physiochemical processes such as sorption of matrix diffusion. The evolution of the NaBr plume will continue to be monitored through ground-water sampling and geophysical measurements.