INITIATING A SOLUTE TRANSPORT EXPERIMENT IN A LARGE MAINE PEATLAND

Dispersive mixing in peatlands, driven by lateral flow, may drive solute fluxes vertically from the mineral soil upward to the peat surface and thereby explain observed geochemical and vegetation patterns in large peatlands. Longitudinal and transverse dispersivities of only 0.5 and 0.05 m, respectively, are sufficient to supply solutes to the peat surface in the absence of upward ground-water flow. To evaluate dispersive mixing in peatland systems, we have initiated a tracer test in the Southern Complex of Caribou Bog (Bangor, Maine). The long-term water-level trend, measured over a three-year period in the Southern Complex, appears to be relatively constant, with similar water levels recorded during the same season in each year. Vertical flow reversals occur from spring to late summer with upwelling from the mineral sediments into the peat in the spring and with downward flow into the mineral sediments in the summer. The concentration of 40 liters of NaBr solution injected into a 7.6 cm diameter PVC pipe screened over a 0.5 m interval decreased from about 15 ppm to 2.0 ppm bromide in the injection well over a two month period. Bromide remained below the detection limit (0.002 ppm) in hydraulically down gradient monitoring wells located 2 m from the injection well. Non-invasive electrical conductivity imaging indicates minimal displacement of bromide away from the injection location over the same period. The evolution of the NaBr plume will continue to be monitored through ground-water sampling and three dimensional electrical imaging.