COMPUTER SIMULATION OF A SALINE TRACER TEST IN CARIBOU BOG, MAINE

Electrical resistivity measurements and ground-water sampling and chemical analysis have been used to monitor a saline tracer injected into Caribou Bog, a 2200 hectare peatland located in central Maine. Field data were compared to solute transport simulations to aid in the interpretation of anomalous results.

In June 2001, 40 liters of NaBr solution were injected into a peatland through a 7.6 cm diameter pipe slotted from 1.0 to 1.5 meters below ground surface. The concentration of bromide in the injection well has dropped from about 25.0 mmol/l (June, 2001) to about 0.03 mmol/l (May,2004). Surface and cross-borehole electrical resistivity data indicate that the majority of the tracer stayed within a meter of the injection well during the first two years of the tracer test. Low concentrations of bromide have been detected in a well cluster located about 1.8 meters from the injection well. At this location, bromide concentrations were first measured in May, 2002 (.001 mmol/l) and have slowly increased to about .004 mmol/l in May,2004. In contrast to the slow migration of the tracer through the peat, the decrease in saline tracer observed in the injection well is much faster than was anticipated. Ground-water discharge rates based on physical hydrogeologic measurements, the point dilution method, and tracer movement differ by almost an order of magnitude.

A ground-water flow and solute transport program has been created using Python, an interpreted computer language to further evaluate the tracer test data. Initial simulations based on the advection-dispersion equation produced a poor match to field data. The rapid drop in bromide concentration in the tracer well and slow migration through the peat was reproduced in computer simulations that included matrix diffusion. This suggests that peat behaves as a dual-domain medium and that solutes are being pulled into and trapped in portions of the peat where ground-water flow is relatively stagnant.