TWO DECADES OF HYDROGEOPHYSICAL MEASUREMENTS IN CARIBOU BOG (MAINE, USA) INVESTIGATING GROUNDWATER FLOW AND FREE-PHASE (METHANE) GAS

Geophysical and hydrological data collected in Caribou Bog (Maine, USA) over the past two decades have been used to 1) characterize the morphology of the peat basin, 2) identify an esker buried beneath the peat, 3) assess groundwater flow patterns within this peatland, and 4) monitor biogenic free-phase gas movement and release from this peat deposit. In this presentation, past work in Caribou Bog is reviewed and ongoing efforts to integrate data sets into computer simulations of peatland dynamics are introduced. Data collection has included the installation of monitoring wells to measure hydraulic head and hydraulic gradients (with particular emphasis on the impact of the esker deposit), collection of GPR data to assess the morphology of the peat basin and free-phase gas content at different depths within the peat column, and autonomous resistivity monitoring to quantify changes in the free-phase gas content over short time intervals (days to weeks). Current work focuses on using gas traps, moisture probes, and measuring hydraulic head fluctuations to better define the depths that free-phase gas occurs and the timing of ebullition events. Gas traps are monitored with time-lapse photography to record the timing of free-phase gas release from different peat depths. Anomalous hydraulic head fluctuations, apparent in plots of hydraulic head data collected at 2 minute intervals, are hypothesized to be related to the movement and release of biogenic gas. Hydraulic head data sets are being evaluated using different mathematical approaches (wavelets, short time FFT's), as well as simple visual inspection of data plots, in an effort to quantify patterns in ebullition within Caribou Bog.