Illuminating Permeable Mineral Soil Groundwater Seepage Pathways Feeding Peatland Pools Using Thermal and Electrical Conductivity Signatures

Abstract

Wetland environments are well documented to contain unique hydrogeomorphic subsystems that benefit from nutrient and temperature regimes provided by upwelling groundwater sources. Matrix seepage and preferential flow can both serve as groundwater inputs that control carbon-cycling within these environments. Recent work in a northern boreal peatland of Maine illuminates parallel dynamics to other wetland environments, with matrix seepage and preferential flow pathways (PFPs) identified and quantified proximal to peatland pools. PFPs around the peatland pools have been interpreted as peat pipes, known to transport nutrients within the peat matrix. Thermal signatures surrounding the peatland pool sources were mapped using point temperature measurements, handheld thermal imagery, and airborne thermal infrared mapping. Electrical geophysical methods were deployed to image the structure and stratigraphy of the underlying mineral sediments to delineate the source of focused upwelling around the peatland pools. Ground-penetrating radar (GPR) surveys show discontinuities in the impermeable glacio-marine clay controlling the hydrogeomorphic development of the peatlands studied. These mineral soil discontinuities in the GPR surveys, interpreted to be regional glacial esker deposits, are located proximal to the overlying peatland pools. Electromagnetic induction surveys were deployed to map the bulk electrical conductivity structures associated with the near-surface geology beneath the peatland pools. Point specific conductance measurements were taken at identified zones of thermal anomalies to further validate contrasts between peat pore water and mineral soil groundwater in the peatlands. Water samples were collected at the seepage sites and analyzed for iron and manganese trace elements to support the hypothesis that upwelling occurs from permeable glacial esker deposits. Focused groundwater inputs into peatlands may define a key hydrogeomorphic development process for peatland pool systems and the surrounding ecology. Further, these inputs could have implications for carbon-cycling, building on the established regional relationship between groundwater flow and carbon transport. Illuminating the focused groundwater flowpaths and interpreting their hydrogeologic origins may serve as a basis for future carbon-cycling exploration within peatlands at novel, fine-scales.