Title

Geochemical characterization of groundwater flow processes in a large patterned peatland

Date of Award

2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Donald I. Siegel

Keywords

Peatlands, Groundwater flow, Pore water, Geochemical mixing

Subject Categories

Earth Sciences

Abstract

For several decades, scientists have studied the hydrology and geochemistry of Northern peatlands with respect to their influence on potential climate change. The complex hydrology and geochemistry in peatlands lead to unique vegetation, which sequesters carbon in peat soils. Peatlands also creates biogeochemical conditions that induce release of greenhouse gases carbon dioxide and methane. Much of what we know about peatland hydrology and ecology evolved from over 30 years of research in the Glacial Lake Agassiz Peatlands (GLAP), MN. Here, gaseous methane, produced at depth, was first identified as a major potential contributor of methane flux to the atmosphere, along with diffusive transport to the atmosphere. The deep methane forms at the interface between downward moving groundwater under raised bogs and upwards moving groundwater from underlying mineral soils. It remains uncertain how groundwater upwells into these peatlands, keeps the peat saturated, and sustains vegetation growth, even under drought conditions. Moreover, seasonal variability in peatland hydrology controls dissolved organic carbon (DOC) mineralization, consequently releasing or sequestering greenhouse gases.

In my dissertation, I have investigated three aspects of the peatland geochemical puzzle that bear on solute movement in large circumboreal peatlands and alter hydrology within peat due to methane formation. For this work, I sampled pore water from seven bog-fen complexes in the study area over a period of three years for geochemical and isotopic analyses.

In chapter 1, I report the results of synnoptic studies of the pore water chemical and isotopic composition in multiple bog-fen complexes found in different hydrogeologic settings. This study was designed to better characterize the geochemical mixing and sources of pore water in peat. In chapter 2, I addressed the effect of decadal scale seasonal wet-dry cycles on the geochemical and isotopic composition of peat pore water and net changes in the rate of deep methanogenesis. In the 3 rd chapter, I heuristically assessed how recharging acidic water and upwelling minerotropic water affect the pore water composition. For this, I built a module in a geochemical computational program PHREEQC, incorporating organic acid dissociation constants to assess the acid-base equilibrium of pore waters in peat.

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