Using hydrologic measurements to investigate free-phase gas ebullition in a Maine peatland, USA

C E Bon,L Slater,A S Reeve,X Comas

doi:10.5194/hess-18-953-2014

Abstract

Abstract. Northern peatlands cover more than 350 million ha and are an important source of methane (CH4) and other biogenic gases contributing to climate change. Free-phase gas (FPG) accumulation and episodic release has recently been recognized as an important mechanism for biogenic gas flux from peatlands. It is likely that gas production and groundwater flow are interconnected in peatlands: groundwater flow influences gas production by regulating geochemical conditions and nutrient supply available for methanogenesis, while FPG influences groundwater flow through a reduction in peat permeability and by creating excess pore water pressures. Water samples collected from three well sites at Caribou Bog, Maine, show substantial dissolved CH4 (5–16 mg L−1) in peat waters below 2 m depth and an increase in concentrations with depth. This suggests production and storage of CH4 in deep peat that may be episodically released as FPG. Two min increment pressure transducer data reveal approximately 5 cm fluctuations in hydraulic head from both deep and shallow peat that are believed to be indicative of FPG release. FPG release persists up to 24 h during decreasing atmospheric pressure and a rising water table. Preferential flow is seen towards an area of relatively lower hydraulic head associated with the esker and pool system. Increased CH4 concentrations are also found at the depth of the esker crest, suggesting that the high permeability esker is acting as a conduit for groundwater flow, driving a downward transport of labile carbon, resulting in higher rates of CH4 production.

Highlights

Northern peatlands cover ∼ 10 % of land north of 45◦ N (3 % of Earth’s surface) and contain about one-third of all soil carbon (Gorham, 1991; Wigley and Schimel, 2000; Rydin et al, 2006)
Headspace in deep wells fitted with gas traps contained up to 3300 ppm CH4, but showed no displacement, indicating CH4 concentrations in wells are derived from diffusion, while shallow to medium wells showed no detectable concentrations of CH4
free-phase gas (FPG) may be released during initial installation of monitoring wells, the installation of monitoring wells at this study site has little to no effect on long-term gas pressure regimes in the peatland subsurface and does not allow FPG trapped in peat pore spaces to escape

Summary

Introduction

Northern peatlands cover ∼ 10 % of land north of 45◦ N (3 % of Earth’s surface) and contain about one-third of all soil carbon (Gorham, 1991; Wigley and Schimel, 2000; Rydin et al, 2006). Net carbon accumulation rates in northern peatlands have been modeled at 76 Tg C yr−1 and rates of CH4 have been modeled at 46 Tg CH4-C yr−1, contributing 5–10 % of total terrestrial CH4 flux to the atmosphere (Gorham, 1991). These numbers will likely need revision with the emerging importance of free-phase gas (FPG) emissions from peatlands. The two major constituent FPGs produced in the peatland subsurface are CO2 and, the focus of this study, CH4 (Tokida et al, 2007b)

Methods

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Discussion

Conclusion