NS24A-01 INVITED
The Acid-Base Balance Between Organic Acids and Circumneutral Ground Waters in Large Peatlands
Organic acids supply most of the acidity in the surface waters of bogs in peatlands. Yet, the fundamental geochemical properties of peatland organic acids are still poorly known. To assess the geochemical properties of typical organic acid assemblages in peatlands, we used a triprotic analog model for peat pore waters and surface waters in the Glacial Lake Agassiz Peatlands, optimizing on charge balance and calibrated to estimates of mole site density in DOC and triprotic acid dissociation constants. Before the calibration process, all bog waters and 76% of fen waters had more than +20% charge imbalance. After calibration, most electrochemically balanced within 20%. In the best calibration, the mole site denisty of bog DOC was estimated as ~0.05 mmol/mmol C., approximately 6 times smaller than that for fen DOC or the DOC in the fen deeper fen peats that underlie bogs. The three modeled de-protonation constants were; pKa1 = ~3.0, pKa2 = ~4.5 and pKa3 = ~7.0 for the bog DOC, and; pKa1 = ~5.2, pKa2 =~ 6.5 and pKa3 = ~7.0 for the fen DOC. Bog DOC, behaves as a strong acid despite its small mole site density. The DOC in bog runoff can therefore theoretically acidify the surface waters in adjacent fens wherever these waters do not receive sufficient buffering alkalinity from active groundwater seepage.
NS24A-02 INVITED
An Overview of Gas Ebullition and Bog Breathing (Mooratmung) in Northern Peatlands
Current estimates of methane emissions from northern peatlands are largely based on flux measurements from chambers and eddy covariance towers that are not well suited for sampling ebullition (i.e. bubbling), which varies in time and space. However, even the older literature contains evidence for methane ebullition manifested by Wil o" the Wisp or bubbling bog pools. In contrast, the older reports of small topographic oscillations on peatlands were related to changes in water storage rather than gas volume. In 1981-82 we measured topographic oscillations exceeding 10 cm on a raised bog in Minnesota that were associated with an anomalous zone of overpressure at 1 m depth. We later used a local GPS network to measure topographic oscillations at a nearby bog and fen site that exceeded 36 cm both vertically and horizontally over a period of 12 hours. These oscillations occurred synchronously with short depressuring cycles in a persistent zone of overpressure in the deeper peat. MRI imaging indicated that this zone of abnormal pressure was produced by the buildup of biogenic gas bubbles trapped under confining layers that episodically ruptured to release large volumes of gas to the atmosphere. We calculated that the 3 largest ebullition events in 1997 released 136 g methane per square meter, a value that exceeded the annual average value measured in chambers by an order of magnitude. These high flux values were supported by independent hydrogeologic methods at our site and also by TDR at a site in Canada by another group. As a result methane emissions from peatlands may be dominated by ebullition, which may represent an underreported source for atmospheric methane.
NS24A-03
Evaluation of ground-water sampling and resistivity data to monitor a saline tracer test in a Maine peatland.
Ground-water sampling and geophysical resistivity measurements 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. In this area within the peatland, hydraulic gradients of $5 \cdot 10^{-4}$ and hydraulic conductivities of about $10^{-3}$ cm/m were measured, suggesting (assuming n$_e$=0.1) a ground-water velocity of about 1.5 m/year. 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). These concentration data have been evaluated using the point dilution method, and the point dilution analysis indicates a ground-water velocity of about 18 m/year. Electrical resistivity measurements imaged the majority of the tracer 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. Ground-water flow rates based on physical hydrogeologic measurements, the point dilution method, and monitoring the tracer migration differ by more than an order of magnitude. To further evaluate the tracer test, ground-water flow and solute transport has been simulated. 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 accounts for the anomalous monitoring data.
NS24A-04
The use of Geophysical Techniques to Monitor the Evolution and Spatial Distribution of Biogenic Gasses in Peat Soils
Considerable uncertainty exists regarding the mechanisms of formation and the spatial distribution of biogenic gasses in peat soils. In this work we investigate the utility of two geophysical measurements, resistivity and ground penetrating radar (GPR), to image the evolution and spatial location of gas formation in peat blocks. Two blocks were extracted from a large freshwater peatland in Maine, transported to the laboratory within 36 hours, and kept at 5$\deg$C until the experiment began. Geophysical measurements were made as temperature was increased 3$\deg$C d$^{-1}$ from 5$\deg$C until 23$\deg$C was reached and held constant. One block was instrumented with 5 electrode arrays (1 cm electrode separation) and the second block was constructed to permit high-frequency GPR measurements (1.2 GHz frequency). Methane emissions and surface peat deformation were monitored concurrently by using a portable methane detector and elevation rods respectively. The results demonstrate that geophysical measurements can provide unique information on the spatial distribution of biogenic gasses forming in peat soils. These findings also have implications for the monitoring of temporal behavior of biogenic gas emissions to the atmosphere from peatlands and its impact on global warming.
NS24A-05
Radar Monitoring of Hydrology in Maryland's Coastal Plain Wetlands: Implications for Predicted Climate Change and Improved Management
Wetlands provide important services to society but wetlands in the Mid-Atlantic United States are at high risk for loss, with forested wetlands being especially vulnerable. Hydrology (i.e., flooding and soil moisture) controls wetland function and extent, but it may be altered due to changes in climate and anthropogenic influence. Wetland hydrology must be better understood in order to predict and mitigate the impact of these changes. Broad-scale forested wetland hydrology is difficult to monitor using ground-based and traditional remote sensing methods. C-band synthetic aperture radar (SAR) data could improve the capability to monitor forested wetland hydrology, but the abilities and limitations of these data need further investigation. This study examined: 1) the link between climate and wetland hydrology; 2) the ability of ENVISAT ASAR (C-HH and C- VV) data to monitor inundation and soil moisture in forested wetlands; 3) limitations inherent to C-band data (i.e., polarization and plant phenology) when monitoring forested wetland hydrology; and 4) the accuracy of forested wetland maps produced using SAR data. The study was conducted at the Patuxent Wildlife Research Center in Maryland, U.S.A. This study showed that: 1) climate was highly correlated with wetland inundation; 2) significant differences in C-VV and C-HH backscatter existed between forested areas of varying hydrology (uplands and wetlands) throughout the year; 3) C-HH backscatter was better correlated to hydrology than C- VV backscatter; 4) correlations were stronger during the leaf-off season; and 5) maps produced using SAR data had relatively high accuracy levels. Based on these findings, I concluded that hydrology is influenced by climate at the study site, C-HH data should be able to monitor changes in hydrology throughout the year, and C-band SAR has the potential to increase the ability to map forested wetlands. The methods developed have the potential to fill the need of managers for increased hydrologic information and improved forested wetland maps.
NS24A-06
Low Atmospheric Pressure Triggers Methane Ebullition From Peatland
Peatland is widely regarded as a significant source of atmospheric CH$_4$, a potent greenhouse gas. At present, most of the information on environmental emissions of CH$_4$ comes from infrequent, temporally discontinuous ground-based flux measurements. Enormous efforts have been made to extrapolate measured emission rates to establish seasonal or annual amounts using relevant biogeochemical factors, such as water table positions or peat temperatures, however, little is known about the variation of CH$_4$ flux in a timescale of individual flux measurement. Once produced in the waterlogged peat sediment, CH$_4$ can be emitted into the atmosphere through any of the three following pathways: diffusion of dissolved CH$_4$, transport via vascular plants, or release of CH$_4$-containing gas bubbles, i.e., ebullition. Although there is some circumstantial evidence for the sudden release of accumulated gas from peat, no direct evidence has been presented. Consequently, some controversy exists over the occurrence of ebullition itself as well as its significance as a mechanism for CH$_4$ transport from peat to the atmosphere. A 90-hour field study was undertaken at an ombrotrophic peatland located in Hokkaido, Japan (141$^\circ$48'E, 43$^\circ$19'N), to determine whether or not CH$_4$ ebullition into the atmosphere from peat soil occurs, and, if it does, to identify the factors that control it. We found that reductions in air pressure enlarged the volume of the gas bubbles stored in waterlogged peat and caused eventual ebullitions of the free- phase CH$_4$ from peat into the atmosphere. As as result, the CH$_4$ flux can change by two orders of magnitude within a matter of tens of minutes. Contributions of the ebullition to the total CH$_4$ flux during the measurements were significant (50-64%). These results clearly indicated that field campaigns must be designed to cover this rapid temporal variability caused by ebullition, which may be especially important under intemperate weather. Process-based CH$_4$ emission models should also be modified to include air pressure as a key factor for the control of ebullient CH$_4$ release from peatland.