Biogeochemistry of a New England Sphagnum bog.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil Engineering, 1977.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING.Bibliography: leaves 150-160.Ph.D

Persistence of bubble outlets in soft, methane-generating sediments

Sediments submerged beneath many inland waterways and shallow oceans emit methane, a potent greenhouse gas, but the magnitude of the methane flux to the atmosphere remains poorly constrained. In many settings, the majority of methane is released through bubbling, and the spatiotemporal heterogeneity of this ebullition both presents challenges for measurement and impacts bubble dissolution and atmospheric emissions. Here we present laboratory-scale experiments of methane ebullition in a controlled incubation of reconstituted sediments from a eutrophic lake. Image analysis of a 0.14 m2 sediment surface area allowed identification of individual bubble outlets and resolved their location to ∼1 cm. While ebullition events were typically concentrated in bursts lasting ∼2 min, some major outlets showed persistent activity over the scale of days and even months. This persistence was surprising given the previously observed ephemerality of spatial structure at the field scale. This persistence suggests that, at the centimeter scale, conduits are reopened as a result of a drop in tensile strength due to deformation of sediments by the rising bubbles. The mechanistic insight from this work sheds light on the spatiotemporal distribution of methane venting from organic-rich sediments and has important implications for bubble survival in the water column and associated biogeochemical pathways of methane.National Science Foundation (U.S.) (1045193)United States. Department of Energy (DE-FE0013999

Geometric Design of Scroll Expanders Optimized for Small Organic Rankine Cycles

The application of organic Rankine cycles (ORCs) for small scale power generation is inhibited by a lack of suitable expansion devices. Thermodynamic and mechanistic considerations suggest that scroll machines are advantageous in kilowatt-scale ORC equipment, however, a method of independently selecting a geometric design optimized for high-volume-ratio ORC scroll expanders is needed. The generalized 8-dimensional planar curve framework (Gravesen and Henriksen, 2001, “The Geometry of the Scroll Compressor,” Soc. Ind. Appl. Math., 43, pp. 113–126), previously developed for scroll compressors, is applied to the expansion scroll and its useful domain limits are defined. The set of workable scroll geometries is: (1) established using a generate-and-test algorithm with inclusion based on theoretical viability and engineering criteria, and (2) the corresponding parameter space is related to thermodynamically relevant metrics through an analytic ranking quantity fc (“compactness factor”) equal to the volume ratio divided by the normalized scroll diameter. This method for selecting optimal scroll geometry is described and demonstrated using a 3 kWe ORC specification as an example. Workable scroll geometry identification is achieved at a rate greater than 3 s⁻¹ with standard desktop computing, whereas the originally undefined 8-D parameter space yields an arbitrarily low success rate for determining valid scroll mating pairs. For the test case, a maximum isentropic expansion efficiency of 85% is found by examining a subset of candidates selected the for compactness factor (volume expansion ratio per diameter), which is shown to correlate with the modeled isentropic efficiency (R² = 0.88). The rapid computationally efficient generation and selection of complex validated scroll geometries ranked by physically meaningful properties is demonstrated. This procedure represents an essential preliminary qualification for intensive modeling and prototyping efforts necessary to generate new high performance scroll expander designs for kilowatt scale ORC systems.United States. Environmental Protection Agency (SU 83436701

Effects of Eutrophication and Runoff on Arsenic Cycling in an Urban Lake

Urban lakes are important recreational and natural resources that add to the quality of life for city residents. Unfortunately, urban watersheds often contribute contaminants to these lakes, including organic chemicals, metals, nutrients, and pathogens. Nitrogen and phosphorus are very high in urban and suburban runoff, mostly as a result of animal waste and fertilizers, although leaky sewage systems may also contribute. These nutrients promote plant and algal growth in urban lakes, ultimately resulting in hyper-eutrophic conditions. Eutrophication, in turn, may affect the cycling and mobility of contaminants, such as arsenic and other toxic metals. Spy Pond, located in Arlington, Massachusetts, was recently discovered to be heavily contaminated with arsenic of unknown origin. Surface sediment concentrations above 2,500 ppm have been measured. Subsequent investigations have also revealed that total arsenic levels in the overlying hypolimnetic waters reach over 150 ppb. However, the two interconnected basins that constitute Spy Pond have been found to differ by an order of magnitude in the concentrations of arsenic found in hypolimnetic waters. The goal of this study is to determine the mechanisms responsible for the differences in arsenic mobility in the two basins of Spy Pond, and how this may impact the potential for minimizing human and ecological arsenic exposure. Based on differences in the concentrations of chemical constituents (e.g. iron, sulfur, conductivity, etc.) measured in each basin, we hypothesize that the greater arsenic concentrations found in the bottom waters of the South Basin of Spy Pond are caused by the combined effects of eutrophication, differences in the Fe/S ratio of the two basins, and the physical and chemical impacts of salts in highway runoff

A differential pressure instrument with wireless telemetry for in-situ measurement of fluid flow across sediment-water boundaries

© 2009 The Authors. This article is distributed under the terms of the Creative Commons Attribution (3.0) License. The definitive version was published in Sensors 9 (2009): 404-429, doi:10.3390/s90100404.An instrument has been built to carry out continuous in-situ measurement of small differences in water pressure, conductivity and temperature, in natural surface water and groundwater systems. A low-cost data telemetry system provides data on shore in real time if desired. The immediate purpose of measurements by this device is to continuously infer fluxes of water across the sediment-water interface in a complex estuarine system; however, direct application to assessment of sediment-water fluxes in rivers, lakes, and other systems is also possible. Key objectives of the design include both low cost, and accuracy of the order of ±0.5 mm H2O in measured head difference between the instrument’s two pressure ports. These objectives have been met, although a revision to the design of one component was found to be necessary. Deployments of up to nine months, and wireless range in excess of 300 m have been demonstrated

Nonequilibrium clumped isotope signals in microbial methane

Methane is a key component in the global carbon cycle with a wide range of anthropogenic and natural sources. Although isotopic compositions of methane have traditionally aided source identification, the abundance of its multiply-substituted “clumped” isotopologues, e.g., 13CH3D, has recently emerged as a proxy for determining methane-formation temperatures; however, the impact of biological processes on methane’s clumped isotopologue signature is poorly constrained. We show that methanogenesis proceeding at relatively high rates in cattle, surface environments, and laboratory cultures exerts kinetic control on 13CH3D abundances and results in anomalously elevated formation temperature estimates. We demonstrate quantitatively that H2 availability accounts for this effect. Clumped methane thermometry can therefore provide constraints on the generation of methane in diverse settings, including continental serpentinization sites and ancient, deep groundwaters.National Science Foundation (U.S.) (EAR-1250394)National Science Foundation (U.S.) (EAR-1322805)Deep Carbon Observatory (Program)Natural Sciences and Engineering Research Council of CanadaDeutsche Forschungsgemeinschaft (Gottfried Wilhelm Leibniz Program)United States. Dept. of Defense (National Defense Science and Engineering Graduate Fellowship)Neil & Anna Rasmussen FoundationGrayce B. Kerr Fund, Inc. (Fellowship)MIT Energy Initiative (Shell-MITEI Graduate Fellowship)Shell International Exploration and Production B.V. (N. Braunsdorf and D. Smit of Shell PTI/EG grant

A conduit dilation model of methane venting from lake sediments

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 38 (2011): L06408, doi:10.1029/2011GL046768.Methane is a potent greenhouse gas, but its effects on Earth's climate remain poorly constrained, in part due to uncertainties in global methane fluxes to the atmosphere. An important source of atmospheric methane is the methane generated in organic-rich sediments underlying surface water bodies, including lakes, wetlands, and the ocean. The fraction of the methane that reaches the atmosphere depends critically on the mode and spatiotemporal characteristics of free-gas venting from the underlying sediments. Here we propose that methane transport in lake sediments is controlled by dynamic conduits, which dilate and release gas as the falling hydrostatic pressure reduces the effective stress below the tensile strength of the sediments. We test our model against a four-month record of hydrostatic load and methane flux in Upper Mystic Lake, Mass., USA, and show that it captures the complex episodicity of methane ebullition. Our quantitative conceptualization opens the door to integrated modeling of methane transport to constrain global methane release from lakes and other shallow-water, organic-rich sediment systems, and to assess its climate feedbacks.This work was supported by the U.S. Department of Energy (grants DE‐FC26‐06NT43067 and DE‐AI26‐05NT42496), an NSF Doctoral Dissertation Research grant (0726806), a GSA Graduate Student Research grant, and MIT Martin, Linden and Ippen fellowships

Ephemerality of discrete methane vents in lake sediments

Methane is a potent greenhouse gas whose emission from sediments in inland waters and shallow oceans may both contribute to global warming and be exacerbated by it. The fraction of methane emitted by sediments that bypasses dissolution in the water column and reaches the atmosphere as bubbles depends on the mode and spatiotemporal characteristics of venting from the sediments. Earlier studies have concluded that hot spots—persistent, high-flux vents—dominate the regional ebullitive flux from submerged sediments. Here the spatial structure, persistence, and variability in the intensity of methane venting are analyzed using a high-resolution multibeam sonar record acquired at the bottom of a lake during multiple deployments over a 9 month period. We confirm that ebullition is strongly episodic, with distinct regimes of high flux and low flux largely controlled by changes in hydrostatic pressure. Our analysis shows that the spatial pattern of ebullition becomes homogeneous at the sonar's resolution over time scales of hours (for high-flux periods) or days (for low-flux periods), demonstrating that vents are ephemeral rather than persistent, and suggesting that long-term, lake-wide ebullition dynamics may be modeled without resolving the fine-scale spatial structure of venting.National Science Foundation (U.S.) (1045193)United States. Department of Energy (DE-FE001399