Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2017 / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 125-138). / Tropical peatlands in Southeast Asia have sequestered carbon over thousands of years and are an important global carbon stock. In natural peat swamp forests, high water levels inhibit decomposition due to anoxic conditions. However, they are being rapidly deforested and drained, releasing stored carbon to the atmosphere. In this thesis, we investigate the carbon dioxide and methane fluxes from both pristine and degraded peat swamp forests in Borneo using field measurements, modeling and remote sensing. We first study methane fluxes from natural peatlands. We use an isotope-based mass transport model to evaluate the extent of methane production, transport and oxidation. We find an order of magnitude more methane is produced than surface fluxes suggest. This dissolved methane is transported belowground to the rivers and streams draining peatlands. However, much of this methane is oxidized before reaching the atmosphere. We then study CO₂ emissions from peatlands. At the local scale, we use automated soil respiration chambers to assess how CO₂ emissions depend on temperature and water table. At a regional scale, we use remote sensing to investigate carbon losses due to peatland degradation. Drainage of peatlands enables peat decomposition and results in subsidence of the land surface. We track this subsidence using InSAR satellite data and use it to quantify regional CO₂ emissions. The spatial resolution of our technique allows us to uncover correlations with past and present land uses and peatland hydrology. / by Alison May Hoyt. / Ph. D. / Ph.D. Massachusetts Institute of Technology, Department of Civil and Environmental Engineering

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2017 / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 137-144). / The Persian Gulf (also known as the Arabian Gulf) is a shallow marginal sea located in a region of Southwest Asia with arid climate. Because of this aridity, evaporation from the Gulf is extremely high (1.84 meters per year) and far exceeds the sum of freshwater inflow to the Gulf, mainly from precipitation and some from river flow. A residual circulation exists between the Gulf and the Indian Ocean such that saline water flows into the Gulf from the Indian Ocean in order to balance the freshwater deficit due to high evaporation rate; and since evaporation only removes freshwater, the salt associated with this inflow from the Indian Ocean is removed in dense saline water that naturally sinks to lower layers and exits the Gulf into the Indian Ocean. / The Gulf is also the source of seawater and sink of hypersaline effluent (brine) for many desalination plants representing about 50% of the world's seawater desalination capacity, because many of the countries on the Gulf have increasing human populations but no renewable freshwater resources. It has been recognized long ago that the impact of brine discharge on the Gulf cannot be discussed separately from the dynamics of the residual circulation, but to date there is no basin scale environmental analysis for the impact of seawater desalination activity on the Gulf. A coupled Gulf-Atmosphere Regional Model that simulates the residual circulation between the Gulf and the Indian Ocean is used to investigate the impact of desalination on the salinity of the Gulf. Satellite data on sea surface temperature and water surface elevation are used to constrain the model, and to analyze trends in the Gulf. Four main contributions of this thesis are emphasized. / First, the equilibrium state variables describing the Gulf are identified, including a new estimate of Gulf basin salinity of ~~ 40.5 - 41g/kg. Second, it is shown that desalination has minimal impact on salinity averaged at the basin scale but significant impact at regional scale. Changes in regional salinities are used to quantify the impact of desalination in different regions within the Gulf. Third, a specific criterion for placement of brine discharge systems in the Gulf that minimizes regional salt accumulation is proposed in order to guide planning and design of future seawater desalination plants, as well as management of existing plants. Lastly, trend analysis shows a small trend in sea surface temperature (~~2°C during 1993 - 2014), most likely caused by climate change, but slight trends in Gulf basin salinity that are likely due to natural variability. / by Hamed D. Ibrahim. / Ph. D. / Ph.D. Massachusetts Institute of Technology, Department of Civil and Environmental Engineering

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2017 / Cataloged from PDF version of thesis. / Includes bibliographical references. / Polychlorinated biphenyls (PCBs), which were a widely used group of chemicals before their manufacture was banned in 1979 in the United States, still contaminate numerous aquatic systems. Although there are ongoing efforts to remediate these sites, our ability to implement effective remediation strategies has been hindered by a limited understanding of how PCBs are transported in the environment as well as how they bioaccumulate into aquatic animals. Investigations of contaminated sites often rely on sediment measurements since PCB concentrations are highest in the sediment phase. However, previous research has shown that sediment concentrations are a poor predictor of contaminant fate and effects, whereas freely dissolved water concentrations were able to predict the extent of bioaccumulation and the onset of toxicological effects more accurately. / Additionally, the concentration gradient between the porewater (i.e., the interstitial water in sediment beds) and the overlying water is thought to be a major driver in PCB transport from the sediment bed. Passive samplers, which are made of polymers like polyethylene (PE), accumulate PCBs when they are deployed into environmental media, can be used to estimate freely dissolved water concentrations. When used in the laboratory by mixing with sediment samples, they provide a measure of the sediment-water equilibrium porewater concentration. When deployed in the field, they can provide measures of the in situ porewater and overlying water concentrations. In this work, PE samplers were deployed at two Superfund sites contaminated with PCBs to investigate if in situ porewater and sediments were at equilibrium with each other. / In situ porewater concentrations were consistently found to be lower than equilibrium concentrations at both sites, an effect that was likely caused by water flow through the sediment bed. At one of the sites, measurements of a geochemical tracer also showed that the exchange of porewater with overlying water was occurring and that the transport of PCBs from sediments was affected by more than just the concentration gradient. Lastly, the sorptive disequilibria between environmental media and the spatial heterogeneity of contamination were shown to affect the extent of bioaccumulation in aquatic animals at one of the sites. / by Jennifer Nicole Apell. / Ph. D. / Ph.D. Massachusetts Institute of Technology, Department of Civil and Environmental Engineering

Thesis: S.M. in Transportation, Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 95-98). / Although automated data collection systems have been in existence long enough to be the subject of extensive research, they continue to transform both the agency and customer side of public transportation. Transit system performance is defined along three dimensions: the supply of trains or buses; the demand from passengers; and the product of the two, service performance. This research takes a broad view of these dimensions, and explores several means of measuring each one, ways in which they interact, and how this information can be valuable for a transit agency. This thesis focuses specifically on the London Underground, but the intent is that the types of analysis described herein can be applied to any rail transit system with similar data resources. The research has three parts. First, a methodology is developed to estimate passenger volumes on the portion of a line between two adjacent stations within a defined time interval. This approach can be implemented using data from either smart card or entry and exit gates. It relies upon the outputs of London's Rolling Origin Destination Survey to infer passenger route choice. The second component identifies the possible causes of poor service performance in terms of both supply and demand, and defines a framework for examining supply and demand data to identify which causes are contributors. This research suggests that a better understanding of capacity constraints can be gained by jointly analyzing AVL-based capacity measures, AFC-based demand measures, and AFC journey times as a measure of service performance. Finally, this thesis explores the possibility of using smart card data in real time to estimate system state. This metric is defined as passenger accumulation, a measure of the number of passengers on a given portion of a line in real time. Building from the approach developed in the first section, this work designs a method to determine accumulation in real time at detailed levels of spatial granularity. The accumulation metric is then compared to Transport for London's existing tools to assess whether accumulation can provide value as a real-time indicator of system state. Based on this analysis and current data availability constraints it cannot be concluded that passenger accumulation provides a reliable indicator of real-time system state. / by Lauren Tarte. / S.M. in Transportation

Thesis: S.M., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 44-46). / This paper presents an experimental study of the behavior of oil plumes in ambient stratification and a mild current, in which the flow is classified as stratification-dominated. Experiments are conducted in an inverted framework by releasing a continuous stream of dense glass beads from a carriage towed in a salt-stratified tank. Non-dimensional particle slip velocity UN ranged from 0.1 to 1.9. While particles of all sizes were affected by the stratification, only those with UN less or equal to 0.5 were observed to enter the intrusion layer. The spatial distributions of beads, collected on a bottom sled towed with the source, present a Gaussian distribution in the transverse direction and a skewed distribution in the along-current direction. Dimensions of the distributions increase with decreasing UN. The spreading relations can be used as input to far-field models describing subsequent transport. The average particle settling velocity, Uave, was found to exceed the individual particle slip velocity, Us, which is attributed to the initial plume velocity near the point of release. Additionally, smaller particles exhibit a "secondary plume effect" as they exit the intrusion as a swarm. The secondary effect becomes more prominent as UN decreases. These findings might explain the observations from the 2000 Deep Spill field experiment where oil was found to surface more rapidly than predicted based on Us. An analytical model predicting the particle deposition patterns, was developed based on findings above and validated against experimental measurements. The model estimates near-field oil transport under the Deepwater Horizon spill conditions, with and without chemical dispersants. / by Dayang Wang. / S.M.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1999. / Includes bibliographical references. / At present, very little is known about the mechanisms that control flow and transport through wetland deposits. Therefore, the objective of this research was to investigate the factors that influenced flow in wetland soils. To accomplish this, Column Experiments, run on a specialized permeameter, were conducted on both re-sedimented and undisturbed wetland soil specimens. During these experiments, sodium chloride (NaCl) tracer was injected into the soil specimen, and its breakthrough was monitored concurrently with other parameters, such as flow velocity and hydraulic gradient. Subsequently, the breakthrough data collected during the Column Experiments were fit using both the One-Region and Two-Region transport models, and the fit results were analyzed and compared to the geotechnical data collected for the soil. The data collected during the experimental program indicate enormous complexity in the mechanisms controlling flow and transport through wetland soils. From their analysis the following observations were made: First, even though wetland soils are considered to be two-region soils, having both an effective and an immobile porosity, the One-Region model was able to describe Sodium Chloride breakthrough in the soil. This indicates that the NaCl tracer was not interacting with the immobile region of the specimen. Second, the results demonstrated that wetland soil hydraulic conductivity is highly variable and sensitive to volume of flow. In fact, hydraulic conductivity was seen to decrease irreversibly by up to 6% per pore volume of flow. It was also found that hydraulic conductivity was sensitive to increases in pore water salt concentration, and to the flushing out of salts from wetland specimens. Finally, it was observed that, for the most part, large changes in hydraulic conductivity did not correspond to changes in the specimen's effective pore size or pore distribution. In fact, unless salt concentrations were increased drastically, the effective pore space remained invariant over an order of magnitude change in soil hydraulic conductivity. This suggests that changes in soil hydraulic conductivity might be due to increases or decreases in the number of flow channel constrictions in a specimen. From the results of this research it is hypothesized that the number of flow channel constrictions increased when flow and a decrease in salt concentration mobilized organic and mineral particles, which collected and clogged narrow pore throats along the flow channels. It is also hypothesized that the number of flow channel constrictions decreased when increases in pore water salt concentration causect organic fibers along the flow channel walls to coil. / by Lana A. Aref. / Sc.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1999. / Includes bibliographical references (p. 713-732). / Drilled shafts are widely used to transfer heavy structural loads (both axial and lateral) through the overburden soil to the underlying rock mass. It is important to consider the effect of discontinuities when designing drilled shafts in rock. This thesis develops (1) a new design method for axially loaded rock-socketed shafts; (b) new design methods for laterally loaded rock-socketed shafts; and (c) a method for assessing the three dimensional (30) geometric characteristics of discontinuities. The new design method for axially loaded rock-socketed shafts concentrates on the prediction of the ultimate side shear resistance and the end bearing capacity. New relations have been developed for both the side shear resistance and the end bearing capacity. Both developments are derived from the analysis of ex.tensive databases. For laterally loaded rock-socketed shafts, a simplified design method and a detailed design method were developed. The simplified design method assumes that the rock mass is isotropic and uses an elasto-plastic continuum model to predict the load-displacement response. This model can consider drilled shafts in rock overlain by soil and it was validated by comparing its results with available theoretical solutions and field test data. The detailed design method considers the effect of anisotropy of jointed rock masses. The key components of this method are the 30 geometric characterization of discontinuities and the estimation of the anisotropic defonnation and strength properties of jointed rock masses. The 30 characterization of discontinuities uses the observed trace data in a finite circular window (on an exposed outcrop or at the bottom of a shaft hole) to determine the corrected trace length distribution, the discontinuity size (diameter) distribution, the total number of discontinuities in an objective volume, and finally the fracture tensor which describes the intensity and orientation of all discontinuities in a rock mass. The method for estimating the anisotropic elastic deformation properties uses the superposhion-principle and adopts an equivalent continuum approach. Hill's criterion is used to describe the anisotropic yield/failure of jointed rock masses. The method was then applied to examine the effect of anisotropy on the lateral deformation of rock-socketed shafts. Also, design charts were developed for considering the effect of anisotropy when predicting the elastic defonnation of laterally loaded rock-socketed shafts. Finally, the applicability of the recommended design methods is discussed and a design example is presented. / by Lianyang Zhang. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1999. / Includes bibliographical references. / Sulfate reducing bacteria are believed to be the most important methylators of mercury in aquatic systems. Methylation may be controlled by the availability of mercury to these bacteria and therefore may depend on mercury speciation in sulfidic water ... / by Jennifer Ayla Jay. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1999. / Includes bibliographical references. / The goal of this thesis is to study the relationship between the community structure of marine microorganisms and nutrient availability. To this end, size spectra of microbes were studied over a range of nutrient regimes, both natural and manipulated. Three transects in the Atlantic provided a natural range of nutrient environments, especially because they captured seasonal variations. The transects encompassed Sargasso Sea, Gulf Stream, and coastal waters, during winter, spring, and summer. Nutrient regimes ranged from surface waters of the Sargasso Sea during stratified periods (low-nutrient), to deeply mixed waters in all three regions of the transects during winter and spring (high-nutrient). Complementing natural variations in nutrients, two experiments were used to study the effects of enrichment on size structure. An in situ iron-enrichment experiment conducted in the equatorial Pacific (IronEx II) provided a unique opportunity to monitor changes in community structure following increased nutrient availability. In a second experiment in the Sargasso Sea, enrichments with nitrogen and phosphorus were conducted in bottles, because one or both are commonly thought to be limiting in this region. In order to carry out the goal of this thesis, which depended on the use of flow cytometry to characterize bacterio-, pico-, ultra-, and nanophytoplankton, advancements were made in methods for enumerating a wide range of cell sizes and for estimating cell size from forward angle light scatter. In addition, because ambient concentrations of inorganic nitrogen and phosphorus are exceedingly low (<10nM) in the Atlantic, especially during stratified periods, low-level determinations of these nutrients were made to compliment the analyses of community structure. Size structure varied systematically, although not necessarily as a function of nutrient availability. Two parameters were explored: 1) spectral slope, which indicates the relative contribution of large and small cells to total biomass, and 2) spectral shape, or adherence of the spectra to relationships explained by a power law. The relative ranking of the slopes from specific regions of the transects remained constant throughout different seasons. Shapes ranged from discontinuous to those which adhered to a power law. It is hypothesized that only microbial systems with abundant nutrient inputs and, perhaps, reduced grazing pressure, have smooth spectra whose shapes conform to power laws. / by Kent Keller Cavender-Bares. / Ph.D.

Thesis: M. Eng., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 39-43). / A new inlet for studying the composition of mixed-phase clouds - the phaSe seParation Inlet for Droplets icE residuals and inteRstitial aerosols (SPIDER) - is described. SPIDER combines an omni-directional inlet, a Large-Pumped Counterflow Virtual Impactor (L-PCVI), a flow tube as evaporation chamber, and a Pumped Counterflow Virtual Impactor (PCVI) to separate droplets, ice crystals, and interstitial aerosols for simultaneous sampling. Verification tests of each individual component of SPIDER were positive, as was the result of investigating that the components work together as a whole setup without flow blockage or choking. SPIDER was installed at Mt. Washington Observatory (MWO), a mountain top research facility in the White Mountains, for a two-week field campaign. There, SPIDER showed promising performance as a field instrument and provided first data that suggest its capability of separating distinct cloud particles. Future design improvements of SPIDER are suggested along with potential locations for field measurements. / by Libby Koolik. / M. Eng.