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Inputs of DOC in precipitation were low and increased with the passage of rainfall through different canopies. Throughfall, stemflow, leachates from A horizons and litterfall were identified as sources of DOC, while B and C horizons in upland areas provide a sink. Throughfall and stemflow displayed high temporal variability in DOC concentrations, while soil leachates and peat waters exhibited strong seasonal patterns. DOC concentrations in throughfall, stemflow and A horizons were highest in the predominantly coniferous site. In the fall, DOC concentrations from A horizons in the deciduous site were significantly higher than those from the coniferous site. / Factors influencing DOC in peat waters are: (1) peat thermal regime, (2) water chemistry, and (3) water table position. Large storms ($>$30 mm precipitation) appear to be the primary factor influencing exports of DOC in streamflow, particularly following dry antecedant soil moisture conditions. Slow rates of water movement through compact deep peats ($>$60 cm depth) and adsorption of DOC in B and C horizons of this catchment obstruct exports of DOC, which over the 5.5 month study period, were minimal in comparison to inputs.

Under iron (Fe3+)-reducing conditions where aqueous Fe2+ and unreduced solid Fe3+-oxides commonly coexist, soil Fe2+ oxidation has been shown to be coupled with nitrate (NO3-) reduction. One possible secondary reaction is the involvement of NO3- and nitrite (NO2-) with Fe-oxide minerals found in many natural environments. Yet, spectroscopic measurements and kinetic data on reactivity of NO3- and NO2- with Fe-containing oxide minerals such as goethite (a-FeOOH), and magnetite (Fe3O­4) are not found in the literature. The reactivity of goethite and magnetite with NO3- and NO2- was studied over a range of environmentally relevant pH conditions (5.5-7.5) with and without added Fe2+(aq) under anoxic conditions. Laboratory experiments were conducted using stirred batch experiments and reaction products were analyzed using ion chromatography (IC), gas chromatography (GC), ultraviolet visible near infrared spectroscopy (UV-VIS-NIR), x-ray diffraction (XRD), scanning electron microscopy (SEM), Mössbauer, and Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. Nitrate removal by goethite and magnetite was much slower when compared with NO2-. There was a pH-dependence in the reduction of NO2-, and the initial rate of NO2- removal was nearly 2 and 8 times faster at pH 5.5 than at pH 7.5 by magnetite and goethite, respectively. Nitric oxide (NO) and nitrous oxide (N2O) were identified as products when NO2- has reacted with magnetite, whereas N2O is the major reaction product in the experiment with goethite. In comparison to experiments containing magnetite or goethite alone, addition of Fe2+ greatly accelerated the NO2- removal rate. Wet chemical experiments combined with the Mössbauer study reveals that NO2- reduction to NO and subsequently to N2O by magnetite occurs via a heterogeneous electron transfer process. ATR-FTIR and diffuse reflectance spectroscopy (DRS) results from the studies with goethite indicate that NO2- was removed from solution by adsorption in a surface complex involving the oxygen atoms, and a portion of the nitrite is reduced to NO and N2O. This study suggests that under anaerobic conditions soil and sediments that contain goethite, magnetite, and other Fe3+-oxides can catalyze abiotic NO2- reduction and the kinetics data from this study can be used to predict the NO2- removal under such conditions.

<p>Methane is an important greenhouse gas, and an ideal target for greenhouse gas emissions reductions. Unlike carbon dioxide, methane has a relatively short atmospheric lifetime, so reductions in methane emissions could have large and immediate impacts on anthropogenic radiative forcing. A more detailed understanding of the global methane budget could help guide effective emissions reductions efforts.</p><p>Humans have greatly altered the methane budget. Anthropogenic methane sources are approximately equal in flux to natural sources, and the current atmospheric methane concentration is ~2.5 times pre-industrial levels. The advent of hydraulic fracturing and resulting increase in unconventional natural gas extraction have introduced new uncertainties in the methane budget. At the same time, the next few decades could be a crucial period for controlling greenhouse gas emissions to avoid irreversible and catastrophic changes in global climate. Natural gas could provide lower-carbon fossil energy, but the climate benefits of this fuel source are highly dependent on the associated methane emissions. In this context of increasing uncertainty and growing necessity, quantifying the impact of natural gas extraction and use on the methane budget is an essential step in making informed decisions about energy.</p><p>In the work presented here, I track methane in the environment to address several areas of uncertainty in our present understanding of the methane budget. I apply the tools of methane analysis in a variety of environments, from rural groundwater supplies to an urban atmosphere, and at a range of scales, from individual point sources to regional flux. I first show that carbon isotopes of methane and co-occurrence of ethane are useful techniques for differentiating a range of methane sources. In so doing, I also show that leaks from natural gas infrastructure are a major source of methane in my study area, Boston, MA. I then build on this work by applying the same methane carbon isotope and ethane signatures to partition methane flux for the Boston metro region. I find that 88% of the methane enhancement in the atmosphere above Boston is due to pipeline natural gas. </p><p>In the final portion of this thesis and the two appendices, I move from the distribution side of the natural gas production chain to extraction, specifically addressing the potential impacts from hydraulic fracturing in my home state of North Carolina. I combine the methane source identification techniques of the previous sections with additional geochemical analyses to document the pre-drilling water quality in the Deep River Triassic Basin, an area which could be drilled for natural gas in the future. This data set is unique in that North Carolina has no pre-existing commercial oil and gas extraction, unlike other states where unconventional gas extraction is currently taking place. This research is, to my knowledge, the first to examine the hydrogeology of the Deep River Basin, in addition to providing an important background data set that could be used to track changes in water quality accompanying hydraulic fracturing in the region in the future.</p> / Dissertation

Phosphorus, a mineral nutrient, is an essential element in aquatic systems. It is only available for biological activity in the form of orthophosphate and soluble restrictive phosphate. Eutrophication, caused by nutrient enrichment, is a problem in many freshwater systems, which results in increased algal blooms, anoxic conditions, and consequently, biodiversity loss and ecosystem failure. Low dissolved oxygen levels trigger the release of sediment bound phosphorus, which reinforces eutrophication. Nutrients in aquatic systems are provided by point and non-point sources and these sources can be affected by several factors, including population, land-use, and climate change. There are many long-term historical phosphorus studies on rivers, but there are very few that are conducted on the Grand River watershed and none that look at factors that might be driving the phosphorus loadings. The Grand River watershed, located in Ontario, Canada, is a highly agricultural watershed with a growing population of approximately one million. It has experienced eutrophication, which has led to excessive production of cyanobacteria and regions of hypoxia. In this study, historical phosphorus concentration data (Total Phosphorus, Soluble Reactive Phosphorus, and Particulate Phosphorus) in five sites along the Grand River were analyzed temporally and spatially from 1965 to 2010 in the upper, middle, and lower parts of the watershed. The Particulate Phosphorus was calculated by subtracting SRP from TP. Several other data such as climate, land-use, geology, and population were also explored and considered as possible factors that may have influenced the trends over time. TP, SRP, and PP average flow weighted concentrations and fluxes were calculated in 2-6 year intervals. SRP load was higher prior to the early 1970???s, declined in the 1970???s, was more stable in the 1980???s and 1990???s, and increased in the 2000???s. The initial decrease in SRP in the early 1970???s was likely due to the phosphorus ban in detergents in 1973 that was implemented over several years. The constant SRP loadings in the 1980???s and 1990???s, despite population and urban development growth, may have been due to upgrades in waste water treatment plants during that time period. The recent increase in phosphorus in more recent years coincides with a large increase in the number of livestock in the 2000???s and population growth. SRP and PP loads increase from upstream to downstream regions are likely due to nutrient accumulation by the river. The higher loads and concentrations of SRP in the CGR is expected because the region is highly urbanized and contains most of the tile drainage in the watershed.

Geologic carbon storage (GCS) is a crucial part of a proposed mitigation strategy to reduce the anthropogenic CO2 emissions to the atmosphere. During this process, CO2 is injected as super critical carbon dioxide (SC-CO2) in confined deep subsurface storage units, such as saline aquifers and depleted oil reservoirs. The deposition of vast amounts of CO2 in subsurface geologic formations may ultimately lead to CO2 leakage into overlying freshwater aquifers. Introduction of CO2 into these subsurface environments will greatly increase the CO2 concentration and will create CO2 concentration gradients that drive changes in the microbial communities present. While it is expected that altered microbial communities will impact the biogeochemistry of the subsurface, there is no information available on how CO2 gradients will impact these communities. The overarching goal of this dissertation is to understand how CO2 exposure will impact subsurface microbial communities at temperature and pressure that are relevant to GCS and CO2 leakage scenarios. To meet this goal, unfiltered, aqueous samples from a deep saline aquifer, a depleted oil reservoir, and a fresh water aquifer were exposed to varied concentrations of CO2 at reservoir pressure and temperature. The microbial ecology of the samples was examined using molecular, DNA-based techniques. The results from these studies were also compared across the sites to determine any existing trends. Results reveal that increasing CO2 leads to decreased DNA concentrations regardless of the site, suggesting that microbial processes will be significantly hindered or absent nearest the CO2 injection/leakage plume where CO2 concentrations are highest. At CO2 exposures expected downgradient from the CO2 plume, selected microorganisms emerged as dominant in the CO2 exposed conditions. Results suggest that the altered microbial community was site specific and highly dependent on pH. The site-dependent results suggests no ability to predict the emerging dominant species for other CO2exposed environments. This body of work improves the understanding of how a subsurface microbial community may respond to conditions expected from geologic carbon storage and CO2 leakage. This is the first step for understanding how a CO2 altered microbial community may impact injectivity, permanence of stored CO2, and subsurface water quality. .

Microbial activity occurs year-round in Arctic soils, including during the winter when soils are frozen. From 2004 to 2008 I monitored soil microbial and nutrient dynamics in low Arctic wet and dry sedge meadows near Churchill, Manitoba. I documented a consistent annual pattern in which soil microbial biomass (MB) and soil nutrients peak in late winter, and decrease during the early stages of spring thaw, remaining in low abundance during the summer. Based on a series of experiments, resource shortages do not appear to be the cause of the microbial decline, as has been hypothesized. Observations and theoretical considerations regarding soil physical properties indicate that this decrease is driven by the influx of liquid water at thaw that brings about a rapid change in the chemical potential of water, leading to cell lysis. I have used 15N isotope tracing to show that inorganic nitrogen is taken up very quickly at thaw by the roots of the dominant plant, Carex aquatilis. This represents a critical window of opportunity for these plants, as nitrogen remains abundant only for a short time. The described annual pattern was pronounced in wet sedge sites, but some inter-annual variation is evident, for example a post-thaw soil nitrogen pulse in 2006, and low winter MB in 2008. In the dry sedge meadow, fluctuations in MB and nutrients were dampened relative to wet sites, and the annual pattern was variable, particularly after 2006. Over four years, peak winter values of soil MB and nutrient variables declined in both wet and dry sites, and this could be related to a drying trend. This work improves our understanding of the controls on decomposition and primary productivity in a system that is experiencing climate warming and increased precipitation. Changes to hydrology, carbon and nitrogen cycling, and primary productivity will have further effects on vegetation communities and higher trophic levels, including several species of migratory birds.

The Changjiang River provides the main source of sediment and terrestrial derived organic carbon (OC) to the Changjiang large delta-front estuary (LDE) in the East China Sea (ECS). This study analyzed bulk OC, biomarkers including lignin and plant pigment, black carbon (BC) on ECS sediments sampled in winter 2009 and 2010 in order to study the OC cycling under the influence of natural and anthropogenic disturbance. Low-oxygen tolerant foraminiferal microfossils were analyzed in another two sediment cores to study the historical hypoxia events in the Changjiang LDE. Bulk carbon to nitrogen (C/N) ratio and stable isotope δ13C in the surface sediment samples indicated a mixture source of terrestrial, deltaic and marine derived OC. Refractory BC and reworked marine OC seemed to comprise most of the OC pool with older, less reactive signatures as deduced from ∆14C, and BC analyses. Winter wind/wave energy and hydrodynamic sorting had a substantial winnowing effect on surface sediment OC redistribution. As a result, the highest lignin concentration shifted to the south during the 2010 cruise after the summer flood event. In addition, algal inputs from local deltaic lakes due to eutrophication and/or lateral transport likely caused the observed lack of benthic-pelagic coupling of pigment concentrations between the surface sediments and the water column after the summer flood in 2010. For the down-core sediment, the mass accumulation rate distribution followed the dispersal pathway of the ECS sediment. Terrestrial and marine derived OC showed significant spatial and temporal distribution. Lignin rich materials were better preserved in sediments closer to the coast while offshore sediments tended to be composed of lignin-poor, degraded OC, that were likely hydrodynamically sorted to a long distance during transport. Besides eutrophication, plant pigments indicated that marine-derived OC was mostly deposited in the sediment mixed layer with decay in the underlying sediment accumulation layer. The total OC standing stock since 1900 is approximately 1.62±1.15 kgC m^-2, about 1/10 of the total OC stock in all the middle and lower lakes in the Changjiang catchment. There has been an increase in the number of hypoxic bottom water events on the Changjiang LDE over the past 60 yrs indicated from the increases in low-oxygen tolerant foraminiferal microfossils due to excess deposition of OC and summer stratification.

Selenium (Se) is a micronutrient, yet elevated Se can be toxic to aquatic organisms. The range of Se concentrations within which Se uptake goes from insufficient to toxic is very narrow. It is thus important to understand the Se biogeochemical cycle in aquatic systems. In this thesis, the study focuses on changes in Se speciation during uptake by green algae. An optimized method was adopted to quantify and speciate Se in water using flow-injection atomic fluorescence spectroscopy coupled with high-pressure liquid chromatography. Details on the method are given here. For the uptake experiments, the uptakes of four Se species (selenite (Se-IV), selenate (Se-VI), selenocystine (Se-Cys) and selenomethionine (Se-Met)) by the green algae Chlamydomonas reinhardtii were compared. This thesis reports that the algae take up higher amounts of organic Se than inorganic Se. Selenomethionine (Se-Met) had the most rapid uptake, during which Se-Cys was produced. For all experiments, Se-IV was produced and found to sorb onto the algae cells, revealing that Se-IV is an important intermediate compound. Mass balance calculations revealed that more than 90% of Se was lost during uptake, probably to the atmosphere. This study also investigated the release of Se during algae decay to simulate the fate of Se during early-diagenesis. Selenium-rich algae cells were mixed with estuarine sediments at the sediment–water interface in a series of column incubations experiments. During the 7-week incubations, Se speciation was measured at the water–sediment interface and in pore water samples. We found that all the Se released to the pore water was in the form of Se-Cys. Although preliminary, these results highlight the key role of organic-Se species in the biogeochemical cycle of Se in the aquatic environment.