Accumulation and fate of mercury in an Everglades aquatic food web

This project examined the pathways of mercury (Hg) bioaccumulation and its relation to trophic position and hydroperiod in the Everglades. I described fish-diet differences across habitats and seasons by analyzing stomach contents of 4,000 fishes of 32 native and introduced species. Major foods included periphyton, detritus/algal conglomerate, small invertebrates, aquatic insects, decapods, and fishes. Florida gar, largemouth bass, pike killifish, and bowfin were at the top of the piscine food web. Using prey volumes, I quantitatively classified the fishes into trophic groups of herbivores, omnivores, and carnivores. Stable-isotope analysis of fishes and invertebrates gave an independent and similar assessment of trophic placement. Trophic patterns were similar to those from tropical communities. I tested for correlations of trophic position and total mercury. Over 4,000 fish, 620 invertebrate, and 46 plant samples were analyzed for mercury with an atomic-fluorescence spectrometer. Mercury varied within and among taxa. Invertebrates ranged from 25–200 ng g −1 ww. Small-bodied fishes varied from 78–>400 ng g −1 ww. Large predatory fishes were highest, reaching a maximum of 1,515 ng−1 ww. Hg concentrations in both fishes and invertebrates were positively correlated with trophic position. I examined the effects of season and hydroperiod on mercury in wild and caged mosquitofish at three pairs of marshes. Nine monthly collections of wild mosquitofish were analyzed. Hydroperiod-within-site significantly affected concentrations but it interacted with sampling period. To control for wild-fish dispersal, and to measure in situ uptake and growth, I placed captive-reared, neonate mosquitofish with mercury levels from 7–14 ng g−1 ww into field cages in the six study marshes in six trials. Uptake rates ranged from 0.25–3.61 ng g−1 ww d −1. As with the wild fish, hydroperiod-within-site was a significant main effect that also interacted with sampling period. Survival exceeded 80%. Growth varied with season and hydroperiod, with greatest growth in short-hydroperiod marshes. The results suggest that dietary bioaccumulation determined mercury levels in Everglades aquatic animals, and that, although hydroperiod affected mercury uptake, its effect varied with season. ^

Human impacts on pig frog (Rana grylio) populations in south Florida wetlands: Harvest, water management and mercury contamination

This study investigated how harvest and water management affected the ecology of the Pig Frog, Rana grylio. It also examined how mercury levels in leg muscle tissue vary spatially across the Everglades. Rana grylio is an intermediate link in the Everglades food web. Although common, this inconspicuous species can be affected by three forms of anthropogenic disturbance: harvest, water management and mercury contamination. This frog is harvested both commercially and recreationally for its legs, is aquatic and thus may be susceptible to water management practices, and can transfer mercury throughout the Everglades food web. ^ This two-year study took place in three major regions: Everglades National Park (ENP), Water Conservation Areas 3A (A), and Water Conservation Area 3B (B). The study categorized the three sites by their relative harvest level and hydroperiod. During the spring of 2001, areas of the Everglades dried completely. On a regional and local scale Pig Frog abundance was highest in Site A, the longest hydroperiod, heavily harvested site, followed by ENP and B. More frogs were found along survey transects and in capture-recapture plots before the dry-down than after the dry-down in Sites ENP and B. Individual growth patterns were similar across all sites, suggesting differences in body size may be due to selective harvest. Frogs from Site A, the flooded and harvested site, had no differences in survival rates between adults and juveniles. Site B populations shifted from a juvenile to adult dominated population after the dry-down. Dry-downs appeared to affect survival rates more than harvest. ^ Total mercury in frog leg tissue was highest in protected areas of Everglades National Park with a maximum concentration of 2.3 mg/kg wet mass where harvesting is prohibited. Similar spatial patterns in mercury levels were found among pig frogs and other wildlife throughout parts of the Everglades. Pig Frogs may be transferring substantial levels of mercury to other wildlife species in ENP. ^ In summary, although it was found that abundance and survival were reduced by dry-down, lack of adult size classes in Site A, suggest harvest also plays a role in regulating population structure. ^

Design and fabrication of two switched superconductivity microstrip hairpin filters using series micro-electro-mechanical systems (MEMS) switches

Series Micro-Electro-Mechanical System (MEMS) switches based on superconductor are utilized to switch between two bandpass hairpin filters with bandwidths of 365 MHz and nominal center frequencies of 2.1 GHz and 2.6 GHz. This was accomplished with 4 switches actuated in pairs, one pair at a time. When one pair was actuated the first bandpass filter was coupled to the input and output ports. When the other pair was actuated the second bandpass filter was coupled to the input and output ports. The device is made of a YBa2Cu 3O7 thin film deposited on a 20 mm x 20 mm LaAlO3 substrate by pulsed laser deposition. BaTiO3 deposited by RF magnetron sputtering in utilized as the insulation layer at the switching points of contact. These results obtained assured great performance showing a switchable device at 68 V with temperature of 40 K for the 2.1 GHz filter and 75 V with temperature of 30 K for the 2.6 GHz hairpin filter. ^

Temperature dependence and characterization of gold thin film actuated membranes for radio frequency microelectromechanical switches

Over the last 10 years, the development and the understanding of the mechanical properties of thin film material have been essential for improving the reliability and lifetime in operation of microelectromechanical systems (MEMS). Although the properties of a bulk material might be well characterized, thin-film properties are considerably different from those of the bulk and it cannot be assumed that mechanical properties measured using bulk specimens will apply to the same materials when used as a thin film in MEMS. For many microelectronic thin films, the material properties depend strongly on the details of the deposition process and the growth conditions on its substrate. ^ The purpose of this dissertation is to determine the temperature dependence of a gold thin film membrane on the pull down voltage of a MEMS switch as the temperature is varied from room temperature (300 K) to cryogenic temperature (10 K). For this purpose, an RF MEMS shunt switch was designed and fabricated. The switch is composed of a gold coplanar waveguide structure with a gold bridge membrane suspended above an area of the center conductor which is covered by a dielectric (BaTiO3). The gold membrane is actuated by an electrostatic force acting between the transmission line and the membrane when voltage is applied. ^ Material characterization of the gold evaporated thin film membrane was obtained via AFM, SEM, TEM and X-ray diffraction analyses. A mathematical relation was used to estimate the pull down voltage of the switch at cryogenic temperature and results showed that the mathematical theory match the experimental values of the tested MEMS switches. ^

Biogeochemical factors affecting the fate and transport of mercury in the terrestrial environment at Oak Ridge, Tennessee

Mercury (Hg) contamination problem in the United Sates has been an important issue due to its potential threat to human and ecological health. This thesis presents a study of two Hg-contaminated sites along the East Fork Poplar Creek (EFPC) at Oak Ridge. The top soils from the terrestrial areas, along with the soils from three vertical soil horizons at the EFPC bank were sampled and analyzed for total-Hg (THg), methyl-Hg, total-organic-carbon (TOC), and pH. The stream bank soils were also analyzed for the stable-Hg-isotopes (198Hg, 199Hg, 200Hg, 201Hg, and 202Hg). Furthermore, some of the soil samples (n=7) from the same study sites were investigated for phytoavailability of mercury as measured by degree of Hg translocation in aboveground biomass of Impatiens walleriana plants grown in the soils.^ The results showed a significant difference (p<0.001) in THg concentrations for the forest soils (42.40±4.98 mg/kg) and the grassland soils (8.71±2.30 mg/kg). The higher THg and methyl-Hg concentrations were commensurate with the higher TOC content in the soils (p<0.001). Also, the THg concentrations for the upstream site was higher (129.08±34.14 mg/kg) than the downstream site (24.31±3.47 mg/kg). The two sites also differed in their stable Hg isotope compositions (p<0.001 for δ199Hg). The stable isotope analysis indicated the increased level of mass dependent isotopic fractionation with increasing depths along the EFPC bank. The difference between the two study sites was also prominent in case of the Hg uptake by the plants, with higher Hg uptake from the upstream soils compared to that from the downstream soils. A significant correlation, r=0.93 p<0.01, was observed between the Hg uptake and the soil-THg concentrations. THg was higher in the leaves (1161.87±310.01 μg/kg) than in the flowers (206.13±55.23 μg/kg) or the stems (634.54±403.35μg/kg). ^ The level of Hg contamination increased with decreasing distance from the point source and was highly influenced by plants/microbes, soil-organic-content, and Hg-speciation. The isotopic study indicated the existence of an additional Hg source in the EFPC watershed, possibly atmospheric Hg-deposition. These findings are worth taking into account while planning any Hg remediation effort and developing Hg loading criteria as per the National Pollutant Discharge Elimination System (NPDES) Program.^

An integrated flow and transport model to study the impact of mercury remediation strategies for East Fork Poplar Creek watershed, Oak Ridge, Tennessee

An integrated flow and transport model using MIKE SHE/MIKE 11 software was developed to predict the flow and transport of mercury, Hg(II), under varying environmental conditions. The model analyzed the impact of remediation scenarios within the East Fork Poplar Creek watershed of the Oak Ridge Reservation with respect to downstream concentration of mercury. The numerical simulations included the entire hydrological cycle: flow in rivers, overland flow, groundwater flow in the saturated and unsaturated zones, and evapotranspiration and precipitation time series. Stochastic parameters and hydrologic conditions over a five year period of historical hydrological data were used to analyze the hydrological cycle and to determine the prevailing mercury transport mechanism within the watershed. Simulations of remediation scenarios revealed that reduction of the highly contaminated point sources, rather than general remediation of the contaminant plume, has a more direct impact on downstream mercury concentrations.

Efficient asynchronous scheduling algorithms for cost-effective buffered crossbar switches

Buffered crossbar switches have recently attracted considerable attention as the next generation of high speed interconnects. They are a special type of crossbar switches with an exclusive buffer at each crosspoint of the crossbar. They demonstrate unique advantages over traditional unbuffered crossbar switches, such as high throughput, low latency, and asynchronous packet scheduling. However, since crosspoint buffers are expensive on-chip memories, it is desired that each crosspoint has only a small buffer. This dissertation proposes a series of practical algorithms and techniques for efficient packet scheduling for buffered crossbar switches. To reduce the hardware cost of such switches and make them scalable, we considered partially buffered crossbars, whose crosspoint buffers can be of an arbitrarily small size. Firstly, we introduced a hybrid scheme called Packet-mode Asynchronous Scheduling Algorithm (PASA) to schedule best effort traffic. PASA combines the features of both distributed and centralized scheduling algorithms and can directly handle variable length packets without Segmentation And Reassembly (SAR). We showed by theoretical analysis that it achieves 100% throughput for any admissible traffic in a crossbar with a speedup of two. Moreover, outputs in PASA have a large probability to avoid the more time-consuming centralized scheduling process, and thus make fast scheduling decisions. Secondly, we proposed the Fair Asynchronous Segment Scheduling (FASS) algorithm to handle guaranteed performance traffic with explicit flow rates. FASS reduces the crosspoint buffer size by dividing packets into shorter segments before transmission. It also provides tight constant performance guarantees by emulating the ideal Generalized Processor Sharing (GPS) model. Furthermore, FASS requires no speedup for the crossbar, lowering the hardware cost and improving the switch capacity. Thirdly, we presented a bandwidth allocation scheme called Queue Length Proportional (QLP) to apply FASS to best effort traffic. QLP dynamically obtains a feasible bandwidth allocation matrix based on the queue length information, and thus assists the crossbar switch to be more work-conserving. The feasibility and stability of QLP were proved, no matter whether the traffic distribution is uniform or non-uniform. Hence, based on bandwidth allocation of QLP, FASS can also achieve 100% throughput for best effort traffic in a crossbar without speedup.

Reduced organic sulfur: Analysis and interaction with mercury in the aquatic environment

Reduced organic sulfur (ROS) compounds are environmentally ubiquitous and play an important role in sulfur cycling as well as in biogeochemical cycles of toxic metals, in particular mercury. Development of effective methods for analysis of ROS in environmental samples and investigations on the interactions of ROS with mercury are critical for understanding the role of ROS in mercury cycling, yet both of which are poorly studied. Covalent affinity chromatography-based methods were attempted for analysis of ROS in environmental water samples. A method was developed for analysis of environmental thiols, by preconcentration using affinity covalent chromatographic column or solid phase extraction, followed by releasing of thiols from the thiopropyl sepharose gel using TCEP and analysis using HPLC-UV or HPLC-FL. Under the optimized conditions, the detection limits of the method using HPLC-FL detection were 0.45 and 0.36 nM for Cys and GSH, respectively. Our results suggest that covalent affinity methods are efficient for thiol enrichment and interference elimination, demonstrating their promising applications in developing a sensitive, reliable, and useful technique for thiol analysis in environmental water samples. The dissolution of mercury sulfide (HgS) in the presence of ROS and dissolved organic matter (DOM) was investigated, by quantifying the effects of ROS on HgS dissolution and determining the speciation of the mercury released from ROS-induced HgS dissolution. It was observed that the presence of small ROS (e.g., Cys and GSH) and large molecule DOM, in particular at high concentrations, could significantly enhance the dissolution of HgS. The dissolved Hg during HgS dissolution determined using the conventional 0.22 μm cutoff method could include colloidal Hg (e.g., HgS colloids) and truly dissolved Hg (e.g., Hg-ROS complexes). A centrifugal filtration method (with 3 kDa MWCO) was employed to characterize the speciation and reactivity of the Hg released during ROS-enhanced HgS dissolution. The presence of small ROS could produce a considerable fraction (about 40% of total mercury in the solution) of truly dissolved mercury (< 3 kDa), probably due to the formation of Hg-Cys or Hg-GSH complexes. The truly dissolved Hg formed during GSH- or Cys-enhanced HgS dissolution was directly reducible (100% for GSH and 40% for Cys) by stannous chloride, demonstrating its potential role in Hg transformation and bioaccumulation.

Legacy and Fate of Mercury and Methylmercury in the Florida Everglades

Mass inventories of total Hg (THg) and methylmercury (MeHg) and mass budgets of Hg newly deposited during the 2005 dry and wet seasons were constructed for the Everglades. As a sink for Hg, the Everglades has accumulated 914, 1138, 4931, and 7602 kg of legacy THg in its 4 management units, namely Water Conservation Area (WCA) 1, 2, 3, and the Everglades National Park (ENP), respectively, with most Hg being stored in soil. The current annual Hg inputs account only for 1−2% of the legacy Hg. Mercury transport across management units during a season amounts to 1% or less of Hg storage, except for WCA 2 where inflow inputs can contribute 4% of total MeHg storage. Mass budget suggests distinct spatiality for cycling of seasonally deposited Hg, with significantly lower THg fluxes entering water and floc in ENP than in the WCAs. Floc in WCAs can retain a considerable fraction (around 16%) of MeHg produced from the newly deposited Hg during the wet season. This work is important for evaluating the magnitude of legacy Hg contamination and for predicting the fate of new Hg in the Everglades, and provides a methodological example for large-scale studies on Hg cycling in wetlands.

Audit pricing, reporting quality, and auditor switches in the post -SOX period

The Sarbanes-Oxley Act represents an important watershed event in the history and regulation of the accounting profession. In this dissertation, I develop arguments as to why we can expect differences in auditor behavior before and after SOX and empirically test if indeed there were differences in auditor behavior before and after SOX. My dissertation consists of three essays. For the three essays, I investigate issues related to auditor independence, audit pricing, the impact of auditor changes in the post-SOX period. The motivation for the first part of my research comes from the SEC's assertions that there are differences between types of non-audit services in terms of their potential to adversely impact auditor independence. The first part of my dissertation empirically validates the SEC's assertions that auditors would be more conservative in those instances where the tax and other non-audit services fee ratios are high but not when the audit-related fee ratio is high. The second part of my study examines if auditors are less likely to "low ball" their audit fees in the period after SOX than in the period preceding SOX. Legislators, regulators, and the media have expressed concerns that auditors "low ball" the fees for initial year audits and that such low-balling can lead to reduced audit quality. I find that there is significant initial year audit fee discount in pre-SOX period and but the fee discount does not hold in post-SOX periods. The third part of my dissertation examines the association between auditor switches and auditor conservatism. I find that a large portion of Big 4 clients switch to non-Big 4 auditors and there is no significant evidence indicating that successor auditors are more conservative in the post-SOX period.

Development of an enhanced hydro-geochemical model to address mercury-speciation fate and transport in aquatic environments

An awareness of mercury (Hg) contamination in various aquatic environments around the world has increased over the past decade, mostly due to its ability to concentrate in the biota. Because the presence and distribution of Hg in aquatic systems depend on many factors (e.g., pe, pH, salinity, temperature, organic and inorganic ligands, sorbents, etc.), it is crucial to understand its fate and transport in the presence of complexing constituents and natural sorbents, under those different factors. An improved understanding of the subject will support the selection of monitoring, remediation, and restoration technologies. The coupling of equilibrium chemical reactions with transport processes in the model PHREEQC offers an advantage in simulating and predicting the fate and transport of aqueous chemical species of interest. Thus, a great variety of reactive transport problems could be addressed in aquatic systems with boundary conditions of specific interest. Nevertheless, PHREEQC lacks a comprehensive thermodynamic database for Hg. Therefore, in order to use PHREEQC to address the fate and transport of Hg in aquatic environments, it is necessary to expand its thermodynamic database, confirm it and then evaluate it in applications where potential exists for its calibration and continued validation. The objectives of this study were twofold: 1) to develop, expand, and confirm the Hg database of the hydrogeochemical PHREEQC to enhance its capability to simulate the fate of Hg species in the presence of complexing constituents and natural sorbents under different conditions of pH, redox, salinity and temperature; and 2) to apply and evaluate the new database in flow and transport scenarios, at two field test beds: Oak Ridge Reservation, Oak Ridge, TN and Everglades National Park, FL, where Hg is present and is of much concern. Overall, this research enhanced the capability of the PHREEQC model to simulate the coupling of the Hg reactions in transport conditions. It also demonstrated its usefulness when applied to field situations.

Areal and vertical distribution of total soil mercury and total phosphorusin the southern half of water conservation area 3-A, Everglades, Southern Florida

Total soil-mercury and phosphorus concentrations were determined in 64 sites in the southern half of Water Conservation Area 3A, an area of approximately 500 km2 . Surface soil-Hg concentrations ranged from 117 to 300 ng-g-1;total phosphorus concentrations range from 350 to 850 pg~g-1. No consistent north-south or east-west trends are found in the mercury or phosphorus surface concentrations when they are normalized to soil bulk density. Nine sites were used for the determination of the vertical distribution of soilmercury. Vertical profiles of soil-Hg revealed decreasing concentrations with depth and correlated well with phosphorus in soil profiles. Mercury concentrations in soil profiles may be interpreted as an increase in the rate of deposition of mercury in the region in recent decades and/or as postdepositionalmobilization of mercury to surface layers.

A model of the hydrothermal system at Casa Diablo in Long Valley, California, based on resistivity profiles and soil mercury analyses

A description and model of the near-surface hydrothermal system at Casa Diablo, with its implications for the larger-scale hydrothermal system of Long Valley, California, is presented. The data include resistivity profiles with penetrations to three different depth ranges, and analyses of inorganic mercury concentrations in 144 soil samples taken over a 1.3 by 1.7 km area. Analyses of the data together with the mapping of active surface hydrothermal features (fumaroles, mudpots, etc.), has revealed that the relationship between the hydrothermal system, surface hydrothermal activity, and mercury anomalies is strongly controlled by faults and topography. There are, however, more subtle factors responsible for the location of many active and anomalous zones such as fractures, zones of high permeability, and interactions between hydrothermal and cooler groundwater. In addition, the near-surface location of the upwelling from the deep hydrothermal reservoir, which supplies the geothermal power plants at Casa Diablo and the numerous hot pools in the caldera with hydrothermal water, has been detected. The data indicate that after upwelling the hydrothermal water flows eastward at shallow depth for at least 2 km and probably continues another 10 km to the east, all the way to Lake Crowley.

Mercury interactions with suspended solids at the Upper East Fork Poplar Creek, Oak Ridge, Tennessee

A water quality model was developed to analyze the impact of hydrological events on mercury contamination of the Upper East Fork Poplar Creek, Tennessee. The model simulates surface and subsurface hydrology and transport (MIKE SHE and MIKE 11) and it is coupled with the reactive transport of sediments and mercury (ECOLAB). The model was used to simulate the distribution of mercury contamination in the water and sediments as a function of daily hydrological events. Results from the model show a high correlation between suspended solids and mercury in the water due to the affinity of mercury with suspended organics. The governing parameters for the distribution of total suspended solids and mercury contamination were the critical velocity of the stream for particle resuspension, the rates of resuspension and production of particles, settling velocity, soil-water partition coefficient, and desorption rate of mercury in the water. Flow and load duration curves at the watershed exit were used to calibrate the model and to determine the impact of hydrological events on the total maximum daily load at Station 17. The results confirmed the strong link between hydrology and mercury transport.

An Integrated Flow and Transport Model to Study the Impact of Mercury Remediation Strategies for East Fork Poplar Creek Watershed, Oak Ridge, Tennessee

An integrated flow and transport model using MIKE SHE/MIKE 11 software was developed to predict the flow and transport of mercury, Hg(II), under varying environmental conditions. The model analyzed the impact of remediation scenarios within the East Fork Poplar Creek watershed of the Oak Ridge Reservation with respect to downstream concentration of mercury. The numerical simulations included the entire hydrological cycle: flow in rivers, overland flow, groundwater flow in the saturated and unsaturated zones, and evapotranspiration and precipitation time series. Stochastic parameters and hydrologic conditions over a five year period of historical hydrological data were used to analyze the hydrological cycle and to determine the prevailing mercury transport mechanism within the watershed. Simulations of remediation scenarios revealed that reduction of the highly contaminated point sources, rather than general remediation of the contaminant plume, has a more direct impact on downstream mercury concentrations.