Lakes and reservoirs exhibit a number of mixing and transport mechanisms. Understanding the transport is crucial to understanding and predicting constituent and density structures. Transport in waterbodies can be natural, such as seiche-induced boundary mixing or advectively-driven inflows. Hypolimnetic oxygenation using bubble-plumes also leads to enhanced mixing. Whether natural or plume-induced, increased mixing will alter the waterbody properties. Conversely, the density structure affects the behavior of plumes as well as inflowing and outflowing water. For example, stratification resulting from impounding a river can result in nutrient and suspended solids retention. Similarly, operation of plumes can induce mixing in the hypolimnion, resulting in warming, increased nutrient transport, and resuspension of settled particles. Modeling is extremely useful in determining the effects of dams on water quality constituents, enhanced transport, and the performance of mitigation techniques, such as hypolimnetic oxygenation. In this work, a variety of modeling techniques are used to evaluate natural and man-made mixing mechanisms. These include simple temperature and mass budgets, a two-dimensional lake model, and a two-phase plume model. A bubble-plume and plume-enhanced mixing was studied in Lake Hallwil. It was found that the plume-lake interaction was much more complex then previously expected, and knowledge of the seiche- and plume-enhanced near-field was necessary to accurately model the plume performance. A two-dimensional lake model was then coupled with a linear-plume model to accurately predict not only the plume performance, but also the plume-enhanced mixing in Spring Hollow Reservoir. The same two-dimensional lake model, used in conjunction with data analysis, demonstrated that the Iron Gate I Reservoir was not a significant sink for suspended solids, with only the large, adjacent side bay (Orsova Bay) thought to be the permanent sink. Furthermore, significant stratification did not develop, preventing substantial primary productivity. While the impoundment did change the water quality characteristics, the extent is much less than previously expected. The modeling methods presented here and the coupled plume-reservoir model should be useful tools for the design, modeling and greater understanding of bubble-plumes and other transport-related phenomena in lakes and reservoirs. / Ph. D.

The purpose of this research was to apply a discrete-bubble model to predict the performance of several hypolimnetic oxygenators. The model is used to predict the oxygen transfer rate in a hypolimnetic oxygenator based on the initial bubble size formed at the diffuser. The discrete-bubble model is based on fundamental principles, and therefore could also be applied to other mass transfer applications involving the injection of bubbles into a fluid. The discrete-bubble model has been applied to a linear bubble-plume diffuser, a full-lift hypolimnetic aerator and the Speece Cone with promising results. The first step in this research was to investigate the principals of bubble formation at a submerged orifice, bubble rise velocity and bubble mass transfer. The discrete-bubble model is then presented. The model traces a single bubble rising through a fluid, accounting for changes in bubble size due to mass transfer, temperature and hydrostatic pressure. The bubble rise velocity and mass transfer coefficients are given by empirical correlations that depend on the bubble size. Bubble size is therefore recalculated at every increment and the values for the bubble rise velocity and mass transfer coefficients are continually updated. The discrete-bubble model is verified by comparison to experimental data collected in large-scale oxygen transfer tests. Finally, the discrete-bubble model is applied to the three most common hypolimnetic oxygenation systems: the Speece Cone, the bubble-plume diffuser, and the full-lift hypolimnetic oxygenation systems. The latter being presented by Vickie Burris in her thesis, <i>Hypolimnetic Aerators: Predicting Oxygen Transfer and Water Flow Rate</i>. / Master of Science

The biogeochemical cycles of iron (Fe) and manganese (Mn) in lakes and reservoirs have predictable seasonal trends, largely governed by stratification dynamics and redox conditions in the hypolimnion. However, short-term (i.e., sub-weekly) trends in Fe and Mn cycling are less well-understood, as most monitoring efforts focus on longer-term (i.e., monthly to yearly) time scales. The potential for elevated Fe and Mn to degrade water quality and impact ecosystem functioning, coupled with increasing evidence for high spatiotemporal variability in other biogeochemical cycles, necessitates a closer evaluation of the short-term Fe and Mn cycling dynamics in lakes and reservoirs. We adapted a UV-visible spectrophotometer coupled with a multiplexor pumping system and PLSR modeling to generate high spatiotemporal resolution predictions of Fe and Mn concentrations in a drinking water reservoir (Falling Creek Reservoir, Vinton, VA, USA) equipped with a hypolimnetic oxygenation (HOx) system. We quantified hourly Fe and Mn concentrations during two distinct transitional periods: reservoir turnover (Fall 2020) and initiation of the HOx system (Summer 2021). Our sensor system was able to successfully predict mean Fe and Mn concentrations as well as capture sub-weekly variability, ground-truthed by traditional grab sampling and laboratory analysis. During fall turnover, hypolimnetic Fe and Mn concentrations began to decrease more than two weeks before complete mixing of the reservoir occurred, with rapid equalization of epilimnetic and hypolimnetic Fe and Mn concentrations in less than 48 hours after full water column mixing. During the initiation of hypolimnetic oxygenation in Summer 2021, we observed that Fe and Mn were similarly affected by physical mixing in the hypolimnion, but displayed distinctly different responses to oxygenation, as indicated by the rapid oxidation of soluble Fe but not soluble Mn. This study demonstrates that Fe and Mn concentrations are highly sensitive to shifting dissolved oxygen and stratification and that their dynamics can substantially change on hourly to daily time scales in response to these transitions. / Master of Science / Iron and manganese are chemical elements that occur in many freshwater systems. Although they are naturally-occurring, high concentrations of iron and manganese can have negative effects on drinking water quality as well as the health of aquatic ecosystems. In temperate regions, iron and manganese can accumulate in the bottom waters of lakes and reservoirs during the summer months, but generally remain at low levels during the fall through spring. This seasonal cycle has been previously documented, but few studies have investigated the ways in which iron and manganese concentrations in a lake or reservoir change over shorter periods of time, such as hours or days. Recent advances in technology to measure chemical elements in the environment have allowed scientists to observe chemical fluctuations of other elements over relatively short time periods, which suggests that iron and manganese could potentially exhibit similar trends. In this study, we used an advanced sensor system to make hourly measurements of iron and manganese concentrations in a drinking water reservoir and observe how they changed during two time periods: in the fall of 2020, as the reservoir was transitioning from summer to winter, and in the summer of 2021, when oxygen was added to the bottom waters to improve water quality. Our observations indicate that iron and manganese concentrations in the reservoir waters were highly variable over short time scales and that they can change dramatically in as little as 24 hours, especially during transitional periods. We also successfully demonstrated the ability of our advanced sensor system to monitor these hourly changes, which could have many benefits for drinking water management and understanding metals cycling in freshwater systems.

This study addressed the geochemical drivers of Mn removal, including pH, alkalinity and the presence of mineral particles. We conducted laboratory experiments and field monitoring at two drinking water reservoirs in southwestern Virginia – Falling Creek Reservoir (FCR) and Carvins Cove Reservoir (CCR). In laboratory experiments in pH and alkalinity-adjusted nanopure water solutions, we observed substantial Mn removal within 14 days only under high pH conditions (pH≥10). In experiments with high pH and moderate to high alkalinity (> 80 mg/L CaCO3), near-total Mn removal occurred within 2 hours, at a rate of 0.25 mg/L-1 hr-1. Mn removal occurred alongside precipitation of microscopic (<5 μm diameter) and macroscopic (>100 μm diameter) particles. Elemental analysis of particles with energy-dispersive X-ray spectroscopy (EDS) supports their identification as Mn(IV) oxides (MnOx), which suggests Mn removal driven by oxidation. Elevated alkalinity in high pH solutions promotes Mn oxidation by maintaining high pH through buffering, which sustains conditions favorable for Mn oxidation. Our results also suggest sorption of Mn and mineral-catalyzed Mn oxidation by Mn oxides formed through oxidation by dissolved oxygen. In experiments using filtered and unfiltered water from the two reservoirs, we observed significant Mn removal in experiments with unfiltered water, suggesting that particles may remove Mn by catalyzing oxidation or nucleating Mn oxide precipitation. Mn removal occurred at 0.05 d-1 in unfiltered FCR water and 0.002 d-1 in unfiltered CCR water. We observed no Mn removal in filtered water from either reservoir. Scanning electron microscope (SEM) and EDS of visible particles from reservoir water experiments suggests that quartz and clay minerals present in the water column may nucleate or catalyze Mn oxide formation. Overall, this research shows that Mn removal under HOx operation is influenced by a variety of factors, including pH, alkalinity and suspended particles. / Master of Science / Elevated concentrations of manganese (Mn), a naturally occurring contaminant, can impair drinking water quality in several ways – by introducing poor taste and smell, staining pipes and appliances, and potentially harming the health of young children. Hypolimnetic oxygenation (HOx) is a novel water treatment method deployed in lakes and reservoirs to control water column contamination of metals and nutrients, including Mn. By pumping oxygen into lakes and reservoirs, HOx systems create conditions favorable for Mn removal from the water column. Previous work in two southwestern Virginia drinking water reservoirs documented differences between sites in how effectively HOx systems are able to remove Mn. These reservoirs have significant differences in their chemical profiles – most notably in pH and alkalinity, which suggests a role for background water chemistry in influencing removal rates in lakes and reservoirs with HOx systems. We used laboratory experiments to simulate the effects of pH and alkalinity on Mn removal rates in oxygenated lakes and reservoirs. We observed substantial Mn removal within 14 days under high pH conditions (pH 10-11) and negligible removal in solutions at or under pH 8. In experiments with pH 10-11 and alkalinity over 80 mg/L, near-total Mn removal occurred within 24 hours. During the 24 hour removal window, we observed yellow-brown discoloration of our experimental solutions within 12 hours, followed by formation of loosely aggregated brown to black particles. Microscopy and elemental analyses indicate that initial discoloration occurs due to formation of 1-2 μm wide manganese oxides with needle-like crystals. The visible aggregates are also manganese oxides. Based on mineral characterization and the time series of Mn removal observed in our experiments, we believe that initial formation of Mn oxides creates a positive feedback loop in solutions of pH 10-11 and alkalinity over 80 mg/L. Mn oxides promote further Mn oxide formation by facilitating conversion of Mn in solution into forms that easily settle from water. Observations of particulate formation and solution chemistry in filtered vs. filtered reservoir water from FCR and CCR supports a pivotal role for particles in facilitating Mn removal. Our research addresses the impacts of water chemistry Mn removal in drinking water, and improves understanding of Mn cycling in natural freshwaters.

When properly designed, hypolimnetic aeration and oxygenation systems can replenish dissolved oxygen in water bodies while preserving stratification. A comprehensive literature review of design methods for the three primary devices was completed. Using fundamental principles, a discrete-bubble model was first developed to predict plume dynamics and gas transfer for a circular bubble-plume diffuser. This approach has subsequently been validated in a large vertical tank and applied successfully at full-scale to an airlift aerator as well as to both circular and linear bubble-plume diffusers. The unified suite of models, all based on simple discrete-bubble dynamics, represents the current state-of-the-art for designing systems to add oxygen to stratified lakes and reservoirs. An existing linear bubble plume model was improved, and data collected from a full-scale diffuser installed in Spring Hollow Reservoir, Virginia (U.S.A.) were used to validate the model. The depth of maximum plume rise was simulated well for two of the three diffuser tests. Temperature predictions deviated from measured profiles near the maximum plume rise height, but predicted dissolved oxygen profiles compared very well to observations. Oxygen transfer within the hypolimnion was independent of all parameters except initial bubble radius. The results of this work suggest that plume dynamics and oxygen transfer can successfully be predicted for linear bubble plumes using the discrete-bubble approach. To model the complex interaction between a bubble plume used for hypolimnetic oxygenation and the ambient water body, a model for a linear bubble plume was coupled to two reservoir models, CE-QUAL-W2 (W2) and Si3D. In simulations with a rectangular basin, predicted oxygen addition was directly proportional to the update frequency of the plume model. W2 calculated less oxygen input to the basin than Si3D and significantly less mixing within the hypolimnion. The coupled models were then applied to a simplified test of a full-scale linear diffuser. Both the W2 and Si3D coupled models predicted bulk hypolimnetic DO concentrations well. Warming within the hypolimnion was overestimated by both models, but more so by W2. The lower vertical resolution of the reservoir grid in W2 caused the plume rise height to be over-predicted, enhancing erosion of the thermocline. / Ph. D.

The Occoquan Reservoir is part of the largest indirect potable reuse systems in the United States. It in an important water supply source for the Northern Virginia area, as well as, an ecological and recreational area. Furthermore, the Occoquan Reservoir protects the water quality of the Chesapeake Bay because it acts as a trap for sediments and pollutants. Continuous water quality monitoring and evaluation is critical to preserve this important water resource. Reservoir water quality can be affected by the delivery of pollutants from point and nonpoint sources, potentially causing problems such as eutrophication, excess salinization, presence of compounds that affect human and aquatic health. Different management strategies have been implemented at the Occoquan Reservoir to nutrient loading into the reservoir and address eutrophication issues, including nitrate addition to hypolimnetic waters and installation of a hypolimnetic oxygenation system. The goal of this study is to assess how current management strategies implemented in the Occoquan Reservoir have affected the water quality from 1973 to 2019, with particular emphasis on the data since 2003. This analysis of the Occoquan Reservoir and its tributary watershed includes the evaluation of hydrometeorological data and morphometric characteristics; establishment of long-term trends for water quality constituents; and determination of the trophic state of the reservoir. Data from water samples from four different stations located at the Occoquan Reservoir and four stations located throughout the Occoquan tributary watershed were analyzed for nutrients, principal ions and metals, synthetic organic compounds (SOCs), and other water quality parameters. Long-term water quality trends were determined using Mann-Kendall test and relationship between constituents was evaluated using Principal Component Analysis (PCA). Trophic state of the reservoir was assessed using Carlson's Trophic State Index (TSI), Vollenweider Model, and Rast, Jones, and Lee's Model. Results indicate the Occoquan Reservoir is a eutrophic waterbody. However, the nitrate management strategy and the installation of the hypolimnetic system have improved reservoir water quality, reducing concentrations of nutrients and metals. / Master of Science / The Occoquan Reservoir is part of the largest indirect potable reuse systems in the United States. Indirect potable reuse refers to the planned discharge of reclaimed water into a water supply source, such as a reservoir or lake. The Occoquan Reservoir also serves as an ecological and recreational area, and serves to protects the water quality of the Chesapeake Bay because it acts as a trap for sediments and pollutants. To protect the different ecosystem services that the reservoir provides, it is critical to continuously monitoring and evaluate its water quality. Reservoir water quality can be affected by the delivery of pollutants from industrial and municipal waste discharges (point sources), as well as, from urban and agricultural runoff (nonpoint sources). Contaminants include nutrients (such as nitrogen and phosphorus), ions, metals, and synthetic organic compounds (SOCs) that can affect human and aquatic health. Different management strategies have been implemented at the Occoquan Reservoir to reduce load of pollutants into the reservoir, particularly to reduce concentrations of nutrients, as excessive nutrients can degrade water quality. Two strategies implemented are the addition of nitrogen, in the form of nitrate, and the installation of an oxygenation system at the reservoir bottom waters. The goal of this study is to assess how current management strategies implemented in the Occoquan Reservoir have affected the water quality from 1973 to 2019, with particular emphasis on the data since 2003. This analysis of the Occoquan Reservoir and its tributary watershed includes the evaluation of the hydrological, meteorological, and morphometric characteristics of the Occoquan Reservoir and Watershed; establishment of long-term trends for water quality constituents; and determination of the productivity (trophic state) of the reservoir. Data from water samples from four different stations located at the reservoir and four stations located throughout the watershed were analyzed for nutrients, principal ions and metals, SOCs, and other water parameters indicative of water quality. Statistical analyses were employed to determine long-term water quality trends (Mann-Kendall test) and relationship between constituents (Principal Component Analysis - PCA). The trophic state of the reservoir was assessed using three methods: Carlson's Trophic State Index (TSI), Vollenweider Model, and Rast, Jones, and Lee's Model. Results indicate the Occoquan Reservoir is eutrophic, or highly enriched with nutrients and productive. However, management strategies employed have improved the water quality and the reservoir continues to improve, though at a slow rate.

Bubble plume oxygenation systems are commonly used to mitigate anoxia and its deleterious effects on water quality in thermally stratified lakes and reservoirs. Following installation, increases in sediment oxygen flux (JO2) are typically observed during oxygenation and are positively correlated with the bubble plume gas flow rate. Studies show that JO2 is controlled by the thickness of the diffusive boundary layer (DBL) at the sediment-water interface (SWI), which is in turn controlled by turbulence. As a result, JO2 can be quite spatially and temporally variable. Accurately predicting oxygenation-induced JO2 is vitally important for ensuring successful oxygenation system design and operation. Yet despite the current understanding of physical and chemical controls on JO2, methods for predicting oxygenation-induced JO2 are still based on empirical correlations and factors of safety. As hypolimnetic oxygenation becomes more widely used as a lake management tool for improving and maintaining water quality, there is a need to move from the current empirically based approach to a mechanistic approach and improve the ability to predict induced JO2. This work details field campaigns to investigate and identify appropriate models of oxygen supply to the SWI and oxygen demand exerted from the sediment, with the intent to use these models to predict oxygenation-induced JO2. Oxygen microprofiles across the SWI and near-sediment velocity measurements were collected in situ during three field campaigns on two oxygenated lakes, providing simultaneous measurements of JO2 and turbulence. Field observations show that oxygenation can increase JO2 by increasing bulk hypolimnetic oxygen concentrations, which increases the concentration gradient across the SWI. Oxygenation can also enhance turbulence, which decreases the DBL thickness and increases JO2. Existing models of interfacial flux were compared to field measurements to determine which model best predicted the observed JO2. Models based on the Batchelor scale, friction velocity, and film-renewal theory all agree reasonably well with field observations in both lakes. Additionally, the oxygen microprofiles were used to fit a transient model of oxygen kinetics in lake sediment and determine the appropriate kinetic model. Oxygen microprofiles in both lakes can be described using zero-order kinetics, rather than first-order kinetics. The interfacial flux and sediment kinetic models are incorporated into a coupled bubble plume and 3-D hydrodynamic lake model, allowing for spatial and temporal variation in simulated JO2. This comprehensive model was calibrated and validated to field data from two separate field campaigns on Carvin's Cove Reservoir, Virginia. Simulated temperature profiles agreed quite well with field observations, while simulated oxygen profiles differed from observed profiles, particularly in the bottom 1 m of the water column. The model overestimates oxygen concentrations near the sediment, which results in higher simulated JO2 than was observed during the field campaigns. These discrepancies are attributed to oxygen-consuming chemical processes, such as oxidation of soluble metals, which are not accounted for in the hydrodynamic model. Despite this, the model is still able to capture the impact of bubble plume operation on JO2, as simulated JO2 is higher when the diffusers are operating. With some additional improvements to the water quality modeling aspects of the model, as well as further calibration and validation, the model should be able to reproduce observed JO2 provided oxygen concentrations near the SWI are accurately reproduced as well. The current work is an attempt to push toward a comprehensive lake oxygenation model. A comprehensive model such as this should improve the ability to predict oxygenation-induced JO2 and lead to improvements in the design and operation of hypolimnetic oxygenation systems. / Ph. D.