Anthropogenic freshwater salinization, which is caused by numerous human activities including agriculture, urbanization, and deicing, impacts an estimated 37% of the contiguous drainage area in the United States. High salt concentrations in brackish and marine environments (~1,500 – 60,000 µS cm-1) influence aquatic bacteria. Less is known about the effects of freshwater salt concentrations (≤ 1,500 µS cm-1) on bacterial ecology, despite the pervasiveness of freshwater salinization. Bacteria perform many fundamental ecosystem processes (e.g. biogeochemical cycling) and serve as indicators of human health risk from exposure to waterborne pathogens. Thus, to understand how salt pollution affects freshwater ecosystems, there is a critical need to understand how freshwater salinization is impacting bacterial ecology. Using a series of controlled mesocosm experiments, my objectives were to determine how (1) survival of fecal indicator bacteria (FIB), (2) the diversity of native freshwater bacterial communities, and (3) bacterial respiration and nutrient uptake rates responded across a freshwater salinity gradient of different salt profiles. Survival rates (t90) of Escherichia coli, the EPA recommended freshwater FIB, increased by over 200% as salinity increased from 30 to 1,500 µS cm-1. Survival rates were also significantly higher in water with elevated Mg2+ relative to other base cations, suggesting that different salt sources and ion profiles can have varied effects in FIB survival. Thus, freshwater salinization could cause accumulating concentrations of FIB even without increased loading, increasing the risk of bacterial impairment. Diversity of native bacterial communities also varied across a freshwater salinity gradient, with a general increase in species richness as salinity reached 1,500 µS cm-1. Community variability (β-diversity) was greatest at intermediate salinities of 125 – 350 µS cm-1 and decreased towards the upper and lower extremes (30 and 1,500 µS cm-1, respectively). These diversity patterns suggest that osmotic stress is an environmental filter, but filtering strength is lowest at intermediate salinities causing a change from more deterministic to more stochastic assembly mechanisms. Different salt types also produced distinct bacterial community structures. Lastly, bacterial respiration doubled as salinity increased to 350 – 800 µS cm-1, revealing a subsidy-stress response of bacterial respiration across a freshwater salinity gradient. Corresponding changes in nitrogen and phosphorus uptake increased N:P ratios in ambient water, especially in mesocosms with elevated Ca2+, which could affect nutrient limitation in salinized streams enriched with Ca2+. Bacterial community structure based on Bray-Curtis dissimilarity was not correlated to pairwise changes in respiration rates but was linked to net nitrogen and phosphorus uptake after five days. Collectively, these results establish that freshwater salinization alters bacterial ecology at the individual population, whole community, and ecosystem process scales. Further, different salt types (e.g., CaCl2, MgCl2, NaCl, KCl, sea salt) had varying effects on bacteria at all levels and should be considered when predicting the effects of salinization on freshwater ecosystems. Developing more nuanced salt management plans that consider not only amount, but different types, of salts in freshwaters could help improve our ability to predict human health risk from waterborne pathogens and mitigate/ reduce salinity-induced impacts to freshwater ecosystem processes and services. / Doctor of Philosophy / Humans rely on streams, rivers, and lakes for many services including transportation, recreation, food, and clean drinking water. Despite our reliance on freshwater ecosystems, human activity has significantly degraded freshwater resources worldwide. Recently, salt pollution caused by human activity on land, known as freshwater salinization, has emerged as a widespread water quality issue. Numerous human activities including agriculture, urbanization, resource extraction, and deicing have increased freshwater salt concentrations in 37% of the United States' contiguous drainage area. Large changes in salinity (i.e. from freshwater to oceanic salinities) are known to affect bacteria that perform many important ecological functions, such as nutrient cycling and water purification, while the effects of smaller changes in salinity more typical within the freshwater range are unknown. I used controlled laboratory experiments to determine how freshwater salinization affects (1) survival rates of Escherichia coli, (2) diversity of native bacterial communities, and (3) bacterial nutrient cycling. My results revealed that freshwater salinization can significantly increase how long E. coli survive in freshwater. E. coli are used to detect the presence of waterborne pathogens and reduce human health risk. Thus, freshwater salinization might reduce the reliability of E. coli as an indicator of waterborne pathogens as well as increase concentrations of bacterial that are potentially harmful to human health in freshwater. Additionally, freshwater salinization affected bacterial diversity by altering the ways in which bacterial communities form. In general, the number of bacterial species present increased as salinity reached the upper freshwater limit, but communities were most variable at intermediate freshwater salt concentrations. These diversity patterns suggest that different salt concentrations can either cause or reduce stress in bacteria, resulting in significantly different bacterial communities. Lastly, moderate increases in freshwater salt concentrations doubled bacterial respiration and nutrient uptake rates. Bacterial respiration influences how energy flows through ecosystems, and freshwater salinization could potentially alter this process. Different salt types also had different effects of bacterial ecology. Collectively, my results establish that freshwater salinization impacts bacteria at the individual, community, and ecosystem levels.

Stormwater detention basins are commonly used in the Eastern United States to temporarily store and attenuate stormwater runoff, and also serve as habitats for native and exotic plants. However, during winter, these basins receive saline runoff from road salt application, which contributes to Freshwater Salinization Syndrome (FSS). Since limited research has connected direct measurement of soil and stormwater salinities to biodiversity and phytoremediation potential of salt-tolerant plant species, this thesis aimed to fill this gap. We selected a set of detention basins draining mostly pervious areas, parking lots, or roads in Northern Virginia and measured temporal variations in stormwater and soil salinities, depth profiles of soil salinities, plant community composition, and plant tissue ion concentration. The results indicated elevated levels of sodium, chloride, electrical conductivity (EC), and exchangeable sodium percentage (ESP)/sodium adsorption ratio (SAR) in soil and stormwater after road salt application during winter, followed by a decrease during the growing season for basins draining parking lots and roads. A subsequent increase at the end of the season was observed for all site types. While some stormwater samples exceeded toxicity thresholds, most soil samples did not exceed their respective thresholds nor reach saline or sodic conditions, and native and exotic plant species of both salt-sensitive and salt-tolerant classifications were observed at almost all sites, although proportions of each varied by site type. Tissue analysis of select plants revealed ionic concentrations that generally coincided with observed soil and stormwater concentrations at each major site type. These findings have implications for future detention basin planting regimes to mitigate FSS, and the thesis discusses native plants found to provide the most benefit for phytoremediation. / Master of Science / Stormwater detention basins are commonly used in the Eastern United States. They slowly release stormwater runoff and serve as habitats for native and exotic plants. However, during winter, these basins receive saline runoff from road salt application. This contributes to Freshwater Salinization Syndrome (FSS). Limited research has connected direct measurement of soil and stormwater salinities to biodiversity and plants' ability to uptake salts, so this thesis aimed to fill this gap. A set of detention basins draining mostly pervious areas, parking lots, or roads in Northern Virginia were selected. Next, stormwater and soil salinities over time, depth profiles of soil salinities, plant community composition, and plant tissue ion concentration were measured. The results showed higher levels of standard salinity benchmarks in soil and stormwater after road salt application during winter, followed by a decrease during the growing season for parking lot and road sites. A final increase in the fall was observed for all site types. While some stormwater samples were toxic to plants, most soil samples were not toxic, saline, or sodic. Also, native and exotic plant species of both salt-sensitive and salt-tolerant classifications were observed at almost all sites, but proportions of each varied by site type. Plant tissues contained ionic concentrations that reflected observed soil and stormwater concentrations at each site type. These findings can inform future detention basin planting regimes to mitigate FSS. The thesis also discusses native plants that provide benefits for phytoremediation.

Inland freshwater salinity is rising worldwide and threatens the quality of our water resources, a phenomenon called the freshwater salinization syndrome (FSS). Simultaneously, the practice of indirect potable reuse (IPR) that augments critical water supplies with treated wastewater to enhance water security presents complexities in water quality management. This dissertation explores the complex interplay between FSS and IPR in the Occoquan Reservoir, an important drinking-water source in the Mid-Atlantic United States, within its diverse environmental, social and political contexts. Using extensive data collected over 25 years, this research quantifies contributions of multiple salinity sources to the rising concentration of sodium (a major ion associated with the FSS) in the reservoir and the finished drinking water. These sources encompass two rapidly urbanizing watersheds, a sophisticated water reclamation facility and the drinking water treatment utility. The novel application of unsteady transit time theory reveals that stream salinization can be linked to watershed salt sources using stream water age as a master variable and provides a real-time prediction model for sodium concentration in the reservoir. These results identify substantial opportunities to mitigate sodium pollution and help set the stage for stakeholder-driven bottom-up management by improving the predictability of system dynamics, enhancing knowledge of this social-ecological system and supporting the development of collective action rules. / Doctor of Philosophy / The global rise in freshwater salinity, termed as the freshwater salinization syndrome (FSS), poses a significant threat to water quality in our freshwater resources. The practice of indirect potable reuse (IPR), which involves reusing treated wastewater to supplement and secure our water supplies presents significant challenges in managing water quality. This dissertation delves into the intricate relationship between FSS and IPR, focusing on the Occoquan Reservoir-a vital drinking water source in the Mid-Atlantic United States-within its multifaceted environmental, social, and political contexts. This study uncovers the contributions of various sources of salinity to rising sodium ion concentrations (a key FSS-associated ion) in the reservoir and in finished drinking water. Sodium ions are contributed by road salts, chemicals used in water and wastewater treatment, commercial and industrial discharges, household products (e.g., laundry detergents) and human excretion. An innovative approach of examining the age of water in the stream and in the reservoir outflow enables us to trace origins of salinity within the watershed and predict the concentration of sodium ions in the reservoir, respectively. These findings reveal promising avenues for effectively addressing sodium pollution at this site. Furthermore, this research underscores the significance of convergence research, bringing diverse stakeholders together to develop collaborative strategies to manage freshwater salinization using a bottom-up approach.

Increased salinity in freshwater systems – also called the Freshwater Salinization Syndrome (FSS) – can have far-ranging implications for the natural and built environment, agriculture, and public health at large. Such risks are clearly on display in the Occoquan Reservoir – a drinking water source for roughly one million people in the northern Virginia/ National Capital Region. Sodium concentrations in the Occoquan Reservoir are approaching levels that can affect taste and health. The Reservoir is also noteworthy as a flagship example of indirect potable reuse, which further adds complexity to understanding the sources of rising levels of sodium and other types of salinity. To help understand the role residential discharges might play in salinization of the Occoquan Reservoir, a suite of laundry detergent products was identified based upon survey data collected in the northern Virginia region. The ionic compositions of these products were then characterized using ion chromatography and inductively coupled plasma-mass spectrometry to quantify select ionic and elemental analytes. Sodium, chloride, and sulfate were consistently found in appreciable amounts. To comparatively characterize the laundry detergents, principal component analysis was employed to identify clusters of similar products. The physical formulation of the products was identified as a marker for their content, with dry formulations (free-flowing and encapsulated powders) being more enriched in sodium and sulfate. This result was corroborated by comparing nonparametric bootstrap intervals for individual analytes. The study's findings suggest an opportunity wherein consumer choice can play a role in mediating residential salt inputs in receiving bodies such as the Occoquan Reservoir. / Master of Science / Many streams, rivers, and other freshwater systems have become increasingly salty in recent decades. A rise in salinity can be problematic, stressing aquatic life, corroding pipes, and even enhancing the release of more pollutants into the water. This phenomenon, called Freshwater Salinization Syndrome, can threaten such systems' ability to serve as sources of drinking water, as is the case for the Occoquan Reservoir in northern Virginia. Serving roughly one million people, the Reservoir is notable for being one of the first in the country to purposely incorporate highly treated wastewater upstream of a drinking water supply. Despite the Reservoir's prominence, the reasons behind its rising salt levels are not well understood. This study sought to understand the role that individual residences could play when household products travel down the drain and are ultimately discharged into the watershed. Laundry detergents are potentially high-salt products. A survey of northern Virginian's laundry habits was conducted to understand local tastes and preferences. Informed by the survey, a suite of laundry detergents was chemically characterized to measure salt and element concentrations. The detergents were found to have notable amounts of sodium, chloride, and sulfate in particular, with sodium being the most abundant analyte in every detergent. However, not all detergents were equally salty; statistical tools revealed that dry formulations (such as powdered and powder-filled pak detergents) contributed more sodium and sulfate, among other things. This study's findings suggest that laundry detergents could be contributing to Freshwater Salinization Syndrome in the Occoquan Reservoir, and that local consumers' choice of detergents could make a difference.

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