Stormwater biofilters are low-impact development systems that are typically designed to quickly remove stormwater from surfaces for flood control with limited capacity to remove dissolved pollutants. Although certain amendments such as biochar, iron filings, and compost are added to biofilters to increase removal of pollutants, their pollutant removal capacity is unreliable due to various reasons including variable removal capacity of adsorbent media, exhaustion of adsorption sites, and environmental conditions. Adsorbed pollutants accumulate in the biofilter media and reduce the further removal of pollutants in stormwater. Replacement of exhausted biofilter media is cost-prohibitive. An alternate strategy is in-situ regeneration of the biofilters as a non-intrusive method to replenish exhausted biofilter media. This dissertation aims to improve the understanding of the fate and transport of emerging pollutants such as perfluoroalkyl substances (PFAS) and legacy pollutants such as heavy metals and pathogens in subsurface systems and use the improved understanding to develop methods to artificially or naturally regenerate the pollutant removal capacity of filter media to limit their exhaustion rate in stormwater biofilters.The dissertation consists of 5 research chapters. Chapter 2 critically examines the PFAS concentrations in suspended particles or colloids in the environment and shows that the suspended particles can adsorb and transport significant amounts of PFAS in surface water, subsurface soil, and even air. Chapter 3 proves that the fluctuations in groundwater flow can release colloids from PFAS-contaminated aquifer soil, which can carry PFAS. Therefore, removing soil colloids from groundwater samples through filtration or centrifugation can underestimate the PFAS concentration in groundwater. Chapter 4 examines the role of weathering cycles in the transport of PFAS in the subsurface and shows that dry-wet and freeze-thaw cycles can increase the release of colloids and associated PFAS from the subsurface into the groundwater. Chapter 5 demonstrates that in-situ injection of cationic polymers could regenerate exhausted biofilter media and improve their ability to remove PFAS from stormwater without clogging the biofilter. Chapter 6 offers a more natural method for regenerating the pathogen removal capacity of biofilters by utilizing the inherent toxicity of heavy metals to pathogens. Overall, the dissertation improves the understanding of the interactions between pollutants, microorganisms, and natural colloids in the solid-liquid interfaces and the effect of environmental conditions on these interactions in subsurface systems such as stormwater biofilters. The knowledge is useful to design biofilters, which are more efficient in removing legacy and emerging pollutants.

Wetlands provide many critical ecosystem functions including cycling of elements such as nitrogen, sulfur, and carbon while providing other functions such as water treatment and flood management in urban areas. Climate change and related climate extremes such as wildfires have the potential to alter the ecosystem balance provided by wetlands. For instance, many wetlands are located downstream of wildfire-affected areas and receive stormwater runoff carrying wildfire residues. The deposition of the wildfire residues may affect some of the functions of the wetlands. Yet, no studies to date examine the effect of wildfire residues on the capacity of wetlands to reduce nitrate and sulfate. To investigate the effect of wildfire residues on microbially mediated denitrification and sulfate reduction, batch experiments were set up in the lab using wetland sediments, water, and wildfire ashes. Results reveal that the presence of wildfire residues accelerated both denitrification and sulfate reduction initially. While the denitrification rate remained consistent following repeated exposure to nitrate-rich water, sulfate reduction rates became slower following each exposure to sulfate-rich water. An increase in nitrate and sulfate reduction in the presence of wildfire residues was attributed to the potential change in water chemistry and microbial community. The presence of wildfire residues increased salinity, dissolved organic carbon, and the concentrations of nitrate and sulfate leached from wildfire residues— all of which could have affected the microbial reduction rate of nitrate and sulfate. Analysis of SUVA showed a possible increase in aromatic DOC in pore water. Analysis of functional genes also confirmed the higher abundance of denitrifying genes in wildfire batches following one week in a field experiment, but by three weeks, denitrifying genes were insignificant in both the field and the lab. Overall, the results suggest that wetlands could provide an effective barrier to removing excess sulfate and nitrate released from wildfire residues or other sources following the deposition of wildfire residues.

Microplastics are conveyed by stormwater and accumulate in stormwater treatment systems such as biofilters, where they are exposed to sunlight and natural weather conditions such as drying, freezing, and wetting cycles. Thus, the mobility of microplastics through biofilters could depend on the interactive effects of the weather conditions. Yet, how weathering of microplastics under UV light affects their mobility during freeze-thaw cycles has not been evaluated. This study estimates to what extent UV weathering could affect the rate of microplastic transport through sand filters during freezing and thawing cycles. To compare the mobility of unweathered and weathered microplastics based on their concentration at different depths, PET microplastic particles weathered to different degrees under a UV light were deposited on sand-packed columns and subjected the columns to freeze-thaw cycles. Our results confirm that an increase in exposure to UV light alters the surface properties of microplastics, particularly contact angle, leading to increased surface hydrophilicity. Additionally, the depth distribution of microplastics varies with weathering of microplastics, with most weathered microplastics move farthest into subsurface. We attribute these results to a combination of changes in surface properties due to weathering and changes in interaction of weathered plastics with either ice or water interfaces, resulting a net increase in downward mobilization of microplastics over time. The results imply that UV weathering, in conjunction with the freeze-thaw cycle process, could substantially increase the mobility of microplastics in subsurface soil, and should be considered when predicting the transport of microplastics in subsurface systems.

PFAS site remediation studies often overlook stormwater as a potential source of PFAS contamination. This review article looks at the transport and distribution of these anthropogenic chemicals in stormwater, to identify the species and extent of distribution of these PFAS molecules, and then brings a comparison of different adsorbents (Biochar, Granulated Activated Carbon, Ion-Exchange Resins) in stormwater biofilters as a remediation technique. We found biochar and GAC to have an extremely variable adsorption capacity (over 3 orders of magnitude) and IX Resins to have the highest PFAS adsorption. These were attributed to several factors including soil chemistry, PFAS species present, and weather conditions. However, from a cost-benefit analysis perspective IX Resins were found to be almost 9x more expensive than traditional adsorbents. Hence, either a particular type of adsorbent or a combination of sorts has been suggested, and different factors that affect the adsorption of each of these amendments has been discussed to make an informed decision.

Rapid detection of microplastics is critical to understand the extent of microplasticcontamination in the environment and identify appropriate mitigation strategies. However, current methods for isolating and counting microplastics from environmental samples can take several hours to days and their efficiencies may vary widely. Herein a rapid method to detect microplastics is described, which involves staining them with Nile Red before taking picture using a mobile phone fitted with add-on camera. The method is capable of detecting and quantifying microplastics as small as 10 μm and identifying microplastics among other debris, thereby eliminating the need for time-consuming digestion steps using hazardous chemicals. The method is effective over a wide range of light-colored plastic polymers, including polystyrene, polypropylene, and polyethylene terephthalate. This field deployable method can provide accessible and user-friendly tool to even citizen scientists and non-specialized labs to detect microplastics in environmental samples.

Wildfires are becoming more frequent and intense due to climate change, especially in the southwestern US. Wildfires occur in vulnerable urban areas prone to intense droughts and extreme floods, exacerbating water scarcity issues. Polluted runoff from wildfires can contaminate the limited surface water available for drinking water. To protect downstream water quality, green infrastructure such as stormwater biofilters could be implemented to remove pollutants from post-fire runoff. However, the impacts of wildfire residues on their functions remain unknown. This dissertation aims to understand how green infrastructure can mitigate wildfire impacts on water quality and how to design innovative wildfire-resilient stormwater biofilters.This dissertation specifically addresses four research questions. (1) How and to what extent are wildfires impacting surface water quality? (2) How do wildfire residues affect the physical and chemical functions of stormwater biofilters? (3) How do wildfire residues affect the biological function of stormwater biofilters? (4) Can waste-derived amendments remove pollutants from wildfire residues? Finally, a study explored whether a cohort-based research experience can increase STEM engagement and persistence. To address these questions, I used a combination of field experiments, bench-scale laboratory experiments with column setups, physical-chemical processes, and geochemical and spectroscopic techniques, as well as quantitative and qualitative research methods. Natural stormwater was collected from Ballona Creek in Los Angeles, CA, and wildfire residues were collected from the Santa Monica Mountains, CA. Results showed that wildfires can have a lasting impact on water quality, releasing pollutants into surface water even years after the wildfire. Deposited wildfire residues can negatively affect the infiltration capacity of biofilters by clogging filter media but do not affect their pollutant removal capacity. Wildfire residues did not appear to impact the germination and growth of plants, although sensitivity to wildfire residues varied among species. Drinking water treatment residuals (WTR) enhanced biofilter capacity to remove wildfire-associated pollutants such as phosphate released from fire retardants. Engaging undergraduate students in cohort-based research fostered their science identity, increasing STEM retention, particularly among underrepresented minority students. Overall, the results inform when, where, and how green infrastructure may be used to reduce the negative impacts of wildfires on water quality.

Roadside biofilters can transform the nation’s millions of miles long road infrastructure into an integrated stormwater treatment system. However, unlike traditional biofilters, roadside biofilter media may need to be compacted to maintain required soil stability. Thus, it is critical to examine how amendments used in biofilters behave under compaction. The objective of this work is to examine the effect of the size of biochar, a carbonaceous soil amendment, on the performance of biochar-augmented biofilters subjected to compaction. To examine the effect of biochar particle size on hydraulic properties and contaminant removal capacity of biofilters, a mixture of sand (0.6- 0.8 mm) and biochar (5% by weight) with three size ranges (150 μm < d < 833 μm, 833 μm < d < 1180 μm and 1180 μm < d < 2000 μm) was packed in plastic columns (5.1 cm I.D. and 30.5 cm media length) using compaction energy similar to Standard Proctor method, and subjected to intermittent infiltration of stormwater. Results showed that the particle size of biochar did not significantly affect the quantity of particles released during intermittent infiltration of stormwater, and the quantity of biochar released was insignificant compared to the biochar remained in the biofilter. To examine the dominant mechanism of particle release after compaction, biochar particles of different sizes were coated with a dye (acridine orange), subjected to compaction, and the dye concentration in the released biochar particles was compared against the concentration in packed biochar from where the fine particles were released from. The results showed that disintegration, not abrasion, is the dominant mechanism for biochar release under compaction, and disintegration is particularly prominent when biochar size was small. Biofilters with larger biochar size exhibited the highest initial hydraulic conductivity and the least potential to clogging. However, biofilters with high particle sizes had the lowest E. coli removal capacity. Overall, these results indicate that the negative impact on compaction can be mitigated by using the size of biochar similar to or greater than the size of other biofilter’s media such as sand.

Stormwater biofilters, particularly woodchip biofilters, have been used to remove nitrate from stormwater, but their performance is expected to decrease under extreme weather conditions such as prolonged drying and high rainfall intensity, which are expected to be more frequent during climate change. The objective of this study is to examine the effect of biochar amendment on nitrate removal by woodchip biofilters subjected to increasing antecedent drying conditions and rainfall intensity. The experiments were designed to test the following hypothesis: the addition of biochar would increase the resiliency of woodchip-amended biofilter by enhancing the physical, chemical, and biological processes that support the removal of nitrate. Biochar-amended woodchip biofilters were packed with a homogeneous mixture of woodchips and biochar at 0, 5, 10, or 20% biochar volume in plastic columns (5.1 cm diameter, 61 cm height). Stormwater spiked with nitrate was injected through biofilters, and the effluent was collected by a raised outlet (30-cm submerged zone) to enhance denitrification. Antecedent drying duration was varied between 1 d to 8 d, and hydraulic residence time (HRT) was varied between 0 to 20 h to examine the effect of drying duration and high rainfall intensity on denitrification.

Results showed that biochar improved denitrification potential of woodchip biofilters. This improvement is attributed to changes in pore water chemistry, such as a decrease in dissolved oxygen (DO) concentration and an increase in dissolved organic carbon (DOC) trapped in pore water, and an increase in robust denitrifying biofilm supported on biochar particles, which has higher surface area than woodchips. Increase in antecedent drying duration had a net positive impact on denitrification, and the addition of biochar further increase nitrate removal during drying period. Antecedent drying periods helped replenish denitrification capacity of biofilters by increasing the dissolution of DOC and decreasing DO. Overall, the results suggest that addition of biochar could increase the resiliency of woodchip biofilters for denitrification during high intensity rainfall expected during climate change—the conditions at which the performance of woodchip biofilters typically deteriorates quickly.

Microplastics are released from terrestrial surfaces into the atmosphere where they are conveyed via wind and may be deposited, remain in the atmosphere, or disperse across long ranges. Because of this high atmospheric contamination, inhalation is one of the primary exposure pathways for microplastics. Inhalation exposure is controlled by two key factors: a) the quantity of microplastics available to be emitted from terrestrial environments to the atmosphere, and b) the factors that influence the potential of this emission occurring. The reported concentrations of microplastics on terrestrial surfaces vary widely based on monitoring methodology and characteristics of the capture region, making it difficult to estimate potentially emitted microplastics. Concentrations of traditional atmospheric aerosols such as particulate matter (PM10, PM 2.5), NOx, SOx, and heavy metals, vary based on region characteristics such as climate, land use, socioeconomic vulnerability, and urban sources including landfills, wastewater treatment plants, and hazardous waste sites. However, microplastics have unique properties: low density and high hydrophobicity, which may alter their emission mechanisms and spatial distribution. Thus, it is unknown how fluctuation in these exposure factors could affect the emission potential of microplastics, influencing concentration levels in the atmosphere. Additionally, the fundamental emission mechanisms and factors that could affect the emission potential of microplastics into the atmosphere are unclear. A wide body of literature models the atmospheric emission of particulate pollutants; however, no model to date adapts these traditional emission models by accounting for the unique properties of microplastics. The overall objective of the dissertation is to improve the mechanistic understanding of microplastic emission into the atmosphere, and the factors which may affect the exposure of human populations to higher levels of emitted microplastics in the atmosphere.The dissertation consists of six research chapters. Chapter 2 analyzes the global distribution of deposited airborne microplastics in terrestrial environments to estimate the relative importance of climate and land use, proving that the climate has a larger impact than land use on the abundance of deposited airborne microplastics. Chapter 3 compares the abundance, composition, and size of microplastics found in sediment cores and the water column in the Gulf of Mexico, proving that most microplastics have been deposited in the last two decades. Chapter 4 estimates the potential variability of the abundance of airborne microplastics on leaves in Los Angeles as a function of leaf height, leaf surface hydrophobicity, and land use, confirming a limited scope to use leaves as biomonitoring systems for urban atmospheric microplastics due to high uncertainty. Chapter 5 estimates the relative importance of socioeconomic status or proximity to known sources of microplastics on deposited airborne microplastics, revealing the difficulty of predicting exposure risk and the ubiquitous spatial distribution of microplastic abundance in the atmosphere. Chapter 6 analyzes wind-borne sediments collected from wind tunnel experiments on biosolid-applied agricultural fields, finding preferential emission of microplastics from the soil when compared to the background soil. A theoretical model was constructed considering microplastics’ low density and high hydrophobicity which verifies this experimental data and predicts a substantial underestimation of the wind events which exceed the fluid threshold necessary to emit microplastic from biosolid-amended solids. Chapter 7 quantifies the adhesion force between plastics and sand revealing a significant inverse relationship between ultraviolet (UV) weathering time and relative humidity (RH), which implies microplastic’s emission potential into the atmosphere could be increased as residence time in the environment increases, or in more humid environments. Overall, the results help to: a) quantify the fundamental forces affecting microplastic emission, b) outline the factors that may influence the emission potential of microplastics, and c) identify research needs to better predict microplastic inhalation exposure.

Access to water is critical for societal development. Urban areas, where more than half of the world’s population currently lives, are projected to increase to 70% by 2050. This growth indicates that the water scarcity issue in urban areas will get worse unless alternative water resources are utilized. Stormwater may serve as an alternative water source, but stormwater often contains many contaminants including pathogens, heavy metals, motor oils, nutrients, pesticides, herbicides, polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), per- and polyfluorinated substances (PFAS), microplastics, and other emerging organic contaminants. To manage and treat stormwater in urban areas, stormwater control measures (SCM) or green infrastructures have been used. However, traditional stormwater control measures are highly unreliable. The performance variability of SCM is due to varying stormwater, weather conditions, and design factors. To reduce the performance variability and make SCM more reliable, the development of climate resilient is required, which will result in increased water security in urban areas. In stormwater treatment systems, the resilience concept involves four central elements: stressors, indicators of resilience, metrics and the intervention. In this dissertation, I researched about each of the four central elements of resilience for stormwater treatment systems in order to develop climate resilient SCM. I researched about the stressor elements in Chapter 2 and 3. In Chapter 2, I showed that both design and local climate can explain nitrate removal variability by critically analyzing data reported on the international BMP database for nitrate removal by four common types of SCM: bioretention cells, grass swales, media filters, and retention ponds. In Chapter 3, I analyzed 7,421 data collected from 19 retention ponds across North America showed that FIB removal in retention ponds is sensitive to weather conditions or seasons, but temperature and precipitation data failed to describe the variable FIB removal. In Chapter 4 and 5, I quantified the performance variability of SCM through metrics. In Chapter 4, I examined how post-wildfire runoff containing burned residues affect the transport and survival of indicator bacteria, resulting in changes in the microbial quality of surface water and subsurface soil. In Chapter 5, I demonstrated how the deposition of wildfire residues could increase methane emissions in wetland sediments by up to 56%, but the emission depended on the amount of wildfire residues deposited. Finally, in Chapter 6 I researched about the last resilience concept: intervention or mitigation strategies. I showed in Chapter 6 that biochar’s capacity to remove pathogens from stormwater can vary by orders of magnitude, but the usage of machine learning techniques can predict biochar’s performance based in their commonly reported properties: surface area, carbon content, ash content, and volatile organic carbon content. This dissertation advances the science applied to climate resilient stormwater treatment systems.