The primary focus of this research project is to determine the availability of current “baseline” data and assess changes in stormwater runoff Total Suspended Solids (TSS). In addition, how does stormwater management represent bias in the Charlotte-Mecklenburg area? The 2040 Charlotte Comprehensive Plan and Unified Development Ordinance (UDO) which was implemented in June 2023, would likely yield an increase in impervious surfaces resulting in increased sediment pollutants associated with stormwater runoff. An increase in runoff will place adjacent aquatic systems at risk for increased contaminants. The areas at greatest risk are those within lower elevations and flood prone areas, combined with areas that often experience the most environmental injustice. Each watershed within Charlotte has several Best Management Practice Facilities (BMPs) charged with monitoring and mitigating sediment buildup. Existing renderings show how the amount of BMPs in low income and high percentage of minority areas reflects inequality. The three main watersheds of concern are the McAlpine creek watershed, Little Sugar Creek watershed and Briar Creek watershed; chosen because they display a broad socioeconomic gradient and cover several land use types. Data implementation comes from open source databases and is symbolized in ArcGIS to demonstrate the demographic trends within the Charlotte-Mecklenburg area. During analysis, we will further use ArcGIS and R to run regression analyses on water quality and socio-demographic data. The resulting statistics will show us data trends that can reveal data voids, need for more management practices, or which communities experience the most surveying disparities. The discussion of concern per the Charlotte UDO plans can be informed by this project. 

Broadly defined by the EPA as “inorganic and organic particles,” sediment is involved in many of earth’s systems (U.S. EPA 2022). However, this review will focus on urban aquatic environments where particle size and composition determine sediment function. Larger bedded sediments may provide habitat for aquatic species, whereas the finer sediment particles remain suspended in the water column resulting in a host of environmental ramifications. These include increased turbidity, increased water temperature, and pollutant transport. In urban waterways, excessive sediment can also cause aesthetic concerns and human health risks.  It is unknown whether sediment pollution disproportionately impacts certain populations. However, as a widespread and high-visibility pollutant that doesn’t require in-depth testing or analysis, sediment is a prime candidate for investigations regarding community impact. 

The state of North Carolina has long recognized sediment as a major pollutant. The Sedimentation Control Act of 1973 first enacted requirements for land-disturbing activities related to construction and road maintenance that include mitigation measures to limit on-site erosion. Despite these efforts, sediment remains a top pollutant. The North Carolina Department of Environmental Quality (NCDEQ) and Charlotte-Mecklenburg Stormwater Services currently identify sediment as a primary concern in local waterways (Charlotte-Mecklenburg Stormwater Services 2022, NCDEQ). In addition to erosion control requirements during construction, post-construction ordinances outline regulation for Stormwater Best Management Practices (BMPs) that control the volume and, in some cases, the quality of runoff from impervious surfaces. Jurisdiction varies for the enforcement of these ordinances, with many towns and cities using individual stormwater management policies which meet the NCDEQ’s requirements. Within Charlotte, a combination of city and county ordinances dictate erosion control during construction and stormwater control post-construction for new development. 

Charlotte’s extensive Unified Development Ordinance (UDO) is a comprehensive document providing development guidelines aligning with Charlotte’s 2040 Comprehensive plan. Passed in August 2022, it will officially go into effect on June 1, 2023 (Charlotte’s Unified Development Ordinance, 2022). One of the document's primary goals is to facilitate equitable growth. The 2040 Comprehensive Plan outlines four equity metrics, including Environmental Justice and a vulnerability to displacement overlay (2040 Comprehensive Plan). The Environmental Justice metric includes 1) tree canopy, 2) impervious surface, 3) proximity to heavy industrial uses, 4) proximity to major transportation infrastructure, and 5) floodplain. All five of these measurements relate to stormwater quantity and/or quality. More specifically, impervious surfaces, industrial areas, and transportation infrastructure are all sediment sources within urban waterways. 

The UDO intends to promote equitable and sustainable development through updated stormwater ordinances. However, no studies have investigated how this increased development and new standards may impact the distribution of sediment pollution in Charlotte’s urban waterways. Some key changes to the Post-Construction Stormwater Ordinances in the UDO include: more standardized requirements for all land-use classifications (industrial, commercial, and residential), redeveloped Built-Upon Area (BUA) is not subject to stormwater control requirements, lower BUA trigger for stormwater control, and designation of maintenance responsibility to the property owner (Charlotte’s Unified Development Ordinance 2022). Housing and development density is another focal area of the UDO that aims to provide more affordable and accessible housing. From a stormwater perspective, high-density development has more stringent requirements, such as BMPs designed for “85% removal of TSS'' and “70% removal of phosphorus.” Low-density development only requires adequate drainage (Charlotte’s Unified Development Ordinance 2022). Therefore, population density and housing typology will significantly impact the spatial distribution of stormwater BMPs.

Historical patterns of redlining and ongoing gentrification in the Southern U.S. have put many minority communities at a disproportionate risk for environmental harm, partly due to placement near Locally Unwanted Land Use areas (LULUs) (Debbage, 2019). This land classification results from the proximity of hazardous waste facilities, landfills, and other undesirable land use types. These patterns of risk burden can also apply to stormwater-related risks due to 1) neighborhood placements within flood zones, and 2) inequitable placement of stormwater BMPs (Debbage 2019, Garcia-Cuerva et al. 2016, Hasala et al. 2020, Scarlett et al. 2021). An expansive study of the Charlanta Megaregion evaluated the flood risk for vulnerable populations, finding “non-Hispanic black, Hispanic, and below poverty populations 31%, 42%, and 14% more likely to reside in areas at risk for flooding, respectively” within the Megaregion (Debbage 2019). Communities experiencing frequent flooding are more likely to face economic burdens from property damage and suffer from health effects from contact with stormwater pollutants. Increased sediment loads can increase the propensity for localized floods by clogging storm drains and sewer systems. After flooding events, drinking water quality is also at risk, which may correlate with sediment loads. For example, Gaffield et al. (2003) found an association between drinking water turbidity and waterborne disease occurrences following storm events (Gaffield et al. , 2003). The authors suggest stormwater BMP implementation to improve water quality before it reaches the water treatment facility, thereby reducing treatment costs and risk of illness.

Despite the higher stormwater-related risks faced by marginalized communities and the evidence that green infrastructure can help mitigate water quality and community health impacts, barriers remain in the equitable placement of BMPs [including Green Stormwater Infrastructure or GSI]. This phenomenon may be partly due to the uncoordinated placement of these facilities that results from a lack of standardized regulations and a combination of public and private involvement. However, compared to commonly used environmental justice metrics, the relationship between stormwater BMPs and minoritized communities is not as straightforward. A 2023 study in Baltimore, Maryland found contrasting patterns of BMP distribution (Solins et al., 2023). “Regulatory BMPs were negatively correlated with Black population percentage, while voluntary BMPs were positively correlated with “ both in low-income areas with predominantly Black populations and in high-income areas with predominantly white populations” (Solins et al., 2023). These results highlight the challenge in developing an equitable framework for the placement of dispersed stormwater facilities. Stormwater BMPs include such a broad class of infrastructure that vary in treatment area and efficiency, making it difficult to quantify benefits. 

Another lens of finding the patterns between environmental injustice is by examining specific land uses and what trends come about from detailing what is a desirable place to live vs not. The Rowangould study examined demographics of areas with high Annual Average Daily Traffic (AADT) which found that only 19.3 percent of all Americans live next to high volume roads, but overall 27.4% non-white citizens live in these areas. The data also detailed that the average median household income near these roadways was $1221 less than the US average median. Additionally, the areas that ranked high in race-income disparity were also congruent with roadway proximity. The range of severity in the disparities based on the race-income relationship vary throughout the country and are more commonly found in the United States South  (Rowangould, 2013). Water quality, health concerns and housing discrimination issues can be factors resulting from this trend. 

Every community requires a different approach for a plan of action but it is noted that when it comes to stormwater infrastructure, the typical course of action has been to copy and paste a method that worked in one area and test its effectiveness. Bias and marginalization of communities comes with this method that is rooted in decision making that has no basis in numbers. Participatory planning and survey taking methods combined with GIS uses are the most accurate way to get a sense of what a community needs spatially (Talen, 2000). Oftentimes the most cost effective choice or least intensive choice is picked when this happens in the “top down” processing approach. The use of “bottom up” processing would allow for customized plans of action based on location specific data and comparable to treating the wound at the source instead of killing everything with radiation. GIS processing using this method would be crucial for implementing Green Infrastructure Indexes that operate off of need based work (Heckert & Rosan, 2016). 

Community reception is another barrier to equitable placement, due to the association of green infrastructure with gentrification, maintenance, and a lack of participatory planning. A 2021 study examined the association between the installation of BMPs (referred to as SCMs in this publication) and the displacement of minoritized groups.  Black populations faced a 4.1% decline following high levels of BMP installation (Chan et al., 2021). The study also found a positive correlation between BMP concentrations and predominantly black neighborhoods in Washington D.C.  This finding doesn’t align with other studies indicating that these communities are underserved by stormwater infrastructure. One plausible explanation for this trend is the increased development rates, higher-density housing, and industrial areas near these communities. These land use types often have increased stormwater control requirements compared to the lower-density residential areas typical of the suburbs. It may also be a location-specific phenomenon, exemplifying the complexity of factors that impact BMP placement and demographic responses. An analysis of this kind has not been conducted in Charlotte. 

Despite assumptions that disadvantaged people are less concerned about environmental issues and less receptive to sustainable action, a Charlotte study found that POC, women, and the less educated tend to be the most concerned about stormwater issues and the most willing to participate in stormwater management (Scarlett et al., 2021). These results suggest that developing avenues for community involvement would help increase the agency of these individuals. It should be noted that this study used “willingness to participate” rather than the traditional metric of “willingness to pay,” which is financially biased. Two other North Carolina studies focusing on the Walnut Creek Community in Raleigh built off this idea. Both studies propose participatory planning combined with hydrological analysis to choose BMP locations (Garcia-Cuerva et al. 2016, Hasala et al. 2020). This approach allows for community-driven BMP placement that also meets hydrological requirements. 

Multi-benefit planning using open-source data in Geographic Information Systems (GIS) has been proposed by researchers as another method for facilitating equitable GSI (Heckert and Rosan 2018, Porse 2018). One of the challenges of GSI is it’s inherently decentralized nature which presents challenges such as “implementation not only on public properties and rights-of-way but also on privately owned properties” and “buy-in from myriad residents” (Heckert and Rosan, 2018). Utilizing available data regarding both hydrology and community characteristics can help create a more systematic and thorough approach to implementing GSI. These tools could also be used to better educate stakeholders to understand the need for GSI as well as help city governments identify areas to invest. 

While none of these studies focus on sediment, it is one of the biggest polluters in Charlotte-Mecklenburg waterways (Charlotte-Mecklenburg Stormwater Services). It is also one of the most easily recognizable by the community since it is often visible to the naked eye and impacts surface water, making it a potential target pollutant for community education and participatory planning.  

Suspended sediments in urban waterways pose public health and ecological risks. Several contaminants are known to adhere to sediments, including heavy metals, organic material (OM), pathogens, and contaminants of emerging concern (CECs). The issue is primarily an urban one, worsened by sprawl and development. Roads, industrial sectors, construction, and pesticide application act as pollutant sinks, while impervious surfaces increase runoff and decrease infiltration. Some excess runoff ends up in stormwater Best Management Practices (BMPs), leading to the accumulation of contaminated sediments, while the remainder travels to local waterways, impacting water quality. 

The toxicity of sediment-bound pollutants in aquatic systems depends not only on the concentration of particles present in sediment but also the bioavailability, which is dictated by the solubility of the pollutant. Solubility is a function of both the composition of the contaminant and the properties of the water. The low pH and high oxidation-reduction potential (ORP) of urban rainwater are associated with the leaching of particle-bound pollutants such as alkalinity, dissolved ions, metals, nutrients, and DOM from road-deposited sediments (Dissolved Organic Matter), increasing bioavailability and potential for environmental harm (Wang 2017, Kim et al. 2021). 

Identifying the source of sediment contaminants found in nonpoint stormwater runoff is a hurdle for effective management and mitigation of risk. Charters et al. (2020) attempted to isolate the effect of surface type on runoff water quality by taking samples directly from specific sites. Two findings were of particular relevance. First, the authors concluded that “all roads and carparks produced high TSS concentrations,” echoing the aforementioned studies that focused on Road Deposited Sediments (RDS) as a primary contributor of sediment in stormwater systems. Second, they found a high level of dissolved metals, suggesting that a better understanding of particle partitioning is needed to improve pollutant management techniques to target both dissolved and particulate contaminants, which are associated with sediment transport (Charters et al. 2020).  The best management practices for roadside areas are most commonly Downslope Highway Stormwater Control Measures (SCMs) known as Filter Strips, Swales, and PFCs; Permeable friction courses. Strips have been proven to lower concentrations of TSS by at least 50% and Swales have shown to remove between 33 and 49% in several studies. Evaluation of each area’s steepness and access to maintenance service plays into choosing which one is the best fit (Winston Ryan J. et al., 2012). A 2017 study proved the effectiveness of downslope SCMs by measuring TSS and particle size of sediment in various elevations of North Carolina in different proximities to rainfall. R software was then used to calculate the distributions of each particle size for each site. Sources of particles specific to roadside sediments are from tire wear, pavement erosion, and vehicle components like brake pads (Winston R. J. & Hunt W. F., 2017). These types of BMPs are commonly referenced as the most simple in design but this doesn't necessarily mean they are most simple in upkeep. Turf grass in the strips and swales has to be mowed routinely but never after a precipitation event greater than 20mm. This means climate and seasonality are important to take into account. Firm delegation of who and how the upkeep is assigned is needed for public spaces such as highways and an annual evaluation of effectiveness and function is recommended (Blecken et al., 2017). Permeable Modular pavement (PMP) as opposed to typical Hot Mix Asphalt (HMA) was implemented in Auckland, New Zealand in a 200m strip of active roadway and proved that it was an effective tool for limiting non point source transportation related pollutants. The study’s 2 year time span that data was collected, the local waterways water quality was not compromised and contaminants like zinc and copper were able to be significantly limited (Fassman Elizabeth A. & Blackbourn Samuel D., 2011). Additionally, a BMP that works in favor of sloped open landscape is the use of riffles or pools that use sand filtration and hold water in steps down the slope which are not exclusive to but make for easy implementation next to open roadsides. The effectiveness is proven but there is a conclusion pointing to the need for testing the pools combined with new vegetation filtration such as surrounding filter strips that could provide a combination of two BMP types and better results (Cizek et al., 2018). 

Several U.S. studies found elevated concentrations of pollutants in sediment samples taken from stormwater ponds. The most concerning contaminants included copper, zinc, cadmium, nickel, and PFAS chemicals, though specific contaminants and concentrations varied depending on the scale  (Crane 2018; McNett 2011). In the context of urbanization, land cover and land usage are among the most relevant factors to sediment pollution. In Minneapolis, Crane et al., (2018) found higher concentrations associated with industrial and commercial catchments, suggesting that land use type contributes to contaminant levels (Crane 2018). Infiltration and retention BMPs trap and store sediment, which must be removed or flushed from the system as part of routine maintenance. One maintenance method includes the excavation of sediments from BMPs and repurposing them as a land cover. This method effectively removes the sediment from the hydrologic system but increases the likelihood of human contact. Alternatively, the sediment may travel to waterways where it can still impact aquatic health and drinking water quality. A North Carolina study found that over half of BMP sampling sites contained contaminant levels in the sediment that exceeded established guidelines for aquatic health but fell within the acceptable range for land-use application (McNett and Hunt 2011). 

Studies indicate that sediments accumulated in stormwater BMPs contain increased contaminant levels compared to the ambient concentrations in nearby soil (McNett and Hunt 2011). Parallel studies conducted in the Minneapolis Metropolitan area and across North Carolina investigated land use effects on sediment pollutants. In both studies, zinc concentrations were higher in commercial catchment areas compared to residential (McNett and Hunt 2011; Crane et al., 2018). Additional testing in Minnesota determined that “4-nonylphenol, six BDEs (28+33, 47, 99, 100, 154, and 209), and total PBDEs” were significantly elevated in commercial and industrial areas (Crane et al., 2018). The more limited contaminants tested in North Carolina did not yield a statistically significant difference. The elevated pollutant concentrations in stormwater sediment pose a concern for BMP maintenance. One maintenance method includes excavation and repurposing as a land cover which effectively removes the sediment from the hydrologic system but increases the likelihood of human contact. Alternatively, the sediment may travel to waterways where it can still impact aquatic health and drinking water quality. In the North Carolina study, over half of BMP sampling sites contained contaminant levels in the sediment that exceeded established guidelines for aquatic health, however, all were within the acceptable range for land-use application (McNett and Hunt 2011). In Minnesota, sediments from stormwater ponds with elevated carcinogenic contaminants warrant restricted disposal (Crane et al., 2018). The authors of both studies suggest more extensive testing before land use application.

In addition to heavy metals and toxic chemicals, stormwater sediments may aid the transport of pathogens. An early study by Schillinger and Gannon (1985) found that bacterial adhesion to sediments ranged from 16-47% depending on the strain of the pathogen and the growth medium. These results suggest that suspended solids may act as a transport mechanism that increases bacterial concentrations in surface water. The authors also conclude that bacterial accumulation in bottom sediments poses health concerns for “recreation and shellfishing” (Schillinger and Gannon 1985).  A more recent study expanded these findings to develop a model for sediment-bacteria transport. Bai and Lung (2005) incorporated a resuspension calculation to more accurately predict the “contributions of fecal bacteria from the sediment bed” during exposure to high shear stresses (Bai and Lung 2005). Urbanization will likely increase resuspension due to the increased flow rates from the impervious surface. Under these scenarios, sediment-bound bacteria may worsen already compromised water quality.

Lastly, there is mounting evidence that urban stormwater sediments contain significant amounts of microplastics, considered a CEC due to a lack of knowledge regarding their health and environmental impacts. Liu et al. (2019) found microplastics to be “prevalent” among stormwater sediments in the seven retention ponds sampled in Denmark. The amounts of microplastics varied between sample locations, but a cause could not be identified (Liu 2019). More research is needed in this area to determine the associations between land use and microplastics in stormwater sediments. 

The landscape alterations associated with urbanization disrupt the natural transport of sediments within watersheds. Increased impervious surfaces and stormwater control measures (SCMs) alter sediment sources and sink, leading to the problem of both “too much and too little” sediment in urban aquatic systems (Russell et al. 2019). Excess sediment builds up in SCMs, creating maintenance issues such as clogging. As a result, only 40% of coarse sediments reach waterways which causes a lack of ecologically and morphologically important bedload sediment (Russell et al. 2019).  Converse to the deficits of coarse-grained bedload sediment, fine-grained suspended sediments tend to increase in urban waterways during and after development. For example, a study from Virginia Beach found elevated TSS levels in industrial and open space catchments. This led to the hypothesis that “patchy vegetative cover within open space land use and exposing its runoff to bare earth may cause high TSS EMCs from open space, and high TSS EMCs from industrial lands may be caused by the low quality of road surfaces (asphalt) and high truck traffic within the catchment” (Nayeb Yazdi et al. 2021). Findings such as these highlight the complexity of sedimentation issues in urban catchments. Although open space areas feature primarily pervious surfaces, without proper vegetation to stabilize soils and runoff control mechanisms to reduce erosion, they will remain vulnerable to erosion and cause increased sediment loading. 

The challenges of identifying a causal relationship between land cover and TSS loading were reinforced by a 2022 study by Praskievicz. Multiple land use predictor values were used, including National Land Cover Database (NLCD) class, impervious surface area, and tax parcels. However, the authors found no significant correlation between these land cover metrics and suspended-sediment concentration (Praskievicz 2022). The author discusses the pitfalls of using broad categories such as impervious surfaces due to the wide range of “possible characteristics that potentially affect hydrology, geomorphology, water quality, and ecosystems” (Praskievicz 2022). Remote sensing raster data is another challenge, as the lower resolutions may not accurately capture variable land use on a specific plot or within a catchment (Praskievicz 2022). While this study did not find significant associations with land use type, all the catchments included were located in developed areas. More dramatic land conversions, such as from forest or agricultural areas to industrial or residential sectors, would likely have a more distinct impact on water quality. 

A review study conducted by O’Driscoll et al. looked at the effects of urbanization on watershed hydrology in the Southern U.S. The authors identified Total Impervious Area (TIA) as the primary land use determinator of hydrology.  While the review looked at multiple aspects of watershed hydrology, the most applicable to the focus of this study included their findings on channel geomorphology, sediment transport, and water quality. The authors found significant “spatial and temporal variability” in geomorphological response to urbanization (O’Driscoll et al., 2010). While most studies included in the review associated larger channels with urbanization due to increased flow velocity, others observed increased aggradation and deposition, especially in areas downstream of erosion (O’Driscoll et al., 2010). Generally, a pattern of increased sediment loads and erosion was associated with increased TIA, however, various other factors must be considered, such as legacy land use, stormwater connectivity, soil type, and land use type. Similarly, water quality varied within urban areas, and it was difficult to identify correlations due to compounding influences. Most of the studies reviewed came to similar conclusions: urbanization negatively impacts water quality. One study identified 7% as the TIA threshold at which water quality degradation began (O’Driscoll et al., 2010). 

Another study looked more specifically at patterns between watershed development and water quality in Durham, North Carolina. Carle et al. (2005) used publicly available water quality data from Durham Stormwater Services supplemented with grab samples for more in-depth testing. They found TSS levels significantly correlated with household density, percent of connected impervious surface area, stormwater outfall density, and natural watershed features. Due to the collinearity of these variables, the exact relationships are difficult to ascertain. However, when all other variables are held constant in the model, densely developed watersheds and those with hydric soils were found to have the highest TSS levels (Carle et al. 2005). 

Sediment management is separated into two categories with some overlap: prevention and removal. Both areas focus on reducing the quantity of fine sediment in waterways. Studies indicate that in-situ sediment management, such as erosion control and street sweeping can help prevent sediments from building up and traveling into waterways, as construction sites and road surfaces are two of the largest contributors to sediment loads in urban environments (Guo et al. 2021, Kim 2021). Extensive sediment control during construction is mandated and regulated by the North Carolina Department of Environmental Quality (NCDEQ). While more research is needed regarding the efficacy of street sweeping, it shows some promise as a method to remove RDS and prevent the associated contaminants from reaching nearby waterways. 

Stormwater BMPs provide the primary mode of suspended sediment removal. However, BMPs vary in size and mechanism of treatment, leading to large variations in effectiveness. A 2021 study utilized rainfall-runoff modeling and in-stream dynamic modeling to predict the effectiveness of various decentralized BMPs (referred to as GSI in this study). They found that stormwater detention basins outfitted with additional infiltration were more effective than other BMPs types at removing sediment (Beganskas 2021). Another study examined the use of Regenerative Stormwater Conveyance, RSCs that used sand filtering systems in small pools, riffles and weirs combined with vegetation for natural filtration. The reduction in flooding was successful as well as reduced sediment movement. The suggestion after the study is to increase effectiveness by increasing low vegetation in and around the implementation sand filtration pools (Cizek et al., 2018)

Summary 

Urbanization changes the sediment dynamics within watersheds leading to issues of too much fine sediment and too little bedload sediment. Fine sediment pollution in urban waterways poses a risk to aquatic environments and human health. Currently, sediment management consists of erosion control in construction sites and stormwater BMPs. While these measures help reduce the impact of excess sediment, developed areas still tend to experience more sediment pollution than undisturbed waterways. While numerous studies have been conducted on the environmental justice considerations of stormwater management and green infrastructure, it is unknown how vulnerable populations are specifically affected by sedimentation.