The idea of “accidental” wetlands is gaining traction as a potentially valuable component of various ecosystems. These are water bodies that “result from human activities, but are not designed or managed for any specific outcome” [1
]. As the moniker implies, these “accidental” systems arise on their own, often in vacant lots or various low spots where stormwater or irrigation runoff collect. In a study of the South Platte River basin, for example, 89% of the extant wetlands exist because of various irrigation conveyances [2
]. Accidental wetlands typically form in low-lying abandoned or underutilized landscapes with poor drainage [3
]. Flooding often occurs in these areas due to a lack of stormwater management and sediment transport processes, which contribute to repeated sediment deposition, allowing wetland plants to grow and establish a habitat [3
]. Brooks et al. [4
] documented that in arid, urban areas, effluent from human sources can account for up to 90% of streamflow during dry periods. Any wetland-like areas on those rivers exist because of human activity. Although they are not yet heavily studied, there is evidence that these accidental wetlands offer benefits to rival constructed versions [1
]. In fact, Palta et al. [3
] argue that “accidental wetlands may provide more services than designed environments because the latter are commonly over-designed for a limited set of specific functions.” There is also evidence that small wetlands, which many accidental wetlands are, can be as or more effective than larger ones [7
]. Although they are just beginning to be intentionally documented and studied, Palta et al. [3
] offer evidence highlighting the ubiquity of accidental wetlands and their potential to offer diverse benefits. In the limited literature available, researchers have found that accidental wetlands can remove pollutants, mitigate heat, store groundwater, restore surface-mined lands, and provide the social benefits of additional green space in urban settings [3
In these few known studies, accidental wetlands were shown to help with pollutant remediation, and a key pollutant in cold urban areas is road salt. Like accidental wetlands, in recent years researchers have paid increasing attention to de-icing salt in stormwater runoff as a key issue for stream quality, especially in urban areas. Although there are numerous salt sources in urban environments, including water-softening, septic field discharge, natural rock weathering, and wastewater treatment plants [12
], the dominant source is road salt applied to the road surface in order to melt snow and ice [13
]. The annual road salt application in the United States in 2014 was about 56.5 million metric tons, costing about $
1.18 billion [15
]. Elevated salt levels in waterways contribute to negative consequences, including contaminating drinking wells [16
] and various ecosystem impacts. In their review of the literature, Hintz and Relyea [17
] found that salt negatively affects all freshwater species, but that the level at which salt becomes a concern is highly variable across species.
There are complex relationships between surface and groundwater systems, such that salt remains available for long periods of time as it travels between surface and groundwater systems [18
]. Furthermore, numerous studies have focused on how effective various “green infrastructure” (e.g., rain gardens, bioretention systems) are in cold climates (see Kratky et al. [21
] for a review). Retention systems capture salt as it runs off and can slow its path to waterways [18
]. Retention systems do not, however, eliminate salt or retain it for long periods. Studies have found that salinity remains a key stream stressor even in restored streams and in those with stormwater management efforts in place [23
]. In addition to its own negative consequences, salt can mobilize various metals and lower plants’ uptake of metals, thereby reducing the effectiveness of bioretention [21
]. Cockerill et al. [20
], however, did find that retention had the potential to reduce salinity spikes, which can reduce the long-term salt levels in a given hydrologic system. Flattening the curve, as it were, and alleviating those salt spikes, may over time lower the summer pulses of salt into that system. Given Hintz and Relyea’s [17
] findings on the variability in how much salt triggers negative consequences, reducing salinity spikes and reducing summer inputs may reduce the effects on some aquatic species.
Although researchers are studying accidental wetlands and the negative effects of road salt on urban waterways, as far as we know, these two subjects have not been assessed in concert. We offer a conceptual study addressing the potential for accidental wetlands to ameliorate salinity levels. Because constructed wetlands offer highly variable results depending on scale, lifespan, and local hydrologic conditions [28
], paying more focused attention to sites where wetlands form themselves may offer a cheaper and equally or more effective alternative means of lessening road salt impacts. Though no wetland will completely remove the salt from an urban stream system, there is still potential to improve urban water quality by documenting and encouraging accidental wetlands.
The project documented here is a testimony to the value of paying attention to local environments and the role of serendipity in research. Copeland [32
] describes serendipity in science as “the intersection of chance and wisdom” and notes that the value of the serendipitous process or event can only be assessed in hindsight. In our case, authors Anderson and Cockerill initially noticed a “wetland” forming in a concrete culvert on the Appalachian State University campus and joked about its potential value. As we continued to observe the accumulation of sediment and increased vegetation, we began thinking that the site actually warranted more focused attention. At that point, author Maas was seeking a project and began to intentionally explore the effect of that wetland on road salt contamination for a headwater stream in an urban setting. In searching for background information to guide this more intentional study, we encountered the premise of “accidental wetlands” and recognized that we had one.
Once intentionality was established, this study focused on these research questions:
Can flow through the shallow subsurface and chloride transport processes through accidental wetlands reduce the magnitude of saline peaks arriving at an adjacent stream?
Can flow through the shallow subsurface and chloride transport processes through accidental wetlands delay the arrival of a salt plume at an adjacent stream?
Boone Creek and the accidental wetland investigated in this study are located in the small yet heavily urbanized town of Boone, North Carolina (Figure 1
). Boone Creek is a headwater tributary of the South Fork New River, located within the Blue Ridge Mountains of northwestern North Carolina [20
]. The town of Boone has high runoff ratios that result in flashy, flood-prone streams due to heavy urbanization near the stream. Although the total catchment area has 24.3% impervious surface cover, much of the more natural land use occurs on the mountain slopes near the catchment boundaries (Figure 2
). Within 25 m of Boone Creek, the impervious surface cover ranges from 1% to 75% [33
]. The Town of Boone has about 19,500 residents (U.S. Census Bureau, Quick Facts, https://www.census.gov/quickfacts/fact/table/boonetownnorthcarolina,US/PST045219
, accessed 25 May 2021) and Appalachian State University (ASU) adds an additional 20,000 students to the local population (Appalachian State University Facts, https://www.appstate.edu/about/
accessed on 25 May 2021). The stream is in a mountainous area, with a total catchment relief of nearly 500 m, although the main channel gradient is fairly low, at 2%. The stream has previously had trout populations and retains a state classification as a “trout stream” [34
]. Other tributaries to Boone Creek have gradients of greater than 10%. The catchment of the basin considered in this study has an area of 5.2 km2
Boone Creek (Figure 1
) runs through several culverts, with the longest being about 600 m long [20
]. The area experiences 89 cm of snow per year, requiring 6 × 105
kg (600 metric tons) of deicing salt applied to the impervious surfaces on average annually [12
]. Electrical conductivity data, converted to equivalent chloride, indicate that the stream regularly violates the US Environmental Protection Agency’s (EPA) chronic threshold of a four-day average chloride concentration, exceeding 230 mg/L, and also exceeds the acute threshold of a one-hour average chloride concentration, exceeding 860 mg/L on a regular basis in the winter salting season [20
]. Between 1 July 2014 and 1 July 2015, Boone Creek exceeded the EPA’s chronic threshold for 10% of the time, or 36 days, and exceeded the acute threshold for the equivalent of 8.5 days [20
Our accidental wetland site is unique because it was created in a constrained concrete channel and did not form on an existing substrate. Subsequently, there is no direct connection between the wetland and the natural groundwater reservoir. We are treating the accidental wetland as an isolated “aquifer” that is disconnected from the natural groundwater flow system. Throughout the rest of this paper, our references to groundwater flow and solute transport are solely related to this process in the small accidental wetland that formed in the concrete culvert that ultimately feeds water to Boone Creek through seepage at its downgradient end. The only sources of water to the accidental wetland are from runoff from impervious surfaces or direct precipitation. Figure 3
shows the total area of the drainage basin to the accidental wetland, delineating pervious areas and impervious areas, such as buildings and sidewalks. The 30-year normal precipitation in Boone as measured at the BOONE 1 SE, NC US weather station between 1981 and 2010 is 1338 mm, with roughly equal precipitation during each season (https://www.ncdc.noaa.gov/cdo-web/datatools/normals
, accessed on 21 April 2021). As Figure 4
a–f show, the 600 m culvert containing Boone Creek is open for five meters to allow stormwater to enter from a concrete conveyance that drains part of campus, as well as part of the Town of Boone.
The focus of this study was not on how accidental wetlands form, because the exact timing for this specific accidental wetland was unclear and did not factor in our findings. It may have originated in 2011–2012 from large storm events, carrying significant sediment into the culvert. By 2015, authors Anderson and Cockerill observed that the channel had become a heavily vegetated wetland (Figure 4
b), and we began observing it regularly. Up to that point, we had paid little attention to the site and the few photos available before 2015 show spotty vegetation in 2005 (Figure 4
a) and we observed some sediment accumulation along one edge by 2012. Since 2015 the system evolved to include diverse vegetation throughout the entire channel and a meandering “stream” through the vegetation.
A storm event in October 2017 deposited a large amount of sand/soil at the culvert end of the wetland. At that time, authors Maas and Anderson decided to assess what was happening more formally. They installed three wells and an electrical conductivity logger (HOBO Conductivity Data Logger, U24-001) along the length of the wetland in 2017. The monitoring wells showed large fluctuations in water levels throughout the year, with high levels during intense storm events during warm months and during the wet and cold winter. In September 2018 the university removed all of the vegetation and accumulated sediment in the accidental wetland as part of flood control measures on campus (Figure 4
Our study demonstrates that accidental wetlands help remediate salt-laden stormwater runoff by increasing the residence time of the salt and delaying its arrival to a receiving stream, thereby decreasing the peak concentration of the salinity pulse but not the overall quantity of salt that is delivered to the stream.
Our field-sampled data support this conclusion. The median and mean values at the seepage face at the end of the wetland were lower in concentration than those at the upper end of the wetland, where salt-rich runoff entered the wetland during salt events. Our groundwater modeling and solute transport simulations also support this conclusion in a comprehensive way. The accidental wetland slows the delivery of salt-laden runoff, thereby lowering simulated salinities by up to 94 percent and lagging the timing of the peak concentrations by up to 45 days. This type of retention system does not remove the salt, but by slowing down its migration to the adjacent stream, it can lower the chance of chronic chloride violations and greatly reduce the potential for acute salinity spikes. This distinction between acute and chronic is important to stream health in the same way that dosage is relevant to any toxin. As noted previously, in their review of salinity impacts, Hintz and Relyea [17
] found significant variability across species and across salinity levels. For example, they found that some species adapt to chronic salinity levels, although acute spikes are lethal. Because road salt will continue to enter streams, finding ways to at least lower the intense spikes of salt should be beneficial. If enough accidental wetlands are encouraged within urban watersheds, rather than removed from them, this could have a positive influence on the overall water quality in streams.
Furthermore, sensitivity analyses with the model showed that hydraulic conductivity values typical of sands are ideal in order for this to take place. This was the primary material in the accidental wetland at our study site and served as the base case for our simulations. It is safe to assume, however, that materials with properties too far outside of this range, such as gravels or clays, would not be effective. The hydraulic conductivity of highly-permeable materials such as gravels would allow rapid migration through the accidental wetland, preventing much of a reduction in peak concentrations or an increase in lag times. Conversely, silts and clays would have so little permeability that surface flow would dominate and would allow the quick delivery of the salt-laden urban runoff to the stream.
We think that there are likely other accidental wetlands similar to the one studied here in other environments; that is, a small scale, concrete substrate wetland forming in an area “designed” for another purpose. The formation of multiple accidental wetlands along a watershed would retain a higher percentage of the road salt, thus decreasing the concentration of the salinity pulses and preventing higher salinities in streams themselves. We can potentially improve the water quality of our urban streams by watching for and then encouraging accidental wetlands in multiple areas throughout a watershed.
4.1. Other Accidental Benefits
In addition to salinity concerns, accidental wetlands also influence other dynamic hydraulic issues such as stream flashiness and temperature surges in urban environments. Stream flashiness is caused by high percentages of impervious surfaces in the watershed, especially in the lower reaches in the Town of Boone and on the ASU campus, and results in rapid stream-stage increases and reversed-gradient events, which drive road salt into riparian aquifers along the length of urban stream reaches. These processes lead to both chronic and acute stream contamination over much longer time frames than what occurs in natural, unurbanized streams [42
]. In our study location, urban, flashy runoff provides a significant source of contamination to the riparian aquifer along Boone Creek, a focus of years of urban hydrology research, and these groundwater–surface water interaction processes are especially important in relation to the chronic and acute chloride contamination of Boone Creek.
When stormwater runoff encounters an accidental wetland, the amount of runoff entering the stream decreases and lags according to the travel time that the wetland requires. Temperature surges occur when heated runoff enters a stream and quickly increases the stream temperature [20
]. Depending on the formation of the accidental wetland, any detained water has time to cool, thereby reducing the temperature of hot runoff in temperature surges. If enough of these wetlands form, temperature surges will perhaps be greatly reduced or eliminated within a watershed. Long-term average stream temperatures are also improved by accidental wetlands via natural shading [3
]; in fact, several small trees, along with other knee-high vegetation, did grow in the accidental wetland we studied. In addition to reducing impacts from salt, our case suggests that accidental wetlands in urban areas can reduce the effects of urban stream syndrome by mitigating flashiness and heat.
4.2. Management Implications
As already noted, campus facilities management removed the accidental wetland in 2018. Removing this wetland aligns with historic management approaches as well as continued perceptions about stormwater, flooding, and vegetated waterways. Traditional stormwater management has focused on shunting water as quickly as possible to the nearest stream to avoid flooding. Although new approaches, including constructing wetlands, bioswales, and retention systems, have been implemented, there remains a lack of understanding and misperceptions about these kinds of management techniques. In his overview of the literature, Everett [43
] concluded that while there is some public awareness of alternative management approaches, it is not yet mainstream knowledge. People tend to focus on the amenity value or the aesthetics of entities like wetlands or bioswales rather than on their function. In some cases, this creates a negative feedback loop, in which people do not understand what the infrastructure is designed to do—or in this case what it accidentally does—and they therefore do not manage or maintain it appropriately, which leads to further negative views as aesthetics and/or function decline.
In our Boone Creek case, a misperception that the wetland was reducing the channel’s ability to handle runoff and thereby increasing flooding risk was the rationale to remove it. This mistaken perception potentially reduced, rather than improved, both flood management and water quality. Because all of the vegetation and sediment was removed, runoff entering the culvert now goes straight into the stream, carrying any salt with it (Figure 2
f). Because accidental wetlands arise without intention and are not part of explicit management plans, they can be ephemeral and can be quickly removed. If paying attention to when and where accidental wetlands form became part of overall urban stormwater management and planning, it could offer reduced flooding, improved water quality, and economic benefits, as the wetland costs nothing to establish.
The accidental formation of a wetland in a cold, urban environment inspired us to study the impacts of this system on mitigating road salt. Returning to our research questions for this conceptual study, we conclude that the accidental wetland decreased the peak concentrations of salt and chloride contamination and delayed the arrival of salt to the stream. Simple numerical simulations demonstrated that the saline pulse from urban runoff was delayed by up to 45 days and peak concentrations declined by up to 94%. Although the salt is not removed from the system, its arrival was delayed and the peak concentrations were lowered, ultimately improving water quality and potentially reducing negative consequences for some aquatic species.
We also think that accidental wetlands are likely to serve several functions and potentially offer multiple benefits to stream quality. For example, our model provided a conservative estimate regarding peak salinity reduction because we did not include uptake by plants. Future research assessing the role of vegetation and/or soil in contributing to reducing salinity impacts in accidental wetlands is warranted. Additionally, our results suggest that accidental wetlands may be effective in lowering runoff-induced thermal pollution due to the delay in the runoff entering the stream. Although this delay does not actually remove the salt, it most likely does reduce the temperature, offering a potentially significant improvement in cold-water streams. Again, more detailed analyses of this potential contribution from accidental wetlands would be valuable. Our work highlights the potential for accidental wetlands to improve water quality issues in urban streams. The only way to decrease salinization issues in cold regions is to discontinue the use of road salt during winter storm events. Because of safety concerns, totally eliminating salt is not practicable and therefore, we need to better understand and explore reactive measures to retain salt and delay its delivery into streams. Our study builds on previous work, suggesting that accidental wetlands cost nothing but offer multiple benefits, which are potentially as valuable as some stream restoration or intentional stormwater retention measures.