Effectiveness of roadside vegetated filter strips and swales at treating roadway runoff: a tutorial review

Alex R. Boger ^a, Laurent Ahiablame† *^a, Esther Mosase ^ab and Dwayne Beck ^c
aDepartment of Agricultural and Biosystems Engineering, South Dakota State University, 1400 N. Campus Dr., Box 2120, Brookings, SD 57007, USA. E-mail: alex.boger@sdstate.edu; laurent.ahiablame@sdstate.edu
^bDepartment of Civil and Environmental Engineering, South Dakota State University, Crothers Engineering Hall, Brookings, SD 570006, USA. E-mail: esther.mosase@sdstate.edu
^cDakota Lakes Research Farm, South Dakota State University, 21310 308th Ave, Pierre, SD 57501, USA. E-mail: dwayne.beck@sdstate.edu

First published on 27th February 2018

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Water impact Vegetated filter strips and swales are often used for roadside ditch management to convey storm runoff and improve water quality. This article synthesizes information from selected publications to document their effectiveness at removing sediment, nutrients and metals from storm runoff in roadside ditches. The data show that the practices are more effective at removing particulate pollutants than dissolved pollutants.

1. Introduction

Water pollution in the United States (US) has been regulated since the 1940s.^1,2 The Clean Water Act (CWA) of 1972 evolved from the Federal Water Pollution Control Act of 1948 as the prime federal law in the United States to guide the way that water quality standards and programs should be developed and established.^3,4 With the objective to restore and maintain the chemical, physical, and biological integrity of the nation's waters, the CWA sets the stage for preventing point and nonpoint pollution source pollutants, giving public water utilities assistance in improving water quality, as well as maintaining wetland health.⁵ The CWA brought a massive decrease in water pollution throughout the nation, including a decrease in municipal pollutant loadings into waterways.^4,6 Despite the massive strides gained from the 1972 CWA amendments, waterways remained polluted in many locations across the nation, mainly due to contamination from nonpoint source pollutants.^4,6

Nonpoint source pollution became the target of the next wave of water quality management.^4,6 In early 1990s, states, territories and authorized tribes were required to start inventorying their waterways by level of impairment with respect to supporting beneficial uses, leading to development of planning tools such as the total maximum daily load (TMDL) program^4,6 for maintaining water quality standards. A TMDL is a measurement of the maximum amount of a pollutant that a water body can receive without exceeding water quality standards.^7,8 Total maximum daily loads often lead to analysis of the sources of pollutants in order to develop restoration and protection methods for lowering these inputs to impaired waterways.⁷

Stormwater management is at the core of nonpoint source pollution management.^9,10 Storm runoff carries considerable amounts of pollutants and suspended solids to waterways from non-permeable surfaces in urbanized areas.^11,12 While the United States Environmental Protection Agency (USEPA) controls the listing of impaired waters and process of TMDL development, implementation of TMDLs is relegated to states or local governments,¹³ which use a suite of best management practices (BMPs) such as vegetative BMPs to protect water quality.^8,14 Vegetative BMPs rely on the bulk of grass to slow down the flow of water, giving time to pollutants to fall out of suspension, allowing the vegetation to uptake nutrients and other pollutants from the water column.^8,15,16

Vegetated FS's and GS's are popular vegetative BMPs used along roadways to slow storm runoff, trap sediment and pollutants, storage, and promote infiltration (e.g.ref. 17–20). Research showed that removal of pollutants in VFS's and GS's can also be accomplished through processes bio-chemical processes such as plant uptake and microbial activity.^21–23 Vegetated FS's and GS's are part of low impact development (LID) stormwater control strategies.^24,25 The use of LID BMPs to mitigate adverse effects of increasing urbanization and impervious surfaces has gained worldwide popularity (e.g.ref. 26–29). Pioneered in Prince George County in Maryland, US,³⁰ the concept evolved into various denominations such as LID in Canada, sustainable urban drainage systems (SUDS) in the UK, water sensitive urban design (WSUD) in Australia, and recently The Sponge City in China (e.g.ref. 31–36). LID BMPs are proven site-scale practices used to manage storm runoff quantity, control flood, improve water quality, enhance natural habitat, and improve community aesthetics and livability (e.g.ref. 27, 30, 37 and 38).

The effectiveness of VFS's and GS's has been presented in several studies that showed a wide range of pollutant removal efficiencies and load reductions.^14,21,22,39 Although these studies provided evidence that the use of VFS's and GS's in roadside ditches is very popular, many knowledge gaps exist in regard to their effectiveness. This tutorial review summarized selected publications and reports that directly demonstrate the potential impacts of roadside VFS and GS on downstream water quality. The specific objectives of the study were to (1) present a synthesis of selected literature on water quality in roadside ditches maintained with VFS and GS, and (2) discuss implications for downstream water quality, and factors that affect the performance of roadside VFS's and GS's. The roadside ditches examined in this study received storm runoff from state roads, highways, and interstates. Water quality constituents summarized include TSS, nutrients (i.e. nitrogen (N), phosphorous (P), and chloride (Cl⁻)), trace metals including lead (Pb), copper (Cu), zinc (Zn), iron (Fe), cadmium (Cd), chromium (Cr), nickel (Ni). This study focused solely on pollutant concentrations and removal (%) to document the effectiveness of these practices; however, constituent mass loading and load reduction is generally considered the more meaningful metric in evaluating LID BMP performance and crediting watershed efforts to meet TMDL requirements and to ensure NPDES (National Pollutant Discharge Elimination System) permit compliance.⁴⁰

2. Performance of vegetated filter strips and swales for roadway runoff treatment

2.1. Nutrient removal

Total suspended solids, total Kjeldahl N (TKN), nitrate (NO₃-N), nitrite (NO₂-N), organic N, total N (TN), total P (TP), organic P, dissolved P, and Cl⁻ concentrations in water samples collected from roadside ditches at various geographic locations show a wide range of variability (Table 1). For the literature examined, TSS, TKN, and NO₃-N show the highest variability, with values ranging from 0 to 1200, 0.04 to 10, and 0 to 8 mg L⁻¹, respectively. This is likely driven by differences in study sites and upland activities at the sites. Total N and organic N also show variability in the data compiled, with ranges of 0 to 65 mg L⁻¹ and 0.1 to 4 mg L⁻¹, respectively. It should be noted some of the values reported for NO₃-N contain NO₃-N + NO₂-N. Removal of TSS, TKN, NO₃-N, NO₂-N, and total N also shows high variability (Table 1). For example, removal of TSS, TKN, and TN range from −129 to 98%, −106% to 77%, and −26% to 86%, respectively. Reduction in NO₃-N range from −25 to 89, and −13 to −8% for organic N. The variability in percent reduction may be imparted to variations in soil, precipitation, antecedent moisture conditions and subsequent surface runoff events received by the roadside environment.^15,22,41 The negative removal reported for some N constituents was likely due to the increase of these contaminants along the ditches. First flush effect may be the cause of negative reductions, as elevated amounts of pollutant build ups in roadsides over time can decrease the reduction capacity of the BMPs, leading to high concentrations of pollutants carried downstream during the beginning of rainfall events after some a dry period.²¹

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Phosphorous, a prevalent environmental pollutant, is common in roadside waters (Table 2). Phosphorous in roadside ditches originates mainly from agricultural fertilizer runoff, manure, and decomposition of vegetation from mowing and maintenance activities. Total P shows variability across studies, with concentrations ranging from 0 to 10.7 mg L⁻¹. Organic P and dissolved P show less variability, with ranges of 0.279 to 0.58 mg L⁻¹ and 0.01 to 10 mg L⁻¹. Removal of the various forms of P vary between storm events and studies (Table 2). Removal of total P varies highly by study, with percent reduction values ranging from −218 to 99%.

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Chloride is another common pollutant in roadsides, especially in the northern US, due to road deicing activities. Concentration of chloride ranges from 0 to 5590 mg L⁻¹, with percent reduction values ranging from −4410 to −78%. The negative values reported for Cl⁻ removal are likely due to high concentrations of roadway deicing salts in the water entering roadside environment, leading to higher downstream Cl⁻ concentrations.^42,43

2.2. Trace metal removal

Metal pollution is prevalent in roadsides due to the variety of metals involved in construction and maintenance of motor vehicles that utilize the roadways. Data summarized on the different metals show wide ranges (Tables 3 and 4). Total Zn concentrations show high variability across studies, with concentrations ranging from 0 to 550 μg L⁻¹. Total Pb and Cu concentrations also vary between 0 and 150 μg L⁻¹, and 0 and 160 μg L⁻¹, respectively. Reduction of total Pb ranges between −186 and 67%, between −35 and 93% for total Zn, and from −288 to 81% across the studies examined. Variability in concentration is also seen in dissolved Zn, Pb and Cu, with ranges of 3 to 120 μg L⁻¹, 1 to 39 μg L⁻¹, and 1 to 58 μg L⁻¹, which are translated into −86 to 32%, −61 to 13%, and 375 to 31% reductions (Table 4).

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3. Discussion

3.1. Implications for downstream water quality

Water entering roadside ditches either infiltrates into the underlying soil or travels through the ditch in the form of direct runoff to downstream water bodies, implying that elevated pollutant levels in roadside ditch water are potential water quality threats for both surface water and shallow groundwater.⁵²

Sediment loading in roadside ditches is common from ditch bottom erosion and sediments and the surrounding landscape.⁵³ Transport of sediment in roadside ditches mainly occur during extreme events.^53,54 These large events result in high flow rates in which finer sediment particles are to be transported downstream out of the practice.^45,48 With a 110 m long roadside GS in Södra Hamnleden, Sweden, Bäckström et al.^45,48 noted that particles larger than 25 μm were effectively trapped in the swale under different real rainfall and runoff events. Deletic's⁵⁵ also found a considerable proportion of sediment particles which were larger than 57 μm in retained in grassed swales, while retention of finer particles (smaller than 5.8 μm) was not high, with the potential to reach downstream water bodies. Increased sediment loading can compromise the ecological integrity of aquatic environments, affecting water quality physically, chemically and biologically. Moreover, sediments often carry organic matter, nutrients, and chemicals that can adversely affect water quality.

Nitrogen pollution is a wide spread problem in water bodies throughout the world, causing eutrophication (nutrient enrichment of an ecosystem),^45,56–58 and human health concerns.^59–61 Sources of N pollution include agricultural runoff, atmospheric deposition, septic tank leaching, construction site runoff, and runoff from abandoned mines.^61,62 A study performed in Maryland on two grassed swales closely examined the mechanisms influencing N removal in roadside waters.²² In this study, removal was attributed mainly to infiltration and sedimentation.²² Most N pollution in roadsides tends to be in dissolved forms, because of this reduction of N pollution through mainly results in a greater reduction in load than concentration.²² One component of TKN is organic N. Due to its particulate nature, organic N is generally effected by filtration and sediment trapping ability of BMP vegetation.²² The negative values shown for contaminant reduction in the literature examined suggest that elevated levels of these pollutants in the water are likely carried downstream.²¹ Negative removal of organic N and TKN can be attributed to the presence of organic matter in the top materials in the roadside ditches, which is likely the result of frequent mowing of the ditches as well as leaf litter from nearby plants.^22,41

Phosphorous loading to downstream waters also lead to eutrophication which stimulates an explosive bloom of algae.^60,63 Organic P, commonly held in particulate form, represents approximately on median 90% of the sum of P losses from roadside ditches.⁴⁴ Phosphorous removal is mainly driven by sedimentation, with approximately up to 70% of total P being bound to particulates.^22,64 Vegetated filter strips and GS are less efficient in attenuating dissolved P concentrations in storm runoff (Table 2), especially in water with low concentration levels and less SS particles, mainly because P tends to be attached to smaller particles which generally remain in suspension longer than larger particules.²² A close examination of the studies discussed reveals that reduction of organic P may be attributed to the ability of the soil to sorb P, driving by high cation exchange capacity.⁶⁵

Chloride mainly enters the roadside ditch environment in the form of salt from road deicing activities during the winter.^22,43 This could lead to increase in Cl⁻ levels in roadside ditches, with the potential to contribute elevated downstream Cl⁻ concentrations.^22,43 Chloride can be toxic to flora and fauna in surface waters,^66,67 and can disrupt stratification in meromictic lakes,⁶⁸ leading to increased mass loadings of metals from groundwater sources into the lake.⁶⁹ As a conservative solute, Cl⁻ attenuation in VFS's and GS's is mainly controls by infiltration process.^22,70–72 However, infiltration is dominant at the beginning of the storm when field capacity is not reached. As soil saturation increases with the rainfall duration, infiltration decreases.^73,74 Thus, Cl⁻ concentrations in roadway runoff tend to be high, and even higher in roadside ditches treated with vegetative BMPs.²² Although Cl⁻ deposition in roadside ditches is mostly seasonal, discharges of Cl⁻ polluted water continues all year round as Cl⁻ is a conservative solute and tends to stay in roadsides for long periods of time after deposition.^22,72

Metals are common in roadside ditch water (Tables 3 and 4). They usually come from natural deposition of materials from motor vehicles, and deposition from common roadside features like guard rails and signs. Heavy metals in surface waters can cause massive die-offs of microorganisms and bioaccumulate in fish and other aquatic animals, leading to risk of metal toxicity up the food chain.⁷⁵ Metals can also adversely affect human health, causing organ failure, retardation in children, lung disease, and a myriad of other health issues.⁷⁶ Most metal removal in roadside ditches can be attributed to sedimentation and filtration processes as proportions of the dissolved forms are rather small based on the studies examined.²²

3.2. Driving factors for contaminant removal in vegetated filter strips and swales

Five main factors appear to influence roadside VFS and GS performance. These include soil type, slope, treatment length of BMP, vegetation type, and vegetation cover.^8,14,41,45 Soil type and slope are the least human-controlled of these factors as they are mostly determined by site characteristics. Soil type impacts the performance of roadside BMPs by controlling the movement of water into and through the soil. In a study on two VFS's in Florida, dry soils with good drainage and high infiltration rates showed high removal of total metal, N, and P loads.⁴⁴ Gentle slopes usually lead to higher infiltration rates by slowing the flow of water through the ditch, allowing more time for infiltration.¹⁴ Steeper slopes increase the speed of water moving through the ditch and give less time for water to infiltrate into the underlying soil.¹⁴ Steep ditch slopes also restrict the time needed for suspended particles to fall out of suspension from the water column, leading to a decrease in the effectiveness of roadside BMPs.¹⁴ Yu et al.,¹⁴ based on monitoring of two swales, one in Virginia and one in Taiwan, found that average pollutant removal in the studied swales varied between 14 and 99% for TSS, COD, TN and TP for a slope of less than 3%; except for TP for which slope was not found to influence the removal. The slope of a site affects contaminant removal by determining the speed of flow and residence time of water in the BMP. Yu et al. (2001)¹⁴ recommended using slopes less than 3% to warrant maximum treatment efficiency of swales.

When eight VFS's, each with various treatment distances were evaluated in California over a two-year period, Barrett et al.⁸ found that pollutant reductions were achieved with combinations of treatment width, slope, and vegetation coverage. VFS with less than 10%, 10 to 35%, and 35 to 50% slopes required treatment widths of 4, 5, and 9 m with more than 80% vegetation coverage to attain irreducible minimum concentrations for constituents which were affected by the vegetated treatment.⁸ Treatment length of BMP also influences the functions of these roadside VFS's and GS's. Treatment length directly influences the residence time and storage of water in the BMP.^77,78 Longer is the treatment length longer is the water residence time in the ditch, providing more interactions between vegetation and pollutants, which in turn result in higher pollutant removal.^14,47 For example, Yu et al. (2001)'s study showed that the swale length is the most important factor driving its performance. A swale length of over 100 m was proven very efficient in removing pollutants from roadway runoff.¹⁴

Vegetation is another important factor that can influence the performance of VFS's and GS's. Optimal plant species for use in roadside ditches should be flood tolerant to maintain critical biomass height and density in waterlogged conditions to foster maximum performance.⁷⁹ A greenhouse study conducted on 20 flood-tolerant plant species showed as much as a 20 fold difference in pollutant attenuation among the examined plant species, with genera Carex, Melaleuca, and Juncus producing the highest pollutant reductions, and Leucophyta, Microlaena, and Acacia producing the lowest reductions.⁷⁹ Vegetation density is closely connected to the species of plant present. Six roadside GSs monitored over two years in central Texas showed that reduction in TSS increased rapidly when vegetation density increased over 90% coverage.⁴¹ Barrett et al.⁸ also reported rapid increases in removal of TSS with increased vegetation density over 80% coverage. Dense vegetation improves filter strip and swale functions by creating more storage, roughness, obstruction in the ditch, allowing more time for the water to infiltrate, more time for pollutants to fall out of suspension, and more time for plants to uptake pollutants present in the water.^14,77,78

4. Conclusions

The studies reviewed reveal that VFS's and GS's were generally efficient at attenuating TSS. While the high variability in the data compiled does not depict a clear pattern, it seems that the total forms of metals followed by total nutrients were more reduced by the VFS's and GS's than their respective dissolved fractions. The studies reported many negative pollutant removals for the various pollutants examined, which are likely due to pollutant build ups and purge cycles as well as their continuous deposition in roadside ditches. Between rainfall events, pollutants are continuously accumulated in roadside ditches. The longer is the dry periods between rainfall events the higher is the likelihood of pollutant accumulation in roadside ditches. When installing BMPs such VFS and GS, numerous factors including vegetation type, vegetation density, ditch slope, BMP area, BMP length, and rainfall regime should be considered to attain maximum pollutant removal benefits. Dense vegetation in wider and longer vegetative filter strips and swales treatment areas with gentle slopes appear to promote good treatment efficiency.

The high variability in pollutant removal values examined from the compiled literature make it difficult to compare the performance of vegetative BMPs across studies. The studies generally vary in parameters considered, sites characteristics, and study approaches. The limited available information pertaining to water quality impacts of roadside vegetative BMPs (in general) suggests that the topic is still not well documented in the scientific information system. A wide range of questions including the effects of slope in the BMP, BMP treatment length and width, vegetation type, vegetation height, and mowing frequency of vegetation on downstream water quality need to be further explored. Future research efforts could consider for example the effects of grass types on water quality in roadside ditches by comparing the performance of swales and strips planted in native and non-native grasses. These efforts have a huge potential to support selection and maintenance of VFS and swale vegetation for roadside ditches.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are thankful to H.G. Buffet Foundation and Dakota Lakes Research Farm for financial support. In-kind support for this project was provided by the Department of Agricultural and Biosystems Engineering at South Dakota State University.

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Footnote

† Current address: University of California, Division of Agriculture and Natural Resources, 9335 Hazard Way, Suite 201, San Diego, CA 92123, USA. Email: lmahiablame@ucdavis.edu, lmahiablame@ucanr.edu

This journal is © The Royal Society of Chemistry 2018