Sustainable Stormwater Management and Bioretention: An Overview of Reviews of the Last 10 Years

Abstract

Extreme rainfalls caused by climate change are a growing worldwide threat to the urban environment. Nature-based solutions (NBS) employ soil and vegetation to manage and treat stormwater while ensuring extensive ecosystem services. In the last decades, these solutions, such as Rain Gardens, Green Roofs, Vegetated Swales, and Constructed Wetlands, have been implemented worldwide under different names. This study is a systematic overview of reviews focusing on the last 10 years of sustainable stormwater management literature. First, a general bibliometric and topic analysis highlights trends and core themes addressed by the reviews. Then, the article delves into bioretention, analyzing water quantity and quality regulation as a function of design choices on media and vegetation. Including an internal water storage zone and using amendments such as biochar and water treatment residuals are relevant, sustainable features to target water pollution and hydrologic functioning. Vegetation, too, has a prominent role. Nevertheless, only the most recent reviews address the species’ selection, highlighting a significant research gap.

1. Introduction

Sustainable stormwater management is a set of policies and practices developed in many regions of the world as a response to two main combined phenomena: climate change and urbanization. Due to climate change, the last six decades showed increased heavy rainfall events worldwide, involving both dry and wet regions [1]. In Europe, these extremes will invest not only warm temperate zones [2] but also the whole Mediterranean region. In this area, the decrease of small and moderate storms is leading to a cumulative reduction of the total rainfall amount, while at the same time, precipitations concentrate on repeated extreme phenomena [3]. Consequently, due to soil sealing and settling along rivers and coasts, urbanized areas are increasingly affected by the risk of flooding. Urban floods disrupt quotidian life, cause economic losses, and, in the most severe cases, are responsible for fatalities [4]. In particular, impervious surfaces alter the natural hydrological processes, producing high runoff loads, which are challenging to manage due to the reduced available spaces [5,6,7].

Urban drainage management is being addressed with novel approaches that target biodiversity, climate regulation, and water supply, together with a more sustainable water quantity and quality regulation [8]. For instance, the Horizon 2020 report of the European Commission expresses the need for “Nature-based solutions or re-naturing cities”, to combine climate change mitigation and adaptation with sustainable urbanization enhancement [9]. Nature-based solutions (NBS) rely on natural processes and effectively address climate-related critical issues, through low-tech and low-maintenance infrastructures, which can offer a cheaper and more durable alternative to traditional means [10].

Nature-based stormwater management has been developed starting in the last two decades of the XX century [8,11], as an alternative or complement to conventional sewer systems [12,13,14]. This change of perspective responds to the failure of traditional stormwater management. The increased rainfall frequencies and intensities undermined the sustainability and reliability of grey stormwater infrastructures [15,16], especially in rapidly growing populated areas where the network’s upgrade is complex and expensive [12]. Grey infrastructures also diminish the ecosystem services potentially provided by stormwater, such as groundwater recharge through infiltration, and often convey a significant load of pollutants [17].

The new practices arose in different countries under diverse names, reported in

Table 1. Main international definitions of Sustainable Stormwater Management.

Practice	Acronym	Geographical Range	Origin	Definition/Main Focus
Low-Impact Development	LID	USA, New Zealand	1980s	From stormwater management practices mimicking natural hydrologic processes to the whole set of stormwater treatment practices;
Best Management Practice	BMP	USA, Canada	1980s	From urban drainage techniques to the broader set of pollution prevention practices;
Green Infrastructures	GI	USA	1990s	From landscape design practices, maximizing green spaces and ecological benefits, to the narrower sense of stormwater management practices, directed to runoff and water quality control;
Green Stormwater Infrastructures	GSI	USA	1990s	Similar to GI but more specific, e.g., used in Seattle;
Blue Green Infrastructures	BGI	USA	-	Similar to GI and GSI but focused, in a broader sense, on the presence and management of water and water bodies in the urban and peri-urban environment;
Integrated Urban Water Management	IUWM	-	1990s	Complex system combining the management of groundwater, wastewater, stormwater, and water supply;
Sustainable Urban Drainage Systems	SUDS	UK	1990s	Techniques and technologies to drain stormwater and surface water, more sustainable than conventional solutions, involving water quantity and quality regulation;
Water Sensitive Urban Design	WSUD	Australia	1990s	Processes of control and enhancement of the urban water system; design integration within the landscape;
Sponge City	-	China	2000s	Set of urban-scale design practices based on the “six-word principle”: infiltrate, detain, store, cleanse, use, and drain water; inspired by the SUDS principles and techniques;
Low-impact Urban Design and Development	LIUDD	New Zealand	2000s	Program focusing on pollution reduction and ecosystem health;
Active, Beautiful, Clean Waters Program	ABC	Singapore	2010s	Program combining drainage and water storage functions of waterways and reservoirs with the creation of recreational spaces;
Nature-Based Solutions	NBS	-	-	Cost-effective, resilient solutions inspired and supported by nature, providing multiple benefits to the environment and the community;
Urban Flood Resilience	-	-	-	Set of operations, design, and planning, to minimize urban flooding risk through water system management, also providing further benefits;

, denoting differences in scopes and technologies [14], and requiring a holistic approach to address key concepts and principles of NBS [18]. Through the implementation at different scales, they increase water infiltration, surface retention, and evapotranspiration, also enhancing water quality. The solutions range from microscale infrastructures close to runoff sources to large-scale areas to be flooded during intense rainfall events [5]. The first are bioretention systems, green roofs, permeable pavements, filter strips, vegetated swales, detention–retention systems, infiltration systems, and rainwater harvesting systems [19]. Besides runoff reduction, groundwater recharge, and water purification, most of these infrastructures provide a multiplicity of ecosystem services, or “nature’s contributions to people” (NCP) in the most recent definition [20]. The presence of vegetation enhances biodiversity while mitigating temperatures and air pollution, offering recreational possibilities to the community [17,21].

Different authors testify to the growing interest in these topics, whose bibliometric analyses show a significant increase in literature production in the last years. Ref. [22] researched the theme of stormwater runoff management within the two decades 2001–2021. The most significant phase started in 2014, with two thematic clusters of articles, the first treating infrastructures’ effectiveness and the second its enhancement through modeling tools. Ref. [23] studied NBS in the period 2010–2020, finding that the major increase started in 2015, the release year of the United Nations’ 2030 Agenda for Sustainable Development. Ref. [18] considered the publications on NBS from 2016 to 2022, of which 18% resulted in reviews, and 61% were quantitative research. Ref. [24] assessed the literature on LID from 2002 to 2022, evaluating data on its performance and resilience.

This review intends to address the recent scientific literature production on Sustainable Stormwater Management (SSM) through the lens of an overview of reviews, also called an umbrella review. Therefore, reviews on the topic were the only production considered. The aim of this type of systematic, qualitative review is highlighted by [25]. Umbrella reviews are intended to summarize the evidence from different interconnected research syntheses on a specific theme, when there is already a consistent production of this kind of literature. This compendium supports further research and can inform decision-makers, or, in this case, designers, on the scientific progress on the topic. First, a synoptic view of research themes and findings on SSM is provided. Then, the review delves into the most researched and promising type of infrastructure, analyzing the main elements influencing its design.

The present article analyses the scientific production of the decade 2014–2024 since the antecedent literature is reduced (32 reviews in the period 2000–2013, applying the keywords presented in Supplementary Material S1) and since it is aimed at addressing the latest research updates. The review answers two main questions. The first one (Q1), treated in the first part of the results section, is: “What are the trends and topics in SSM reviews, and which infrastructures emerge as the most treated?”. This survey pointed out that bioretention is the most prominent theme of the current literature. Therefore, all the reviews found on this topic were examined in detail. In this second phase, the other question (Q2) was answered: “What are the findings influencing the design, specifically media and planting, of the main SSM infrastructure assessed?”. The subsequent analysis presents the main indications on the physical elements of bioretention, in the second part of the results. This section treats general architecture, media and amendments, and plant selection, considering their quantitative and qualitative effects on the received stormwater.

2. Materials and Methods

The article structuring and research process were based on the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) protocol. The use of the protocol was aimed at standardizing the contents and clarifying the research and analysis process. PRISMA is a reporting guideline initially developed for the medical field, reflecting the methodological advances in the identification, selection, appraisal, and synthesis of scientific studies. It includes standardized checklists, explanations, and flow diagrams applicable to quantitative and qualitative analysis. In 2020, it was found to be cited in over 60,000 reports [8]. The flow diagram of the research phase can be found in Supplementary Material S1.

2.1. Studies’ Research

The studies’ research was performed in November 2024 on the Web of Science platform, limiting the results to peer-reviewed reviews from 2014 to 2024. For a detailed process description, please refer to Supplementary Material S1.

The first research (R1) targeted SSM practices using their main international synonyms [8,26], which are reported in Table 1. The title screening identified three groups of thematic clusters, which are reported in Table S1 of Supplementary Material S3. Based on the Group 1 cluster (SSM infrastructures), a second set of research (R2.1–9) was performed. The results of R1 and R2 were joined. Based on the first screening, 317 reviews were included (group A) and grouped into the thematic clusters of Table S1. The most represented infrastructural type, “bioretention”, was chosen for the full-text screening. After the full-text reading, for the final inclusion (group B), only the reviews that included design indications, notably on media and vegetation, were considered.

2.2. Studies’ Analysis

2.2.1. Group A (SSM): Bibliometric and Topic Analysis

To answer Q1 (what are the trends and topics in sustainable stormwater management reviews? Which is the main class of infrastructures assessed?), Group A reviews were analyzed; titles, abstracts, and keywords were considered. The results are presented in Section 3.1.1 and Section 3.1.2. The complete list of Group A reviews is reported in the Supplementary Material S1.

• Temporal and geographic range. Based on the information exported from the Web of Science platform, the temporal and geographic ranges of the reviews were extracted using Excel software.
• Authors and keywords occurrence and co-occurrence. The occurrences and co-occurrences of authors and keywords for the whole reviews’ group A were analyzed using VosViewer 1.6.20 software (using the RIS file exchange format).
• Terms occurrences. A second, more specific set of analysis was performed in VosViewer to delve into the topics assessed, considering each cluster of reviews indicated in Table S1 separately. As the Group 3 macro-topics (Ecosystem Services, Models and Tools, Planting Design, Water Quality) overlap with Group 1 and Group 2, a broader grouping (Group 3 *) was created to assess them across all the articles (see

  Table 2. Authors with the highest number of publications in Group A reviews.

  Author	Institution	N. Publications	Total Link Strength
  Wang, Mo	Guangzhou University, China	6	31
  Fletcher, Tim D.	The University of Melbourne, Australia	5	39
  Balasubramanian, Rajasekhar	National University of Singapore, Singapore	4	16
  Biswal, Basanta Kumar	National University of Singapore, Singapore	4	16
  Chui, Ting Fong May	The University of Hong Kong, Hong Kong, China	4	3
  Li, Jianjun	Guangzhou University, China	4	24
  Lucke, Terry	University of the Sunshine Coast, Australia	4	10
  Moore, Trisha L.	Kansas State University, USA	4	18
  Sun, Chuanhao	Guangzhou University, China	4	22
  Vogel, Jason R.	University of Oklahoma, USA	4	18

  , total nr. of reviews). Based on these clusters (Group 1,2,3 *), the text report files were extracted from the Zotero software and imported into VosViewer, where the words’ occurrences were measured. Words with at least three repetitions were considered to achieve at least 50 words to be analyzed, from which the terms in

  Table 3. Number of reviews by thematic clustering.

  			Number of Reviews
  Group 1.	SSM Infrastructures	Abbreviation	By Title	From Other Classes	Total
  1.1	Bioretention	BR	32	40	72
  1.2	Green Roofs and Facades	GRF	25	43	68
  1.3	Permeable Pavement	PP	10	27	37
  1.4	Rain Gardens	RG	5	21	26
  1.5	Rainwater Harvesting	RH	9	22	31
  1.6	Stormwater Ponds	SP	6	22	28
  1.7	Swales	SW	6	11	17
  1.8	Trees	TR	6	23	29
  1.9	Wetlands	WL	16	27	43
  		Total	115
  			Number of Reviews
  Group 2.	SSM Practices	Abbreviation	By Title	From Other Classes	Total
  2.1	Blue-Green Infrastructure	BGI	6		6
  2.2	Flood Management	FM	12		12
  2.3	Green Infrastructure	GI	37		37
  2.4	Green Stormwater Infrastructure	GSI	13		13
  2.5	Low-Impact Development	LID	25		25
  2.6	Mixed Infrastructures	MI	3		3
  2.7	Nature-Based Solutions	NBS	18		18
  2.8	Sponge City	SC	14		14
  2.9	Stormwater Management	SM	30		30
  2.10	Sustainable Urban Drainage Systems	SUDS	7		7
  2.11	Water Sensitive Urban Design	WSUD	5		5
  		Total	170
  				Number of Reviews
  Group 3.	Models, Design and Services	Abbreviation	By Title	From Other Classes	Total
  3.1	Ecosystem Services	ES	8	18	26
  3.2	Models and Tools	MT	8	8	16
  3.3	Planting Design	PD	4	7	11
  3.4	Water Quality	WQ	35	39	74
  		Total	55

  were extracted.
• Relevant topics. The reviewer performed a third screening of abstracts and keywords. Three general (water quantity regulation, water quality treatment, other ecosystem services provisioning) and design (general design aspects, media and substrates, vegetation) categories, and two other categories (bibliometric analysis, other topics) were chosen and researched through the presence of related relevant terms within the abstracts’ texts. The results are shown in

  Table 4. Most repeated words within Group A articles (VosViewer analysis). Titles, keywords, and abstracts were analyzed. The list of abbreviations is presented in Table 1, Table S1, and Table 3.

  Group 1.	Abbreviation	Main Themes	Main Infrastructures
  1.1	BR	plant, medium, nitrogen	bioretention system, bioretention
  1.2	GR	runoff, substrate, plant	green roof
  1.3	PP	technology, environment, hydrological balance	permeable pavement, porous asphalt pavement
  1.4	RG	climate change, plant, herbaceous plant	rain garden, green infrastructure
  1.5	RH	technology, biotechnology, social aspect	rainwater harvesting, stormwater harvesting
  1.6	SP	biodiversity, city, ecosystem function	stormwater pond, urban pond
  1.7	SW	engineering function, maintenance, pollution	bioswale, wet swale, grass swale
  1.8	TR	city, urban forest, infiltration	tree, street tree, green infrastructure
  1.9	WL	wastewater, plant, reuse	wetland, floating treatment wetlands (FTWs)
  Group 2.	Abbreviation	Main Themes	Main Infrastructures
  2.1	BGI	benefit, biodiversity, flood	blue green infrastructure, green infrastructure, green roof
  2.2	FM	urban flood resilience, runoff, climate change	green infrastructure, infiltration trench, permeable pavement
  2.3	GI	city, planning, environment	green infrastructure, urban green infrastructure, greenery system
  2.4	GSI	contaminant, industrial area, active mobility	green stormwater infrastructure, green infrastructure, green roof
  2.5	LID	runoff, urbanization, climate	low-impact development, best management practice, bioretention
  2.7	NBS	climate change, city, mitigation	Nature-based solutions, wetland, green roof
  2.8	SC	China, city, flooding	sponge city, pavement
  2.9	SM	climate change, city, urban area	urban stormwater management, green infrastructure, LID, rain garden
  2.10	SUDS	climate, soil, urbanization	suds, sustainable urban drainage system, sustainable drainage system
  2.11	WSUD	mitigation strategy, sewer overflow, urban water cycle	wsud, water sensitive urban design
  Group 3.	Abbreviation	Main Themes	Main Infrastructures
  3.1	ES	biodiversity, climate change, community	green roof, tree, wetland, pond
  3.2	MT	runoff, real-time control, sensing	LID, green infrastructure, green roof
  3.3	PD	vegetation, city, species	bioretention system, green infrastructure, green roof
  3.4	WQ	plant, contaminant, medium	bioretention system, wetland, green roof

  .

2.2.2. Group B (Bioretention): Findings on Design Aspects

To answer Q2 (What are the findings influencing the design of the main SSM infrastructures, specifically media and planting?), group B reviews were analyzed; the full text was considered. First, general information on bioretention definition and quantitative and qualitative effects is reported. Then, a section is dedicated to media, which is expanded in the following paragraphs specifically dedicated to media amendments. The second major component of bioretention systems, vegetation, is treated in the Section 4. Table S2 indicates the type of data reported in the articles in tables to support the reader in literature consultation.

3. Results

3.1. Group A Analysis: Sustainable Stormwater Management

3.1.1. Bibliometric Analysis

The publications’ temporal analysis (Figure 1) shows a growing trend during the whole decade 2014–2024, with a significant increase in the last 4 years. Half of the total number of reviews were edited in the last 3 years (2022–2024), reaching 61% when including also 2021.

The publications are geographically distributed, as reported in the chart (Figure 2), based on the institution address related to each article. In total, 49 countries are reported. The three central countries for literature production are the USA (with 86 reviews), China (with 71 reviews), and Australia (with 53 reviews). In the European area, the leading publishing country is England which has 14 reviews, followed by Germany, France, Italy, and Spain. The 341 papers selected (Group A) count 1149 authors, of which 119 appear in two or more distinct documents.

The 10 most represented authors, appearing in at least four documents, are reported in Table 2. The analysis confirms the prominent role of China, notably Guangzhou University, followed by Australia and the USA, and highlights the relevance of Singapore’s researchers.

The keywords individuated in VosViewer are 211, of which only 37 are repeated twice or more. Excluding the researched term “stormwater” and “stormwater management”, the most relevant keywords are “bioretention”, “green infrastructure”, and “green roofs”, followed by “climate change”, and then “ecosystem services”, “green roof”, “street trees”, “urbanization”, and “wastewater”.

The most recent publications are related to the terms “sponge city”, “nature-based solutions”, and “blue-green infrastructures”, which are connected to “water sensitive urban design”. Also, these reviews are often bibliometric analysis. “Climate change” is an issue mostly addressed in the last years’ publications, in connection with “mitigation strategies” and “extreme rainfalls”. “Green roofs” and “bioretention” main literature dates to 2020–2022, on average, and continue to be key issues. “Runoff quantity” and “runoff quality” are both treated in the latest reviews, but many topics related to water purification were already present in the first years considered. “Wetlands” and “nature-based treatment systems” are recent topics which address water quality regulation. “Simulation models” are other key issues of the newest reviews.

The clusterization resulted in 20 groups. Bioretention (BR) recurs in four clusters (as “bioretention”, “bioretention systems”, “bioretention system”, and “biofiltration”) in conjunction with both water quality and quantity regulation, and in the two cases with plant selection (as “plant-media interactions”, “vegetation”, “plant”, and “species selection”). The central theme is water quality, the core aspect of five clusters (67 elements) and recurring in seven clusters. Water quantity assessment is the core theme of three clusters and recurs in five clusters. Other thematized clusters are dedicated to sustainability, resilience, and ecosystem services (two clusters); urban elements and infrastructures, such as swales, trees, and permeable pavements (two clusters); climate change and nature-based solutions (one cluster); benefits and life cycle analysis (one cluster); wetlands (one cluster).

3.1.2. Topics Assessed

Based on the titles’ screening, the articles were thematically grouped as described in Table S1 (Supplementary Material S3) to proceed with more specific research of the reviews’ core themes. The results are presented in Table 3 and confirm the prevalence of BR systems and Green Roofs (GRs) on the other types of infrastructures. Also, the GI group is the most consistent, followed by the general definition of Stormwater Management and the LID category. BGI and WSUD resulted as the least represented among the reviews analyzed. Water quality regulation emerges as a prominent topic of stormwater management, with 74 reviews dedicated.

The terms’ occurrences, based on the thematic groups (highlighted in bold characters) in Table 3, are presented in Table 4. The words are reported in order of occurrence. In Group 1, plants are a relevant topic of BR systems, Green Roofs (GRs), Rain Gardens (RGs), and Constructed Wetlands (WL), together with medium and substrates. Literature on Permeable Pavements (PP) and Rainwater Harvesting (RH) focuses more on technology. At the same time, Swales (SW) and Constructed Wetlands are primarily aimed at water treatment and reuse, and Stormwater Ponds (SP) at enhancing biodiversity. In Group 2, climate change and climatic aspects are recurring topics, together with mitigation strategies and flood protection, and the relation with the urban environment and its planning. Biodiversity emerges in BGI literature. Group 2’s most cited infrastructures are GRs, but BR systems and RGs are also present. In Group 3, biodiversity emerges as the leading ecosystem service provided, real-time control as a relevant technological feature, and vegetation adaptation to cities’ conditions as a priority in plant selection, also having a contaminant control function together with substrates.

Table 5. Group A reviews’ contents by title, keywords, and abstract analysis. QN = water quantity regulation; QL = water quality regulation; ES = other ecosystem services; DE = general design aspects; ME = media; VE = vegetation; BI = bibliometric analysis; OT = other main topics. The list of abbreviations is presented in Table 1, Table S1, and Table 3.

		Percentage on the Total Number of Reviews								Number of Reviews
Gr.	Abbreviation	QN	QL	ES	DE	ME	VE	BI	OT	QN	QL	ES	DE	ME	VE	BI	OT
1.1	BR	34%	78%	13%	19%	56%	47%	3%	-	11	25	4	6	18	15	1	-
1.2	GR	40%	28%	20%	20%	36%	48%	8%	36%	10	7	5	5	9	12	2	9
1.3	PP	70%	80%	30%	50%	-	10%	-	10%	7	8	3	5	-	1	-	1
1.4	RG	60%	60%	60%	-	60%	80%	20%	20%	3	3	3	-	3	4	1	1
1.5	RH	44%	67%	-	22%	11%	11%	-	56%	4	6	-	2	1	1	-	5
1.6	SP	83%	67%	67%	33%	-	50%	-	17%	5	4	4	2	-	3	-	1
1.7	SW	67%	100%	17%	50%	-	-	-	50%	4	6	1	3	-	-	-	3
1.8	TR	67%	-	50%	17%	17%	33%	17%	-	4	-	3	1	1	2	1	-
1.9	WL	31%	81%	13%	-	31%	69%	-	-	5	13	2	-	5	11	-	-
									Total	82	51	41	32	9	9	20	61
Gr.	Abbreviation	QN	QL	ES	DE	ME	VE	BI	OT	QN	QL	ES	DE	ME	VE	BI	OT
2.1	BGI	67%	50%	33%	-	-	17%	17%	17%	4	3	2	-	-	1	1	1
2.2	FM	67%	-	17%	42%	8%	8%	8%	33%	8	-	2	5	1	1	1	4
2.3	GI	30%	16%	46%	14%	5%	5%	14%	35%	11	6	17	5	2	2	5	13
2.4	GSI	23%	15%	8%	23%	8%	-	15%	38%	3	2	1	3	1	-	2	5
2.5	LID	56%	44%	4%	20%	4%	8%	16%	28%	14	11	1	5	1	2	4	7
2.6	MI	33%	-	-	33%	-	-	-	67%	1	-	-	1	-	-	-	2
2.7	NBS	50%	44%	39%	11%	6%	11%	11%	44%	9	8	7	2	1	2	2	8
2.8	SC	79%	21%	36%	29%	7%	-	-	43%	11	3	5	4	1	-	-	6
2.9	SM	43%	37%	10%	17%	-	-	17%	37%	13	11	3	5	-	-	5	11
2.10	SUDS	57%	57%	14%	29%	29%	14%	-	43%	4	4	1	2	2	1	-	3
2.11	WSUD	80%	60%	40%	-	-	-	-	20%	4	3	2	-	-	-	-	1
									Total	17	40	11	5	15	8	2	11
Gr.	Abbreviation	QN	QL	ES	DE	ME	VE	BI	OT	QN	QL	ES	DE	ME	VE	BI	OT
3.1	ES	50%	25%	100%	-	13%	-	-	-	4	2	8	-	1	-	-	-
3.2	MT	75%	38%	-	13%	-	-	-	100%	6	3	-	1	-	-	-	8
3.3	PD	25%	-	50%	-	25%	100%	-	-	1	-	2	-	1	4	-	-
3.4	WQ	17%	3%	11%	37%	6%	11%	6%	9%	6	35	1	4	13	4	2	3
									Total	17	40	11	5	15	8	2	11

shows a more in-depth analysis of the macro-topics assessed. First, 27 bibliometric reviews were found, mainly on GI, LID, and Stormwater Management (SM). Group 1 mainly focuses on water quality treatment (except for SP and trees). At the same time, in Group 2, the core theme is water quantity regulation, being also the main focus of Group 3, except for the “water quality” (WQ) class. Other ecosystem services are assessed in RGs, SPs, and trees literature and are also a focus of GI, NBS, and WSUD. Vegetation is assessed in 66 reviews, mainly in Group 1, specifically on BR, RGs, GRs, and WLs. Media in BR have a major role and are treated more than plants; in RGs, they are still a relevant topic, while in WLs, they are secondary.

3.2. Group B Analysis: Bioretention

3.2.1. Bioretention Definition and Applications

Definition and Description

Bioretention (BR) is the main topic assessed by the literature on SSM. Studies demonstrate that bioretention is one of the most promising technologies for runoff volume reduction and water quality improvement, being the most cost-effective integrated management practice for filtering stormwater pollutants [6,27,28,29]. BR components are the vegetation layer, the growth media or substrate, the drainage layer, and often an underdrain [30,31,32,33]. BR is preferable for its versatility and provides aesthetics, biodiversity, and urban heat island (UHI) mitigation in the urban environment [27,31,34].

In a broader sense, BR systems can be swales, tree pits, curb extensions, rain gardens, and planters [35]. In a narrower sense, BR systems are often known as “rain gardens” (RGs) [27,33,34,35,36,37], but the two infrastructures can slightly differ [31], as in many cases RGs are smaller and less engineered [37]. RGs are primarily defined as shallow, planted depressions meant to collect runoff, “dry” or “wet”, depending on the possibility of infiltration into the ground, and sometimes also built-in containers [38].

Ref. [39] point out that BR can be easily adapted to different small urban spaces and is a valuable solution for retrofitting them. Ref. [34] report BR maintenance costs of 6 USD/m², though highly variable according to the system-specific needs, and construction costs of 109–227 USD/m²; for RGs, ref. [40] describe a significantly lower cost of 68 USD/m² (25 USD/ft²). Instead, ref. [41] considered the results of LCCA, showing the environmental sustainability of BR in comparison with other treatment facilities, and a cost range of 1.31–1.55 USD/m² of impervious drainage area.

Water Quantity and Quality Regulation

BR attenuates peak flow and reduces runoff volume through infiltration, exfiltration, and evapotranspiration (ET), promoting the natural hydrological processes [27,34]. It is capable of significantly reducing the runoff volume generated by small storm events and capturing most of it [6,42], demonstrating good performances also with moderate storms [27], but high-intensity rains with higher return periods determine BR efficiency decrease and overflow [6,13].

The hydrological performance is measured using different metrics, reported by [43], such as runoff volume reduction and hydraulic conductivity K, often given as saturated conductivity or Ksat. The performance, indeed, is shaped not only by design choices (such as ponding depth, media permeability, and presence of an underdrain) but encompasses location and average annual precipitation, impervious-to-pervious ratio, permeability, subsoil conditions, vegetation, and potential ET [32,42]. Design parameters of the Sustainable Stormwater Management Model (SWMM) and the model HYDRUS-1D for the media are reported by [44].

Overall, BR hydrological performance is high, but the magnitude of storm events severely impacts it [34,45,46]. Ref. [6] found a broad efficiency range between 14% and 100% (water not overflowed by the system), with most values between 60 and 90%, but they also highlight the difficulty of comparing the RGs analyzed. Ref. [29] report two volume reduction ranges, one from field studies (50–98%) very close to that of [39] and the other from laboratory studies (11–71%). The latter is lower due to simulated precipitation’s higher (5-year) return period. Ref. [47] tested a rain garden for 3 years in a semi-arid climate, finding an average runoff volume reduction of 53%, decreasing over time; the intense and concentrated summer precipitations might explain the lower efficiency.

Among design choices, the confluence ratio (BR surface/impervious catchment area), which strongly impacts hydrological effectiveness, is highly variable, with recurring values between 5% and 20% [6]. Ref. [48] findings suggest a minimum ratio of 2% to reduce overflow, while a ratio of 20% can reduce the runoff volume by 90% or more over 5 years. According to their results, ref. [29] recommend a higher range, between 8% and 25%. For the less investigated arid and semi-arid climates, ref. [35] report a study suggesting a 6–8% confluence ratio. Furthermore, BR systems, such as tree box filters, can be integrated with harvesting systems or connected via trenches to increase the storage capacity and get the same efficiency at a lower ratio [49].

Overall the water storage capacity is more dependent on the surface of the infrastructure than on its depth, as it influences both subsurface and surface storage [46]. The literature suggests adopting a ponding depth equal to or superior to the product of infiltration rate and 24 h, as this avoids the completion of a mosquito breeding cycle [29]. The presence of an Internal Water Storage Zone (IWSZ), which is treated in Section IWSZ and Other Configurations, is the second influential factor in volume reduction after the infiltration rate of the media, significantly reducing overflow; its depth influences the water storage capacity [29]. Also, the surrounding soil’s conductivity has to be addressed when the system is designed for infiltration. An underdrain is necessary with low infiltration rates, e.g., less than 13 mm/h [44,50]. Drains are usually contained in the gravel layer at the system’s base, which is a relevant element for the water storage capacity and can be successfully thickened to address intense precipitations [46].

BR literature is primarily dedicated to pollutant removal mechanisms and performance, and overall, BR systems are effective in stormwater pollution mitigation [51]. However, standard BR media efficiently removes solids and particulate but not dissolved pollutants, requiring, therefore, the use of specific soil amendments [42]. Media hydraulic conductivity regulates the residency time of water within the infrastructure and, therefore, the contact time between substrate and pollutants [31,52]. For example, using amendments with fast pollutant removal kinetics is recommended with high infiltration rates [31]. With its design and position, a reduced flow outlet valve can regulate and prolong the water stay within the medium and increase its movement through the sides and bottom of the cell, enhancing contaminants removal [29].

In conclusion, the ratio between impervious and BR surfaces should range between 2% and 25%, and the greater it is, the better the performance; runoff volume reduction is influenced by the BR surface and by the cell’s depth, which can be expanded with an IWSZ. Drains are required when the surrounding soil has a low Ksat. Infiltration rates and amendments should be carefully balanced to address pollutants’ removal.

Table S2 (Supplementary Materials S4) describes the type of data reported in the articles in the form of quantitative and qualitative comparative tables, which detail the surveyed experiments about water quantity and quality regulation.

3.2.2. Media

Ref. [30] treat in detail BR media, describing a typical composition, blended to reach the design hydraulic conductivity, as follows: mineral fraction, being sand (35–90%), silt (0–55%), clay (0–25%); organic fraction (3–40% by volume); secondary amendment. Within the analyzed case studies, sand was always present (30–89%), followed by native soil (25–75%), and compost, which should be carefully considered because of the risk of leaching. For [29], a typical mix consists of silty loam (40–60%), sand (30–50%), and organic matter (10–30%). Mulch, responsible for adequate infiltration and attenuation of pollutant load, is also included by [46].

Media depth is a highly relevant design variable that should consider the groundwater level, the cost-effectiveness, including excavation expenses, and the media composition [29]. For instance, [53] suggest having soil at least 1 m deep, which might interfere with the water table. In their RGs survey, [6] found depths ranging from about 0.5 m to more than 3 m, but primarily within 1.5 m.

Water Quantity Regulation

Saturated hydraulic conductivity (Ksat) is the main parameter to control infiltration and depends mainly on the media composition [6]. Some authors [30,31] suggest manuals’ values ranging from 25–50 mm/h to 200 mm/h, e.g., 33 mm/h for loamy mixtures to 160 mm/h for sand mixtures. Ref. [6] report higher values, as recommended by other manuals. Their comparative study of RGs found a broad interval from 1.71 mm/h to 8172 mm/h but with most values within 120 mm/h. The variability is explained by different measurement techniques and small testing areas [6].

Studies on hydraulic conductivity fluctuation over time are controversial and show different decrease rates and speeds; furthermore, lab-based tests (accelerated loading tests) do not consider the role of plants and maintenance regimes [52]. However, the larger initial conductivities are strongly impacted, e.g., with a 1/3 decrease in 50 weeks [44], suggesting different solutions, such as using coarser materials [54].

Materials choice is challenging as it determines the balance between infiltration and water retention properties [6], but the infiltration process also depends on other factors, for instance, soil compaction. A topsoil or a BR media layer of at least 0.5 m depth is recommended to avoid media compaction [48]. Furthermore, some jurisdictions require not mechanically compacting the media and the use of specific machinery to reduce the compaction of the newly installed media [30]. Excavation techniques and tools that increase porosity and promote exfiltration, such as the rake technique, should be preferred while raking or stripping the topsoil is needed to counteract clogging [27,39]. Ref. [27] also suggest considering clogging in the design phase through a safety factor in Ksat calculation. Instead, ref. [46] report a simple solution to low infiltration rates of the media, which is drilling holes to reach the base gravel layer, and filling them with crushed stones and sand.

Overall, manuals are the reference for Ksat, which should range between 25 and 200 mm/h; the design value should take into account its initial decrease, which severely impacts higher conductivity. Proper soil manipulation is required to avoid compaction.

Water Quality Regulation

Traditionally designed BR is associated with high removal rates of total suspended solids (TSS) of 78% or more, not influenced by rainfall characteristics and increasing with system age [29,30,55]. With a high sediment content, though, to limit clogging issues, ref. [31] recommend the use of pretreatments such as vegetated buffer strips.

The presence of an anoxic layer, low Ksat, and longer water retention time favoring complexation correspond to better phosphorous (P) removal performances [37]. Ref. [27] report a total phosphorous (TP) reduction range from 50%, found in arid and semi-arid contexts, to 97.2%, corresponding to a typical soil mix of 20% compost, 50% sand, and 30% topsoil. Media amendments are particularly relevant for P removal and will be treated in the following paragraphs.

Field and laboratory experiments have shown efficacies of 70% nitrogen (N) removal or more, denitrification being the primary mechanism, and better results at low hydraulic loading rates [33]. Denitrification happens in anaerobic conditions, primarily within an IWSZ and in the anaerobic microsites in the media close to plants’ roots or within amendments [42,56]. BR hydraulic conductivity increases correspond to a diminished NO_x (nitrates and nitrites) removal, and the proposed range for BR systems, according to different authors, goes from 10–40 mm/h to 200 mm/h, with an optimum corresponding to the intermediate values [33]. Ref. [57] highlight the crucial role of the drying and rewetting cycle, affecting plants, soil, and microbial communities, which influence N dynamics.

BR effectively removes total heavy metals [40,55], e.g., scholars have also found removals of 97% or more [40]. Dissolved ones like Fe and Ni, though, are, in some cases, exported by the system [30]. The top filler layer (10–20 cm) gives the best results, and the main interactions involve clay particles and media amendments [29], with sorption being especially related to humic substances [58]. Ref. [58] point out the importance of media depth, as it regulates the number of sorption sites.

Regarding the removal of organic compounds, specifically hydrocarbons, a relevant feature is the presence of a mulch layer [55]. Between organic compounds, polycyclic aromatic hydrocarbons (PAHs) are efficiently trapped by bioretention cells [40]. Ref. [59] found that microplastics (MPs) removal ranges from 76% to 95%; sand is the leading media component responsible for the treatment, and the efficacy increases with greater media depths. MPs > 100 μm, though, were not investigated. Furthermore, most authors agree on the high BR pathogen removal rates, even though [30] argue that they can vary by more than one order of magnitude.

In conclusion, while TSS are almost always efficiently removed, longer water retention and anoxic or anaerobic sites favor the removal of nutrients. At the same time, the organic fraction of the top layer intercepts heavy metals, mulch hydrocarbons, and sand MPs. Since stormwater composition is complex, though, ref. [60] points out the need to use different materials, each chosen to target one or more specific pollutants. The most common media amendments will be treated in Section 3.2.3. These materials can be mixed or used in a sequence of “treatment trains” in different steps.

3.2.3. BR Modifications to Enhance Treatment Performance

Besides its hydrological relevance, most authors agree that the Internal Water Storage Zone (IWSZ) is the main feature favoring denitrification, being also a cost-effective solution [29]. The presence of an IWSZ also increases TSS removal and particle-bound P filtration [29] and the degradation of organic oxidized pollutants [58]. For [58], the optimal configuration for multiple pollutants’ removal is a multizone or multilayer design, combining saturated and specifically amended zones. Amendments integrate the missing capacity of dissolved pollutants removal and diminish the risk of leaching [30]; they are also chosen according to their impact on infiltration rates [44]. Further investigations, though, are still required to assess the long-term performance, the relationship between amendments and vegetation, and the best application modes [29,31].

Preferably, amendments should target a specific pollutant and be low-cost, readily available, long-lasting, and able to support plant life [30]. Carbon-based amendments biodegrade and consequently have to be periodically replaced, but significantly increase the media sorption capacity, while inorganic amendments are readily available, long-lasting materials [30,31]. Ref. [44] point out the growing interest in reusing materials, such as solid waste for compost and biochar preparation or construction waste, already highlighted by [54] for Singapore. The amended layer depth is a highly influential factor to be chosen according to nutrient loading and required bed lifespan [58].

In conclusion, the IWSZ, media amendments, saturated zones, and their combination in multilayer designs have the highest impact on treatment performance. Amendments’ choice should also be based on their durability and sustainability.

IWSZ and Other Configurations

The IWSZ can be included within the design or realized by retrofitting, typically using a PVC elbow that elevates the underdrain [30,31,56]. This saturated zone supports plants’ growth and survival [33], significantly increasing and prolonging ET [6]. Therefore, it is a relevant resource in arid and semi-arid contexts [7]. However, it is irrelevant to plants’ health in tropical climates thanks to the high precipitation rates [29].

In the literature, authors found a depth range from 100 to 350 mm [6] and from 150 to 600 mm [61], with the 600 mm depth significantly increasing the NO_x removal, as it guarantees the contact time needed for the process [56]. Furthermore, it is important to incorporate in the IWSZ a carbon source (1–5% in volume), such as woodchips, mulch, or newspaper, as an electron donor and energy source for microbial activity [29,32,56]. Ref. [29] report a promising long-term performance of 80% NO_x decrease, even though [32] argue that current field-scale experiments’ efficacies are inconsistent. Instead, ref. [44] point out the need to assess the potential adverse effects of long-term operations, such as pollutant accumulation. Furthermore, ammonium and phosphate leaching are reported over time in different case studies [54].

For [62], the most effective N removal solution combines the IWSZ and the soil media amendment with additives. Furthermore, the IWSZ can be implemented within the linear infrastructures called bioswales to enhance N removal, even though their sloped bottom limits the treated volume [63]. The creation of alternated saturated and unsaturated zones can substitute the IWSZ and can remove up to 91% NO_x, enhanced with biochar amendment to a 95% removal [56]. For instance, placing specific amendments at the system’s base can create an anaerobic zone, while in the upper layer, they support plants’ growth and remove other pollutants [46]. Another recent alternative design is the biphasic rain garden, composed of a sequence of separated saturated, aerobic, and infiltration zones, favouring different reactions and groundwater recharge [44,64]. Ref. [64] found that biphasic RGs showed a 40–60% removal capacity of nitrates, while [44] reported a 91% removal capacity.

Overall, the IWSZ is relevant for N removal and plants’ growth and survival, especially in arid contexts. N removal is maximized by combining an IWSZ and proper amendments or creating one or more saturated zones within the media. Biphasic rain gardens are an effective alternative design exploiting a sequence of diversified cells.

Amendment with Biochar

Biochar is the leading media amendment considered in the analyzed literature. It is a carbonaceous pyrolysis product whose properties are influenced by the raw material and the production temperature; e.g., higher temperatures increase surface area and porosity [65,66]. It is highly interesting as it is low-cost, stable, and can be regenerated. Furthermore, it enhances the infrastructure’s sustainability since it can be prepared by reusing local waste materials, such as plant or animal material, industrial waste, and sludge [30,31]. Because of its porous nature, it can be an alternative to compost [6], enhancing water retention and infiltration and increasing Ksat in soil and clay soil [36,65,66]. High-pyrolysis temperatures correspond to a significant water-holding capacity functional for delaying peak flows during storm events [66].

Biochar is appreciated for its high pollutant removal capacity, which involves different physiochemical processes. Nutrient removal depends on its characteristics, size, and quantity, enhancing cation exchange capacity and microbial growth [33,65]. Biochar-based biofilters were found to remove from 45% to 94% of TP and from 32% to 61% of total nitrogen (TN) [33]. Wood biochar obtained by gasification can remove 85% nitrates, while biochar produced with low-temperature pyrolysis better removes ammonia [66], being also more suitable for toxic metal interception [30,46,66]. For [66], though, its average efficacy for metals remains limited, while other scholars found a removal capacity of 33–80% for Zn, 49–100% for Cu, and 27–100% for Cd [33]. Ref. [46] report cases of biochar combined with other materials, significantly enhancing nitrates and metals removal, but modifications of biochar deserve further investigation [33,65]. Over time, metals seem not to accumulate over the recommended threshold [52], and scholars found a lifetime of biochar-based biofilters of approximately 15 years [36].

Organic contaminants and MPs removal rates are high but variable [29], rising with small particle biochars [33,65]. High-temperature biomass treatment is more suitable for remediation [66] as it maximizes hydrophobicity [30], but scholars also suggest mixing biochars to intercept a broader range of organic compounds [33]. MPs’ filtering efficacy increases with the dimension of their particles [29]. Biochar also enhances E. coli adsorption and straining [30,66], with smaller particles and high-temperature pyrolysis giving the best results, as the removal can nearly reach 100% [65].

In conclusion, biochar is a sustainable alternative to compost, offering a high removal of pollutants. When produced with low-temperature pyrolysis, it efficiently removes ammonia and metals, while high temperatures favour organic substances and pathogens remediation. Mixing and modifying biochars can intercept more compounds.

Other Amendments

Besides biochar, coconut coir and zeolites are the lower-cost, most efficient amendments in nutrient removal; coconut coir removes up to 90% of nutrients, and the absence of P leaching makes it preferable to compost [30]. Further optimal amendments are Fe and Al oxides, as they enhance P adsorption ability and adsorb NO_x compounds [29,50,67]. Therefore, possible P-adsorbing materials are water treatment residuals (WTR), lime and alum sludge, iron-enriched sand, and fly ash [30,42,58]. Ref. [58], for example, suggest using iron-enriched sand in the base layer of a two-stage BR system. [54] suggest the use of WTR for their stability and absence of leaching. WTR are highly efficient and sustainable since they are a locally available waste material; further investigations are required on the potential of metals leaching [68], and blends perform differently [50]. Ref. [69] report different case studies using fly ash. Ref. [67] analyzed different media’s P adsorption capacities and concluded that the stronger they are, the shallower the layer depth required.

Regarding N, amendments such as zeolites, coal slag, vermiculite, and perlite reduce ammonium through ion exchange [29], while organic ones (e.g., plants’ detritus) bind nitrates [42]. Using carbon-rich additives, such as newspaper or wood chips, mixed within or as a layer below the filter media also creates anoxic conditions favouring denitrification [56]. For instance, shredded newspapers removed more than 99% of nitrates, even though less common than biochar [30], while the amendment with wood chips inhibits nitrogen leaching [64]. Organic amendments such as compost are particularly relevant for metals sorption [58].

Zeolites, coconut coir, and iron-based amendments are the lower-cost solutions for metals removal; both natural and synthetic zeolites remove more than 90% of Zn, Cu, Pb, and Cd, while coconut coir is particularly effective for Zn and Cd, up to 90% removal [30]. Granular-activated carbon enhances plants’ uptake of organic compounds [55], while peat moss can remove 82–100% of biocides and up to 95% of polycyclic aromatic hydrocarbons or PAHs [30]. Besides biochar, zeolites and iron-based amendments are lower-cost amendments for pathogens’ removal; e.g., sand coated with iron oxides can remove almost 100% E. coli [30].

Overall, zeolites and coconut coir are low-cost solutions for removing both nutrients and metals. Fe and Al oxides are confirmed to be crucial for nutrients’ adsorption and particularly P removal. WTR can be used, which are sustainable, local, low-cost materials. Organic amendments (newspaper, wood chips) can create anoxic conditions for N removal and are efficient for metal sorption.

3.2.4. Vegetation

Ref. [35] conclude that vegetation is the most important component of BR. Besides influencing both stormwater quality and quantity, plants also offer multiple other services, including an aesthetic function, providing habitats for wildlife, mitigating UHI, and improving air quality [31,35,70]. Ref. [35] synthesize vegetation’s beneficial effects on the systems’ efficiency in two groups: above-ground benefits (settling of particles, surface erosion and compaction prevention, weeds control, flow and infiltration enhancement) and below-ground benefits (soil permeability and water retention maintenance, contaminants’ uptake, organic matter provision, support to the microbial community). These services are supplied by the foliage, through evapotranspiration and water interception, and by the root system, which modifies the soil structure, absorbs water, and biochemically interacts with its surroundings.

Water Quantity Regulation

Plant roots promote infiltration and water retention by counteracting clogging over time, maintaining porosity thanks to root expansion and turnover [42,52]. Quantitatively, alterations and seasonality effects on root growth, more marked in spring and summer, were assessed [48]. Ref. [48] found that the roots can modify Ks within a range from −143% to 1085%, and root decay can increase Ks up to two orders of magnitude. To prioritize infiltration, thick, fleshy, and deep roots are preferred. Little dimensional variations can have a strong influence. For instance, scholars found that a minimum root diameter of 1 mm promotes the formation of macropores [35,48], which are crucial for infiltration and soil water detention and retention [7]. Low herbaceous vegetation is less effective than tall herbaceous vegetation, while shrubs and trees exhibit the best performance [48]. To maximize infiltration, trees with a deep root distribution should be preferred to medium- and shallow-rooted ones unless there are limitations to roots’ expansion typical of the urban environment [49]. For instance, many filtration-only RGs, limited by an impermeable layer’s presence, require shallow roots and high adaptability in the rhizosphere [7].

Water interception of the vegetation is another influential phenomenon shaping plants’, particularly trees’, hydrological performance [35]. It can have a primary role and varies across seasons [46]. Furthermore, vegetation’s ET replenishes the media water storage capacity [42]. When prioritizing stormwater volume reduction, therefore, high ET rates should be a relevant selection criterion [49,71]. Ref. [71] found a significant volume reduction interval, ranging from 19% to 84%, even though higher values correspond to laboratory studies. Ref. [6] found that ET removes from 5% to 19% of rainfall received by the rain garden, while other scholars addressed transpiration only, finding an interval of 7–17% stormwater reduction [72]. Ref. [31] report a range of values from 3 to 8 mm/day of evapotranspiration for herbaceous plants, slightly exceeded by that of shrubs.

Quantitatively, higher plants and vegetation formed by structurally complementary species give the best results in terms of ET [6]. Furthermore, ref. [73] point out that combining species differing in functional type can provide year-round management of stormwater inputs. To maximize ET effects, ref. [72] suggest using broad-leaved plants, which are also relevant for interception, and species with high biomass and water-use efficiency. E.g., trees with greater total leaf area and larger mature size contribute the most to the system’s hydrological functioning [49]. On the other hand, ref. [49] point out that high ET rates could exacerbate drought stress, therefore the balance between these two aspects should be carefully considered, together with the possibility of providing irrigation.

To sum up, infiltration is maximized using plants with thick or fleshy and deep roots, such as woody species and tall herbaceous vegetation, e.g., prairie mixes; roots at least one millimeter thick promote macropore formation. Evapotranspiration is favoured by broad-lived species, higher plants, and structurally complementary vegetation.

Water-Quality Regulation

Plants significantly reduce stormwater pollution [7], through phytodegradation, phytoextraction, phytovolatilization, and rhizosphere interactions [31]. Plants, though, also indirectly influence pollutant removal, by altering water movement paths and speed. E.g., preferential flow paths limit the interactions between contaminants and media [42]. Relevant functional traits to consider are the roots’ dimension, length, and density [62]. Overall, for [35], the most efficient groups in toxic substance removal are Poaceae, Cyperaceae, Myrtaceae, and Asteraceae. Ref. [72], in addition to mixing different plants, propose plant rotation and intercropping to enhance general stormwater treatment.

Ref. [74] studied the role of vegetation in slowing down water and favouring solid particles’ sedimentation within bioswales. Vegetation also stabilizes the mulch and media layers, enhancing the removal of particle-bound pollutants, such as metals, organic contaminants, and P [46]. Furthermore, plants effectively improve the retention of nutrients through direct uptake and degradation and indirect effects on the substrate and microbial community [31,32,58]. The highest plant density maximizes nutrient processing while offering better weed control [35]. Besides assimilation, root respiration and the readily available C provided by plants favour denitrification, while ET favors unsaturated conditions for ammonification and nitrification [42]. To avoid nutrient release, the vegetation should be periodically harvested [58], a particularly important operation to prevent P leaching [55].

The prairie mixture is an example of a plant community with high nutrient removal capacity [7]; ref. [35] provide a list of Poaceae and other taxa for maximizing nutrient uptake. The same scholars report a study showing that trees have the highest nutrient accumulation in their dry biomass, but also another study in which herbaceous species like Panicum virgatum exceeded trees’ performance. Ref. [69] report that grasses, or the use of mixed vegetation, outperform the systems planted with trees and shrubs only.

Vegetation biomass and fast growth correlate to N removal, while root biomass and slow growth to P removal [69]. Fibrous log roots with high biomass are effective at P removal [72], while nitrogen uptake is enhanced by high growth rates, extensive root systems, deep soil penetration, and high plant biomass [29,31,42]. However, ref. [55] suggest using a root system not penetrating the media depth entirely. Tolerance to nutrients should also be considered for species selection [72], and usually corresponds, together with elevated salt tolerance, to an effective N removal [33]. Mycorrhizae can enhance organic and inorganic N removal, increase P retention, and reduce P leaching [42,75].

Metals uptake can be passive or active when metabolic processes are involved [58], and varies according to the phytoremediation capacity, the metals’ bioavailability, and the plant’s part [29,31,35]. Zn, Cu, Mn, and Ni are directly absorbed micronutrients; plant harvesting can eliminate metals from the system [52]. Plants’ secondary role in metals removal suggests the use of hyperaccumulating species [7], but further studies are required on their application’s usefulness [58] and also selection [55]. Ref. [35] provide a list of Poaceae and Cyperaceae capable of efficient heavy metals removal, together with different Iris species and other flowering taxa.

Plants are also responsible for uptaking organic substances, especially low-weight molecular PAHs, for stimulating the rhizosphere microbiological activity and introducing organic matter into the system, enhancing sorption [55]. To enhance organic compounds’ removal, [58] suggest using high plant densities and phreatophytic or deeply rooted species. Plants slowing infiltration are preferred for pathogens’ removal [55]. Ref. [35] report a list of suitable species removing E. coli, primarily monocotyledons, such as the highly adaptable Miscanthus sinensis Andersson.

In conclusion, high planting densities maximize the processing of nutrients and promote weed control. Vegetation with fast growth, high plant biomass, and an extensive root system (e.g., the prairie mixtures) enhance N uptake; slow growth and high-biomass, fibrous roots promote P assimilation. Hyperaccumulating species deserve further investigation and applications for metals’ absorption.

Taxa and General Selection Criteria

Besides pollutants and salt stress tolerance, adaptation to fluctuating water regimes and especially to dryness is crucial, as bioretention substrates are designed to drain fast, and the urban context is subject to the increasing effects of climate change [31,55]. Plants, therefore, should be capable of fast absorption of the water draining through the system [55]. Ref. [76] highlight the moisture content’s temporal and spatial variability in BR systems, to which the species should respond with adequate survival rates and aesthetics [55].

For [7], knowing the species’ natural habitat can help understand their adaptability to the design conditions. Ref. [35] agree with this position, but argue that the physiological optimum should also be considered, and the response to artificial conditions can be different. Ref. [35], then, point out that species from floodplains are typically selected for the wettest ponding area, but the range of species could be broader. They report the performance of terrestrial plants such as Carex species and annual flowering taxa, as well as a list of flood- and drought-resistant shrubs. Furthermore, mosquito-repelling vegetation should be selected for the ponding area [46].

Interestingly, ref. [35] also propose taxa for arid contexts and list salt-tolerant and salt-sensitive species. For tropical climates instead, [61] suggests not using grasses and sedges due to their potential invasiveness. However, abundant vegetation growth, such as that of Switchgrass (Panicum virgatum L.), could also provide biomass for energy production [55], while species could also be selected for food provisioning [39]. For example, [35] report different cases of vegetable rain gardens.

Species diversity can maximize the benefits provided, minimize the impact of losses, and increase resilience to stress factors; it enhances biodiversity and, through diversified functional traits, the overall provisioning of ecosystem services [7,35,55]. For example, ref. [48] showed that wild-type plants and taxonomically diverse plant communities are better at enhancing infiltration, particularly American native prairie grasses and rich perennial mixes.

Ref. [35] point out that most studies used native taxa, which provide an extended range of benefits and easier cultivation, e.g., local climate adaptation and resistance to pests and diseases. The use of natives is also suggested by [32] for their better N removal, but [76], which confronted manuals’ recommendations with empirical evidence, claim that proof of superior technical performances is lacking. However, ref. [55] report that native species can maximize habitat provision and avoid the risk of invasiveness [7]. On the other hand, exotics can extend the flowering period, better respond to climate change, and support a large part of pollinators [7].

Ref. [7] report that all plant lifeforms are used in run-through rain gardens with a surface runoff collection. In more arid climates with subsurface water collection, phanerophytes (trees and shrubs, especially phreatophytes), together with the more diffused hemicryptophytes (herbaceous perennials), are the most represented taxa. Poaceae, Cyperaceae, Juncaeae, and Myrtaceae families are preferred in rain gardens with filtration only, prioritizing functionality over visual appeal [7].

For [70], herbaceous plants are preferable as they offer greater diversity, landscape coherence, and ornamental effect than woody plants, effectively managing runoff and soil stability with low costs and maintenance regimes. Trees, instead, have been poorly investigated in their use in bioretention, although possessing great potential, as they combine conspicuous ecological benefits (such as climate and air quality amelioration) with a long life span [31].

Spontaneous vegetation can better adapt to fluctuating water regimes, compaction, and pollution. Again, its removal should rely on comparing functional traits’ effectiveness and reduced maintenance costs with potential invasiveness and aesthetic acceptance [48]. Overall, careful consideration should be given to species’ ecological niches, cultural connotations and seasonal appeal, economic convenience, and ease of plantation and maintenance [70].

In their RGs survey, ref. [6] found a broad species diversity; the recurring taxa are presented in

Table 6. Most represented taxa in literature.

Authors	Description	Families and Species
[6]	Recurring taxa in the surveyed RGs	Genera Carex, Iris, Eupatorium, Hemerocallis; species Echinacea purpurea (L.) Moench, Panicum virgatum L.
[7]	Most represented taxa in the surveyed BR literature	Juncus effusus L., Panicum virgatum L., and Carex appressa R.Br.
[35]	Most successful families and relevant species in BR literature	Monocotyledons: sedges—Cyperaceae (with Carex spp.); grasses—Poaceae (Miscanthus sinensis Andersson., Panicum virgatum Muhl., Phragmites australis (Cav.) Steud.); rushes—Juncaceae (Juncus spp.). Dicotyledons: Asteraceae (Aster nova angliae L., Rudbeckia spp., Liatris spicata L., Echinacea purpurea L.); Myrtaceae (Melaleuca ericifolia Andrews, Leptospermum continentale Joy Thomps)
[70]	Sponge City initiative’s main families	Asteraceae, Poaceae, Liliaceae, Crassulaceae

. Ref. [35] report an extended range of tested species, organized by flooding and drought resistance, contaminants’ removal, ice-salt tolerance, Nordic and xeric climate adaptation, medium-term performance, erosion control, and biodiversity enhancement. Ref. [7] provide a complete list of the taxa (391) encountered in their extensive BR literature survey. Ref. [70] report that the leading families used in the Sponge City initiative. The corresponding plant categories, flowering perennials, ornamental grasses and sedges, and aquatic species, reflect the RGs’ humidity gradient.

Overall, waterlogging and drought resistance are core aspects that should be related to climate and plants’ position in the BR system; the species’ choice can partially rely on their ecology. Species diversity can be enhanced with a rich herbaceous mix or using woody species. Indeed, native species reduce the risk of invasiveness and have local adaptations, but exotics can better respond to climate change in urban scenarios. Also, the spontaneous flora might be exploited for its hardiness.

Plants and Soil

A good soil water-holding potential reduces the dependency on irrigation and supports a wide variety of vegetation. Ref. [53] suggest incorporating organic substances and exploring the aggregates alternatives to sand to provide plants with water and nutrients. A bi-layer system might be a design solution, with suitable growing media on top and filtering media at the base [46]. Furthermore, an IWSZ can be included in the design, or an irrigation system, such as sub-irrigation, might be necessary [53]. Another solution can be irrigating the BR cell with harvested runoff from adjacent permeable pavements [31].

Also, to retain humidity, mulch should cover the whole bioretention surface with a 2–4 inches depth, and hardwood mulch is recommended to prevent floating [31]. In tropical climates, it is unnecessary, and organic mulch can lead instead to scouring and floatation [61], and pollutants’ leaching [46], therefore suggesting the use of non-biodegradable materials [46].

Biochar enhances vegetation growth and, consequently, transpiration rates through water and nutrient retention; the type and quantity of nutrients depend on the feedstock and production temperature. It also reduces toxicity and plant uptake of trace elements and beneficially influences the rhizosphere microorganisms [66]. Coconut coir and biochar enhance soil fertility, while iron-based amendments can provide micronutrients to plants; fly ash effects are controversial, as it can, on the one hand, release micronutrients, on the other hand, increase soil pH and release heavy metals [30].

Mycorrhizae can augment plants’ tolerance to salinity, temperature, and water stresses, enhancing photosynthesis and plants’ aesthetics. Mycorrhizal plants perform better under dry conditions thanks to the increased access to soil micropore water; furthermore, the water relation of plants is positively influenced by different related mechanisms, like increased P absorption [75]. Soil fauna, too, has great relevance in contributing to plants’ survival and performance and often has a positive interaction with mycorrhizal fungi. These organisms store N, C, and P and release them for plants’ uptake through decomposition and degrade leaf litter, releasing nutrients, while bioturbation enhances root density and growth [77].

In conclusion, the media’s water retention and supply should be carefully considered when selecting species; a bi-layer system might provide an adequate growth substrate, but irrigation might be necessary. Biochar and other organic substances enhance soil fertility and structure, while mycorrhizae could substantially reduce plants’ water stress.

4. Discussion

First of all, the current study’s limitations should be considered. Firstly, the research only relies on a single research platform (Web of Science), which might not be exhaustive of the whole literature. Furthermore, a single researcher screened the results. Secondly, it addresses a specific type of literature, reviews, which are based on research criteria that might not encompass the entirety of the articles and topics. On the other hand, this kind of survey, with its specific boundaries, was the target of this article. Thirdly, the topics’ assessment of SSM literature is only based on the titles, keywords, and abstracts reading, not on the full-text assessment. This only allows a partial reading of the contents, which might not reflect the extended contents. The broad survey, though, allows an adequate overview of SSWM literature worldwide.

Refs. [16,18,22,23,78,79] confirm the leading role of the USA in SSM literature. The same authors focus on the recent interest in climate change issues, the Sponge City concept, Blue-green infrastructures, and NBS [16,22,80]. The recent trends in SSM literature are also consistent with those of [80,81], which highlight the emerging role of China in this research field and the leading role of their researchers in publications. [80] also reports as core themes BR systems and, as one of the main newest topics, GRs. Refs. [18,78] describe the emerging interest in ecosystem services and particularly biodiversity, which is testified by the broad literature found in the current reviews survey. Interestingly, a recurring word, “resilience” [16,18], was not found in the present analysis.

Regarding BR systems, the review encompasses the multiplicity of BR design factors, both biotic and abiotic, which are often addressed separately or in a very specific way in the review literature. For instance, refs. [7,35,42,48,53,70,76] focus on the role of vegetation, only in some cases [42,48,53] extending the topic to the media and their interaction. The current article, together with a synthesis of the research on plant species, treats the general design of facilities and media concerning water quantity and quality regulation. E.g. from the water quality perspective, it integrates more specific studies, such as those of [62,67,82] on nutrient removal or those of [33,36,65,66] on biochar, with more comprehensive ones on multiple substrates and different pollutants’ removal [29,30,32,50,58,63,64,74,83,84].

In general terms, ref. [31] argue that adequate optimization studies of BR components are still lacking, particularly in field and long-term assessments. The broad ranges between the results observed in the current study partly confirm this observation and often make the reviews scarcely supportive of possible implementations. Indeed, field assessment and long-period studies, which should be the most “operative” references, result in a minority. For example, ref. [60] highlights that the short duration of most experiments does not allow the researchers to know when the materials reach saturation and have to be replaced, therefore, their cost-effectiveness. This is a relevant aspect affecting the financial sustainability of BR, which, as a LID practice, is supposed to be a low-cost facility. Amendments, though, compensate for their potentially higher costs with fewer filters to reach law standards [36].

Consistently with [44], the research results also highlight the need and difficulty of considering all the biotic and abiotic factors that can modify performance over time in the initial design target. At the same time, different designs, which are often highly empirical, together with precipitation patterns and their interaction, determine the difficulty of comparing different studies, as reported by [6]. Nevertheless, the rich and diversified data collection reported in the analyzed literature forms a conspicuous reference database for designers.

From the hydrological point of view, many authors report biofilters’ failure to deal with stormwater runoff loads, potentially worsened by climate change causing heavier rainfalls and more prolonged droughts, and challenging to compensate within the urban context through oversizing the infrastructure [46,48,54]. On the other hand, a consensus about their efficiency in dealing with minor and medium storms emerges, which makes BR a viable solution for managing stormwater. Also, the research highlights possible solutions consistent with [46]. It suggests combining BR systems with the other SSM infrastructures, such as green roofs, rainwater harvesting, and permeable pavements, enhancing runoff volume management and the water storage capacity, promoting a more water-wise configuration.

Overall, information on substrates and amendments is abundant and has grown in the last few years. Nevertheless, as reported by [31], media performance is still critical to assess and often inconsistent with the design regarding infiltration rates, mainly due to clogging and water retention capacity. Indeed, the primary results concern water purification, not hydrological performance. Ref. [6] observes that even though saturated hydraulic conductivity (Ksat) is the main parameter to control infiltration, there is no consensus about the optimal infiltration rate range to reach with the mix. Indeed, different values can be found in literature and manuals, determining a range that can be challenging in the design phase. Still, there is consensus on overestimating the value to compensate for the initial physiological decrease in performance.

As already said, the lack of field tests of media mixes’ performances in pollutant removal is critical as laboratory performance differs from actual conditions, which are influenced by hydraulic residence time, and competition from other stormwater runoff constituents [30,60]. Ref. [83] highlight the inconsistency of results in nutrient removal and the leaching potential of BR cells, which are some of the main criticalities to be carefully addressed. According to [57], the behavior and fate of nitrogen (N) in BR are challenging to assess and most of the studies remain a qualitative analysis, with nitrogen leaching mechanisms remaining unclear. The broad ranges in the reviewed studies confirm this uncertainty, but many promising results are worth further implementation and studies.

Biochar is appreciated for its high pollutant removal capacity which involves different physiochemical processes, even though field-scale and long-term studies are still lacking, and the removal can vary by orders of magnitude [46]. Incorporating different materials (such as clay minerals, metal oxides, and organic compounds) can enhance its properties. However, the selection of modified biochars is challenging because of inconsistent literature, suggesting the use of a mixture of them [36,66] and testing before application [46].

Vegetation is a minor topic of the reviews and is primarily treated in general terms, rarely as taxa with their context- or substance-related performances. This reflects a broader research gap in the overall scientific publications, which might be partially explained by the complexity and number of variables involved in plants’ species- and context-specific responses. It might also refer to the scarce participation of ecology and cultivation experts in creating and monitoring laboratory or field-scale BR systems. A few authors, though, have recently collected the taxa listed in BR literature, and this can support further applications and local testing [6,7,35].

Roots determine the trade-off between quantitative and qualitative stormwater management by regulating infiltration and rhizosphere-mediated pollutant removal. Therefore, the correct selection of plant species should rely also on root functional traits [31,48]. [42], though, concluded that the current models are still too reductionist and plant-media interactions are insufficiently addressed, e.g., porosity is calculated based on texture and not on structure, to which plants’ roots contribute. Also, ref. [31] point out that there are few studies on ET, a fact confirmed by [73]. For [7], ET was mainly analyzed in trees, with significant differences among species; also, where and how densely trees are planted influences ET rates [73].

Assessing species’ remediating efficacy is also a considerable research gap [31]. Plants’ nutrient uptake is highly species-specific and depends on the availability of nutrients, wet and dry conditions, and growing or dormancy season. Literature is inconsistent, especially for P removal, and the response to BR stresses is partially unknown [7,42]. Consequently, further specific studies are required to address the species- and context-specific performances in water treatment.

In literature, the rationale of species selection is often less unaddressed [7]. Also, ref. [7] point out that species’ diversity, how they interact in mixtures, and how they provide ecosystem services, are still a consistent research gap. Furthermore, a relevant missing knowledge highlighted by many authors is the species’ performance in tropical climates [6]. This is consistent, for example, with the findings of [7], which mostly surveyed species from temperate climates. The analysis of [7] also highlights how narrow the range of species suitable for the European context is, as the prevailing Chinese and American natives are often invasive, and the Australian species poorly adapt to different regions. This opens a broad research and testing area that should aim to select native species from the European countries based on ecological coherence with the complex set of BR conditions.

Traditional substrates of BR are fast-draining sandy loam or loamy sand with very low water and nutrient retention. In their review, ref. [53] point out that this kind of substrate is one of the primary causes of the many vegetation failures in BR. Therefore, the relationship between plants and soil amendments varies and is partially unresolved [30]. The optimization of the growing medium, with its specific materials balance, is another interesting research field that requires further investigation. Species-specific responses to cultivation conditions deserve in-depth analysis.

5. Conclusions

The article analyzed distribution and topics assessed by the last 10 years’ reviews on SSM. Together with the hydraulic function of the infrastructures, water purification emerges as a priority of scientific research in the field, while biodiversity provision is the leading other service addressed. Bioretention systems are the most researched infrastructures of SSM, conjugating water quantity and quality regulation and multiple ecosystem services.

Confluence ratio, ponding depth, and the inclusion of an IWSZ and drains emerge as crucial choices for the general design of the infrastructure. Oversizing or combining the BR system with other infrastructures can address heavier storms. Media’s Ksat choice should consider its initial decrease; lower Ksat, though, favours nutrient removal. A mix or a sequence of different materials can target the multiple pollutants in stormwater.

The IWSZ is relevant for both N removal, maximized when combined with specific amendments, and plants’ growth and survival. An alternative is creating one or more saturated zones within the media, e.g., using organic amendments, or building biphasic rain gardens. Biochar is a sustainable alternative to compost with high pollutant removal; mixing and modifying biochar can intercept more compounds. Zeolites and coconut coir are low-cost solutions to remove both nutrients and metals, while Fe and Al oxides are confirmed to be crucial for nutrients’ adsorption. For example, WTR can be used as a sustainable, local, low-cost material.

Plants with thick or fleshy and deep roots maximize infiltration; broad-lived species, higher plants, and structurally complementary vegetation favor ET. Planting with high densities enhances nutrient processing and weed control. Slow growth and high-biomass roots promote P assimilation, while fast-growing vegetation with high biomass and an extensive root system enhances N uptake. Hyperaccumulating species deserve further investigation for metals’ absorption.

Species’ choice should consider their waterlogging and drought resistance, relating to their ecology and the BR conditions. It is important to maximize taxas’ diversity, e.g., with a rich herbaceous mix or using woody species. Exotics can better respond to climate change urban scenarios, but natives reduce the risk of invasiveness and have efficient local adaptations. Water availability should be carefully addressed when selecting the species; irrigation might be necessary, while mycorrhizae can enhance plants’ absorption capacity. Organic substances such as biochar enhance soil fertility and structure; a bi-layer system can provide a more adequate growth substrate.

Overall, the analyzed reviews present evidence of BR efficiency and its limitations, depending on complex variables that deserve further field and long-term studies, particularly on vegetation.