Understanding Recent Trends in Global Sustainable Development Goal 6 Research: Scientometric, Text Mining and an Improved Framework for Future Research

Abstract

The fulfilment of Sustainable Development Goal (SDG) 6, concerning water and sanitation, is critical in itself and also conditional for the other 16 SDGs being met. The purpose of this study was to understand the scientific research trajectories, spatiotemporal development, scientific collaboration, ongoing research themes, and gaps related to SDG 6. We propose a coupling of bibliometric and text mining methods in this work, to statistically portray the impact of water research on the accomplishment of SDG 6. Through the Web of Science database, we focused on a single UN SDG goal (i.e., six related publications that were current (2015–2021)). The study was performed on the chosen 289 publications. With the analysis of Keywords Plus, abstracts, titles, as well as author keywords, we looked at the performance of authors, publications, journals, institutions, and nations in terms of publishing. To obtain an insight into the water and sanitation study topic, we used co-citation, co-occurrence, cooperation networks, theme networks and cluster analysis, word dynamics, thematic evolution, and other techniques. We filtered out five distinguishing themes using text mining and showed their temporal trends. The main outcome is that participation, as well as collaboration with countries of the Global South, is still lacking in the SDG 6 research sphere. Therefore, as an insight from this study, we proposed a conceptual framework, the sustainable development of water and sanitation (SDWS) framework, to classify the research domain of water and sanitation regarding its connections to the environment, economy, and society (i.e., sustainable development). The scientometric and text analysis results provide the contemporary state and overview of the water and sanitation research field, whereas the second, conceptual framework section, provides a better understanding of qualitative contents, by revealing the insights gained, as well as the important work to be done in future water and sanitation studies.

1. Introduction

The United Nations (UN) sustainable development goals (SDGs) were created in 2015 and call for cooperative action to achieve peace, prosperity, and well-being for all by 2030. There are 17 goals and 169 targets in the SDGs, which succeed the Millennium Development Goals (MDGs) (8 goals, 1990–2015). The MDGs were (1) eradicate extreme poverty and hunger, (2) achieve universal primary education, (3) promote gender equality and empower women, (4) reduce child mortality, (5) improve maternal health, (6) combat HIV/AIDS, malaria, and other diseases, (7) ensure environmental sustainability, and (8) global partnership for development. Among these, the water issue was included in the 7th MDG.

1.1. On SDG 6

A specific focus on clean water and sanitation was not contained in the MDGs’ eight goals, but was promoted to one of the goals among the SDGs. The 6th goal among the sustainable development goals, prescribed by the United Nations, is to ‘ensure availability and sustainable management of water and sanitation for all’. The eight targets of Sustainable Development Goal 6 (SDG 6) include universal and equitable facilitation of safe and affordable drinking water (6.1); adequate and equitable sanitation and hygiene services, with an end to open defecation (6.2); increase in recycling and safe reuse of wastewater and improvement of water quality by pollution reduction (6.3); increasing water-use efficiency (WUE) across all sectors and reducing water scarcity through sustainable withdrawal and supply of freshwater (6.4); implementation of integrated water resources management (IWRM) at all levels and transboundary water cooperation (TWC) (6.5); protection and restoration of water-related ecosystems (6.6); expansion of international cooperation and capacity-building activities (6.a); and participation of local communities in improvement of water and sanitation management (6.b) (see Table S1 in Supplementary File).

Although 5 years (2015–2020) have passed since the onset of UN SDG 6, very little progress has taken place in this time. As per a report of March 2021 [1], the changes that have taken place are as follows: 1% increase in safely managed drinking water services (6.1.1) and safely managed sanitation services (6.2.1a); 0.9 USD/m3 increase in water-use efficiency (WUE) (6.4.1); 0.2% increase in the level of water scarcity (6.4.2); 5% increase in the implementation of IWRM (6.5.1); 9% increase in the proportion of river basins showing high surface water extent changes (6.6.1); USD 0.27 billion increase in water and sanitation-related official development assistance (ODA) received (6.a.1); and 13 additional countries or areas with a high level of participation by communities (6.b.1). In this period of time, various targets have also remained unchanged: the proportion of the population with a handwashing facility with soap and water available at home (6.2.1b); the proportion of the transboundary basin area with an operational arrangement for water cooperation (6.5.2), etc. The required rate (i.e., the change needed from the current rate) for achieving the various targets of SDG 6 by 2030 is as follows: basic drinking water (×2), safely managed drinking water (×4), basic sanitation (×4), safely managed sanitation (×2), implementation of IWRM (×2).

1.2. On Bibliometrics

There are numerous methods of literature review, such as – narrative (traditional), scoping, systematic, meta-analysis, critical, overview, state-of-the-art, rapid, and bibliometric, etc. There were a few reasons for choosing bibliometrics for this study: (a) it is a quantitative analysis, i.e., a higher degree of reproducibility; (b) based on a broad range of publications, i.e., can ensemble large collection of publications; (c) statistical tools can yield fairly accurate appropriations about research spheres; and (d) diverse tools available to accomplish bibliometric analysis (viz. R, VOSviewer, Python, BibExcel, CiteSpace, Pajek etc.).

A bibliometric study on global research regarding sustainable development and its subareas was performed by Hassan et al. [2], and they suggested the keyword collection approach as useful for analysis in interdisciplinary research fields such as sustainable development. Zhu and Hua [3] found that two countries (the USA and UK) in general occupied dominant positions in sustainable development studies, while China had the highest publication number. Körfgen et al. [4] composed a study to recognize pertinent publications from 13 Austrian universities, to assess the focus areas of SDG-related research in Austria. They have found that the research on SDG 3 and SDG 4 was well represented, although other SDGs (e.g., SDG 1, SDG 14) were less well represented in research domains. A study by Olawumi and Chan [5] revealed the advancement of the research field from the definition related to the concepts in the Brundtland Commission report towards recent sustainability indicators and models. The most substantial input in sustainability research has come from the USA, China, the UK, and Canada. In addition, the prevailing studies belong to the subject categories of environmental sciences, green and sustainable science technology, civil engineering, construction, and building technology. To assess the research trends related to the SDGs, Salvia et al. [6] found a general prominence in the study of SDGs 4, 11, and 13. Meschede [7] suggested that most research arises from the life sciences and biomedicine and social sciences areas, whilst the most predominant SDG was SDG 3. We compiled the related research performed (see

Table 1. Comparative analysis table of recent literature about bibliometric analyses on sustainable development and water resource research.

Author(s)	Overview Period	Focus	Database	Documents
Hassan et al. [2]	2000–2010	sustainable development	Scopus	NA
Zhang et al. [8]	2006–2015	water footprint, virtual water,	WoS	636
Zhu & Hua [3]	1987–2015	sustainable development	WoS	59,926 (special selection-626)
Körfgen et al. [4]	2013–2017	sustainable development goals	IRDS	28,229 (and 3581 projects)
Mubako [9]	2000–2018	blue, green, grey water	WoS	192
Olawumi & Chan [5]	1991–2016	sustainability; sustainable development;	WoS	2094
Martinez et al. [10]	1992–2018	environmental footprint; life-cycle assessment;	WoS	1472
Zhu et al. [11]	2003–2018	water footprint; china; virtual water;	WoS, CNKI	1564
Armitage et al. [12]	2015–2018	sustainable development goals 1, 2, 3, 7, 13, 14	WoS,	500
Ho et al. [13]	1926–2019	sustainable development goal 6; Belgium;	Scopus	5703
Ma et al. [14]	1993–2020	water footprint; virtual water;	WoS	1592
Meschede [7]	2015–2019	sustainable development goals	Scopus, WoS,	4593
Sweileh [15]	2015–2019	sustainable development goal 3	Scopus	18,696
Xie et al. [16]	1996–2018	environmental footprint;	WoS	6680
Adetoro et al. [17]	1987–2019	water footprint, water sustainability, water productivity (WFSP)	Scopus, WoS,	2059
Payumo et al. [18]	1999–2018	millennium development goals, sustainable development goals,	MAG	16,447
Wang et al. [19]	1996–2019	environmental footprint family	WoS	7114
Wu et al. [20]	1986–2019	environmental footprint family (carbon, nitrogen, phosphorus, ozone, PM10, PM2.5, chemical, water, land and biodiversity footprint)	WoS	4352

CNKI = China Knowledge Resource Integrated Database, IRDS = internal research documentation systems of the universities in Australia; MAG = Microsoft Academic Graph, WoS = Web of Science.

).

Provisioning of SDG 6 can also be seen through the lens of safe and just operating space (SJOS), i.e., an ecologically safe and socioeconomically just zone [21]. Hence, the ecological ceiling of water should always be taken into account when delivering WASH services to the population [22,23,24,25,26].

2. Materials and Methods

To shed light on the impact of academics and researchers on the scientific literature regarding SDG 6, in this study, a bibliometric, as well as text analysis, method was applied to assess the research volume and research trends of scientific publications on SDG 6 over the past 6 years (2015–2021), i.e., from the introduction of the UN SDG proposal.

2.1. Literature Search Tool

Bibliometrics pertains to sophisticated quantitative analyses and statistical approaches to evaluate the scientific yield in a certain field. It uses various data incorporated into the publications (as citation, relevant sources and authors, affiliation, country of author, etc.) to provide the quantity, quality, and efficiency of performance of research results. It is, therefore, an apt methodology to gauge the water and sanitation research field. Bibliometrics and text analysis were applied to recognize and weigh the hotspots and tendencies of sustainable development goal 6-related research. The analysis and classification of scientific works and publications related to SDG 6 from 2015 to 2021 are presented in this paper. In the current study, the Web of Science Core Collection, maintained by Clarivate Analytics, was used to retrieve publications on sustainable development goal 6 for that period. We used the ‘bibliometrix’ package in R [27] and VOSviewer software (v. 1.6.16) for the bibliometric analysis. We performed a bibliographic coupling, citation, co-citation, and co-occurrence analysis using VOSviewer. All the remaining work was done in R (4.1.2) using the ‘bibliometrix’ package (3.1.4) with the ‘biblioshiny’ function. It is a Shiny app with a graphic user interface (GUI).

2.2. Literature Search Strategy

The data collection was carried out on 25 March 2021, and all data analyses were carried out based on updates from the same day.

2.2.1. Inclusion Step

The strategy developed was based on the initial construction of a separate search strategy for each possible mention in the spectrum of the definition of sustainable development goal 6. Research that supports national or global analysis, development, or implementation to achieve the whole, or any target, of SDG 6 can be categorized into two types: those works not mentioning the phrase ‘sustainable development goal 6’, and those which explicitly bring up the phrase ‘sustainable development goal 6’. In the current study, we focused on the latter, in which the phrase ‘sustainable development goal 6’ was mentioned in the paper (title, keywords, or abstract). We omitted those works that do mention this in the body of the paper (i.e., from introduction to discussion or conclusion), as there may be some works that do not have a primary focus on work related to SDG 6. In addition, for all readers, only these parts of the research are freely available to read, irrespective of the nature of the publishing source. We searched for ‘SDG 6’ or ‘SDG6’ or ‘sustainable development goal 6’. Any research document in which one of the three words was present in the title, abstract, or keywords was considered SDG 6-related literature.

2.2.2. Exclusion Step

An exclusion step was used supplementary to the search strategy. The purpose of this step was to purge all potentially faulty results. The following criteria were implemented for this step:

(a)

The duration of the study was set from 2015 to 2021 (until 25 March), and all other years were excluded. From this, we can assume that this study has complete coverage of 6 years (2015–2020).

(b)

Upon the second round of manual checking, results that were found to have very little connection with SDG 6 in their research content, were removed.

(c)

Duplicate results were removed.

(d)

Results that were not published in English were removed.

(e)

To comply with our second and third steps of analysis (i.e., text analysis and comprehensive overview), results for which full text was unavailable to us were removed (see Supplementary File S1).

2.3. Text Mining

Text analysis is another suitable procedure for sorting unstructured text data, making it easier to derive high-quality information from a text. The statistical analysis was done in R statistical software, and full-text versions of the 289 documents considered in this study were imported into the software using the ‘pdftools’ package [28]. For preliminary analysis, the full dataset was cleaned of stop-words (common English words, which do not add meaning to the data in this context, such as ‘the’, ‘is’, ‘of’ etc.); all numbers, except 6; and other journal related words such as ‘https’, ‘doi’, ‘org’, etc. (see Supplementary File S1 for full list).

Topic models are algorithms that can find and underline word occurrence patterns using word distributions from a document collection. As an output, it indicates topics that are made of clusters of co-occurring words that follow certain patterns [29]. One such topic modelling method is latent Dirichlet allocation (LDA), which is a very popular unsupervised modelling technique used for topic modelling [30].

3. Results

3.1. Characteristics of Publication Outputs

The number of sustainable development goal 6-related publications went from one in 2015, to 118 in 2020 (29 in 2021, till the date of WoSCC access) (Figure S1). It is safe to anticipate that by the end of 2022, the number of papers published will have skyrocketed. The peak year of scientific production for SDG 6 was 2020 (118). A total of 289 publications were collected from the WoSCC relating to sustainable development goal 6 research, which consisted of 234 articles (80.96%; of this, articles: 222 or 76.81%, book chapter articles: 5 or 1.73%, early access articles: 2 or 0.69%, and proceedings paper articles: 5 or 1.73%), 8 proceeding papers (2.76%), 37 reviews (12.8%; of this, reviews: 35 or 12.11%, book chapter review: 1 or 0.34%, and early access review: 1 or 0.34%), and 10 editorial materials (3.46%; of this, editorial materials: 9 or 3.11%, and early access editorial material: 1 or 0.34%). (Figure S2). The 289 publications originated from 133 sources (books, journals, etc.). The average number of years from publication is 1.8, the average number of citations per document is 6.05, and the average number of citations year⁻¹ document⁻¹ is 1.82. There are 1131 authors and 1271 author appearances in these publications. There are 0.25 documents author⁻¹, 3.91 authors document⁻¹, 4.4 co-authors document⁻¹ and the collaboration index is 4.35. There are 36 authors of single-author documents and 1095 authors of multi-author documents. There are 37 single-authored documents. There are 1024 author keywords and 739 keywords plus. Average article citations were at their highest in 2019 (4.1); followed by 2016 (3.7), 2018 (3.5), and 2020 (2.1). This means the average citations are gradually getting higher.

3.2. Subject Categories

The examination of subject categories can reveal common patterns in current research in the sector. For this, we used categories from the Web of Science. Every publication covered by the Web of Science core collection (WoSCC) was assigned at least one of the subject categories. These come under the column untitled ‘web.of.science.categories.’, which we aggregated into major fields. The SDG 6 related publications belong to 34 categories, of which the top ten broad categories with ≥3 publications contain 250 (86.5%) publications related to sustainable development goal 6. These are environmental sciences (110 or 38.06%); green and sustainable science and technology (53 or 18.33%); water resources (25 or 8.65%); engineering (20 or 6.92%); public, environmental, and occupational health (16 or 5.53%); development studies (9 or 3.11%); environmental studies (7 or 2.42%), computer science (4 or 1.38%); biodiversity conservation (3 or 1.03%); and chemistry (3 or 1.03%). Since engineering, science, and technology-oriented discourses dominate the SDG 6 research, societal and economic aspects regarding the sustainability of water and sanitation are lagging. The chemical or data related research spheres of human–water interactions are also falling short in comparison to other areas.

3.3. Top Output Analysis

3.3.1. Analysis of Sources (Journals)

From the selected 289 publications under consideration, the most relevant sources with ≥5% publication output are Sustainability (8.3%), Science of the Total Environment (6.92%), and Water (6.92%) (Figure 1a). According to our findings, more than 30% of the publications were published in the top five journals, two of which specialize in water research. As a result, the most significant contributions to SDG 6 came from publications that are not just focused on water, but also from broader transdisciplinary journals. Among the sources, as per cumulative production dynamics, the top five are Sustainability; Water; Science of the Total Environment; Journal of Water, Sanitation & Hygiene for Development; and Journal of Environmental Management. Journal of Water, Sanitation & Hygiene for Development had the top position in 2017, yet from 2018 onwards it was surpassed by Sustainability (Figure 1b). This also implies that nearly 70% of publications came from the top three journals. From the reference list, the top five most-cited local sources with ≥100 citations were Science of the Total Environment (337), Environmental Science & Technology (179), Water (165), Journal of Cleaner Production (131), and Water Research (129). This means that most (70%) are interdisciplinary journals. The top five sources with the most local impact (via h-index) were Sustainability (7), Science of the Total Environment (6), Water (6), Journal of Water, Sanitation & Hygiene for Development (5), and International Journal of Environmental Research and Public Health (4) (Figure 1c). However, from total citations (TC), the top five were Journal of Environmental Management (175), Sustainability (168), Water (125), Journal of Industrial Ecology (121), and Science of the Total Environment (99). The richness of the publications, as well as the inclusion of non-water sources, indicate the transdisciplinary approach to embracing and accomplishing SDG 6. Bradford’s law, also known as Bradford’s law of scattering or Bradford distribution, is a pattern that estimates the exponentially decreasing returns of searching for references in journals of science. As per this study, six journals occupy zone 1, or the core area, i.e., journals that are the most frequently cited in the literature of this subject area. These, along with their frequency, are Sustainability (24), Science of the Total Environment (20), Water (20), Journal of Water, Sanitation & Hygiene for Development (14), Journal of Cleaner Production (9), and Journal of Environmental Management (9). Zones 2 and 3 each have 32 and 95 journals (Figure 1d). The dominance of two open access journals in the core SDG 6 research discourse emphasizes the importance of scientific knowledge being freely and openly accessible.

3.3.2. Analysis of Authors

Fractional authorship is a measure that quantifies an individual author’s contributions to a published set of papers, assuming a uniform contribution of all co-authors to each document. The most relevant authors (top three), as per fractionalized publications, are Spijkers O (2.00), Hashemi S (1.75), and Gronwall J (1.5). However, as per published articles, the top five are Rivett MO (8), Kalin RM (7), Bartram J (7), Han M (4), and Qadir M (4) (Figure S3). Among the authors, the top three authors with local impact (via h-index) are Bartram J (4), Bain R (3), and Qadir M (3) (Figure S4). Among the publications considered in this study, the top five authors with the most local citations (LC) are Addison MJ, Bartram J, Borge RM, Budimir L, and Mannix N. Lotka’s law provides the frequency of publication by author patterns in a given field and over a specified period. Here, 91.5% of authors produced only a single document. The contribution of authors with a higher number of publications decreases: 2 (6.3%), 3 (1.4%), 4 (0.5%), 7 (0.2%), and 8 (0.1%) (Figure 2a). This means that, although many authors are working in the SDG 6 research sphere, less than 0.5% have produced more than five publications. It seems that more than 90% of authors are only passing through the SDG 6 field. This is either due to the short period (2015–2021) or that they moved to other research fields that do not principally concern the SDG 6 theme. The top five authors with the most consistent productivity over time were Rivett MO, Bartram J, Kalin RM, Bain R, and Evans B.

3.3.3. Analysis of Affiliations

The ten most relevant global affiliations related to publications on SDG 6 are University of Strathclyde (23, UK), University of Oxford (20, UK), Utrecht University (15, Netherlands), University College Cork (17, Ireland), Swiss Federal Institute of Aquatic Science and Technology (11, Switzerland), University of California-Berkeley (11, USA), University of Leeds (11, UK), Delft University of Technology-TU Delft (10, Netherlands), Seoul National University (10, South Korea), and University of North Carolina (10, USA) (Figure 2b). This implies the majority of relevant associations connected to SDG 6 come from European countries, the United Kingdom, and the United States. Only one university from Asia is within the most relevant globally affiliated institution list. This implies that most of the global south, where the SDG 6 related problems are highly concentrated, is not producing significant research works, at a global standard.

3.3.4. Analysis of Countries

Among all the countries that published SDG 6-related documents, the top five, with >60 publications, are USA (199), UK (179), Netherlands (107), Germany (62), and Australia (61), (Figure 2c). This means that among the top ten most SDG 6-related publishing countries, 50% are from Europe, and 20% are from Asia (China and India) and North America (USA and Canada). This means, except for one (Malawi), almost no countries from South America or Africa have yielded a significant number of publications on SDG 6. The top ten countries for hosting corresponding authors regarding publications on SDG 6 are UK (43), USA (40), Netherlands (25), Australia (14), Germany (13), India (13), Switzerland (12), Italy (10), Canada (9), and Spain (8) (Figure 2d). This means that among the top ten countries hosting the corresponding authors of SDG 6-related publications, 60% are from Europe and 20% are from North America. Among these, as per single country publications (SCP), the top ten, with ≥2 publications, are USA (18, 15%), India (13, 11%), UK (12, 10%), Netherlands (12, 10%), Switzerland (6, 5%), Australia (5, 4%), Canada (5, 4%), South Africa (5, 4%), China (4, 3%), and Germany (3, 3%) (Figure 2e). This means, among the top ten SDG 6-related single-country publishing countries, 40% are from Europe and 20% are from Asia and North America. However, regarding multi-country publication (MCP), the top ten, with ≥2 publications, are UK (31, 19%), USA (22, 13%), Netherlands (13, 8%), Germany (10, 6%), Australia (9, 5%), Italy (9, 5%), Switzerland (6, 4%), Spain (6, 4%), Sweden (5, 3%), and Canada (4, 2%) (Figure 2f). This means that, among the top ten SDG 6-related multi-country publishing countries, 70% are from Europe and 20% are from North America.

Among the countries that published SDG 6-related documents, the top 10 most cited (≥30 citations) countries, as per total citation (TC) count, are UK (280), USA (201), Netherlands (186), Zimbabwe (110), Australia (105), Sweden (93), Italy (90), Germany (90), Switzerland (78), and Japan (43) (Figure 3a). This means, among the top ten SDG 6-related cited countries, 60% are from Europe. However, as per average article citations, the top ten (≥7 citations per average article) are Zimbabwe (110), Kenya (17), Japan (14.33), Pakistan (13), Sweden (11.62), Uganda (11), Indonesia (10), Austria (9.67), Italy (9), and Laos (8). This means that among the top ten SDG 6-related cited articles on average, 60% are from Africa.

3.3.5. Analysis of Publication Documents

From the most globally cited (GC) documents, the top five are Schroeder P, 2019, J Ind Ecol (global citations, GC-121), Gwenzi W, 2017, J Environ Manage (GC-110), Bangert M, 2017, Infect Dis Poverty (GC-61), Bhaduri A, 2016, Front Environ Sci (GC-52), and Mainali B, 2018, Sustainability (GC–49). However, if we consider time (i.e., total global citation, GC per year), the top five are Schroeder P, 2019, J Ind Ecol (global citations per year, GCpy–40.33), Gwenzi W, 2017, J Environ Manage (GCpy-22), Mainali B, 2018, Sustainability (GCpy–12.25), Bangert M, 2017, Infect Dis Poverty (GCpy–12.2), and Scherer L, 2018, Environ Sci Policy (GCpy–11). If we consider local citation (LC), the top five are Rivett MO, 2019, Sci Total Environ (LC-8), Quinlivan L, 2020, Sci Total Environ (LC-5), Mccracken M, 2018, J Hydrol (LC–4), Bartram J, 2018, Npj Clean Water (LC–4), and Geere JA, 2018, J Glob Health (LC–3). However, if we consider the ratio of local to global citations (i.e., LC/GC in %), the top five are Rivett MO, 2019, Sci Total Environ (61.54%), Quinlivan L, 2020, Sci Total Environ (55.56%), Lapworth DJ, 2020, Environ Res Lett (50%), Wiegleb V, 2018, Sustain Sci (50%), and Fleming L, 2019, Sci Total Environ (50%). This means that these academics have gathered more local citations for their work than others, in other words, their works have proved to be significant locally, as well as globally.

The use of reference publication year (RPY) spectroscopy allows us to see which year the oldest references are from; i.e., the 289 publications cite literature from which historical period and how much in that year. In our study, the top ten earliest years of citation (before 1900s) are from 1776 (1), 1789 (1), 1798 (2), 1855 (1), 1858 (1), 1859 (1), 1866 (1), 1884 (1), 1895 (1), 1896 (1), 1905 (1), 1931 (1), 1936 (6), and 1939 (1). Starting from 1951, from each year, at least one citation is found. From 1999, every year has ≥100 citations. The year with the highest citation output is 2017 (1550) (Figure 3b). From the network of GC and LC score coupling, seven clusters are generated. These clusters are composed of 53 (cluster 1), 28 (cluster 2), 71 (cluster 3), 17 (cluster 4), 33 (cluster 5), 26 (cluster 6), and 22 (cluster 7) documents (Figures S5–S8 in Supplementary File S1). From the citation network of countries (Figure S9 in Supplementary File S1), we can see those countries, such as USA, UK, Germany, India, etc. previously connected strongly via citation. However, as the time progress to more recent times, different countries, such as Scotland, Algeria, Spain, Belgium, and Brazil are rising. From the citation network of sources (Figure S11 in Supplementary File S1), it can be seen that Water; Sustainability; and Journal of Water, Sanitation & Hygiene for Development were dominant during 2018–2019. However, after this Science of Total Environment, Remote Sensing of Environment, Environmental Research Letters etc. were emerging.

3.4. Academic Cooperation

Academics and researchers from many nations with similar skills have been shown to interact and contribute significantly to SDG 6 water research through interesting research collaborations. Among the countries with the most multi-country collaborations in SDG 6 publications, the top ten countries, with frequency ≥5, are USA, UK, Malawi, Germany, Netherlands, Australia, Canada, Italy, Spain, India, and Switzerland (Figure 3c). This indicates a higher degree of collaboration between European and North American countries and a moderate level of collaboration between Asian countries.

In the collaboration network of authors, there are six clusters. Cluster 1 contains six authors (Rivett MO, Kalin RM, Phiri P, Addison MJ, Banda LC, and Mleta P). Other clusters and their composing authors are cluster 2 (5 authors), cluster 3 (Han M and Hashemi S), cluster 4 (5 authors), cluster 5 (Giupponi C, Farinosi F, and Gain AK), and cluster 6 (Forio MAE and Goethals PLM). These three clusters (1, 2, and 4) contain most of the authors with multi-country collaboration. In the collaboration network of countries, there are five clusters. Six countries are in cluster 1 (UK, Malawi, Ireland, Thailand, Sri Lanka, and Fiji). Other clusters and their composing countries are cluster 2 (15 countries), cluster 3 (10 countries), cluster 4 (South Korea and Vietnam), and cluster 5 (China, Brazil, Japan, and Portugal). These three clusters (1, 2, and 3) contain most of the countries with higher levels of inter-country collaboration. In the collaboration network of institutions, there are four clusters. Cluster 1 contains three institutes (Univ. Oxford-UK, Univ. Leeds-UK and Univ. N Carolina-USA), cluster 2 contains two institutes (Swiss Fed Inst Aquat. Sci and Technol., Switzerland and Swiss Fed Inst Technol., Switzerland), cluster 3 contains two institutes (Univ. Utrecht, Netherlands and Wageningen Univ., Netherlands), and cluster 4 contains two institutes (Univ. Strathclyde, UK and Univ. Malawi Polytech, Malawi). This means that, either at country, author, or institution scale, there has been academic collaboration and transfer of emergent knowledge for collective solutions between developed nations and developing or least developed countries, or institutions hailing from them.

The process of co-citation analysis entails tracking down pairs of papers that are cited in the same source article. Clusters of study emerge when consistent pairs of papers are co-cited by a large number of authors. These clusters of co-cited works are likely to have the same theme. It is a technique for delving into a scientific field’s cognitive structure. If we consider the sources, there are two clusters, composed of 25 journals in each. Among them, the top five from each cluster, as per betweenness, are Water (58.63), Water Resources Research (45.34), Nature (40.39), Water International (32.6), and Environmental Research Letters (32) in cluster 1; and Science of the Total Environment (152.42), Environmental Science & Technology (55.73), PLOS One (38.34), Water Policy (23.24), and International Journal of Environmental Research and Public Health (21.96) in cluster 2. If we consider the co-citation network of documents, there are five clusters. They contain 10 (cluster 1), 9 (cluster 2), 12 (cluster 3), 13 (cluster 4), and 5 (cluster 5) documents. As per the betweenness, the top three from each cluster are Mainali B, 2018 (291.65), Le Blanc, 2015 (179.05), and Nilsson M, 2016 (127.77) in cluster 1; Kalin RM, 2019 (185.36), Howard G, 2016 (133.56), and Truslove JP, 2019 (75.5) in cluster 2; WHO, 2017a (396.95), WHO-UNICEF, 2017 (129.53), and WHO, 2017b (117.14) in cluster 3; UN, 2018 (244.4), Bhaduri A, 2016 (102.74), and Gain AK, 2016 (35.78) in cluster 4; Bartram J, 2014 (211.35), Satterthwaite D, 2016 (169.37), and Malik OA, 2015 (129.31) in cluster 5 (Figure 3d). When we consider the co-citation network of authors, there are two clusters, composed of 21 (cluster 1) and 27 authors (cluster 2) in each. Among them (excluding institution-endorsed report documents), for betweenness the top five from each cluster are Gleick PH (3.4), Mekonnen MM (1.4), Vorosmarty CJ (1.31), Hoekstra AY (1.15), and Nilsson M (0.95) in cluster 1; Bartram J (112.63), Hutton G (9.63), Bakker K (5.81), Howard G (4.27), and Jimenez A (3.02) in cluster 2 (Figure 3e). Similarly, if we consider the co-citation network of organizations, we can see that the UN, WHO, and UNICEF are the three most important organizations engaged in producing SDG 6 related research documents.

Bibliographic coupling is a similarity measure that uses citation analysis to build a similarity link between documents. Bibliographic coupling occurs when two works in their bibliographies refer to the same third work. This is a sign that the two works are likely to be about the same subject. Two documents are considered to be bibliographically connected if they share one or more citations. The stronger the coupling strength of two documents, the more citations to other writings they share. Similarly, two writers are bibliographically coupled if their cumulative reference lists both contain a reference to the same document, and the degree of their coupling develops as the number of citations to other documents they share increases. We composed the bibliographic coupling of global SDG 6 research for different aspects: authors, countries, documents, and sources (Figures S13–S16 in Supplementary File S1). From the bibliographic network of authors, it is clear that networking has increased to a higher degree, as well as many new authors joining in recent years, especially since 2018–2019, in comparison with 2015–2016. Among them, only a handful, such as USA, UK, India, and Netherland showed higher coupling strength. From the bibliographic network of countries, it is evident that there are two separate phases. Some countries, such as the USA, UK, South Africa, Sweden, India, Canada, Australia, Ecuador, etc. were similar during 2018–2019. However, from 2019 to 2020, many new nations, such as Malawi, Scotland, Brazil, Poland, Iran, Egypt, Belgium, Greece, etc. emerged. The bibliographic coupling network of sources, similarly, shows two phases. From 2018 to 2019, there were Water, Sustainability, International Journal of Hygiene etc. with stronger coupling. However, since 2019–2020, Science of the Total Environment, Journal of Cleaner Production, Remote Sensing of Environment, Frontiers in Environmental Science, Applied Sciences, Utrecht Law Review etc. have been bibliographically networked. In terms of scientific understanding, these findings show that scientific research on attaining SDG targets, progress, or realization of UN-SDG 6 is still very much in the early stages.

3.5. Keyword Analysis and Hotspots

Through the analysis of Keywords Plus (

Table 2. Comparative table of top keywords on sustainable development of water and sanitation research. The frequency of keywords is in the respective brackets.

Ranking	Abstracts	Author’s Keywords	Keywords Plus	Titles
1	data (219)	groundwater (14)	drinking-water (19)	quality (30)
2	quality (213)	water quality (14)	climate-change (18)	drinking (24)
3	drinking (189)	wash (12)	quality (16)	rural (24)
4	access (162)	drinking water (11)	health (15)	urban (23)
5	health (154)	water supply (9)	challenges (13)	monitoring (21)
6	monitoring (151)	Malawi (8)	groundwater (12)	achieving (17)
7	targets (141)	water governance (8)	contamination (10)	supply (17)
8	countries (140)	climate change (7)	impact (10)	challenges (16)
9	services (135)	developing countries (7)	model (10)	health (16)
10	global (123)	Africa (6)	systems (10)	Africa (15)

, Figure S17 in Supplementary File S1), the top ten most frequently used words are ‘management’ (26), ‘drinking-water’ (19), ‘climate-change’ (18), ‘quality’ (16), ‘health’ (15), ‘challenges’ (13), ‘groundwater’ (12), ‘contamination’, ‘impact’, and ‘indicators’ (10 each). From a title analysis (Figure S18 in Supplementary File S1), the top ten are ‘quality’ (30), ‘drinking’ (24), ‘rural’ (24), ‘urban’ (23), ‘monitoring’ (21), ‘management’ (19), ‘supply’ (17), ‘challenges’ (16), ‘health’ (16), and ‘Africa’ (15). From the analysis of the content of abstracts (Figure S19 in Supplementary File S1), the top ten are ‘data’ (219), ‘quality’ (213), ‘drinking’ (189), ‘management’ (183), ‘access’ (162), ‘health’ (154), ‘monitoring’ (151), ‘services’ (135), ‘systems’ (100), and ‘environmental’ (99). Last, through the analysis of authors’ keywords (Figure S20 in Supplementary File S1), the top ten are ‘groundwater’ (14), ‘water quality’ (14), ‘wash’ (12), ‘drinking water’ (11), ‘water supply’ (9), ‘Malawi’ (8), ‘water governance’ (8), ‘climate change’ (7), ‘developing countries’ (7), and ‘Africa’ (6). The keyword analysis found a considerable increase in scientific research related to SDG 6 over the years.

If we wish to delve further into temporal trends, we can use cumulative word dynamics. From the analysis of cumulative word dynamics of Keywords Plus, ‘quality’ was the highest used from 2015 to 2016, but it was exceeded by ‘sanitation’ from 2017 onwards. The consecutive most used keywords were ‘water’, ‘management’, ‘drinking-water’, and ‘climate-change’. The keyword ‘health’ was second most used during 2017–2018, but has since declined to 7th position. The keywords that are increasing are ‘water’, ‘drinking-water’, ‘quality’, ‘challenges’, and ‘groundwater’. On the other hand, the keywords that are decreasing are ‘management’, ‘climate-change’, ‘health’, and ‘contamination’.

A co-occurrence network is a collective connecting of terms based on their matching existence inside a certain unit of text. Using a set of co-occurrence criteria, networks are built by connecting pairs of phrases. Co-occurrence networks are a popular way to visualize possible connections between people, organizations, concepts, and other entities in a text. The building and display of co-occurrence networks has become possible with the introduction of electronically stored text that is text mining compliant. From the analysis of the co-occurrence word network of Keywords Plus (Figure 4a), we can see there are eight clusters. The highest occurring three co-words of major clusters, as per betweenness, are ‘sanitation’ (betweenness = 185.86), ‘water’ (59.92), and ‘groundwater’ (15.57) in cluster 1; ‘challenges’ (160.02), ‘systems’ (58.73), and ‘access’ (17.28) in cluster 2; ‘drinking-water’ (133.03), ‘quality’ (98.08), and ‘contamination’ (81.82) in cluster 3; ‘scale’ (79.51) and ‘nexus’ (38) in cluster 4; ‘governance’ (108) and ‘resources management’ (75) in cluster 5; ‘management’ (195.57), ‘climate-change’ (123.45), and ‘security’ (4.61) in cluster 7. We can see that three clusters (viz. 2, 4, and 6) have a niche orientation. The second can use systems and models such as LCA to deal with various challenges such as access. The fourth one deals with various nexuses (e.g., with energy) at various scales. Another two, ensemble irrigation in river basins (cluster 6) and treatment and removal of wastewater (cluster 8). From the content of abstracts, they are ‘water’ (0.84), ‘goal’ (0.26), ‘study’ (0.23) from cluster 1; and ‘development’ (0.8), ‘sustainable’ (0.76), and ‘sdg’ (0.36) from cluster 2. As we can see, there is not much difference in betweenness, we can easily understand that these two covers varied topics, i.e., as such they are not discernable. From the authors’ keywords (Figure 4b), they are ‘sdg6’ (93.65), ‘sanitation’ (88.75), and ‘wash’ (13.83) from cluster 1; ‘drinking water’ (44), ‘water quality’ (38.3), and ‘groundwater’ (31.79) from cluster 2; ‘water’ (291.42), ‘sustainable development goals’ (291.42), and ‘sdgs’ (38.98) from cluster 3; ‘sustainable’ (78.12), ‘water management’ (4.5), and ‘climate change’ (1.69) from cluster 5; ‘sdg 6’ (286.16), ‘peri-urban’ (39), and ‘water governance’ (4.18) from cluster 6. Among these, the second cluster deals with the quality and availability of groundwater and drinking, specifically related to less developed areas (i.e., rural), countries (i.e., Malawi), or regions (i.e., Africa). Through the analysis of titles, they are ‘sustainable’ (142.82), ‘development’ (133.61), and ‘goals’ (24.07) from cluster 1; ‘water’ (543.53), ‘quality’ (26.95), and ‘sanitation’ (10.89) from cluster 2. Unlike the Keywords Plus, or the author keywords, this covers a wide range of topics and are, hence, not minutely distinct. To sum up, we can see that, at least for the co-word network of the SDG 6 research domain, either Keywords Plus or author keywords yield better insight than titles or the content of abstracts. The co-occurrence network for all keywords has also been prepared (Figure S21 in Supplementary File S1).

3.6. Research Trajectories

In recent years, as per Keywords Plus (

Table 3. Comparative table of trending topics in sustainable development of water and sanitation research. Frequency and peak trending year is given inside brackets.

Ranking	Abstracts	Author’s Keywords	Keywords Plus	Titles
1	data (214, 2020)	sanitation (20, 2020)	drinking-water (19, 2019)	development (89, 2020)
2	quality (205, 2020)	sustainability (19, 2019)	climate-change (18, 2020)	quality (29, 2020)
3	improved (56, 2018)	sustainable (18, 2019)	quality (16, 2020)	urban (22, 2019)
4	cooperation (37, 2018)	sustainable development (16, 2018)	health (15, 2019)	achieving (17, 2019)
5	interactions (35, 2018)	groundwater (14, 2020)	challenges (13, 2020)	services (10, 2018)
6	basic (31, 2018)	wash (12, 2019)	groundwater (12, 2020)	south (8, 2018)
7	projects (30, 2018)	development goals (7, 2018)	impact (10, 2019)	risk (7, 2018)
8	sludge (25, 2021)	water scarcity (6, 2018)	consumption (8, 2018)	data (6, 2018)
9	coli (19, 2021)	rural (5, 2017)	children (6, 2018)	improved (6, 2018)
10	nexus (18, 2021)	water and sanitation (5, 2018)	nexus (6, 2018)	sludge (5, 2021)

, Figure S22 in Supplementary File S1), the top trending topics of each year with the highest frequency were ‘consumption’ (8), ‘sustainability’ (8), ‘children’ (6), ‘nexus’ (6), and ‘resources management’ (5) in 2018. Then ‘sanitation’ (30), ‘management’ (26), ‘drinking-water’ (19), ‘health’ (15), and ‘impact’ (10) in 2019. Lastly, in 2020, they were ‘water’ (29), ‘climate-change’ (18), ‘quality’ (16), ‘challenges’ (13), and ‘groundwater’ (12). From the analysis of authors’ keywords (Figure S23 in Supplementary File S1), the trending words were ‘rural’ (5) in 2017; ‘sustainable development’ (16), ‘development goals’ (7), ‘water scarcity’ (6), and ‘water and sanitation’ (5) in 2018; ‘sustainable development goals’ (59), ‘sustainability’ (19), ‘sdgs’ (18), ‘sustainable’ (18), and ‘wash’ (12) in 2019; and ‘water’ (31), ‘sdg 6’ (29), ‘sanitation’ (20), ‘sdg6′ (16), and ‘groundwater’ (14) in 2020. Using the analysis of titles (Figure S24 in Supplementary File S1), they were ‘services’ (10), ‘south’ (8), ‘risk’ (7), ‘data’ (6), and ‘improved’ (6) in 2018; ‘water’ (164), ‘goals’ (49), ‘urban’ (22), ‘achieving’ (17), and ‘sdgs’ (16) in 2019; ‘sustainable’ (99), ‘development’ (89), ‘sanitation’ (42), ‘sdg’ (31), and ‘quality’ (29) in 2020; and ‘sludge’ (5) in 2021. Lastly, as per the analysis of content of abstract, the trending topics are were ‘census’ (11), ‘justice’ (11), and ‘testing’ (9) in 2017; ‘improved’ (56), ‘cooperation’ (37), ‘interactions’ (35), ‘basic’ (31), and ‘projects’ (30) in 2018; ‘water’ (1400), ‘development’ (449), ‘sanitation’ (337), ‘goals’ (193), and ‘goal’ (174) in 2019; ‘sdg’ (613), ‘sustainable’ (445), ‘sdgs’ (240), ‘data’ (214), and ‘quality’ (205) in 2020; and ‘sludge’ (25), ‘coli’ (19), ‘nexus’ (18), ‘finance’ (13), and ‘suppliers’ (13) in 2021. This gives out an outline of the dynamicity of the focus in SDG 6 research, especially in recent years.

From the thematic network analysis, the basic themes that arise are ecosystem services, impacts, management, sanitation, water, and health (from Keywords Plus, Figure S25 in Supplementary File S1); monitoring, assessment, sdg, quality, and sustainable development goals (from titles Figure S26 in Supplementary File S1); development, sustainable, and water (from abstracts, Figure S27 in Supplementary File S1); groundwater, water quality, drinking water, sdg, management, India, sanitation, sdg, wash, water, and sustainability (from author keywords, Figure S28 in Supplementary File S1). The motor themes that arise are scale, energy, nexus, drinking-water, quality, and groundwater (from Keywords Plus); sanitation, review, and sustainability (from titles); goal and study (from abstracts); and sustainable development goal, monitoring, and citizen science (from author keywords). The emerging themes are framework, sustainability, governance, climate change, policy, and security (from Keywords Plus); services and indicators (from titles); data-based monitoring (from abstracts); sustainable, water governance, and climate change (from author keywords). The niche themes are removal, carbon, risk-assessment, design, fecal sludge management, and resource recovery (from Keywords Plus); challenge, island, management, basin, resources, sdgs, Africa, and governance (from titles); and sustainable development, developing countries, and indicators (from author keywords). It seems that the SDG 6 research domain is going through a paradigm shift. It is changing from a traditional management and monitoring based phase, to a more framework, services-oriented, data-based, governance- and policy-mediated phase.

A collection of themes that have evolved over time and across sub-phases are referred to as thematic development. We have divided the total period in this study (i.e., 2015–2021) into two time slices, 2015–2018 and 2019–2021, to distinguish temporal trends. We can see the themes that were present in the water and sanitation research field (2015–2018) were freshwater, management, irrigation, sustainability, and perspective (from Keywords Plus) (Figure 4c); agriculture, water, Accra, developing countries, and development goals (from authors’ keywords, Figure S29 in Supplementary File S1); potential and management (from the content of abstracts, Figure S30 in Supplementary File S1); and national, monitoring, study, Africa, risk, and systems (from titles, Figure S31 in Supplementary File S1). However, the contemporary themes (2019–2021) that are conspicuous in this research domain are indicators, biodiversity, and groundwater (from Keywords Plus); management and groundwater (from authors’ keywords); results (from abstracts); and management, sanitation, and sdg (from titles). The period from 2015 to 2018 can be characterized as the commencement, or conceptual understanding, while the period from 2019 to 2021 can be termed the adoption and initial implementation. As the decade of action for the SDGs, 2020–2030, this period is expected to show the direction of future research and will aid in identifying actual and prospective research themes on SDG 6. Through historiography, we may learn which works were cited to a greater extent by other works over time (i.e., annually). Local citation score (LCS) and global citation score (GCS) have been used to derive this. Though citation condensation starts to increase from 2018, it reached a high position between 2019–2020.

Multiple correspondence analysis (MCA) is a technique for detecting and elucidating underlying structures in nominal category data. From the conceptual structure map of Keywords Plus generated using MCA, two clusters emerge, in two dimensions (13.07% and 16.83%). Likewise, two clusters in two dimensions (10.07% and 15.28%) for authors’ keywords; two clusters in two dimensions (9.44% and 53.89%) for titles; and two clusters in two dimensions (13.96% and 68.17%) for content of abstracts. These can also be visualized using topic dendrograms (Figure 4d). We created a three-field plot for best-performing affiliations, countries, and research focus on global SDG 6 research (Figure 4e). It shows affiliations on the left side, nations in the middle and research focus keywords on the right side. It matches with the previous trends we uncovered earlier. The developed countries of North America and Europe lead the research on SDG 6, whilst a handful of countries from Asia and Africa come into the lower ranks. This gives a sense that there may be a significant amount of water and sanitation-related problems, which are to be dealt with via the UN SDG 6 framework, being left out. This in turn would increase the burden, especially creating a hindrance towards achieving UN SDG by 2030 for those nations. Some keywords, such as indicators, models, systems, framework etc. which we think as indispensable to solve water and sanitation-related problems (since these are multifaceted multilayered interconnected problems), have been less considered so far. It also seems that water and sanitation-related to two other pillars of sustainability, i.e., economic and societal aspects, are nearly missing from the research focus, at least on a global scale.

3.7. Text Mining

A bigram network plot was constructed using word pairs occurring more than 180 times in the pooled dataset containing all the documents (Figure 5a). This plot depicts both the co-occurring adjacent words in the document, as well the relative abundance. The base cut-off frequency of co-occurring adjacent words was taken 180 times, so as to only highlight the most common bi-gram associations between the words in the entire literature collection. From the plot, it can be seen that the biggest connected nodes network is centered around water, where all terms are connected with a minimum pair frequency occurrence of 180 times. The other isolated smaller terms are not connected with the bigger network because of not satisfying the 180-frequency threshold. Here the arrow connecting individual terms indicates the order of the occurrence of the words, and the variation in opacity indicates the occurrence frequency (darker indicates more and lighter indicates less frequent) of the word pairs.

For topic modelling, further cleaning of the data was carried out. We removed the search terms we used initially for identifying the literature set on SDG 6. The terms ‘Sustainable Development Goal’, ‘SDG 6’ and selectively ‘water’ terms were removed from the dataset to capture the topics discussed under these overarching terms present in every literature work. For selecting the optimal number of topics, we used the ‘ldatuning’ package [31]. To obtain the optimal number of topics suitable for the dataset, we tested it using the ‘Griffiths2004’, ‘CaoJuan2009’, ‘Arun2010’, and ‘Deveaud2014’ methods for a full range of 2 to 80 topics. From the results, it is clear that the most appropriate topic number was five for the dataset predicted by the Deveaud2014 method [32] (Figure 5b). Since all other results, except- Deveaud 2014, did not give clear maxima or minima, the others results were not used for determining the optimal topic number. Therefore, the latent Dirichlet allocation (LDA) topic model was made using five as an optimal topic number. The ‘topicmodel’ statistical package [33] was used for conducting the analysis. The topic distribution over the years was also depicted. (see Supplementary File S1)

As we can see from the results, the major themes of five topics are (1) sanitation and its allocation related, (2) groundwater and its supply system related, (3) sanitation and its characteristic features-related, (4) health connection to water and sanitation-related, and (5) other determining factors of water and sanitation (Figure 5c). From the temporal distribution (Figure 5d), we can see five years had a higher research prevalence of only a single topic. These were 2015 (topic 4), 2016 (topic 1), 2017 (topic 4), 2019 (topic 1), and 2021 (topic 5). Two years showed a higher research prevalence of two topics, 2018 (topic 3 and 4, 4 was predominant) and 2020 (topic 2 and 3, 2 was predominant). This gives us an impression about the dynamic nature of research focus on SDG 6 over the years.

4. Interrelationships of Sustainable Development Goal 6

Going through the literature related to SDG 6 research (marked in yellow in Supplementary File S2), we can conceptualize the links (driving factors and effects) between various targets of SDG 6 and other sustainable development goals. Target 6.1 (access to safe and affordable drinking water) is driven by 11.1 (access for all to adequate, safe, and affordable housing and basic services and upgraded slums), 11.3 (enhance inclusive and sustainable urbanization) and 6.3. Similarly, this target (6.1) influences 2.1 (end hunger and ensure safe, nutritious, and sufficient food), 2.2 (end all forms of malnutrition), and 3.9 (reduce the number of deaths and illnesses from hazardous chemicals and air, water, and soil pollution etc.). Target 6.2 (access to adequate and equitable sanitation and hygiene for all) is influenced by 11.1 and 11.3. While, target 6.2 influences 6.3 and 3.9. Target 6.3 (improve water quality by reducing pollution, increasing recycling and safe reuse) is influenced by 6.1, 6.2, 6.4, 6.5, 9.1 (develop quality, reliable, sustainable, and resilient infrastructure), 9.4 (upgrade infrastructure and retrofit industries to make them sustainable), 11.5 (reduce the number of disasters affecting people), 12.4 (achieve environmentally sound management of chemicals and wastes throughout their life cycle), and 12.5 (reduce waste generation through prevention, reduction, recycling, and reuse). Moreover, target 6.3 influences 3.3 (combat water-borne diseases), 3.9, 6.6, and 14.1 (reduce marine pollution of all kinds from land-based activities). Target 6.4 (water use efficiency) is driven by 6.5, 9.1, 9.4, and 12.2 (achieve sustainable management and efficient use of natural resources). Furthermore, target 6.4 influences 2.4 (ensure sustainable food production systems), 6.3, 8.2 (economic productivity through diversification), 8.4 (global resource efficiency in consumption and production and endeavor to decouple economic growth from environmental degradation) and 11.3. Target 6.5 (integrated water resources management systems) is influenced by 8.2, 9.1, 9.4, and 11.a (support positive economic social and environmental links between urban-rural areas). This target (6.5), in turn, influences 6.3, 6.4, 6.6, 11.3, and 12.5. Target 6.6 (protect and restore water-related ecosystems) is driven by 6.3, 6.5, 14.2, and 15.1 (ensure conservation and sustainable use of terrestrial and inland freshwater ecosystems). This target (6.6), in turn, influences 14.2 (sustainably manage and protect marine and coastal ecosystems to avoid adverse impacts) and 15.1. Target 6.b (strengthen the participation of local communities in improving water and sanitation management) is influenced by 5.5 (ensure women’s participation and equal opportunities for leadership) and 16.7 (responsive and inclusive participatory decision making). Target 6.b influences 11.3. There are many more intricate connections, which also connect the SDG 6 targets to other domains of UN SDGs. For example, utilization of wastewater (6.3) for generation of energy (7.1) via innovation (9.b), contributes to creating new decent employment opportunities (8.5) and economic growth (SDG 8); use of wastewaters (6.3) for food production (2.4 and 12.4) in peri-urban areas etc.

From these, we can ensemble components of sub-sectors (i.e., zones of focus) of SDG 6. Basic access involves 6.1, 11.1, 6.2, and 6.3. Protection of terrestrial ecosystems involves 6.3, 6.6, and 15.1. Protection of marine and water-related ecosystems involve 6.3, 6.6, 14.1, and 14.2. The efficiency of resource utilization involves 6.4, 8.4, and 12.2. Health involves 3.3, 3.9, and 6.3. Availability of water resources involves 6.3, 6.6, 12.5, and 15.1. Engagement of community involves 6. b, 1.4, 5.5, and 16.7. These interrelated SDG targets are important and need research work of more depth, especially regarding the SDG 6 research domain. These intrarelationships (within SDG 6), as well as the interconnectivity of SDG 6 with other SDGs, have also been supported by many other works [34,35,36,37,38].

5. A proposal for Integrated Assessment of Future SDG 6 Research

Some existing frameworks deal with various facets of water, sanitation, and their linkages to sustainability [39,40,41,42,43,44,45,46,47]. Consequently, owing to the comprehensive studies completed related to SDG 6 at the global scale, as well as to synthesize a bridge to minimize the research gaps, we have conceptualized an integrative framework. This framework can be interpreted as the sustainable development of water and sanitation (SDWS) framework (Figure 6). As per the three pillars of sustainable development [47], we can state that water and sanitation are also connected to three main dimensions: environment (i.e., ecology), economy, and society. For water-related research, we can name them (1) water environment or water ecology (environmental aspects of water research), (2) water economy (economic aspects of water research), and (3) water society (societal aspects of water research). Likewise, in the case of sanitation-related research, we can name them (1) sanitation environment or sanitation ecology (environmental aspects of sanitation research), (2) sanitation economy (economic aspects of sanitation research), and (3) sanitation society (societal aspects of sanitation research). We acknowledge the fact that water and sanitation are very intricately connected [48]. Hence, we can also join them together in some circumstances, as per the requirements or scope of research work. As the environment, economy, and society are connected through basic principles of sustainability science, the three areas of water and sanitation research are also connected. In other words, water environment or water ecology, water economy, and water society are connected. Similarly, sanitation environment or sanitation ecology, sanitation economy, and sanitation society are also connected. This creates three interdependent zones of research for water and sanitation. This could also yield a conflict of interest or desirable focus in research. To elucidate, water research is connected to both environment (ecology) and the economy. As we know, the economy is embedded in society, which in turn is embedded in the environment [49]. If we apply the same principle in the case of either water or sanitation, we can design a domain of water and sanitation research, as well as make priorities. The order should be as follows: environment > society > economy. As we have to more efficiently use finite natural resources for our anthropogenic purposes (i.e., to get more social development or economic benefit using a lesser amount of water), the environmental efficiency of water and sanitation should take a higher priority than societal efficiency, which again should have a higher priority than economic efficiency. This constitutes the core component of the framework.

We also need the framework to be multifaceted respective to other connections of water and sanitation. This is necessary because we need to implement and apply the insights generated for fundamental research in water and sanitation. For this purpose, we can connect water and sanitation research for sustainability to six nodes. Each node represents a different aspect of the application or distinguished magnitude of connection in the real world. We have segregated each node into three layers. Though intricately connected, each consecutive layer (from inner to outer ring) represents an increase in magnitude or operational area or complexity. This also means that each element in a ring is inclusive of the related element in its inner ring. These are (1) local → regional → global, (2) production → trade → consumption, (3) individual → institution → community, (4) rural → sub-urban → urban, (5) household → industry → agriculture, and (6) policy → law → governance.

There are three distinct purposes of this framework. First, to effectively include or ensemble almost all of the research work output on the sustainable development of water and sanitation. Second, to classify or categorize the research output into major domains or sub-domains using this framework. This in turn brings about the third target, which is to delve into the spatiotemporal variation of research in water and sanitation, to map out the lack (gap) of research, i.e., where more work needs to be done and in which direction. Moreover, to our knowledge, this framework is one of the first of its kind to comprehensively include almost all of the major types of water and sanitation-related research; we expect this to be deconstructed and redesigned to suit the needs of the future academics in water and sanitation research. We think that, in addition to looking for specific keywords as a marker, one should also qualitatively analyze any SDG 6 research work for more accurate categorization.

6. Conclusions

This research demonstrates how the area of sustainable development target 6 research has progressed in the six years since the UN SDGs were established (2015–2021). An overview of SDG 6 research was presented, including information on authors, institutions, journals, nations, papers, keywords, research developments, and so on. The findings revealed that a quantitative analysis and visualization of information about research publications across a wide range of topics, journals, and nations, mirrored SDG 6 growth rates. We have suggested a method that combines bibliometric analysis with text mining to extract and analyze the scientific knowledge base to solve the concerns that underpin SDG 6. Three countries, the United States, the United Kingdom, and the Netherlands, are at the top of the list of 79 countries that have produced SDG 6 related research publications in terms of total citations (from 41 countries), hosting corresponding authors (from 55 countries), and multi-country publications (MCP) (from 42 countries). This means only a few countries have been engaged in SDG 6 research. In other words, many small, less economically developed countries are still not sufficiently engaged in SDG 6 research, despite water and sanitation-related problems being rampant. Among the 577 affiliations (i.e., institutions) that have generated SDG 6 related scientific publications, three universities hold the top positions: the University of Strathclyde (UK), the University of Oxford (UK), and Utrecht University (Netherlands). Of the 1131 authors engaged in SDG 6 research publications, the most productive are Michael O. Rivett and Robert M. Kalin (both from the University of Strathclyde) and Jamie Bartram (the University of North Carolina at Chapel Hill). Among the 133 sources that have produced SDG 6 related research publications, the most productive are Sustainability, Science of the Total Environment, and Water. Of the 39 countries producing single country publications (SCP) on SDG 6, the USA, India, and the UK are the highest performers. Of the 50 sources of co-citations, the most co-cited sources in SDG 6 are Water, Water Resources Research, and Nature.

There are a few limitations to this study. First, the database used was limited to the Web of Science only; other databases such as Scopus or Google Scholar were not included. We think that Web of Science is the best source for the collection of academic literature, whilst keeping non-peer-reviewed or grey literature at bay. If all of them could have been used simultaneously, this might have increased the number of retrieved SDG 6 related publications. Second, we focused on peer-reviewed literature only, i.e., grey literature was generally not included. Grey literature sometimes offers critical data and reflects national progress regarding SDG 6 implementation. Third, the current study did not distinguish between studies that focused on a single target or a mix of targets under UN SDG 6. Fourth, this study only included peer-reviewed research articles. However, we think that retrieving data from daily newspapers would have yielded a better real-time picture of ongoing conflicts or concerns related to SDG 6, and coupled with this, the study would have been more comprehensive. This would yield a set of arising problems or ongoing concerns on one hand, and what has been, or is being, done to address these on the other hand. When reading the current study, all of these limitations should be considered. Despite this, the current study presents a detailed scenario of the existing scientific research on SDG 6 and identifies gaps where more collaboration between scientists and policymakers is needed. We hope that in the future, further studies will be conducted without these shortcomings.

The SDG 6 research domain to date revolves around a few highlight keywords, such as groundwater, drinking water, health, climate change, water quality, water governance, water supply, contamination, and developing countries, etc. (highest frequency from Keywords plus: 739 keywords, abstracts: 5953 keywords, author keywords: 1024, and titles: 1062 keywords). Although the spatial scale of SDG 6 research is very dispersed, the highest productivity has so far been seen in countries in Europe and North America. Towards the end of our study, we proposed a conceptual framework that depicts connections between sustainable development (in three pillars: environment, economy, and society) with water and sanitation, along with six sub-sectors. We hope that by using this framework, we will be able to better categorize SDG 6 related research works, as well as identify research gaps for the further redesigning of research focusing on the sustainable development of water and sanitation, i.e., SDG 6, globally.