Environmental Science and Pollution Researchhttps://doi.org/10.1007/s11356-022-24370-1

RESEARCH ARTICLE

A bibliometric and visual analysis of contaminant transport modelingin the groundwater system: current trends, hotspots, and future
directions
Abhay Guleria1· Sumedha Chakma1

Received: 9 August 2022 / Accepted: 18 November 2022
© The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2022

Abstract
Contaminant transport modeling (CTM) in the groundwater represents the complex bio-geo-chemical processes in the subsurface system and plays a vital role in designing remediation strategies for contaminated sites. Hence, this study evaluated1955 articles on CTM-based studies published from 2010 to 2022 by utilizing the Scopus® database to provide a quantitative
analysis of current trends and future directions. A systematic approach comprised of a bibliometric survey and visualizationof various networks was conducted with the VOSviewer and Biblioshiny software for quantitative analysis of progress of
CTM research field over the past decade. The hotspots and evolution of themes of CTM were found using Sankey diagrams,
and thematic maps. It was observed that articles were published at an exponential rate over past 5 years. The analysis of
Scopus® database revealed that the average citations per article and the average citations per year per article were 14.7 and2.225. In the past decade, USA published 489 articles and emerged as the leading country based on mutual collaborations
and cooperation with other top 9 most productive countries. Flinders University was ranked in the first position based on
the number of articles, followed by Pacific Northwest National Laboratory. The findings obtained from this study can help
researchers identify exploitation and core areas, to understand better the direction of research of mathematical model development, and to determine hotspots of CTM research field.
Keywords Bibliometric analysis · Groundwater · Contaminant transport modeling · Research hotspots ·Biblioshiny · VOSviewer

Introduction
Groundwater contamination has been seen a major challenging task over past decades due to continuous dischargeof toxic chemicals from industrial sites (Wu and Sun 2016;
Zhang et al. 2017a; Li et al. 2021; Guleria et al. 2022),
due to inefficient usage of chemicals in the agriculturallands (Wu and Sun 2016; Zhang et al. 2018; Sharma et al.
2019; Giri et al. 2021; Salomon and Cavagnaro 2022), andleakage of untreated leachate from open dumping site/unengineered landfills (Wakida and Lerner 2005; Han et al.
2016; Guleria and Chakma 2019; Parvin and Tareq 2021).
Responsible Editor: Marcus Schulz* Abhay Guleria
abhay_guleria@civil.iitd.ac.in
1

Department of Civil Engineering, Indian Instituteof Technology Delhi, Delhi, India

Groundwater protection from contaminated source zone
and/or remediation of contaminated sites are cost-dependentand economic activities that primarily depend upon physical, chemical, and biogeochemical processes in the subsurface environment (Bonvicini et al. 2015; Zhang et al.
2017a; Brusseau and Guo 2018; Guo et al. 2019). Designing appropriate site-specific remediation strategies involves
a large number of economic considerations and possible
scenarios, which is possible by implementing mathematicalmodeling (Bonvicini et al. 2015; Guo and Brusseau 2017;
Yadav et al. 2018). Therefore, contaminant transport modeling (CTM) plays an important role in selecting the mostefficient remediation strategy, which is impossible with anexperimental approach. For a better understanding of CTM
in the research landscape, a quantitative analysis that provides relevant information is required for determining thepast, present, and future trends.
Bibliometric analysis is found to be an efficient tool for
quantifying the measure and research impact of a specific13Vol.:(0123456789)

Environmental Science and Pollution Researcharea of interest in the research field (Zare et al. 2017; Hireet al. 2022). Generally, its emphasis is on the identificationof strengths and weaknesses of research, determinationof leading countries, productive authors, and top journals(Salim et al. 2018; Akbari et al. 2020; Kumar and Goel
2021; Deb and Chakma 2022; Boraah et al. 2022). However, due to emergence of analysis tools like bibliometrix via
Biblioshiny (Aria and Cuccurullo 2017), CiteSpace (Chen
2006; Chen et al. 2010), CitNetExplorer (van Eck and Waltman 2014), SciMAT (Cobo et al. 2012), Pajek (Batagelj andMrvar 2004; Mrvar and Batagelj 2016), and VOSviewer (vanEck and Waltman 2010), bibliometric analysis is also used
for specialized tasks. These include document-citations andco-author network visualization (Kumar and Goel 2021),
cluster analysis (Akbari et al. 2020), keyword co-occurrence
network analysis (Zhang et al. 2017b), and analysis of collaboration between countries and affiliations (Ranjbari et al.
2021), and theme evolution (Liu et al. 2021). In summary,bibliometric analysis can be inferred as an efficient tool tosummarize the trends in the research field over past decadeand draw future directions (Zhang et al. 2017a; Zare et al.
2017; Kumar and Goel 2021; Liu et al. 2021).
Several review studies on CTM and mechanisms affecting
the fate and transport were conducted in the recent years.
Bolster et al. (2019) evaluated the recent advances and applications of anomalous transport models (continuous-time
random walks, fractional advection–dispersion equations,and multi-rate mass transfer). Their study showed that theheterogeneity was accurately described in anomalous transport models at the pore-scale conditions by representative
elementary volume of size m­ m3. However, the applicabilityof anomalous transport models at the field-scale conditionsof size ­km3 has yet to be determined (Bolster et al. 2019).
It is highlighted that the anomalous transport models arenecessary for effective implementation of remediation policies and strategies (Bolster et al. 2019). Several properties,
reactivity, and mobility of nanoscale zero-valent particles
in porous media and their applications in the remediation ofgroundwater and further modeling tools were studied in areview by Tosco et al. (2014). A need of field-scale transportmodels for long-term simulations of nanoscale zero-valent
particles in the subsurface system was highlighted in theirstudy (Tosco et al. 2014). Essaid et al. (2015) reviewed over
500 articles on the fate and transport of organic contaminants in the subsurface and concluded that aquifer chemistry and missing processes govern the bioremediation ofdissolved contaminant plumes. The paradigm shift in theanalytical and numerical modeling from simple 1D/2D
homogeneous systems with equilibrium partitioning towards
3-D modeling of heterogeneous porous systems with ratelimited mass-transfer processes was highlighted by Essaid
et al. (2015). Their study also highlighted that the mixing of
dissolved reactants is dominated by low dispersivity values

13in the groundwater, and sorption and diffusion processes
cause retention of contaminants in the immobile regions,which require a specialized mathematical framework. A
review study emphasizing removal of heavy metal(loids)from groundwater by using reactive media within permeable reactive barrier system was conducted (Song et al.
2021). Their study highlighted the need of implementing
remediation operations via permeable reactive barrier system especially for nano-materials at pilot-scale and fieldscale conditions, as most of the studies were limited tobatch and column experiments (Song et al. 2021). Further,
Pietrzak (2021) conducted a systematic review of most commonly used software for modeling the fate and transport oforganic contaminants in groundwater system. HYDRUS-1D(Simunek et al. 2005), CXTFIT (Parker and van Genuchten
1984; Toride et al. 1995), and StanMOD (Šimůnek and Van
Genuchten 1999) were classified as inverse modeling related
software, while, HYDRUS 2D/3D (Šimůnek et al. 2006),
FEFLOW (Trefry and Muffels 2007) and MODFLOW/
MT3D family of software (Owen et al. 1996; Chiang andKinzelbach 1998; Hydrogeologic 2015; Langevin et al.
2017; Rumbaugh and Rumbaugh 2017) were classified as
direct modeling software based on the review of 140 report
documents/ scientific papers (Pietrzak 2021). It is found thatmost of the inverse modeling software are limited to onedimensional spatial domain (Pietrzak 2021).
In the critical review by Lari et al. (2019), 36 models, forexample, MT3DMS, MIN3P, COMSOL, HP1, OpenGeoSys,
and REMCHLOR, related to natural source zone depletionof light weight non-aqueous phase liquids were studied tounderstand the simulation capabilities of various models toincorporate 21 transformation and transport processes like
capillary effects, multiphase transport, and also on volatilization. Incorporation of microbial, biodegradation, andgeochemical processes under transient conditions at themacro-scale field conditions needs to be carried out (Lariet al. 2019). The simulation capabilities of reactive transport codes viz. PHT3D, OpenGeoSys (OGS), HYTEC,ORCHESTRA, eSTOMP, TOUGHREACT, PHREEQC,
HPx, HYDROGEOCHEM, CrunchFlow, MIN3P, and
PFLOTRAN were reviewed (Steefel et al. 2015). On thebasis of a number of simulation features like Richard’s equation, multiphase–multicomponent flow, multiple continua,
parallelization, and inverse optimization, PFLOTRAN and
OpenGeoSys were observed to have a large number of simulation features in comparison to other codes (Steefel et al.
2015). In another study by Poonoosamy et al. (2021), five
fluid flow and reactive transport codes, namely, OpenGeoSys-GEM, MIN3P-THCm, C­ ORE2D, PFLOTRAN, andTOUGHREACT, were tested for several benchmark problems. Overall, results from all codes were in good agreement; however, small differences in the results were attributed to the difference in the discretization schemes like

Environmental Science and Pollution Researchfinite volume versus finite element and due to differencesin the implementation of the directional dispersion tensor(Poonoosamy et al. 2021). Similarly, Šimůnek et al. (2016)
presented a review of recent advances in the HYDRUS andits several add-on modules. Their review highlighted therequirement of a special software module for effective representation of phenomena like freeze–thaw processes, colloid transport in the vadose zone, and root growth. Also, thestudy highlighted the need for coupling HYDRUS models
with surface water flow-related models to understand thevarious phenomenon in the surface water-vadose zonegroundwater systems in a cohesive manner.
A comprehensive review and assessment of soft computing models such as artificial neural network (ANN),support vector machine (SVM), decision tree (DT), fuzzy
logic (FL) models, self-organizing mapping (SOM), andrandom forest (RF) for the simulation of nitrate transport
in the groundwater system was conducted (Haghbin et al.
2021). Hasan et al. (2021) conducted a comprehensive literature review on groundwater contamination due to hazardous wastes and various aspects of CTM. A brief review ofmathematical models, including analytical and numericalmodeling approaches and software of CTM such as MODFLOW, MT3/MT3DMS, ParFlow, COMSOL Multiphysics,and HydroGeosphere, were presented (Hasan et al. 2021).
Also, a few bibliometric analysis–based studies related towater were also published, such as bibliometric analysis ofintegrated water resource assessment and modeling (IWAM)
carried out by Zare et al. (2017). Their analysis of 13,239
publications of IWAM over the last 45 years revealed theemergence of integrated publications of IWAM, such as
studies related to environment, ecology, groundwater, agriculture, and climate change. A bibliometric analysis on
the water footprint research field based on the 636 articles
published from 2006 to 2015 was conducted and found that
the Journal of Cleaner Production as top journal with 58
articles (Zhang et al. 2017b). Also, a bibliometric analysisof 2182 papers published from 2000 to 2020 on plastic pollution in the water ecosystem was conducted and USA wasobserved as the leading country, while Centre National De
La Recherche Scientifque (CNRS) was found as the leading
affiliation based on the total number of articles published(Kasavan et al. 2021). Goh and See (2021) carried out bibliometric analysis of 142 articles on benchmarking analysisof sewage and water industry, published between 2000 and2019. Their findings from research hotspot analysis suggested to increase the number of studies related to sewage
and water industries in developing countries.
Similarly, a social network and bibliometric survey
analysis of a total of 5486 groundwater remediationrelated publications from 1995 to 2015 were conducted(Zhang et al. 2017a). The bibliometric survey of scientific

literature (4968 papers from the Scopus® database) related
to emerging contaminants published from 2000 to 2019was conducted, and results revealed USA, China, andSpain as the leading countries (Ramírez-Malule et al.
2020). On the basis of authors’ keyword analysis, occurrence of pharmaceuticals and personal care products in thewastewater, and removal of these contaminants via adsorption and oxidation processes were found as hot spots inthe field of emerging contaminants (Ramírez-Malule et al.
2020). It is observed that the above-mentioned studies arelimited to a narrow, restricted single theme. Thus, there isa need to conduct a broad bibliometric survey on CTM andits related themes, which will describe the overall current
scenario and guide future directions. Therefore, this study
aims to conduct the bibliometric analysis of CTM based on
the data collected from the Scopus® database from 2010
to 2022 by considering the authors, journals, affiliations,countries, co-citation factors, and collaboration dynamics. It will help in identifying the hot topics and research
themes for model development in future studies.
The present study provides bibliometric analysis, recent
advances, and future directions of CTM research field.
Thus, it (1) considers the evolution of CTM research fieldfrom 2010 to the present time, 2022; (2) examines themost cited articles and core journals publishing articles
on CTM; (3) provides a list of leading authors, affiliations, and countries; (4) identifies top keywords, different
thematic areas of CTM, and their temporal evolution; (5)identifies the collaboration dynamics of different countries, interconnections between authors, journals, countries, affiliations, co-citation analysis, and (6) identifies
the clustering patterns based on the theme and factorial
analysis.

MethodThe overall methodology of bibliometric survey adopted
in this study is shown in Fig. 1. It includes the collection of publications from the Scopus® database of CTM.
Then after, the implemented inclusion and exclusion criteria are shown, which were used to compile literature
related to area of focus of the present study. A detailed
description of procedure is presented in the below subsections. After compiling the required literature on CTM in
the groundwater, a basic performance analysis of publications database was conducted. Further, science mapping,such as analysis of three-field plots, network visualizationof co-citation, and co-author, were carried out. In the end,
concluding remarks highlighting future directions were
proposed based on bibliometric analysis.

13

Environmental Science and Pollution Research
Fig. 1  Methodology adopted forbibliometric analysis

Data collection strategy
The present study used an extensive research design
approach to CTM literature. Initially, publications of
CTM from the Scopus® database were retrieved to givea broad perspective. Recent advances in the CTM and its

13

applications in real-field-based studies were the overall
themes of the present study. Therefore, only journal articles were retrieved so that recent advances in the mathematical framework of CTM can be determined. Publications
from 2010 through 2022 were considered for the two-tierapproach of bibliometric analysis and network visualization

Environmental Science and Pollution Researchof various parameters. The steps implemented to collect thedata of CTM are shown in Table 1.

Criteria of inclusion and exclusion
Inclusion–exclusion criteria were used to restrict the literature to the area of interest, which is CTM in the groundwater system. In the first step, papers published in English
language only were included, and the rest of the languages
were ignored. In the second step, papers published from2010 through 2022 were considered because focus of thepresent study was to point out the recent development in the
CTM over past decade. Further, only journal articles wereconsidered for bibliometric survey, ignoring review articles,
which generally highlight the pros and cons of already published mathematical models. Book chapters and conference
proceedings were not considered because most of them lackstandard international policies and a robust peer-review process. However, no other exclusion criteria were implemented
for shortlisting. The details of inclusion–exclusion criteria
are shown in Table S1 of supplementary information. After
implementing inclusion–exclusion criteria, 1955 papers
were included for the final phase of the study. One of themain aims of the present study was to make an inclusive
bibliometric analysis of 1955 documents of CTM from 2010
through 2022 and to understand the past, present, and futurescenarios of CTM in the groundwater system.

Research technique and tools used
A bibliometric survey was conducted initially, which mainly
included the identification of leading authors, highly citedarticles, and leading countries and most productive affiliations. Network visualization of co-authors, citation analysis,
keywords mapping, and cluster identification was conductedto comprehensively analyze the collected information of scientific literature on CTM. VOSviewer version 1.6.17 wasused to visualize the network of cited references based on theco-citation and cooperation network of authors (van Eck andWaltman 2010, 2021). Primarily, Biblioshiny R-toolbox wasused to identify the year-wise variation of top journals of

CTM, leading authors, leading countries, and most productive affiliations (Aria and Cuccurullo 2017). The list of top
leading authors, countries, affiliations, and highly cited articles were identified based on the total number of citations,
number of articles published, and average article citation per
year. The evolution of CTM was analyzed based on the treeplot analysis, three-phase field plots, and temporal variationof author’s keywords. The research hotspots in the CTM
were identified by generating thematic maps, and clusters
visualization of subject areas. Also, collaboration dynamics
between countries and co-author network visualization werecarried out using Biblioshiny.

Results and discussion
Analysis of publication pattern in CTM
Temporal distribution of publications about CTM
The CTM database comprises 1955 documents from 2010
to 2022 published in 365 sources. It is observed that the
retrieved papers have 5.18 average years from publication(Table S2 of supplementary information). The value of average citations per document is 14.7, whereas 2.225 average
citations per year per document is observed from the Scopus® database. A total of 93,385 references are cited in the
published articles of CTM. The collected 1955 documents
have 4728 author keywords, 5268 authors, and 7758 times
authors appeared in these papers (Table S2 of supplementary
information). It is observed that only 84 documents are written by a single author, whereas 5184 documents are multiauthored. The collected papers have a value of 2.69 authors
per document with a collaboration index of 2.79.
Figure 2 shows the growing trend of publications of CTM
along with average value of article total citations from year
2010 to 2021. In 2010, 127 articles related to CTM were
published, while 201 articles were published in 2021. A total
number of 1955 articles retrieved from Scopus® shows anincreasing trend in the number of publications with time
(Fig. 2). An increasing growth rate is observed from 2010

Table 1  Steps used for data collection from the Scopus® database
Search steps

Steps used in search

Query on Scopus®

Description

1

Search Keywords

TITLE-ABS-KEY

2
3
4

Year
Source type
Document typeAND PUBYEAR > 2009
AND (LIMIT-TO (SRCTYPE, "j"))AND (LIMIT-TO (DOCTYPE, "ar"))5

Document’s Language

AND ( LIMIT-TO (LANGUAGE, "English"))((("contaminant" OR "solute") AND transport) AND(modelling OR modeling) AND (groundwater OR
aquifer OR subsurface))
Published from 2010 to present considered
Journal ()Article (journal articles searched only, ignoring bookchapter, conference proceeding etc.)
Language “English”

13Environmental Science and Pollution Research
Fig. 2  Variation of publication
statistics of CTM with year

to 2014; then after, a constant growth rate of the numberof publications is observed from 2015 to 2018, and finally,rate increases at a steep slope from 2019 onwards. A total
number of 365 sources published 1955 articles about
CTM (Table S2 of supplementary information). Due to theincreasing trend of publication with time, it is necessary tounderstand the trends in the citation of published papers.
The average total citation (TCi) per an article in 2010 was
29.14 and further dropped to 1.01 in 2021 (Fig. 2). It isevident that the number of publications increases with time,thus causing decay in the average total citation (TCi) per
article. A similar trend was observed in the bibliometric survey study of Interpretive structural modeling (Kumar andGoel 2021). A similar decaying trend is observed for the
average total citation (TCi) per year, as average total citation(TCi) per year decreases from 2.43 in 2010 to 1.01 in 2021
(Fig. 2). Overall, a non-uniform variation of avg. TCi/year
were observed for the past decade.
Most cited published articles
The list of top 10 most influential articles is shown in
Table 2. The total number of citations (TCi) was taken as
one criterion for ranking, followed by the total number ofcitations per year. It is observed that all of top ten articles
were cited more than 110 times, and article by Šimůnek
et al. (2016) ranked in the first position with 443 TCi and73.83 TCi per year. Thus, the article titled “Recent Developments and Applications of the HYDRUS Computer
Software Packages” from “Vadose Zone Journal” wasTable 2  List of top 10 published articles in the CTM
First author of article

TCi

Rank basedon TCi

SYoA

CiY

TCi per year

Rank based on
TCi per year

NTC

ReferenceŠimůnek J
Oude Essink G.H.P
Myers T
Hyman J.D
Brantley S.L
Hammond G.E
Wang K
Detty J.M
Paniconi C
Shen C

443
207
192
181
181
150
125
120
117
117

1
2
3
4
5
6
7
8
9
10

2016
2010
2012
2015
2014
2014
2011
2010
2015
2010

6
12
10
7
8
8
11
12
7
12

73.83
17.25
19.20
25.86
22.63
18.75
11.36
10.00
16.71
9.75

1
6
4
2
3
5
8
9
7
10

27.73
7.10
8.25
9.28
9.12
7.56
5.28
4.12
5.99
4.01

Šimůnek et al. (2016)
Oude Essink et al. (2010)
Myers (2012)
Hyman et al. (2015)
Brantley et al. (2014)
Hammond et al. (2014)
Wang and Wang (2011)
Detty and McGuire 2010)
Paniconi and Putti (2015)
Shen and Phanikumar (2010)

TCi total number of citations, SYoA start year of article, CiY citable years, NTC normalized total citation

13

Environmental Science and Pollution Researchobserved as the most dominant article in the CTM research
field. It is interesting to see that the second-ranked article
titled “Effects of climate change on coastal groundwater
systems: a modelling study in the Netherlands” received
207 total citations (Oude Essink et al. 2010), which isapproximately less than half of TCi received by ranked
one article.
The highest-ranked article by Šimůnek et al. (2016) discussed the simulation capabilities of HYDRUS standard andspecialized modules, for example, implementation of a furrow irrigation scheme in HYDRUS environment, HYDRUS
package for MODFLOW, and impacts of plant water uptake
on the recharge of groundwater. Further, implementationof the freezing and thawing process, colloidal-facilitatedtransport of heavy metal, and precipitation-dissolution
were highlighted in the article (Šimůnek et al. 2016). The
second rank article based on the TCi by Oude Essink et al.
(2010) investigated the impact of a rise in seawater level on
the coastal aquifer system of the Dutch Delta. A variable
density-driven groundwater flow and solute transport modelin 3-dimensional space was used to understand the effectiveness of mitigation measures; however, the article ranked
6th based on TCi per year parameter. The research articles
by Myers (2012) and Hyman et al. (2015) ranked third andfourth on the basis of TCi, respectively. Myers (2012) discussed the MODFLOW-based approach and contaminanttransport associated risk factors from hydraulically fractured
shale to the aquifers. The article by Hyman et al. (2015)
described the details of DFNWorks (three-dimensional discrete fracture networks), which is a single/multiphase flow
and transport model for the fractured system implemented
at millimeter to kilometer scale. The article was found to
be second-ranked based on the TCi per year criterion andalso presented various applications of a developed model,such as for nuclear waste repository, C­ O2 sequestration, andhydraulic fracturing problems (Hyman et al. 2015).

The sixth-ranked article by Hammond et al. (2014)
discussed the simulation capabilities and performanceof parallel flow and contaminant transport simulator viz.
PFLOTRAN on the realistic field-scale problems such ascopper leaching from mineral deposits and uranium complexation under variably and transient groundwater flow conditions. The seventh-ranked article by Wang and Wang (2011)
presented the developed fast characteristic finite difference
method (FDM) to solve the 1-D fractional advection–diffusion equation, and also highlighted the accuracy of method
even for large mesh sizes and time steps. The eighth-ranked
article by Detty and McGuire (2010) discussed the impactsof various landform types on the shallow water table dynamics and hydrological connections. A historical overview of
50 years of research on process-based hydrological modeling
was presented in the ninth-ranked article (Paniconi and Putti2015). The focus of the article was scale effects and nonlinearity in the flow and transport phenomenon, complex
boundary conditions in the numerical model, and integrated
surface/subsurface hydrological models (Paniconi and Putti2015). Further, advancements in the numerical schemes,
parallelization of numerical codes, and various aspects ofcomputational science were also discussed (Paniconi and
Putti 2015). The article by Shen and Phanikumar (2010)is the oldest publication among the top ten articles and isobserved at tenth rank. The article discussed the mathematical framework, testing results, and preliminary applicationsof a process-based distributed hydrologic model (Shen andPhanikumar 2010).
Distribution of publications based on Journals
The top ten sources or journals based on the TCi and number of articles (NoA) with h-, g-, and m-index values areshown in Table 3. Water Resources Research ranked firstin the list based on the total number of citations (TCi) andTable 3  List of top 10 journals (sources) in the CTM
Sr. noSource (journal)

TCi

Rank basedon TCi

NoA

Rank basedon NoAh-Indexg-Index

m-Index

1
2
3
4
5
6
7
8
9
10

Water Resources Research
Journal of Hydrology
Journal of Contaminant Hydrology
Groundwater
Advances in Water Resources
Environmental Science and Technology
Hydrological Processes
Vadose Zone Journal
Hydrogeology Journal
Soil Science Society of America Journal

11,747
4772
3214
2626
2444
2391
1253
999
966
923

1
2
3
4
5
6
7
8
9
10

214
178
153
46
75
32
46
29
75
6

1
2
3
6
4
8
6
9
4
10

39
26
26
12
27
19
16
13
19
3

55
37
40
19
39
31
30
27
25
5

3.250
2.167
2.167
1.333
2.250
1.583
1.333
1.083
1.583
0.250

TCi total number of citations, NoA number of articles

13

Environmental Science and Pollution Researcha number of articles (NoA) published from 2010 through2022. Journal of Hydrology and Journal of ContaminantHydrology ranked second and third with 178 and 153 articles of CTM, respectively. Groundwater ranked in fourth
position on the basis of TCi; however, it ranked sixth position based on the NoA. On the basis of NoA, Advances
in Water Resources and Hydrogeology ranked in fourth
position with 75 articles of CTM from 2010 to present
time period. Overall, it can be stated that the top threepublishing journals of CTM received the highest numberof citations.
Temporal variation of top 10 journals of CTM on thebasis of cumulative occurrences of articles was also analyzed. Water Resources Research remained at the top
based on the cumulative occurrences of articles on CTM
for past decade (Fig. S1 of supplementary information),whereas Journal of Contaminant Hydrology remained at
the second position up to 2018 and then slipped to third
position and second position was attained by Journalof Hydrology after 2018. It is interesting to see that the
cumulative occurrences of articles of only the top threejournals crossed 150 mark in 2022, while cumulative
occurrences of bottom seven journals appeared to be lessthan 100. Furthermore, the annual growth rate of cumulative occurences of the top three journals and bottom sevenjournals observed to be varying at steep and mild slope,
respectively. It shows that the articles related to development of groundwater flow and contaminant transport models and applications of CTM are more likely to be published in the top three journals. Hydrogeology Journal andAdvances in Water Resources shared the fourth position
with 75 cumulative occurrences and then followed by Science of the Total Environment, Environmental Earth Sciences, Water, Hydrological Processes and Groundwater.
Bradford’s law is known as the most prominent model inselecting core journals and determines the spreading patternof bibliographical information across the various journals(Venable et al. 2016; Tsay and Li 2017; Lei et al. 2019).
Core journals are those small number of journals that publish the majority of the articles of a specific research field(Tsay and Li 2017). Results from Bradford’s law analysis
revealed that five journals, viz., Water Resources Research,
Journal of Hydrology, Journal of Contaminant Hydrology,
Advances in Water Resources, and Hydrogeology Journal
fall in zone 1, subsequently known as core journals among
the list of 365 journals that published articles related to the
CTM research field (Fig. S2 of supplementary information).
The list of the top 10 journals based on Bradford’s law isshown in Table S3 of supplementary information, although
365 journals are categorized in three zones. It is observedthat the top 10 journals published 47.93% (937 of 1955) of
the total articles related to the CTM research field from 2010
to present — 2022 (Table S3 of supplementary information).

13

Productive authors in the field of CTM
Several publication’s statistics related to author’s productivity and impacts of top authors were computed via Biblioshiny. The top ten authors in the CTM research field
were categorized based on the total citations (TCi) andnumber of articles (NoA). Based on the Scopus® database from 2010 through 2022, Simunek J. ranked in thefirst position based on TCi; however, he ranked in the second position based on the NoA published (Table S4 of
supplementary information). Simunek J., a Professor of
Hydrology, belongs to the Department of Environmental
Sciences, University of California Riverside, and received
total citations of 758 from 2010 through 2022 and has
an h-index of 11, g-index of 17, and m-index of 0.917.
The second-ranked author Van Genuchten M. T. (based on
the TCi), belongs to the Department of Nuclear Engineering, Federal University of Rio de Janeiro, received a totalof 542 citations and has an h-index of 6. It is observedthat the top two authors (Simunek J. and Van Genuchten
M.T.) of CTM worked together on several articles in collaboration, for example, in the development of HYDRUS1D/2D/3D software for the movement of water, heat, andmultiple solutes in the variably saturated porous media(Šimůnek and Van Genuchten 1999, 2008; Šimůnek et al.
2008), and reviewed the recent developments of HYDRUS
and its specialized add-on modules (Šimůnek et al. 2016).
Simunek J. published a total of 17 articles after 2010,
while Fiori A. published a total of 19 articles on CTM
and ranked first based on the NoA criteria. It is observedthat the top three authors received citations higher than
500 (Table S4 of supplementary information). Overall, all
the top ten authors of CTM received total citations higher
than 300 from 2010 through 2022.
Leading affiliations in the field of CTM
There are 1970 affiliations involved in the CTM research
field. The ranking of top 20 leading affiliations who frequently published in the CTM research field from 2010
through 2022 is shown in Fig. S3. Flinders University
ranked first based on the number of articles (58), followed by Pacific Northwest National Laboratory with 45
articles, and the University of California ranked in third
place with 44 articles from 2010 through 2022 (Fig. S3 of
supplementary information). Overall, it is observed thatthe four affiliations in the top 10 list belong to the USA,
while 3 out of top 10 affiliations belong to China, andthe rest three affiliations belong to Australia, Canada,
and Germany. It is observed that all the top 20 affiliations have published at least 15 articles on CTM from2010 to 2022.

Environmental Science and Pollution Research

Leading countries in the field of CTM
The list of top 10 leading countries involved in the CTM
research field based on TCi, NoA published, and averagearticle citation (AACi) is shown in Table S5 of supplementary information. The most dominant country in the CTM
research field is the USA, with 9714 citations, 1473 articles, and ranked at first position based on the TCi and NoA
criterion (Table S5; Fig. 3a). However, the USA ranked
second based on the average article citation. Germany,with 2225 citations, 353 articles, and 18.70 AACi, ranked
in the second, third, and third positions based on the TCi,
NoA, and AACi criteria, respectively (Fig. 3). Canada, with1950 total citations, ranked in third place; however, on
the basis of NoA, it ranked fourth position in the list. It
is observed that all of the top 10 leading countries havereceived at least 750 citations. Further, it is seen that
the USA published three times as many articles as thesecond-ranked country, China, depicting the dominance of
the USA in the CTM research field for the past 11 years.
The Netherlands, with 764 TCi and 93 NoA, was rated
tenth on the list, but based on the average article citation
value of 29.38, it was ranked first (Fig. 3c).
The list of most influential 20 countries based on the statusof corresponding author in the CTM is shown in Fig. S4. It isobserved that the USA dominated the list based on literaturefrom 2010 to the present. The USA is the only country thathas published more than 400 papers, followed by China (184),
Germany (119), Canada (106), and India (102), as shown in
Table S6. It is interesting to see that the USA published alarge number of papers based on SCP criteria (Fig. S4 of supplementary information); however, based on the MCP ratio,
Australia and Iran appeared in the top two positions (Table S6and Fig. S4 of supplementary information). It indicates that
Australia and Iran have a strong willingness to cooperate indeveloping models in the CTM search field.

Fig. 3  a Country-specific articles production in the CTM, b top cited country, and c average article citation of top-cited country

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Environmental Science and Pollution Research

Co‑occurrence and content analysis
Co‑occurrence analysis of keywords of CTM
The co-occurrence network of author’s keywords in CTM
research field, considering 20 nodes, is shown in Fig. 4.
A circle was used as a networking layout, while InfoMap
was used as a clustering algorithm, and an association was
kept as a normalization method. The strength of the nodeis proportional to the size of node, and thickness of connection line between any two nodes represents the connection strength. The thicker the line between two nodes, thestronger the connection between them. It is observed thatthe “solute transport” dominates the author’s keywords
list, followed by “groundwater,” “contaminant transport,”“numerical modeling,” and “modeling.” The “solutetransport” node (shown in red) has a strong link with the“preferential flow” node, whereas the “groundwater” node
has a significant connection with the “nitrate” node. Also,
small-sized nodes like “heterogeneity” and “groundwaterflow” have a strong link to the “solute transport” node. In
addition, nodes such as “dispersion,” “sorption,” “seawater

intrusion,” “modflow,” and “reactive transport modeling”
occurred in the network, indicating the prominence of such
authors’ keywords in the CTM research field.
The keywords in the published article represent thehigh-level summary of whole text of the article andreflect the core theme of a particular study (Zare et al.
2017; Liu et al. 2021). In this study, a 25-node wordtree diagram is constructed using “Author’s Keywords”
search criteria as shown in Fig. 5. In the tree plot of top
25 authors’ keywords, “solute transport” and “groundwater” appeared in the top 2 positions based on their frequency, as shown in Fig. 5. It is observed that the rangeof words in the tree plot of author’s keywords is wide,
whereas similar types of words appeared in the tree plotwhen search field criterion was “Keywords Plus.” Words
such as “vadose zone” and “heterogeneity” appeared in
the top 10 list of frequent words, depicting their significance in the CTM research field. The emergence ofwords like “inverse modeling,” “modflow,” “preferential
flow,” “Richards equation,” and “reactive transport modeling” shows their increasing importance in the CTM
search field.

Fig. 4  Co-occurrence network of top 20 most influential author’s keywords as per Scopus.® database from 2010 to 2022

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Environmental Science and Pollution Research

Fig. 5  Tree plot of top 25 words based on the author’s keywords search criteria

The temporal variation of top 10 authors’ keywords of
CTM observed based on the cumulative occurrences isshown in Fig. S5 of supplementary information. Fig. S5clearly depicts the evolution of important keywords from2010 to 2022. It is observed that the authors’ keywords“solute transport,” “groundwater,” and “contaminant transport” remained in the top three positions on the basis ofoccurrences for past decade. The top four authors’ keywords
emerged at a high growth rate in 2014, while the growthrate of occurrence of bottom-six keywords were found tobe mild. “Numerical modeling” and “modeling” authors’
keywords were observed in fourth and fifth place, respectively (Fig. S5), depicting the increased importance of keywords related to mathematical modeling for past decade.
The increasing trend of keywords like “heterogeneity” and“vadose zone” shows the significance of important physicsrelated keywords in the CTM field, and we can assume their
growth in the future.
Three field plots on CTM
Three-field plots, also known as Sankey diagrams, are used
to display the interconnections between different attributesof the research database. Sankey diagrams describe theweighted features and help in visualizing the different systems and their conversations (Riehmann et al. 2005; Aria
and Cuccurullo 2017). The sum of influx weights at eachnode is same as the sum of outgoing fluxes weights. In thisstudy, a three-field plot is obtained via Biblioshiny R-programming package, which is utilized to visually describe therelationship between journals, leading authors, countries,
affiliations, author-keywords, and other search fields attributes. The essential elements are presented by rectangulardiagrams with different colors. The number of communications between multiple components of three-field plots
directly depends upon the height of the rectangle (Aria andCuccurullo 2017).
Figure 6 shows the Sankey diagram of CTM literature
representing the association between leading author (left),country (middle), and affiliation (right). The analysis from
Fig. 6 indicated that the authors, namely, Cirpka OA., Zhang
Y., Zheng C., Simunek J., and Prommer H., had the highest relationship with leading countries and affiliations. The
relationship of the top 15 journals with leading countries
and leading affiliations from 2010 to 2022 shows that theUSA, China, and Germany were dominating countries withhighest connections with other two components (Fig. S6).
It is observed that most frequently, papers on CTM were
published in the Water Resources Research, Journal ofContaminant Hydrology, and Journal of Hydrology, whileauthors associated with Flinders University, Pacific Northwest National laboratory (PNNL), and University of California System. Fig. S7 shows the relationship of the top 10
productive authors with leading top 10 countries and top
5 journals publishing papers of CTM. It is observed thatthe top 10 productive authors published articles mostly inthe journals with highest to lowest priority order as Water
Resources Research, Journal of Contaminant Hydrology,
Hydrogeology Journal, Journal of Hydrology, and Advances
in Water Resources.

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Environmental Science and Pollution Research
Fig. 6  Three-field plot of theauthor (left), country (middle),
and leading affiliation (right)

Network analysis
Co‑author network analysis
Figure 7 shows the co-citation network of leading authors of
CTM. The size of the bubble represents the number of publications, while the thickness of the connecting line between two
bubbles shows the linking strength. A co-citation network was
generated considering “Authors” search field, Fruchterman
and Reingold network layout, Louvain clustering algorithm,
50 nodes, and 0.10 as repulsion force. The total number of
4 clusters were identified based on the CTM Scopus® database (Fig. 7). All four clusters were interconnected with eachother. However, strong intra-connections within the cluster
were observed. Leading authors of all the clusters belong to adifferent type of modeling approach. For example, cluster 1,
as shown by red-colored circles, is found to be dominated by
Li, Liu, Wang, Zhang, Chen and worked on the developmentof analytical/semi-analytical solutions for various contaminanttransport models. Blue colored cluster 2, led by Zheng worked
largely on the development of MODFLOW/MT3DMS and its
related modules (McDonald and Harbaugh 1988; Zheng andWang 1999). One of the nodes named Doherty from cluster 2
worked on uncertainty and sensitivity analysis and developed
PEST (Doherty and Hunt 2010; White et al. 2020). Cluster
3, as shown by green colored circles led by van Genuchten,emphasized flow and contaminant transport processes in thevariably saturated porous media and developed HYDRUS
and its related add-on packages (Šimůnek and Van Genuchten
1999, 2008; Šimůnek et al. 2016). Cluster 4 is found to be led
by Bear, Gelhar, and Neuman and included authors who had
developed contaminant transport models. For example, Brusseau, who had developed a multi-processes non-equilibrium
model, is found in cluster 4 (Brusseau et al. 1989). Similarly,

13

Dentz, who had developed several contaminant transport models and implemented OpenFOAM framework, is observedin cluster 4 (Dentz and Berkowitz 2003; Dentz et al. 2015;
Municchi et al. 2021).
Countries network analysis
Figure 8 shows the cooperation network among authors fromdifferent countries. The size of the circle represents the number of collaborations and cooperative countries/regions withreference to a specific country, while line thickness represents the closeness with cooperating countries (Xiao et al.
2022). The authors from the United States have the strongest
cooperation network, as they have collaborated with nearly
every top country in the cooperation network (Fig. 8). On thebasis of size of circle, USA is observed at the top of the list,
followed by China, Canada, Australia, France, India, United
Kingdom, and Italy. Further, it is observed that authors fromUSA and China have strongest connection, as evident fromthe thickness of a line connecting both the countries. The
top five connections based on the closeness between two
countries are observed as USA-China, USA-Canada, USAAustralia, USA-Italy, and USA-United Kingdom. The closeness of the network shows the willingness of the authors
from these countries to cooperate for the development of
CTM field.

Identification of research hot spots using mapping
analysisAnalysis of thematic maps based on keyword mapping
Figure 9 shows the thematic map of authors keywords usedin the CTM Scopus® database. A thematic map is divided

Environmental Science and Pollution Research
Fig. 7  Co-citation network
visualization of authors

into four themes named motor, niche, basic, and emerging
or declining. These themes are generated based on density(development degree) and centrality (relevance degree).
Density represents the strength of internal connections
between all the keywords of research theme, while centrality represents the strength of external connections to otherthemes via utilization of author’s keyword field (Xiao et al.
2022). The motor theme is well developed and vital for the
research field and characterized by a high value of centralityand density. Niche themes are highly developed, having highdensity but very limited connections with other themes withlow centrality. Emerging or declining themes have a low
density as well as low centrality, which shows the marginal
development of the theme. Basic themes are highly connected to other themes with high centrality and are important
for the research field and cover the general topics which canbe beneficial for its overall growth. In the present study,
seven clusters were generated via Biblioshiny with serial
numbers and labeled as solute transport (1), groundwater (2),transport (3), vadose zone (4), groundwater modeling (5),reactive transport (6), and hydrogeology (7). Cluster 1 wasobserved in the basic theme and includes “solute transport,”“numerical modeling,” and “heterogeneity” as most frequentwords, indicating high relevancy to the CTM. Clusters 2
and 5 (as shown in Table S7) include “groundwater,” “contaminant transport,” “modeling,” and “groundwater modeling,” “seawater intrusion,” and “modflow,” respectively,
and fall in the motor themes, indicating as important andwell-developed clusters in CTM. Cluster seven, labeled ashydrogeology, includes “hydrogeology,” “parameter estimation,” “sensitivity analysis,” “geostatistics,” and “dataassimilation” as the most frequent keywords observed in
the emerging theme of CTM. It can be inferred that there
is much more scope for research development in the area ofcluster seven, such as sensitivity and uncertainty analysis ofcontaminant transport model.
Thematic evolution has been analyzed in past studies
using the Sankey diagram, which depicted the evolution
and connection of different themes over a timescale effectively (Aria et al. 2020; Xiao et al. 2022). The theme is13

Environmental Science and Pollution Research
Fig. 8  Research cooperation
network among authors fromdifferent countries obtained
from VOSviewer

Fig. 9  Thematic map showing different clusters based on
author’s keywords

represented by a rectangular box, and the size of the box
denotes how frequently the theme occurred in the databaseof a research field. The connecting line of box between different time levels represents the evolution trace of theme,
and linkage strength of two themes directly depends
upon the thickness of connecting line (Riehmann et al.
2005; Xiao et al. 2022). Thematic evolution maps weredeveloped based on the author’s keywords and keywords
plus searching criteria between two-time levels from2010–2016 to 2017–2022. Figure 10 shows thematic

13

evolution of author’s keywords in the CTM. It is observedthat the “groundwater,” “preferential flow,” “seawater
intrusion,” “Richards equation,” and “solute transport”
keywords dominated the database of CTM from 2010 to2016. However, in recent years, “solute transport” and“porous media” author’s keywords dominated the CTM
from 2017 to 2022. It is interesting to see the additionof new keywords such as “inverse modeling,” “reactive
transport modeling,” and “uncertainty quantification” from2017 to 2022, depicting the paradigm shift in the CTM.

Environmental Science and Pollution Research

Fig. 10  Thematic evolution as per the search field author’s keywords

Collaboration dynamics
The collaboration network of countries represents the frequency of collaborations between countries and regions. The
number and proportion of collaborations are represented byshade and line thickness, respectively. Fig. S8 shows thecollaboration network of CTM, assuming at least two collaborations. It is observed from Fig. S8 that USA dominates
the network. The network between USA and China, with92 collaborations, is observed to be the strongest among all
networks in the CTM research field (Fig. S8 and Table S8).
The network lines from USA to China, USA to Canada,
USA to Germany, and USA to Australia are highly thick ascompared to other network lines, depicting the closeness ofcollaborative network between these countries/regions. Outof top 10 most collaborations between countries, USA collaborated six times with other countries (Table S8), showing
the dominance of USA and their willingness to cooperate in
the field of CTM.
Clustering of subject areas
Clustering of subject areas using a conceptual structure map
can help to identify the specific clusters. A factorial analysis
was conducted using all the three available methods in Biblioshiny, i.e., correspondence analysis (CA), multiple correspondence analysis (MCA), and multidimensional scaling(MDS) method. For the sake of brevity, results from MCA
method are presented here. In the MCA method, conceptual
structure mapping of CTM was done assuming 5 clusters,
50 terms, and a “Keywords Plus” search field (Fig. 11). The
detailed information on clusters is described below:• Cluster 1 ( ) consists of twelve keywords: “soils,”

•

•
•

•

“tracer,” “solute transport,” “computer simulation,”“advection,” “groundwater,” “reactive transport,” “flowmodeling,” “numerical model,” “vadose zone,” “porous
medium,” and “porous materials.”
Cluster 2 ( ) consists of eleven keywords: “water quality,” “hydraulic conductivity,” “groundwater pollution,”“pollutant transport,” “hydrological modeling,” “contamination,” “concentration composition,” “contaminant transport,” “groundwater flow,” “United States,”and “numerical models.”
Cluster 3 ( ) includes “aquifer,” “groundwater
resources,” “aquifers,” and “hydrogeology” keywords.
Cluster 4 ( ) contains eight keywords namely “controlled
study,” “pollutant transport,” “article,” “water pollutants,” “ground water,” “chemical,” “theoretical,” and“priority journal.”
Cluster 5 ( ) contains four keywords namely “water
flow,” “porosity,” “models,” and “simulation.”

Figure 12 shows the topic dendrogram of highly frequent keywords obtained via multiple correspondence
analysis (MCA) method in the Biblioshiny environment.
A topic dendrogram was obtained based on the “Keywords
Plus” search criteria in the CTM research field. A totalof five clusters were obtained, in which clusters 1 and2 involved the most keywords (12 and 11 keywords inclusters 1 and 2, respectively), followed by cluster 4 (8keywords) and cluster 3 and 5. It is observed that cluster
1 paid attention to the research in solute transport and its
governing factors and mechanisms, while cluster 2 dealt
with the broad perspective of groundwater pollution. Also,
cluster 4 was found to be emphasizing controlled study

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Environmental Science and Pollution Research
Fig. 11  Conceptual structure
map based on the MCA method

Fig. 12  Topic dendrogram obtained as per MCA method

and specific sub-areas related to CTM. Cluster 3 is foundto be focusing on hydrogeology and aquifer conditions,while cluster 5 emphasizes the water flow theme. Overall,
hotspots of CTM research field were revealed by criticalanalysis of author’s keywords via generation of thematicmaps, temporal evolution of themes, and conceptual structure mapping.

13

Conclusion and future directions
The present study outlined the current state of the CTM
research field via bibliometric survey and visualization of
networks to get a comprehensive understanding of past andpresent developments of CTM. A total of 1955 articles ofEnvironmental Science and Pollution Research

CTM from the Scopus® database, published from 2010
through 2022, were analyzed using the VOSviewer andBiblioshiny software. The major findings of the study aresummarized as follows:• Water Resources Research, Journal of Hydrology, and

•

•
•

•

•

•

Journal of Contaminant Hydrology were the top threejournals based on the number of published CTM articles
and received the highest total number of citations overthe past 11 years.
Flinders University, Pacific Northwest National Laboratory, and the University of California published the largest number of CTM articles and were leading affiliationsin the CTM research field.
USA published 489 articles on CTM and emerged as theleading country on the basis of mutual collaborations andcooperation networks.
A strong connection between highest dominating author’s
keyword, i.e., “solute transport” with “preferential flow,”“heterogeneity,” and “groundwater flow” was revealed
from co-occurrence network of author’s keywords. It
depicted the significance of process-dominant keywords
in the CTM research field.
The co-citation network of leading authors of CTM
showed the formation of four clusters. Authors in cluster
one worked majorly in the development of analytical/
semi-analytical solutions of flow and contaminant transport equations, while cluster two worked majorly towards
development of the MODFLOW family of software andrelated modules. Researchers from cluster three worked
in the variably saturated porous media research area,
whereas the leading author from cluster four developed
new flow and contaminant transport models.
Keywords clusters analysis categorized the author’s
keywords into seven clusters, in which cluster seven,labeled as “Hydrogeology” ranked at the first position
and falls under emerging theme of CTM. “Hydrogeology,” “Parameter Estimation,” “Sensitivity Analysis,”“Geostatistics,” and “Data Assimilation” were observedin cluster seven, indicating their emergence in the CTM
research field.
New keywords namely “inverse modeling” and “uncertainty quantification” were added from 2017 to 2022,
depicting the paradigm shift in the CTM research field.

The study was restricted to the Scopus® database from2010 onwards and might have missed good papers fromother literature databases before this period. Also, the study
was limited majorly to groundwater systems. In the future,
critical review and bibliometric analysis of model development of flow and contaminant transport involving groundwater-surface interaction and physics-informed machine learning can be considered to understand the broad perspectiveof CTM research field. The findings of a bibliometric andvisual analysis show sensitivity and uncertainty analysis,
geostatistics, and data assimilation as emerging sub-areasof CTM research field, which will motivate researchers andmodel developers to work in these areas. The outcomes ofthe study could help researchers to quickly locate the hotspot
areas of CTM research field.
Supplementary Information The online version contains supplementary material available at https://d​ oi.o​ rg/1​ 0.1​ 007/s​ 11356-0​ 22-2​ 4370-1.
Author contribution Abhay Guleria: conceptualization, methodology, data curation, formal analysis, software, writing — original draft,writing — review and editing. Sumedha Chakma: conceptualization,
supervision, writing — review and editing. All the authors read andapproved the final version of manuscript.
Data availability All data generated or analyzed during this study areincluded in this article and its supplementary files. The detailed excel
sheet files used are available from the corresponding author on reasonable request.

Declarations
Ethics approval and consent to participate Not applicable.
Consent for publication Not applicable.
Competing interests The authors declare no competing interests.

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