A bibliometric analysis on microplastic pollution in groundwater

Publications on microplastics have most widely focused on identifying their presence in the ocean or surface water and have ignored their existence in groundwater systems. The aim of this study was to identify publications and citation trends of microplastics in groundwater. VOS viewer software is done for analyzing the data, and maps are used for showing relationships between countries, journals, authors, and keywords related to the distribution of microplastics in groundwater. This search yielded 695 documents published between the year 2011 to 2022. The most popped-up keyword was “microplastics” (302 occurrences with total link strength of 340) which had strong links to “transport”, “adsorption” and “drinking water”, while the word “groundwater” appeared only 13 times and total link strength was 16. It shows that research on microplastics in groundwater is still limited. The journal Science of the Total Environment publishes the highest number. China is the country with the most publications and the most cited citations. Microplastics research in groundwater development not only analyzes the abundance and characteristics of microplastics, but also the transport processes of microplastics from soil to groundwater, contribution of groundwater to microplastic contamination in drinking water, potential ecological risks, and their impact on human health.


Introduction
Microplastics (MP) are an emerging contaminant of global concern, having been detected in both aquatic and terrestrial ecosystems.It is estimated that around 12 billion metric tons (Mt) of plastic waste will be disposed of in landfills or in the environment by 2050 [1].Systematic results of the literature show that there is still a large gap between freshwater and marine microplastic research.Studies on MP in freshwater are mostly conducted in developed countries such as the United States and Germany, while fewer studies have been conducted in developing countries facing serious plastic pollution.Most microplastic pollution research focuses on rivers and lakes, but other freshwater sources, such as groundwater and reservoirs, have received less attention [2].
Publications on microplastics have mostly focused on identifying their presence in marine or surface waters and have neglected their presence in groundwater systems.The first study to analyze microplastics in groundwater samples from alluvial sediment aquifers, found that the average number of microplastics detected across all sites was 38 ± 8 microplastics/L, ranging from 16 to 97 particles/L polyethylene (PE) and polyvinyl chloride (PVC) in total contributed 59% of the total number of microplastics detected [3].Research in the Jiaodong peninsula of China showed that only a few fibers detected in groundwater samples may have originated from the external environment [4].
Bibliometric analysis is widely used to identify, examine and highlight emerging global research themes and scientific studies in the literature.Quantitative and qualitative evidence from bibliometrics has been used to conduct in-depth reviews of microplastics research progress and trends globally [5].Some of the studies that have been published based on the presence of MPs in the environment include the presence of plastic particles in the environment from nano plastics to microplastics [6], MP and nano plastics in food webs [7], MP as an atmospheric pollutant [8], MP in the marine environment [9,10], interactions between microplastics and microorganisms in the soil environment [11] as well as the impact of physicochemical properties of MPs on human health [12].To the best of the researchers' knowledge, no one has studied MP in groundwater.The purpose of this study is the research trend of microplastics in groundwater.
Data retrieval was carried out on May 5, 2023, and obtained articles as many as 932 documents published from 2011 to 2023.Furthermore, articles processed with VOS Viewer were limited from 2011-2022, and 695 articles were obtained.VOS Viewer software visualizes similarities between different objects [13].Document information that met the requirements including year of publication, language (English), journal, title, author, affiliation, keywords, document type (article), abstract and number of citations were exported into CSV format.VOS viewer (version 1.6.18)was used to analyze Coauthorship, Co-occurrence, and Citation.Furthermore, a review of articles with the keywords microplastics and groundwater simultaneously in the article was conducted.

Publication Outputs
The total number of publications obtained was 695 articles from 2011 to 2022. Figure 1 shows the annual growth trend where the publication increase occurred in 2019 (80%).The number of articles grew slowly with no more than 10 per year before 2019.This study examines trends in microplastic research in groundwater.Based on the search that has been done, there are no articles in this database from 2011-2018 that discuss in detail about microplastic research in groundwater.The article written in 2011 investigated the physical properties of beaches contaminated with plastic fragments and has been cited 259 times [14].Harmonized monitoring of microplastics as one of the emerging pollutants (EPs) in surface water and groundwater has not been achieved and is urgently needed [15].The discovery of microplastics in bottled water [16] The discovery of microplastics in bottled water [16] and its impact on health encourages research on the presence of microplastics in drinking water sourced from groundwater [17][18][19].Viviana Re (2019) studied the potential contamination of groundwater by microfiber plastic.

Top Cited Articles and Most Productive Authors
Analysis of authorship networks with author analysis units is used to determine author productivity and the number of articles cited.The minimum number of documents for each author is 5 documents, so it is known that out of 2,720 eligible authors there are 104 people.Zhang Y is the most productive author with the highest number of citations (Figure 2).Not all authors who have many documents have a large number of citations.Wang J with 17 documents is more than Wang H with 16 documents, but Wang H has more citations (504) than Wang J (400).The top 10 productive authors are all from China.

Figure 2. Performance graph of the top ten most productive author by the number of documents
The author with the highest number of citations is Geissen V. et al (2015), this article is widely cited possibly because it discusses microplastics as one of the emerging pollutants (EPs) providing many challenges to be addressed including chemical analysis and screening techniques, properties and behavior, multi compartment monitoring, risk assessment tools (human health and ecological risk assessment), tools modeling the fate of microplastic, management and policies.Article written by Mintenig et al. (2019) analyzed ground water and drinking water for the presence of microplastics (> 20 μm) using FTIR imaging has been cited 452 times.

Contribution by Countries
695 documents were published by 73 countries.Analysis with a minimum number of documents in one country of 5 documents, there are 37 countries that meet.China is the country with the largest publication percentage of 54.6% (349 documents out of 695 articles).When viewed from the number of citations, a large number of documents does not necessarily have a lot of citations (Figure 3), India

Co-occurance Analysis of Author Keywords
In this section, the overlay visualization is presented, the minimum number of occurrences of keywords is 5, of the 1978 keywords, 83 meet the threshold.The 83 keywords are divided into 11 clusters.Keywords microplastics have occurrences of 302 with a total link strength of 340, while keywords groundwater have occurrences of 13 with a total link strength of 16.This shows that the study of microplastics in groundwater is still limited.The bigger the size of the circle is, the greater the frequency of the keyword.

Figure 5. Overlay visualization map co-occurance keywords for microplastic in groundwater research
Colors refer to different clusters or related topics.The yellower the color of the node in the figure is, the newer the keyword.Some of the keywords that link microplastic and groundwater are shown in Figure 6.Keyword analysis was used to identify research topics and their interrelationships.Microplastics are investigated in various environmental media, but studies in groundwater environments are limited.The presence of microplastics in groundwater is difficult to understand, study and manage, due to their invisibility [20].
A study of MP contamination of groundwater in a Karst area in Illinois USA, found fiber-type microplastics with a concentration of 0.86 -15.2 particles/L, with a size <1.5 mm, type polyethylene, with a color of 65% is blue.It is suspected that these MPs migrated from the surface to the subsurface through sinkholes (focused drainage features), rapidly entering fractures and fissures of the underlying aquifer [17].Polypropylene (PP), polystyrene (PS), PE, and PVC microplastics were detected in all groundwater samples in alluvial sediment aquifers sealed from the atmosphere.The mean size of microplastics was 89 ± 55 m (St.Dev.), ranging from 18 hingga 491 m.The mean number of microplastics detected at all sites was 38 ± 8 microplastics/L, ranging from 16 to 97 particles/L [3].

Figure 6. Network between groundwater keywords and microplastics
The study of microplastic research in groundwater examines not only the source, abundance, and characteristics of microplastics in groundwater but also research on the process of transportation and migration of microplastics in groundwater [22][23][24].Studies on the fate and transport of nano plastics in complex natural aquifer media by batch experiments using column transport found that heteroaggregation of nano plastics with groundwater OM, assisted by Ca 2+ , is a critical process governing particle fate and mobility.The OM layer generated on the nanoplastic surface can take over the role of particle size and surface groups in controlling the nanoplastic process [22].Studies on policy and remediation of microplastics in groundwater have also been conducted, including policy recommendations to limit the amount of plastic waste to reduce microplastic pollution [25].

Conclucion
Bibliometric analysis was applied to highlight the research trend of microplastics in groundwater.The results show that research on microplastics in groundwater has increased in 2019 (80%).The most numerous publications and has the largest link strength is China.The keyword groundwater has 13 occurrences with a total link strength of 16, so the study of microplastic research in groundwater is still broad to be improved.Topics studied include the transfer of microplastics from soil to underground, the contribution of groundwater to microplastic pollution of drinking water, interactions with other contaminants, and what still needs to be studied further is the impact on human health and the environmental remediation process.

Figure 3 .
Li Y. Wang Z. Wang J. Zhang X.Wang H. Wang Y. Wang X. Liu Y. Chen Y Documents Citations has 54 more documents than Australia 38 documents, but the number of citations Australia 85 is better than India only 57.The top 10 most contributions and citations of articles by country.(a) The number of articles by country published.(b) Total citations according to the country 3.4 Productivity in Scientific Journals A total of 116 journals were analyzed based on the largest number of articles discussing microplastics in groundwater, resulteded in 5 journals as shown in Figure 4.The top five journals contributed 49.1% of the total articles and the journal Science of the Total Environment contributed the most articles 118 articles (17%).The highest impact factor is in the Water Research journal at 3,34.

Figure 4 .
Figure 4.The top 5 most document per year by source