Review of Current Issues and Management Strategies of Microplastics in Groundwater Environments

: Microplastic contamination has become widespread in natural ecosystems around the globe as a result of the tremendous rise in plastic production over the last 70 years. However, microplastic pollution in marine and riverine habitats has received more attention than that of terrestrial environments or even groundwater. This manuscript reviews the current issues, potential occurrences, and sources of the emerging problem of microplastic contamination in groundwater systems. The most prevalent types of plastic detected in groundwater are polyethylene and polyethylene terephthalate, and ﬁbers and fragments represent the most commonly found shapes. The vertical transportation of microplastics in agricultural soils can affect groundwater aquifer systems, which is detrimental to those who use groundwater for drinking as well as to microorganisms present in the aquifers. Moreover, this review sheds light on the interlinkage between sustainable development goals and groundwater microplastic contamination issues as part of the strategies for the management of microplastic contamination in groundwater. Overall, this review reveals a lack of interest and a gap in knowledge regarding groundwater microplastic pollution and highlights future perspectives for research in this area.


Introduction
Groundwater is used for various purposes by almost two billion people worldwide, such as for drinking and agricultural, residential, and industrial functions [1]. According to recent reports, groundwater is contaminated by microplastics [1][2][3][4][5]. Plastic products, which have played a vital role in global society since the 20th century due to their unique properties, represent the source of microplastics. Plastic can conveniently be shaped into any design by applying temperature or pressure and these products are immensely useful and ubiquitous in daily life, such as for packing food and other materials. Plastic production has increased from 5 Mt in 1950 to 367 Mt in 2020 [6] (Figure 1). The materials employed to build utensils and other fundamental items are frequently used to depict human history and advancement [7]. Mankind has progressed through the rock, bronze, iron, and copper ages and is now widely considered to be in the digital age. However, an alternative perspective is that mankind is currently in the plastic age [8] owing to the pervasiveness of plastic in human life.
When plastic waste is disposed of in the environment, it degrades into smaller sizes [9][10][11][12]. Plastic pieces with diameters of <5 mm are referred to as microplastics [13,14] and can be divided into primary and secondary microplastics ( Figure 2). Primary microplastics are released directly into natural environments and originate mostly from body and skin care products, industrial wastes, and textile applications [12,[15][16][17]. Secondary microplastics originate from the fragmentation of larger plastic particles into smaller particles that are degraded in the environment due to ultraviolet exposure from the sun as well as from chemical, physical (such as washing machines), and mechanical weathering (such In 2015, approximately 6300 Mt of plastic waste was produced. Of this waste, 9% was recycled, 12% was incinerated, and 79% was disposed of in landfills or natural environments [23,24]. In addition to natural environments, plastic waste has been found in sea salt, table salt [25], and beer [26]. In recent years, there has been increasing research on the presence of microplastic contaminants in marine, river, and lake environments [27][28][29][30][31][32][33][34]; however, little attention has been paid to microplastic contamination of groundwater on a global scale. This could be because groundwater microplastic contamination is still in its early stages [35]. However, it presents an emerging concern [36].  In 2015, approximately 6300 Mt of plastic waste was produced. Of this waste, 9% was recycled, 12% was incinerated, and 79% was disposed of in landfills or natural environments [23,24]. In addition to natural environments, plastic waste has been found in sea salt, table salt [25], and beer [26]. In recent years, there has been increasing research on the presence of microplastic contaminants in marine, river, and lake environments [27][28][29][30][31][32][33][34]; however, little attention has been paid to microplastic contamination of groundwater on a global scale. This could be because groundwater microplastic contamination is still in its early stages [35]. However, it presents an emerging concern [36]. as tidal waves) [18,19]. Most microplastics in the environment are secondary microplastics and they, together with primary microplastics, pose a threat to the environment [4,[20][21][22]. In 2015, approximately 6300 Mt of plastic waste was produced. Of this waste, 9% was recycled, 12% was incinerated, and 79% was disposed of in landfills or natural environments [23,24]. In addition to natural environments, plastic waste has been found in sea salt, table salt [25], and beer [26]. In recent years, there has been increasing research on the presence of microplastic contaminants in marine, river, and lake environments [27][28][29][30][31][32][33][34]; however, little attention has been paid to microplastic contamination of groundwater on a global scale. This could be because groundwater microplastic contamination is still in its early stages [35]. However, it presents an emerging concern [36].  Groundwater microplastic pollution has been less studied than that in other natural environments such as marine, river, aquatic, and soil ( Figure 3). Soil can act as a barrier to groundwater microplastic pollution, which could explain why researchers have not focused on groundwater microplastic pollution [5]. However, soil microplastic pollution has increased in recent years [38][39][40][41][42][43][44] and soil is the most likely route for microplastics to enter the groundwater system, meaning that the rise in microplastic pollution in soil in recent years is a point of concern. Previous studies have identified the vertical transportation of microplastics from soil to groundwater systems [45][46][47][48][49][50] which can lead to significant consequences when groundwater is used for drinking or agricultural purposes.
cused on groundwater microplastic pollution [5]. However, soil microplastic pollution has increased in recent years [38][39][40][41][42][43][44] and soil is the most likely route for microplastics to enter the groundwater system, meaning that the rise in microplastic pollution in soil in recent years is a point of concern. Previous studies have identified the vertical transportation of microplastics from soil to groundwater systems [45][46][47][48][49][50] which can lead to significant consequences when groundwater is used for drinking or agricultural purposes.
Although groundwater pollution can affect human health [51,52], plant species, and underground microorganisms, there are considerably fewer studies on microplastics in groundwater than in soils. Microplastics in groundwater should not be underestimated. They necessitate urgent attention from the scientific community, especially hydrogeology and environmental impact studies, to decrease their negative impact and to estimate their potential threats to the environment and human society. The purpose of this review is to (1) highlight the gaps and challenges in the current literature on microplastic pollution and sources in groundwater, and (2) describe and discuss strategies for the management of microplastic contamination in groundwater systems in the future.

Current Issues in Microplastic Pollution Occurrences and Sources in Groundwater
The horizontal and vertical transportation of microplastics from soil migration, surface runoff from mulching waste, and industrialization and urbanization can lead to the contamination of groundwater systems with microplastics. Groundwater can also be con-  Although groundwater pollution can affect human health [51,52], plant species, and underground microorganisms, there are considerably fewer studies on microplastics in groundwater than in soils. Microplastics in groundwater should not be underestimated. They necessitate urgent attention from the scientific community, especially hydrogeology and environmental impact studies, to decrease their negative impact and to estimate their potential threats to the environment and human society. The purpose of this review is to (1) highlight the gaps and challenges in the current literature on microplastic pollution and sources in groundwater, and (2) describe and discuss strategies for the management of microplastic contamination in groundwater systems in the future.

Current Issues in Microplastic Pollution Occurrences and Sources in Groundwater
The horizontal and vertical transportation of microplastics from soil migration, surface runoff from mulching waste, and industrialization and urbanization can lead to the contamination of groundwater systems with microplastics. Groundwater can also be contaminated with several toxic materials and contaminants from anthropogenic activities [53], thereby placing groundwater resources at risk [54,55]. Notably, groundwater is used in approximately 38% of agricultural and cultivated areas globally [56]. The invisible nature of groundwater makes it difficult to observe and maintain [1]. Groundwater samples from Chennai, India, were reported to contain fibrous and fragment-shaped microplastics [57]. When synthetic microfibers are too small to be filtered by wastewater treatment plants (WWTPs), they can leach into soil via land-applied WWTP's biosolids [38,[58][59][60] (Figure 4) and/or may directly be dispensed as grey water out of a septic tank, creating a conduit for microfibers to infiltrate groundwater systems [1,61]. Median and maximum concentrations of microplastics (microfibers) measuring 6.4 and 15.2 n/L, respectively, were observed in a karst groundwater aquifer system [2]. A low concentration of microplastics measuring 0.0007 n/L was reported in Holdorf, Germany, which was smaller than other microplastic concentrations observed in groundwater around the world [3]. In an alluvial sedimentary unconfined aquifer area (agricultural area), groundwater was found to be contaminated with microplastics with a concentration of 38 ± 8 n/L [5]. Groundwater contamination with microplastics [62] and numerous metals, such as Pb, Cu, Cd, As, Zn, and Mn, has been linked with landfills [63] at Chennai and Tamil Nadu, India. These investigations constitute the most recent contributions to the knowledge on groundwater contamination with microplastics, and distribution comments and remarks have been made and published for those investigations [64,65]. The above studies have provided data indicating that research on groundwater should receive international attention.
Therefore, qualifying and quantifying microplastics in groundwater may require a multi-pronged strategy with careful sampling methods and alternative approaches, making it more complicated than studies of other freshwater environments [66]. There have been a few published research and review papers on groundwater contamination with microplastics ( Table 1). Some of these studies related the problem with soil pollution and determined that soil acts as a potential conduit for microplastics to enter groundwater systems [45,[67][68][69]. There is a possibility that microplastics can reach groundwater situated below agricultural or cultivated land [50] The two most common transport systems are horizontal and vertical transportation [70]; horizontal transport of microplastic in soil mostly occurs via surface runoff and wind erosion [44,71], whereas vertical transportation of microplastic in the soil is mainly influenced by microorganisms and earthworms, which increase the risk of microplastic contamination in groundwater systems [44,59].
PE and PET are the most common microplastic materials in groundwater pollution systems [3,5,69,72,73] and fragments and fibers are the most common shapes ( Table 2). There are five main sources and causes of microplastics in groundwater: landfill leachate, soil migration, wastewater effluent, surface runoff from mulching waste [74], and human activities related to plastic usage and disposal [2,63,73,75]. When compared with groundwater microplastic contamination, the surface water contamination is considerably higher since it has directly been impacted and contaminated by anthropogenic activity (Figure 4). WWTPs and Sewage treatment plants (STPs) serve as pathways for microplastics to enter the surface water when such water sources are located near the WWTP and STP areas [76,77]. PE, polypropylene (PP), and polystyrene (PS) are the most abundant types found in the surface water, and fragments, fibers, and films are mostly common shapes [77,78]. Unlike in groundwater, PET is not abundant in surface water because the density of PET is higher than that of the surface water [79].  Therefore, qualifying and quantifying microplastics in groundwater may require a multi-pronged strategy with careful sampling methods and alternative approaches, making it more complicated than studies of other freshwater environments [66]. There have been a few published research and review papers on groundwater contamination with microplastics ( Table 1). Some of these studies related the problem with soil pollution and determined that soil acts as a potential conduit for microplastics to enter groundwater systems [45,[67][68][69]. There is a possibility that microplastics can reach groundwater situated below agricultural or cultivated land [50] The two most common transport systems are horizontal and vertical transportation [70]; horizontal transport of microplastic in soil mostly occurs via surface runoff and wind erosion [44,71], whereas vertical transportation of microplastic in the soil is mainly influenced by microorganisms and earthworms, which increase the risk of microplastic contamination in groundwater systems [44,59].
PE and PET are the most common microplastic materials in groundwater pollution systems [3,5,69,72,73] and fragments and fibers are the most common shapes ( Table 2). There are five main sources and causes of microplastics in groundwater: landfill leachate, soil migration, wastewater effluent, surface runoff from mulching waste [74], and human activities related to plastic usage and disposal [2,63,73,75]. When compared with groundwater microplastic contamination, the surface water contamination is considerably higher since it has directly been impacted and contaminated by anthropogenic activity ( Figure  4). WWTPs and Sewage treatment plants (STPs) serve as pathways for microplastics to enter the surface water when such water sources are located near the WWTP and STP areas [76,77]. PE, polypropylene (PP), and polystyrene (PS) are the most abundant types found in the surface water, and fragments, fibers, and films are mostly common shapes [77,78]. Unlike in groundwater, PET is not abundant in surface water because the density of PET is higher than that of the surface water [79].

Study (Author)
Study Title [1] Addressing the potential for groundwater contamination by plastic microfibers [4] Existence of microplastics in soil and groundwater in Jiaodong Peninsula [73] Microplastic pollution in soils and groundwater: Characteristics, analytical methods, and impacts [80] Microplastics in the environment: A critical review of current understanding and identification of future research needs [62] Spatial distribution of microplastic concentration around landfill sites and its potential risk on groundwater [5] Microplastic contamination of an unconfined groundwater aquifer in Victoria, Australia [81] Microplastics in the soil-groundwater environment: Aging, migration, and co-transport of contaminants (review) [82] Heavy metal remediation by nano zero-valent iron in the presence of microplastics in groundwater [66] Emerging concerns about Microplastics Pollution on Groundwater in South Korea [45] Microplastic pollution in soil and groundwater (review) [57] Microplastics Pollution Pathways to Groundwater in India [2] Microplastic Contamination in Karst Groundwater Systems [63] Hazardous microplastic characteristics and its role as a vector of heavy metal in groundwater and surface water of coastal south India [50] Plastic in agricultural soils; A global risk for groundwater systems and drinking water supplies (review) [3] Low numbers of microplastics detected in drinking water from ground water sources [84] The occurrence of microplastics in freshwater systems-preliminary results from Krakow (Poland) [85] Fate and transport of microplastics from water sources [86] Identification and Quantification of Microplastics in Potable Water and Their Sources within Water Treatment Works in England and Wales [87] Mapping Microplastic in Norwegian Drinking Water, Atlantic [88] Analysis of microplastic particles in Danish drinking water [89] Metro station free drinking water fountain-A potential "microplastics hotspot" for human consumption [90] Drinking plastics?-Quantification and qualification of microplastics in drinking water distribution systems by µFTIR and Py-GCMS [72] Investigation of microplastic contamination in drinking water from a German city Landfill leachates are mainly responsible for heavy and hazardous metal contamination in groundwater systems [63,91,92]. Microplastics can absorb persistent organic pollutants and metals and may act as a transporter of these hazardous substances in the subsurface water, soil, and/or groundwater [44,84,93]. The leachate pollution index [94] related to groundwater contamination presents a gap in the research and should be investigated in the future. Washing clothes made from synthetic materials can produce microfibers in the wastewater or septic tank effluent, which is a potential source of microplastics (microfibers) in the hyporheic zone, the zone between surface and groundwater [95] and groundwater systems [2,15,57,89].  Notes: PVC, polyvinyl chloride; PA, polyamide; PE, polyethylene; PS, polystyrene; PP, polypropylene; PU, polyurethane; PET, polyethylene terephthalate; ABS, acrylonitrile butadiene styrene; PTT, polytrimethylene terephthalate; PMMA, polymethylmethacrylate; LOD, limit of detection; LOQ, limit of quantification; nd, not described.

Effect of Groundwater Microplastics on Health of Humans, Plants, and Other Species
There has been almost no research on the impacts and effects of groundwater microplastic contamination [45,96]. This reveals a major gap in the research that needs to be filled in the future. To reach the groundwater, microplastics need to be smaller than soil pores, as this allows them to pass through the soil layers [49,81,[97][98][99], which indicates the degradation of larger plastic waste that is buried in soil [100]. Soils contain macropores (>0.08 mm) and micropores (<0.08 mm) which drive cracks, fissures, and fractures [101]. Some external factors such as earthquakes and liquefaction can also play a vital role in shaking down the soil pores. This creates new paths in the groundwater system, posing a hazard. Additionally, if the soil layer is too shallow and the groundwater level is high, there is a higher chance that microplastic can pass the soil horizon and enter the groundwater environments easily. Drinking water from groundwater contaminated with smaller microplastic particles is a major issue [102,103]. Although the direct effects of groundwater microplastics on human health have not been studied, there is evidence that microplastics bear adverse effects on humans, such as contributing to cardiovascular diseases, skin irritation, cancer, reproductive effects, and respiratory and digestive problems [61,[104][105][106][107][108][109].
Research on the effects of microplastics on plants is still in its infancy [73]. According to P. Wanner [50], microplastics are more likely to reach groundwater below farmland or agricultural land. The potential uptake routes mostly occur through the soil and in some farmlands, by the plant roots. Exposing crops to microplastic contaminated groundwater could trigger microplastic uptake throughout plant roots or a change in soil characteristics, both of which could impact plant development [62,110]. Microplastic uptake by microbial activity and plant roots pose a hazard to edible plants ( Figure 4) and can even-tually be distributed up the food chain system [111]. Groundwater contaminated with microplastics is dangerous for use as drinking water or in agricultural processes for human health and is more dangerous than consuming contaminated seafood and fish [1]. In the case of agricultural processes, a microplastic waste cycling system can be developed if groundwater contaminated from the vertical transport of microplastics is used for agricultural and cultivated land. The increase in microplastic contamination in groundwater can impose a destructive effect on groundwater microorganisms. There are several unique faunas in groundwater, such as troglofaunal [112][113][114][115][116] and stygofauna [73], which could be vulnerable to microplastic contamination. However, the exact mechanism underlying how groundwater microplastics affect such faunal species remains unknown and requires additional research.

Strategies for Groundwater Microplastic Management
The study of microplastic contamination in groundwater and strategies for groundwater microplastic management are in the early stages. Reports of groundwater contaminated with heavy metals, arsenic, fluoride, chloride (salinization), coliform bacteria, pesticides, petrochemicals, nitrates, light non-aqueous phase liquids (LNAPL), dense non-aqueous phase liquids (DNAPL), pathogens, and volatile organic compounds (VOCs) [117,118] surfaced prior to the issue of microplastic pollution emerging. Now, this too poses a serious threat to human health and natural environments as with the other pollutants [45]. Strategies to manage microplastic pollution in groundwater should focus on three main factors: (1) preventive measures and developing national and international rules and regulations, (2) remediation of microplastics that have entered groundwater, and (3) increasing social awareness and encouraging the usage of biodegradable plastics. To reduce the severity of microplastic contamination in groundwater, the quantity of contaminants from different sources needs to be controlled [44,119,120].
One example of prevention measures through national policy is the banning of cosmetic products that contain microplastic beads, which represented the major source of primary microplastics in the United States in 2017 [121]. At the same time, several countries in the EU have already banned or imposed taxes on plastic bags as an effort towards plastic reduction [122]. The EU have been implementing restrictions on the usage of both single and multiple-use plastic bags with various strategies depending on the country [121,123]. By 2030, Europe aims to recycle more than half of all plastic waste. All plastic packaging will be reusable or recycled in order to reduce cost and to prevent microplastic [124]. According to Magnusson and Noren [125], microplastic is often found in the receiving water body from WWTPs, and thus, an initiative monitoring system is required. The United Nations launched 17 Sustainable Development Goals (SDGs) in 2015 to maintain human peace and prosperity, eradicate poverty, and safeguard the planet's resources for the future. Among the 17 SDGs, Goal 14 (Life Below Water) is the most relevant to microplastic pollution in the environment (mostly marine) [126]. Although no SDGs directly refer to microplastic contamination in groundwater, some SDGs relating to maintaining the health of aquifers are relevant to microplastic pollution management (Table 3). According to Sinreich [127], different groundwater contaminants, such as heavy metals, arsenic, nitrates, LNAPL, and microplastics, require different remediation methods. There have been no recent studies on the remediation of groundwater microplastic; however, the mitigation of microplastics and other contaminants from groundwater has been studied [121,[128][129][130]. The mitigation of microplastics in groundwater plays a vital role and should be carried out before remediation. Additionally, the mitigation of microplastics from soil and surface water is also helpful in mitigating and resolving microplastic contamination in groundwater systems [45,118,131]. Future research should focus on the remediation of microplastics in groundwater. Another strategy for the management of microplastics in groundwater is using biodegradable plastic material that can be completely degraded either anaerobically or aerobically in the environment [98,132]. The impact of which biodegradable plastic can have on hydrological environments and marine species remains a controversial Water 2022, 14, 1020 9 of 16 topic [133,134]. However, microplastics in the soil can be reduced by using biodegradable plastic in agricultural or cultivated lands [47]. Therefore, using biodegradable plastic can help reduce the potential impact of microplastics on groundwater indirectly since the soil can provide a potential pathway for microplastics to enter the groundwater environment. Biodegradable plastic will continue to have an undeniable and favorable influence on applications that are likely to end up in the environment [132,135]. It is also necessary to develop organized and systematic methods, protocols, and strategies for reducing microplastic contamination in groundwater through local and international governments and/or agencies.

Rank (1-5) of Relevance to Microplastic Pollution in Groundwater
should be carried out before remediation. Additionally, the mitig from soil and surface water is also helpful in mitigating and resol tamination in groundwater systems [45,118,131]. Future research s mediation of microplastics in groundwater. Another strategy for th croplastics in groundwater is using biodegradable plastic material degraded either anaerobically or aerobically in the environment [ which biodegradable plastic can have on hydrological environmen remains a controversial topic [133,134]. However, microplastics in t by using biodegradable plastic in agricultural or cultivated lands biodegradable plastic can help reduce the potential impact of micro ter indirectly since the soil can provide a potential pathway for mic groundwater environment. Biodegradable plastic will continue to h favorable influence on applications that are likely to end up in the e It is also necessary to develop organized and systematic methods, gies for reducing microplastic contamination in groundwater thro tional governments and/or agencies.

Goal Number Link between SDG and the Groundwater Microplastic Contamination
Drinking groundwater contaminated by microplastics could have negative effects on human health and well-being [68,136].
Drinking groundwater contaminated by microplastics could have negative effects on human health and well-being [68,136].

4
Water 2022, 14, x FOR PEER REVIEW Microplastics can be present even in 'clean' drinking water and treated wastewater effluent [137,138] and can thus harm the goal for clean water and sanitation.
The irresponsible production and consumption of plastic can be a big threat to the environment and increase the chance of microplastic contamination in groundwater [139].
Enormous amounts of microplastic are entering marine and freshwater ecosystems and damaging those environments, including groundwater habitats [22,31,33,140].
Terrestrial microplastic pollution, such as in landfills and agricultural soils, affects terrestrial life and can contaminate groundwater as soil is a potential pathway for the vertical transportation of microplastic [45][46][47].

Conclusions
Microplastic contamination is likely to persist for a long time production and use of plastics worldwide and a lack of efficient plas systems. It is expected that the study of microplastic contaminatio continue to grow owing to the susceptibility of groundwater resou Microplastics can be present even in 'clean' drinking water and treated wastewater effluent [137,138] and can thus harm the goal for clean water and sanitation.

3
Water 2022, 14, x FOR PEER REVIEW Microplastics can be present even in 'clean' drinking water and treated wastewater effluent [137,138] and can thus harm the goal for clean water and sanitation.
The irresponsible production and consumption of plastic can be a big threat to the environment and increase the chance of microplastic contamination in groundwater [139].
Enormous amounts of microplastic are entering marine and freshwater ecosystems and damaging those environments, including groundwater habitats [22,31,33,140].
Terrestrial microplastic pollution, such as in landfills and agricultural soils, affects terrestrial life and can contaminate groundwater as soil is a potential pathway for the vertical transportation of microplastic [45][46][47].

Conclusions
Microplastic contamination is likely to persist for a long time production and use of plastics worldwide and a lack of efficient plas systems. It is expected that the study of microplastic contaminatio continue to grow owing to the susceptibility of groundwater resou The irresponsible production and consumption of plastic can be a big threat to the environment and increase the chance of microplastic contamination in groundwater [139].

5
Water 2022, 14, x FOR PEER REVIEW Microplastics can be present even in 'clean' drinking water and treated wastewater effluent [137,138] and can thus harm the goal for clean water and sanitation.
The irresponsible production and consumption of plastic can be a big threat to the environment and increase the chance of microplastic contamination in groundwater [139].
Enormous amounts of microplastic are entering marine and freshwater ecosystems and damaging those environments, including groundwater habitats [22,31,33,140].
Terrestrial microplastic pollution, such as in landfills and agricultural soils, affects terrestrial life and can contaminate groundwater as soil is a potential pathway for the vertical transportation of microplastic [45][46][47].

Conclusions
Microplastic contamination is likely to persist for a long time production and use of plastics worldwide and a lack of efficient plas systems. It is expected that the study of microplastic contaminatio continue to grow owing to the susceptibility of groundwater resou Microplastics can be present even in 'clean' drinking water and treated wastewater effluent [137,138] and can thus harm the goal for clean water and sanitation.
The irresponsible production and consumption of plastic can be a big threat to the environment and increase the chance of microplastic contamination in groundwater [139].
Enormous amounts of microplastic are entering marine and freshwater ecosystems and damaging those environments, including groundwater habitats [22,31,33,140].
Terrestrial microplastic pollution, such as in landfills and agricultural soils, affects terrestrial life and can contaminate groundwater as soil is a potential pathway for the vertical transportation of microplastic [45][46][47].

Conclusions
Microplastic contamination is likely to persist for a long time production and use of plastics worldwide and a lack of efficient plas systems. It is expected that the study of microplastic contaminatio continue to grow owing to the susceptibility of groundwater resou Terrestrial microplastic pollution, such as in landfills and agricultural soils, affects terrestrial life and can contaminate groundwater as soil is a potential pathway for the vertical transportation of microplastic [45][46][47].

Conclusions
Microplastic contamination is likely to persist for a long time due to the increasing production and use of plastics worldwide and a lack of efficient plastic waste management systems. It is expected that the study of microplastic contamination in groundwater will continue to grow owing to the susceptibility of groundwater resources to anthropogenic pressure, the critical role of groundwater in maintaining human activities and natural ecosystems, and since groundwater conservation and management measures are urgently required [1]. The negative impacts of microplastics on the environment, humans, and other species are increasing and extensive studies are required to fully comprehend the incidence, fate, and movement of microplastics in groundwater systems. The problem of microplastics in groundwater is currently in an emerging stage but is steadily growing. This review presents recent and emerging knowledge on microplastic contamination in groundwater. Further research is needed to fill and address key gaps in knowledge, which we identified as follows:

•
More studies are needed to determine the effects of groundwater microplastic contamination on the human body and microorganisms, such as stygofauna and troglofaunal, that inhabit groundwater environments. • Systematic reports and investigations of potential groundwater microplastic contamination with careful sample collection should be carried out in Korea and other developed countries using advanced technology and instruments, such as Raman spectroscopy, Fourier transform infrared spectroscopy, and gas chromatography-mass spectroscopy, in the near future. • Landfill leachates and surface runoff are among the main factors responsible for groundwater microplastic pollution. The leachate pollution index relationship with groundwater contamination represents a gap that needs to be addressed in the future. • Studies should consider the involvement and links between microplastic contamination in groundwater and SDGs and encourage the United Nations to focus on this problem more closely in their future goals as groundwater is used as drinking water and for agricultural purposes.

•
Since soil and surface water are the main potential pathways for microplastics to enter groundwater, further detailed research on the fate of microplastics and the occurrence in different horizons of soil should be conducted. In particular, the study of microplastics in the hyporheic zone, which is the area of contact between surface water and groundwater, will help to understand problems in groundwater.

•
Whether microplastics play a role in the transportation of heavy metals into groundwater systems should be investigated to protect groundwater environments.

•
In groundwater, PE and PET microfibers and fragments are the most common microplastics; therefore, specific remediation and mitigation strategies for these are needed.  Data Availability Statement: Not applicable.

Conflicts of Interest:
The authors declare no conflict of interest.