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 ^[1][53], thereby placing groundwater resources at risk ^[2][3][54,55]. Notably, groundwater is used in approximately 38% of agricultural and cultivated areas globally ^[4][56]. The invisible nature of groundwater makes it difficult to observe and maintain ^[5][1]. Groundwater samples from Chennai, India, were reported to contain fibrous and fragment-shaped microplastics ^[6][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 ^{[7][8][9][10]}[38,58,59,60] and/or may directly be dispensed as grey water out of a septic tank, creating a conduit for microfibers to infiltrate groundwater systems ^[5][11][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 ^[12][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 ^[13][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 ^[14][5]. Groundwater contamination with microplastics ^[15][62] and numerous metals, such as Pb, Cu, Cd, As, Zn, and Mn, has been linked with landfills ^[16][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 ^[17][18][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 ^[19][66]. There have been a few published research and resviearchw 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 ^{[20][21][22][23]}[45,67,68,69]. There is a possibility that microplastics can reach groundwater situated below agricultural or cultivated land ^[24][50] The two most common transport systems are horizontal and vertical transportation ^[25][70]; horizontal transport of microplastic in soil mostly occurs via surface runoff and wind erosion ^[26][27][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 ^[9][26][44,59].

PE and PET are the most common microplastic materials in groundwater pollution systems ^{[13][14][23][28][29]}[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 ^[30][74], and human activities related to plastic usage and disposal ^{[12][16][29][31]}[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 ^[32][33][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 ^[33][34][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 ^[35][79].

Landfill leachates are mainly responsible for heavy and hazardous metal contamination in groundwater systems ^[16][36][37][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 ^[26][38][39][44,84,93]. The leachate pollution index ^[40][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 ^[41][95] and groundwater systems ^{[6][12][42][43]}[2,15,57,89].

There has been almost no research on the impacts and effects of groundwater microplastic contamination ^[20][44][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 ^{[45][46][47][48][49]}[49,81,97,98,99], which indicates the degradation of larger plastic waste that is buried in soil ^[50][100]. Soils contain macropores (>0.08 mm) and micropores (<0.08 mm) which drive cracks, fissures, and fractures ^[51][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 ^[52][53][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 ^{[11][54][55][56][57][58][59]}[61,104,105,106,107,108,109].

Research on the effects of microplastics on plants is still in its infancy ^[29][73]. According to P. Wanner ^[24][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 ^[15][60][62,110]. Microplastic uptake by microbial activity and plant roots pose a hazard to edible plants (Figure 4) and can eventually be distributed up the food chain system ^[61][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 ^[5][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 ^{[62][63][64][65][66]}[112,113,114,115,116] and stygofauna ^[29][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.

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) ^[67][68][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 ^[20][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 ^[26][69][70][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 ^[71][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 ^[72][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 ^[71][73][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 ^[74][124]. According to Magnusson and Noren ^[75][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) ^[76][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. According to Sinreich ^[77][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 ^{[71][78][79][80]}[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 ^[20][68][81][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 ^[48][82][98,132]. The impact of which biodegradable plastic can have on hydrological environments and marine species remains a controversial topic ^[83][84][133,134]. However, microplastics in the soil can be reduced by using biodegradable plastic in agricultural or cultivated lands ^[85][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 ^[82][86][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.