Microplastics contamination of groundwater: Current evidence and future perspectives. A review

Highlights

• Current studies on microplastics (MPs) in groundwater are scarce.

• Structured and standardized approaches to MPs in groundwater are needed.

• The Hydrogeoplastic conceptual Model for aquifers contaminated by MPs is proposed.

• The proposed approach can be a reference for future hydrogeologist research.

Abstract

Groundwater is a primary water source which supplies more than 2 billion people. The increasing population and urbanization of rural areas stresses and depletes the groundwater systems, reducing the groundwater quality. Among the emerging contaminants, microplastics (MPs) are becoming an important issue due to their persistency in the environment. Seepage through the pores and fractures as well as the interaction with colloidal aggregates can partially affect the MPs dynamics in the subsoil, making the detection of the MPs in the groundwater systems challenging. Based on literature, a critical analysis of MPs in groundwater is presented from a hydrogeological point of view. In addition, a review of the MPs data potentially affecting the groundwater systems are included. MPs in groundwater may have several sources, including the atmosphere, the interaction with surface water bodies, urban infrastructures, or agricultural soils. The characterization of both the groundwater dynamics and the heterogeneity of MPs is suggested, proposing a new framework named “Hydrogeoplastic Model”. MPs detection methods aimed at characterizing the smaller fragments are necessary to clarify the fate of these contaminants in the aquifers. This review also aims to support future research on MP contamination in groundwater, pointing out the current knowledge and the future risks which could affect groundwater resources worldwide.

Introduction

Groundwater is the world's most important source of freshwater, supplying more than 2 billion people access to safe water for domestic drinking, as well as agricultural and industrial purposes (Wada, 2016). Increasing population and incremented human activities affect the urbanized plains and valleys with a significant pressure on aquifers. In the last decades, a deterioration of the groundwater resources quality has been observed due to increasing anthropic activities (Famiglietti, 2014), that also lead to widespread emerging contaminants (Lapworth et al., 2012). Among them, the role of Microplastics (MPs) in groundwater is relatively less explored (Re, 2019). MPs are defined as plastic particles smaller than 5 mm (Arthur et al., 2009), prone to be disaggregated in finer fragments (Mintenig et al., 2018). This emerging contaminant is rapidly gaining attention because of its ubiquitous nature, high resistance, and dispersion in the environment (Enfrin et al., 2019). MPs are intentionally manufactured or derived from the fragmentation of larger plastic debris subjected to external forces such as high temperature, friction, and UV exposure (Hale et al., 2020; Wang et al., 2021a). The continuous production and consumption of plastic objects generates an accumulation of MPs in the environment that can contaminate the subsoil (Duan et al., 2021).

Besides their intrinsic toxicity (Zhang et al., 2022), MPs can anyway play the role of source, carriers, and accumulators of other contaminants, and consequently, also have the potential to be used as environmental indicators (Ma et al., 2020; Wang et al., 2021b), providing information about groundwater quality. Thanks to their strong hydrophobicity, absorption capacity, and large specific area, MPs are in turn drivers for other contaminants (Ren et al., 2021a) during the seepage processes. In addition to their own toxic additives (Bradney et al., 2019), MPs tend to adsorb and transport a wide range of organic and inorganic hazardous substances like heavy metals, pesticides, bisphenols, and antibiotics (Li et al., 2019; Godoy et al., 2019; Selvam et al., 2021; Wu et al., 2019; Li et al., 2018).

Despite large scientific interest, publications focusing on MPs occurrence and fate in freshwater resources, and more specifically in groundwater, are only a minor percentage of the total research conducted (Yao et al., 2020). Indeed, it is more difficult to carry out hydrogeological investigations as direct measurements are possible only via water springs, piezometers, and water-wells. In addition, physical and chemical processes occurring in the soil can attenuate the contaminant in space and time (Ruimin et al., 2019; Zhou et al., 2020).

From a theoretical point of view, the suspended solid transport processes can be used as a reference to describe MPs dynamics in groundwater (Keller et al., 2020). According to laboratory studies, several factors (e.g., particle size, type, shape, density) seem to influence the transport of MPs in subsoil (Ren et al., 2021a). Among them, in the frame of hydrogeological assessments at catchment scale, the comparison between the size of the plastic fragments and the pore-throat size of the geological media is a crucial aspect to consider.

The hydraulic parameters of the geological media favor or impede the groundwater flow (Fetter et al., 2018). In fact, during the seepage processes, the subsoil has the capacity to block suspended solids larger than the soil pore-throats and the bottlenecks of the bedrock fractures. Due to the challenging relationship between MPs and groundwater characteristics, and to contribute to this emerging field of investigation, a critical literature review is performed to: i) describe the MPs occurrence in different aquifer types in relationship with the performed detection methods; ii) summarize the possible sources of MPS contamination of groundwater resulting from the interaction with the other ecological spheres; iii) provide an integrated overview of the MPs in the groundwater cycle.