water Review Effect of Physical Characteristics and Hydrodynamic Conditions on Transport and Deposition of Microplastics in Riverine Ecosystem Rakesh Kumar 1 , Prabhakar Sharma 1, * , Anurag Verma 1 , Prakash Kumar Jha 2 , Prabhakar Singh 3 , Pankaj Kumar Gupta 4 , Ravish Chandra 5,6 and P. V. Vara Prasad 2   Citation: Kumar, R.; Sharma, P.; Verma, A.; Jha, P.K.; Singh, P.; Gupta, P.K.; Chandra, R.; Prasad, P.V.V. Effect of Physical Characteristics and Hydrodynamic Conditions on Transport and Deposition of Microplastics in Riverine Ecosystem. Water 2021, 13, 2710. https:// doi.org/10.3390/w13192710 Academic Editors: Lihui An, Li Xu and Lixin Zhu Received: 31 August 2021 Accepted: 25 September 2021 Published: 30 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 School of Ecology and Environment Studies, Nalanda University, Rajgir 803116, Bihar, India; rakesh.kumar.PhD@nalandauniv.edu.in (R.K.); anurag.verma.phd@nalandauniv.edu.in (A.V.) 2 Sustainable Intensification Innovation Lab, Department of Agronomy, Kansas State University, Manhattan, KS 66506, USA; pjha@ksu.edu (P.K.J.); vara@ksu.edu (P.V.V.P.) 3 Ecole Centrale School of Engineering, Mahindra University, Hyderabad 500043, Telanagana, India; Prabhakar.Singh@mahindrauniversity.edu.in 4 Faculty of Environment, University of Waterloo, Waterloo, ON N2L 3G1, Canada; pk3gupta@uwaterloo.ca 5 Department of Soil & Water Engineering, Dr Rajendra Prasad Central Agricultural University, Pusa 848125, Bihar, India; ravish@rpcau.ac.in 6 Centre of Excellence on Water Management, Dr Rajendra Prasad Central Agricultural University, Pusa 848125, Bihar, India * Correspondence: psharma@nalandauniv.edu.in Abstract: Microplastic disposal into riverine ecosystems is an emergent ecological hazard that mainly originated from land-based sources. This paper presents a comprehensive review on physical processes involved in microplastics transport in riverine ecosystems. Microplastic transport is gov- erned by physical characteristics (e.g., plastic particle density, shape, and size) and hydrodynamics (e.g., laminar and turbulent flow conditions). High-density microplastics are likely to prevail near riverbeds, whereas low-density particles float over river surfaces. Microplastic transport occurs either due to gravity-driven (vertical transport) or settling (horizontal transport) in river ecosys- tems. Microplastics are subjected to various natural phenomena such as suspension, deposition, detachment, resuspension, and translocation during transport processes. Limited information is available on settling and rising velocities for various polymeric plastic particles. Therefore, this paper highlights how appropriately empirical transport models explain vertical and horizontal distribution of microplastic in riverine ecosystems. Microplastics interact, and thus feedback loops within the environment govern their fate, particularly as these ecosystems are under increasing biodiversity loss and climate change threat. This review provides outlines for fate and transport of microplastics in riverine ecosystems, which will help scientists, policymakers, and stakeholders in better monitoring and mitigating microplastics pollution. Keywords: microplastics; riverine ecosystem; sedimentation; deposition; settling velocity; plastic density 1. Introduction Nowadays, plastics have been widely used because of their low cost, durability, and resourcefulness in socio-economic sectors, like fishing, industry, tourism, and more [1–4]. Plastic pollution gained attention of the scientific community and has been documented across the globe [5]. Plastic particles with a size of less than 5 mm are known as microplas- tics which have been studied extensively in different ecosystems, such as marine [6–8], wetlands [6,9], rivers [10–12], groundwater [13–15], sub-surface system [14,16,17], at- mosphere [18,19], soil [14,20,21], and remote mountain [22–25]. Microplastics are sub- categorized as primary and secondary depending upon their origin [26], such as products Water 2021, 13, 2710. https://doi.org/10.3390/w13192710 https://www.mdpi.com/journal/water