1 Pathline density distributions in a Null-Space Monte Carlo approach to assess groundwater pathways Christian Moeck* 1 , John Molson 2 , Mario Schirmer 1,3 Contact: Christian.moeck@eawag.ch *Corresponding author 1 Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland 2 Département de géologie et de génie géologique, Université Laval, Québec City, Canada 3 Centre of Hydrogeology and Geothermics (CHYN), University of Neuchâtel, Neuchâtel, Switzerland Conflict of interest: None Keywords: Groundwater flow, Groundwater modeling, Particle tracking, Inverse stochastic modeling, Uncertainty analysis, Water supply protection Article Impact Statement: An inverse stochastic modeling approach with particle tracking provides probability information beyond classical deterministic approaches This document is the accepted manuscript version of the following article: Moeck, C., Molson, J., & Schirmer, M. (2020). Pathline density distributions in a null‐space Monte Carlo approach to assess groundwater pathways. Groundwater, 58(2), 189-207. https:// doi.org/10.1111/gwat.12900