Numerical Investigation of Residence Time Distribution for the Characterization of Groundwater Flow System in Three Dimensions

How to identify the nested structure of a three-dimensional(3D)hierarchical groundwater flow system is always a difficult problem puzzling hydrogeologists due to the multiple scales and complexity of the 3D flow field.The main objective of this study was to develop a quantitative method to partition the nested groundwater flow system into different hierarchies in three dimensions.A 3D numerical model with topography derived from the real geomatic data in Jinan,China was implemented to simulate groundwater flow and residence time at the regional scale while the recharge rate,anisotropic permeability and hydrothermal effect being set as climatic and hydrogeological variables in the simulations.The simulated groundwater residence time distribution showed a favorable consistency with the spatial distribution of flow fields.The probability density function of residence time with discontinuous segments indicated the discrete nature of time domain between different flow hierarchies,and it was used to partition the hierarchical flow system into shallow/intermediate/deep flow compartments.The changes in the groundwater flow system can be quantitatively depicted by the climatic and hydrogeological variables.This study provides new insights and an efficient way to analyze groundwater circulation and evolution in three dimensions from the perspective of time domain.