Groundwater age dating in fractured rock using 4He data

Abstract

Using environmental tracers to date groundwater age in fractured systems is difficult since mass exchange processes between fractures and matrix might mask the actual distribution of groundwater velocity, thus leading to biased results. Here, we propose an analytical solution to date groundwater age in fractured media using 4He data. The solution, which is an extension of existing solutions based on linear accumulation methods, decouples the transport equation in the flowing fracture from the diffusion equation in the rock matrix, under the assumption of steady-state conditions. In the proposed analytical solution groundwater age and 4He concentration are linearly related, and the 4He bulk production rate in matrix, scaled by the inverse of fracture volume fraction, is the proportionality constant. This scaling factor accounts for the underlying mass exchange processes between fracture and rock matrix. Moreover, an Equivalent Continuous Porous Medium (ECPM)-based formulation for the numerical assessment of steady-state 4He levels in fractured media is presented and used to test the performance of the proposed dating technique in realistic conditions. Keywords: Groundwater age, Fractured media, 4He data, Analytical solution