Uncertainty assessment of solute concentration in natural aquifers sampled from observation wells

Abstract

The subsurface environment’s complex heterogeneous structure poses challenges for accurately modeling transport phenomena due to limited data and measurement errors, leading to uncertainties in solute transport predictions. This study proposes a computational framework to semi-analytically compute the cumulative distribution function (CDF) of solute concentration in heterogeneous aquifers. We investigate how hydrogeological heterogeneity and sampling volume affect concentration uncertainty, as measurements are typically taken at observation wells defined by their sampling volume. Our framework estimates the CDF for a conservative solute, considering hydraulic conductivity heterogeneity, local-scale dispersion, and sampling dimensions. The CDF solution is applicable to point-source injections and scenarios with low to moderate heterogeneity. The CDF solution is verified against high-resolution numerical simulations in a 3D heterogeneous aquifer setting. Results show that sampling volume significantly impacts the concentration CDF tails, crucial for evaluating extreme event probabilities, with sensitivity to sampling volume decreasing as travel distance increases.