Abstract
Abstract. Aquifer heterogeneity in combination with data scarcity is a major challenge for reliable solute transport prediction. Velocity fluctuations cause non-regular plume shapes with potentially long-tailing and/or fast-travelling mass fractions. High monitoring cost and a shortage of simple concepts have limited the incorporation of heterogeneity into many field transport models up to now. We present an easily applicable hierarchical conceptualization strategy for hydraulic conductivity to integrate aquifer heterogeneity into quantitative flow and transport modelling. The modular approach combines large-scale deterministic structures with random substructures. Depending on the modelling aim, the required structural complexity can be adapted. The same holds for the amount of monitoring data. The conductivity model is constructed step-wise following field evidence from observations, seeking a balance between model complexity and available field data. The starting point is a structure of deterministic blocks, derived from head profiles and pumping tests. Then, subscale heterogeneity in the form of random binary inclusions is introduced to each block. Structural parameters can be determined, for example, from flowmeter measurements or hydraulic profiling. As proof of concept, we implemented a predictive transport model for the heterogeneous MADE site. The proposed hierarchical aquifer structure reproduces the plume development of the MADE-1 transport experiment without calibration. Thus, classical advection–dispersion equation (ADE) models are able to describe highly skewed tracer plumes by incorporating deterministic contrasts and effects of connectivity in a stochastic way without using uni-modal heterogeneity models with high variances. The reliance of the conceptual model on few observations makes it appealing for a goal-oriented site-specific transport analysis of less well investigated heterogeneous sites.
Highlights
Groundwater is extensively used worldwide as a major drinking water resource and needs to be protected with respect to quantity and quality
We demonstrate the methodology for MADE, a heterogeneous, well investigated research site (e.g. Boggs et al, 1990; Zheng et al, 2011; Gómez-Hernández et al, 2017)
Based on the scale-dependent conductivity modules (Sect. 2.2), we develop different conductivity structures according to the field evidence given the structural data at MADE
Summary
Groundwater is extensively used worldwide as a major drinking water resource and needs to be protected with respect to quantity and quality. Increasing pressure on the quality originates from the intensification of agriculture using agrochemicals (non-point sources) and an increased urbanization with the resulting solid and liquid waste and contaminant spills from industrial applications (point sources). Essential for groundwater protection is the quantitative analysis of the fate and transport of various contaminants in the groundwater body. This can be either for a provisional risk assessment or for the clean-up of an already existing groundwater contamination. Numerical models are common tools to quantify the flow and transport, where partial differential equations are solved using initial and boundary conditions. We restrict ourselves to saturated flow and transport of a dissolved, non-reactive contaminant. The governing equation for its concentration C(x, t) is the advection–dispersion equation (ADE) (Bear, 1972):
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