ON THE PREDICTION OF THE BREAKTHROUGH CURVE TAILS OF TRACERS THROUGH REGIONAL-SCALE ALLUVIAL AQUIFER SYSTEMS

The breakthrough curves (BTC) of conservative tracers through regional-scale alluvial aquifer systems were simulated systematically using 3,000 high-resolution heterogeneity models representing thirty scenarios of hydrofacies structures characterized by five main properties of facies, including facies proportion, mean length, mean length ratio, juxtaposition tendency, and permeability distributions. The simulated late tails of BTCs for solutes in mobile, immobile and total domains were then predicted by the multi-rate mass transfer method proposed by Haggerty and others. The good fit indicates that the late time solute concentration can be approximated by a summation of a small number of exponential functions, and the shape of the late time BTC depends primarily on the thickness and the associated volume fractions of immobile water in “blocks” of fine-grained material. The simulated early tails of BTCs were predicted by a multi-fracture transport approach similar to that proposed by Becker and Shapiro. The predictability of the early BTC tails for individual realizations, however, is limited, due to the difficulty in predicting the exact paths of fast moving particles and the inapplicability of standard stochastic methods to the uniform conductivity in the high-permeable facies. Although the BTC early tail is difficult to predict, the shape of late tail of BTCs for tracers through a wide spectrum of complex alluvial aquifer systems can be accurately approximated using available hydrofacies distributions such as core descriptions, driller's logs, geophysical logs and/or outcrops.