The conceptualization of a channel network through macroscopic analysis of pumping and tracer tests in fractured chalk

A better conceptual understanding of flow and transport in fractured rocks can be gained through macroscopic interpretation of multi-borehole interference and tracer tests. A homogeneous dilution factor, expected in the pumping borehole in a forced-gradient tracer test, was developed based on flow-dimension analysis of an interference test. This expected dilution factor (HEDF) can be compared to the actual dilution factor (DF) inferred from an advection-based interpretation of the tracer test. These analyses were applied to tests performed in inclined boreholes intersecting fractured chalk in the Negev desert, Israel. The flow dimension observed in the interference test, the large differences between the HEDF and the DF calculated from the interference and tracer tests, respectively, combined with independent results of fracture surveys in nearby outcrops, imply that the dominant feature controlling flow at the site is a network of channels. This finding implies that parallel-plate-fracture transport models probably overestimate the role of matrix diffusion where channel flow predominates. A horizontal, homogeneous, isotropic channel dilution factor (HHICDF) was developed as a DF estimator, using interference-test analysis, and provided reasonable estimates of the dilution in the pumping borehole, for two independent tracer tests. This estimator can be used to predict the dilution of a solute injected near a pumping borehole. Finally, a simple method to estimate the contributions of fluxes from different injection boreholes to the pumping borehole in a multi-borehole tracer test is described. Such estimates can improve our understanding of aquifer anisotropy and heterogeneity at investigated sites.