Review PaperDiagnostic tests for conceptualizing transport in bedrock aquifers

- Identification of the correct conceptual model in bedrock aquifers is challenging.
- Aquifers often behave as equivalent porous media for flow but not for transport.
- The major issue is whether there are continuous open fracture pathways.
- 14 diagnostic tests for conceptual model identification are described.

SummaryTransport in bedrock aquifers is complex because there is often substantial flow through fractures, and the apertures and interconnectivity of these fractures are usually uncertain. Single-porosity numerical models often give satisfactory results for simulating flow. However, simulating transport is more challenging and results based on single-porosity assumptions can yield inaccurate results. Seven cases are reviewed where travel times were found to be unexpectedly short. Results show that dual-porosity flow is common, with advective flow through fracture networks and immobile storage in the matrix. However, in some cases a dual- or multiple-permeability (or porosity) approach provides better simulations of aquifer behavior. Fracture porosity of bedrock aquifers is usually <1%, resulting in rapid groundwater velocities in many aquifers. Overestimation of the effective porosity is the most common reason for the overestimation of travel times. Residence times of artificial tracers in bedrock aquifers are typically two to three orders of magnitude less than residence times of environmental tracers because the latter are retarded by matrix diffusion. Fourteen diagnostic tests for determining the appropriate conceptual model for bedrock aquifers are described.