Inferring transport characteristics in a fractured rock aquifer by combining single-hole GPR reflection monitoring and tracer test data

TL;DR

This study combines tracer tests with single-hole GPR reflection monitoring to better characterize solute transport in fractured rock aquifers, providing detailed insights into fracture connectivity, flow paths, and transport parameters.

Contribution

It introduces a novel integrated approach using GPR and tracer data to infer detailed fracture network and transport characteristics in a granitic aquifer.

Findings

GPR signals correlate qualitatively with tracer breakthrough data.

Identified fracture connectivity and flow paths contribute to transport.

Estimated effective transport parameters for individual flow paths.

Abstract

Investigations of solute transport in fractured rock aquifers often rely on tracer test data acquired at a limited number of observation points. Such data do not, by themselves, allow detailed assessments of the spreading of the injected tracer plume. To better understand the transport behavior in a granitic aquifer, we combine tracer test data with single-hole ground-penetrating radar (GPR) reflection monitoring data. Five successful tracer tests were performed under various experimental conditions between two boreholes 6 m apart. For each experiment, saline tracer was injected into a previously identified packed-off transmissive fracture while repeatedly acquiring single-hole GPR reflection profiles together with electrical conductivity logs in the pumping borehole. By analyzing depth-migrated GPR difference images together with tracer breakthrough curves and associated simplified flow and transport modeling, we estimate (1) the number, the connectivity, and the geometry of fractures that contribute to tracer transport, (2) the velocity and the mass of tracer that was carried along each flow path, and (3) the effective transport parameters of the identified flow paths. We find a qualitative agreement when comparing the time evolution of GPR reflectivity strengths at strategic locations in the formation with those arising from simulated transport. The discrepancies are on the same order as those between observed and simulated breakthrough curves at the outflow locations. The rather subtle and repeatable GPR signals provide useful and complementary information to tracer test data acquired at the outflow locations and may help us to characterize transport phenomena in fractured rock aquifers.