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Electrical and electromagnetic methods are extensively used to map electrically conductive tracers within hydrogeologic systems. Often, the tracers used consist of dissolved salt in water, leading to a denser mixture than the ambient formation water. Density effects are often ignored and rarely modeled but can dramatically affect transport behavior and introduce dynamics that are unrepresentative of the response obtained with classical tracers (e.g., uranine). We introduce a neutrally buoyant tracer consisting of a mixture of salt, water, and ethanol and monitor its movement during push-pull experiments in a fractured rock aquifer using ground-penetrating radar. Our results indicate a largely reversible transport process and agree with uranine-based push-pull experiments at the site, which is in contrast to results obtained using dense saline tracers. We argue that a shift toward neutrally buoyant tracers in both porous and fractured media would advance hydrogeophysical research and enhance its utility in hydrogeology.

Plain Language Summary Geophysical techniques allow for nondestructive monitoring of groundwater processes provided that contrasts in physical properties are available through time and space. To achieve this contrast, the common approach is to inject saline water (a so-called saline tracer) into the ground and to subsequently monitor its movement using geophysical measurements. Unfortunately, classical saline tracers are denser (thus heavier) than fresh groundwater, which leads to instabilities and downward movement of the tracer. This implies that the information gained from saline tracer tests might be unrepresentative of natural groundwater conditions. We introduce a saline tracer that has the same density as the ambient groundwater by mixing the saline tracer with ethanol. This enables, for the first time, geophysical imaging of groundwater flow and transport without associated density effects. By considering experiments performed in fractured rock, we demonstrate (1) that previously used dense saline tracers used for geophysical monitoring lead to very strong density effects, (2) that the new tracer solution can be imaged very well using geophysical techniques, and (3) that the recovered tracers (the so-called breakthrough curves) agree well with traditional tracer tests and hydrogeological theory.