Tracer transport in fractured crystalline rock: Evidence of nondiffusive breakthrough tailing

Extended tailing of tracer breakthrough is often observed in pulse injectio n tracer tests conducted in fractured geologic media. This behavior has bee n attributed to diffusive exchange of tracer between mobile fluids travelin g through channels in fractures and relatively stagnant fluid between fluid channels, along fracture walls, or within the bulk matrix. We present a fi eld example where tracer breakthrough tailing apparently results from nondi ffusive transport. Tracer tests were conducted in a fractured crystalline r ock using both a convergent and weak dipole injection and pumping scheme. D euterated water, bromide, and pentafluorobenzoic acid were selected as trac ers for their wide range in molecular diffusivity. The late time behavior o f the normalized breakthrough curves were consistent for all tracers, even when the pumping rate was changed. The lack of separation between tracers o f varying diffusivity indicates that strong breakthrough tailing in fractur ed geologic media may be caused by advective transport processes. This find ing has implications for the interpretation of tracer tests designed to mea sure matrix diffusion in situ and the prediction of contaminant transport i n fractured rock.