The effect of permeability windows (PW) within a large-scale fault on the advective-dispersive, matrix-diffusive transport of a generic non-reactive solute (conservative tracer) from a neighboring, stimulated deep reservoir towards a shallow aquifer is explored by numerically simulating a set of worst-case hydrogeological scenarios. A generic conceptual model comprising reservoir, basement, cap rock, aquifer layers, adjacent to or intersected by a sub-vertical normal fault underlies all scenarios. The large-scale fault is represented as a tight core flanked by relatively permeable damage zones. Within core and/or damage zones, PW (putatively associated with fault reactivation by pore pressure increase during unconventional reservoir stimulation) are assumed; their location, vertical extent and physical properties are varied between scenarios. Somewhat counter-intuitively, PW augmenting is not found to increase the overall solute freight to the aquifer, but, in some cases, solute plumes are found to redistribute from the reservoir-adjacent to the opposite side of the fault zone. Bearings on aquifer monitoring system design, and on some current research projects in the unconventional realm are briefly discussed. Our simulation results should not be mistaken for a prediction of shallow aquifer contamination induced by HF operations. Rather, we are more interested to see whether tracer signals detectable by shallow sampling can be used for fault-zone characterization at depth, or for telling the transport properties of at least some of its compartments. This would nicely complement Shapiro’s (2015) agenda of using induced (micro)seismicity to infer hydraulic properties of the subsurface (with a prominent example at the KTB site), since inverting hydraulic parameters from seismicity and/or from pressure signals (McDermott et al. 2006) leaves fluid transport parameters yet undetermined (Ghergut et al., 2007). This hidden agenda has let us select and combine hydrostratigraphy, fault-zone and PW property ranges yielding non-zero (i. e., instrumentally detectable) tracer shallow signals for the scenarios’ most part, for which we were willing to put up with unrealistically high upward fluxes (ultraconservative scenarios, in the sense of Sauter and Lange 2012). Thus it’s more about seeing the fault zones, than about seeing the tracers. Furthermore, at research sites where large-scale fault systems are not the undesired, hazardous feature, but the very target of a forced-gradient fluid-turnover based exploration (like the KTB site in Germany), tracers are expected to convey the fluid transport properties that could not be told from geophysical and hydraulic signals. On the other hand, we coined the term ENDO-tracers – always in conjunction with a georeservoir-TYPICAL* (and well-defined) hydraulic operation sequence on a (more or less well-characterized, maybe still largely unexplored) georeservoir – in order to flexibly denote some either naturally-present, or artificially-introduced, or a combination of both kinds of tracer species, whose subsurface transport (within/to/from the georeservoir and its neighboring geological formations) is subject only to the forced gradients implied by that georeservoir-typical operation sequence. – I. e., we are not referring to a purportedly conducted tracer TEST with its well-tailored design, usually involving some forced-gradients added, and its well-tailored sizing of tracer quantities, added such as to ensure optimum detection, metering, and inversion of tracer signals. This might sound like a bit too much of sophistication; yet: the rationale emphasizing this distinction from a purportedly conducted tracer TEST is that the ENDO-tracer signals, as generated without further intervention, may – if at all detectable and quantifiable – be quite far-from-optimal for the purposes of georeservoir and/or process characterization; whereas the endeavor to optimize their use – like in a normal tracer TEST – might require hydraulic deviations at odds with the georeservoir’s economic use. The epilogue addresses the (endo-)tracer aided evaluation of stimulation outcome for EGS purposes. [* This is, for instance, massive fluid injection for the purpose of georeservoir stimulation, followed by injection or production of a different kind of fluid]

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