How much lithium can we gain by way of solute co-production from geothermal reservoirs in Central Europe? without asking at what costs, we examine the generic question of solute co-production solely in terms of fluid turnover and residence times, assuming conservative mixing and transport (while a lithium-depleted fluid is being continually re-fed into the reservoir and recirculated within its available flow-paths, lithium replenishment from adjacent rocks likely remains negligible over the project time scales of consideration). We illustrate this for two typical geothermal reservoir settings based on fluid turnover (liquid-phase only) in Central Europe: a petrothermal system in the N-German Sedimentary Basin, and a more aquifer-like reservoir in the Upper-Rhine Rift valley. Can tracer tests enable model-independent predictions of georeservoir output? this question is worth raising particularly as concerns thermal drawdown, and solute co-production. For geothermal reservoirs operated by production/re-injection wells, thermal lifetime (TLT) is usually defined in terms of a temperature drop threshold, and estimated as a function of fluid turnover time (FT) and heat exchange surface-area-per-volume, the former (FT) being hopefully measurable by means of a tracer test, whereas the latter is rather difficult to infer from tracer signals alone. Deriving FT from artificial-tracer signals looks model-independent (formally), but is subject to large-time extrapolation uncertainty (which restores model-dependence). Unlike thermal forecasting, tracer-based prognosis of solute co-production (more precisely, of its lower-bound level, assuming conservative transport by fluid turnover only, non-replenished from adjacent rocks) is not impeded by large-time extrapolation uncertainty, nor by reservoir model and/or parameter ambiguity, since mass output prediction as a function of time requires just knowledge of conservative-tracer fluxes within the forecasting time horizon. Once a tracer test was conducted in accordance with the rules of the art, the reservoir can be treated like a black box with a response function derivable, in a well-defined manner, from the artificial-tracer signal. This approach is adequate for (conservative) solute co-production, but not for heat transport. Tracer test results from a particular Upper-Jurassic (Malm) carbonate aquifer near Munich illustrate the issue with TLT as a poorly-defined function of FT. Tracer signals available to date yield FT in the range of months (still subject to extrapolation uncertainty), and are compatible with both fracture-dominated and aquifer-like representations of reservoir structure; compatible values for the heat exchange surface area span four(!) magnitude orders.

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