Recently, we derived a tracer-based method to forecast the depletion of a co-produced solute during fluid re-circulation in a geothermal reservoir operated by means of a well doublet. This method of Behrens et al. (2022) was deemed model-independent, viz. the measured signals of any conservative artificial tracer from inter-well or inter-horizon circulation tests, conducted under representative flow conditions, can be used to predict the co-production output of any fluid-mined solute, and its gradual depletion during fluid turnover, irrespective of the availability and parametrizing of a reservoir model (up to the effects of depletion-induced water-rock interactions, which are difficult to model in detail, but mostly negligible). We also explained two generic (model-independent) differences between solute depletion rates in petrothermal versus hydrothermal reservoirs, with major implications for the economic efficiency of any solute mining endeavor. Over the past twelve months, at one of the tested geothermal sites, Eastern side of the Upper Rhine rift valley, mid-to-late signals have been recorded, enabling to significantly constrain the uncertainty of lithium output predictions, and to further reduce their dependence on a particular reservoir model. Some ten more tons of lithium per reservoir fluid ToV (turnover volume) may not sound like a dramatic improvement, compared to inferences made (Ghergut et al. 2023) from the incipient ‘nil signal’ information in conjunction with a largely ‘closed-boundary’ reservoir model. The noteworthy aspect, though, is that lithium mid-term depletion can now be re-assessed more robustly, i.e. with a single-parameter focused sensitivity w. r. to reservoir hydrogeology and hydraulics, provided that the forced-gradient inter-well flow field, as established so far, is going to be maintained in the long run. On the other hand, at this stage, the available tracer signal yet cannot ‘anticipate’ how its large-time asymptotic recovery ratio might increase or decrease if the inter-well flow regime would vary significantly. Augmenting the forced-gradient flow rate by some (desired, and cautiously deemed as hydraulically ‘feasible’) 40%–50% would accelerate lithium co-production (proportionally to the flow rate), which, economically, is a sensible thing to do, of course, though it might not significantly augment the total (cumulative) output.

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