| 09 Apr 2026
Managed Aquifer Recharge in Confined Multi-Layer Aquifers: A Scalable Framework for Drought Resilience in Central Europe
Abstract. Managed Aquifer Recharge (MAR), particularly Aquifer Storage, Transfer and Recovery (ASTR), can enhance groundwater resilience in confined multi-layer aquifers under drought stress. We develop an integrated and scalable framework to assess ASTR feasibility by combining (i) meteorological and groundwater drought analysis using the Standardized Precipitation Evapotranspiration Index (SPEI-12) and Standardized Groundwater Index (SGI), (ii) GIS-based multi-criteria decision analysis (MCDA) for recharge site suitability, and (iii) dynamic assessment of surface-water availability using ecological flow thresholds. Applied to the water-stressed Berlin-Brandenburg region, one of Germany’s driest areas, where water supply relies heavily on induced bank filtration and faces emerging deficits. Results show that groundwater levels closely follow climatic conditions, indicating that climate-based drought indices can guide timely ASTR operations. The MCDA identified 62.5 % of the area (2,154 km²) as viable for ASTR. Flow-threshold analysis at 27 gauges showed that high-potential downstream sites could provide mean annual recharge volumes of 1.6–4.3 Mm³, offsetting 6–79 % of local extractions. At the catchment scale, total mean annual available recharge is 18.2–23.0 Mm³. Literature-based cost estimates (€0.37–0.51 m-³) are substantially lower than regional drinking-water production costs (€1.80 m-³), suggesting potential annual savings of €23–41 million.
This is an interesting study addressing a real gap in MAR research for confined aquifers in Central Europe. The integration of three methodological components into a single transferable framework is a good contribution. The manuscript is generally well-written and clearly structured. However, several issues require attention before publication.
Major concerns:
1) The authors report a high pearson correlation between SPEI-12 and SGI with "no lag". This is an interesting finding for a confined multi-layer aquifer system, where pressure propagation rather than direct recharge drives head changes. The manuscript does not sufficiently explain the hydrogeological mechanism behind this near-instantaneous coupling. In confined systems, piezometric responses can be rapid, but this is a different mechanism than moisture storage response observed in unconfined systems. This distinction must be clearly discussed and reconciled with the literature.
It would be more correct, rather to state that the absence of lag between SPEI 12 and SGI "indicates that groundwater levels in this confined aquifer respond almost immediately to climatic anomalies" (Discussion section, lines 487-488) to state that the time scale of the response process of groundwater levels to climatic anomalies is not detectable by the aggregated annual scale of SPEI 12 compared with SGI.
Due to the importance of this statement in discussion and conclusion section, to avoid misinterpreting of results and findings, a montly based analisys using SPEI 1 would be recommended to eventualy detect and quantify lags or strongly confirm the "immediate response of GW to climate variability" statements of the whole manusccript.
2) Section 3.2, Line 200-206: Application of SGI to this context should be explained properly and with more details. Some parts of this paragraph are unclear, for example you state that "a common preprocessing step is to remove non-stationarity" but it's not clear if you proceded this way or not.
3) Globally, a flowchart image of the methodology would enrich methodological section.
4) In section 3.4, regarding the MCDA procedure for selecting site suitabilities for ASTR and MAR, you selected six site-selection criteria and classify their suitability scores. MCDA applications in groundwater reletad Issues, such as groundwater potential recharge evaluation and mapping, are very common in scientific literature of recent Years; often exploring and selecting different ranges of criteria due to case specific needs or constrains. In your case, criteria selection seems to be arbitrary or author-knowledge based, as well as the attributed scores in table 1 and their range values, which can be acceptable only if properly justifyied for the characteristics of your case study and strongly linked to scientific literature and to the purpose of your sudy. In any case, you choises have to be properly framed in the context of contemporary scientific literature on the topic. From both aspects, this methodological section of the manuscript is weak and needs improvements.
5) Moreover, regarding MCDA procedure, you apply equal weighting to all six MCDA criteria. It is well-known that weighting choices strongly influence MCDA outcomes in MAR suitability studies. The manuscript acknowledges this only in passing ("based on literature and expert judgment"). A sensitivity analysis testing alternative weighting schemes is strongly recommended.
6) Section 3.6 on Cost Benchmarking from Literature, the cost-analysis is just mentioned as evaluated "from a review of recent european MAR literature". An economic cost analysis would be very interesting and enrich your work, but it hasn't been investigated further since you state it's beyond the scope of your study. At least, you should properly report a summary table of specific informations (if available) for your case study from metioned literature in this paragraph.
It's important to underline that the economic analysis in your study uses a single literature-derived cost range of €0.37–0.51/m³. This range is derived from Ross and Hasnain (2018) and Sprenger et al. (2017), which are based on heterogeneous international datasets. You acknowledge that site-specific analysis was beyond scope, but the manuscript presents potential savings of €23–111 million/year with a level of confidence that is not warranted given the uncertainty in both cost and volume estimates. At minimum, a clear uncertainty paragraph should be introduced, and potential savings claims in the manuscript and in the abstract should be scaled back and framed explicitly only as order-of-magnitude estimates.
7) You note that the study covers only 33% of the total surface-water network. While this limitation is acknowledged in the Discussion, the scale of this gap is substantial and is used to argue that actual regional MAR potential is likely substantially higher. This extrapolation is speculative without further analysis since the unmonitored 67% could include tributaries with lower flow regimes, higher ecological sensitivity, or no overlap with suitable ASTR zones. This claim should be removed or framed as future research direction to investigate and verify.
Following, other minor concerns:
8) Approach 2 relies on hydroecological thresholds that are described generically. The manuscript does not clarify whether these thresholds were derived from local ecological assessments or transferred from literature.
9)The abstract states results clearly but uses the phrase "offsetting 6–79% of local extractions" without specifying which sites this applies to. Readers may misinterpret this as a catchment-wide statement. Clarify that this refers to specific downstream sites.
10) The study focuses on ASTR (Aquifer Storage, Transfer and Recovery), but the framework description and some results sections frequently use ASR and ASTR interchangeably. Since ASTR involves an additional spatial separation between injection and recovery wells compared to ASR, this distinction carries operational significance and should be used consistently throughout.