the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Sep 2025
Mapping Water Content Dynamics in MAR-SAT systems using 3D Electrical Tomography
Abstract. The growing demand for high-quality water requires sustainable strategies to promote reuse and recycling. Managed Aquifer Recharge systems, particularly Soil-Aquifer Treatment (SAT) systems, have demonstrated effectiveness in improving water quality by reducing contaminants through biodegradation, retention, and sorption. The coexistence of solid, liquid, and gas phases in the unsaturated zone (USZ) enhances adsorption, and retention of pathogens and colloids, while the availability of organic carbon and terminal electron acceptors sustains this zone as critical for biodegradation processes.
During recharge, the hydration-drainage front in the USZ follows a depth-dependent pattern. However, soil heterogeneity causes water to infiltrate through preferential pathways, gradually hydrating the surrounding medium. This behavior is further influenced by system management (recharge strategy, applied flow rate, and/or installation of reactive barriers), which also promotes the development of biofilms. These biofilms facilitate water retention and act as localized microreactors for contaminant biodegradation.
We investigate the influence of recharge strategies (pulsed versus continuous) and the presence of a reactive barrier on the hydration-drainage front in the USZ and biofilm development in two SAT systems. This was achieved using cross-hole electrical resistivity tomography and assessing biofilm formation through extracellular polymeric substances quantification in solid samples collected from the USZ during the recharge episodes.
The study compared two SAT systems: one consisting of fine sand and another with a reactive barrier incorporating of sand, woodchips, compost, biochar, zeolites, and clay. Two recharge episodes were analyzed: one with a continuous flow rate and the other using a pulsed flow rate (while maintaining the same average flow rate across both episodes). Resistivity measurements, associated with the properties of the porous medium and the fluids circulating through it, were collected in the initial dry state and during the recharge, revealing the 3D distribution of the USZ volume during hydration (at the start of recharge) and drainage (when recharge ceased). Over time, these measurements also indicated the potential formation of biofilms in the SAT system. Measurements at the beginning and end of each recharge period capture the 3D evolution of water content.
Results showed that water infiltration occurred through preferential pathways or fingers, creating significant heterogeneity in water content in both SAT systems. The reactive barrier enhanced water retention during dry periods, supporting biofilms development. Furthermore, the pulsed recharge strategy promoted biofilm growth more effectively than the continuous recharge strategy. These findings provide insight into optimizing recharge strategies and media composition to manage system dynamics and, consequently, enhance contaminant removal in SAT systems.
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Status: open (until 10 Dec 2025)
- CC1: 'Comment on egusphere-2025-3994', Alex Furman, 23 Sep 2025 reply
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RC1: 'Comment on egusphere-2025-3994', Alex Furman, 02 Oct 2025
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This comment is identical to that submitted earlier as "community comment"
This is an interesting study that examines a geophysical (ERT) investigation of soil aquifer treatment (SAT). It is well-written, and most of the discussion, especially that related to the core data (electrical), is well-supported and thoroughly discussed. Surprisingly, very little research followed the pioneering work of Haaken et al. Perhaps even more surprising is that the authors also almost ignore it (and even cite it in the wrong context). This is actually my main criticism of this work - it ignores much of the relevant SAT literature, especially the content from the SHAFDAN site in Israel, which is by far the world's largest and has been in operation for over three decades. This criticism by no means suggests that this work is not of value, but it certainly lacks - not only in the introduction part of the manuscript, but perhaps mostly in the discussion
Below are several specific comments
- While not the core of this study, it is about time that researchers stop claiming that adsorption is a key element in SAT. A simple mass balance would show that all the adsorption capacity is used at a relatively early stage of the SAT facility life, and additional adsorption is possible only if there exists a degradation process that ‘empties’ adsorption sites
- L68: Biofilms are not the critical element. Microbes are. Biofilms are the environment that helps microbes perform their functions
- Apparently, the authors did not read Haaken et al. thoroughly…
- Interestingly, the authors practically ignore the vast body of literature that comes from the Israeli SAT experience – the largest of its kind that has operated for about 30 years, and is documented quite well in the literature
- Pulse (it is fine to use a short version, but be consistent and use short or full versions synchronized)
- ~L145. Isn’t the difference in environmental conditions (temperature) a concern?
- Section 2.4. Any petrophysical model to relate resistivity with water content?
- Is a quarter of a second enough to get charge stability for the reactive layer?
- 2-3. These plots are helpful, but zooming in to show a single representative day or two at different periods would be much more interesting
- Figures 5-7 are nice, but difficult to analyze. The authors are encouraged to use some integrative type of analysis that would smooth the noise
- 5through 7.
- L423-426. This is not much more than speculation. Any evidence?
- This is an interesting idea, but it requires much better support. Resistivity ‘hot-spots’ indicate water content. From here to biochemical hot-spots, the distance is quite far. At best, it indicates the formation of biofilm, but there are so many things that can trigger biofilm formation (some of which are related to microbial stress, not to microbial prosperity
- L456 and on. Well, understanding that oxygen monitoring is useful is interesting, but given that this is the only thing that was monitored, it puts it in a different light. What about ORP (just an example) – won’t that be equally supportive? While I do not argue, it isn't easy to conclude here
- The use of biological materials to enhance biochemical activity is not new, but interesting in the context of ERT. Nevertheless, this practice needs to be discussed in terms of long-term activity. It is not trivial to refresh such a layer after its functionality diminishes
Citation: https://doi.org/10.5194/egusphere-2025-3994-RC1 -
AC1: 'Reply on CC1', Cristina Valhondo, 26 Sep 2025
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3994/egusphere-2025-3994-AC1-supplement.pdf
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CC2: 'Comment on egusphere-2025-3994', Xinqiang Du, 21 Nov 2025
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- From an academic classification perspective, Soil-Aquifer Treatment (SAT) is not universally recognized as a subset of Managed Aquifer Recharge (MAR). According to standard definitions, MAR refers to the intentional recharge of water to aquifers for subsequent use or environmental benefit. At the same time, SAT primarily focuses on improving water quality through soil infiltration. Consequently, the term "MAR-SAT" does not represent a commonly accepted implementation scenario in the literature. Given the study’s core focus on unsaturated zone monitoring via ERT, the standalone term "SAT" is sufficiently precise to contextualize the research. The explicit association with MAR is unnecessary unless the authors can demonstrate direct relevance to MAR’s core objectives.
- Line 46-50, the natural pollutant reduction capacity of the porous media can be seen as an additional guarantee to MAR. So, the mentioned regulations are not a constraint for widespread application of MAR. Although the citation supports the conclusion, it is not a reasonable opinion.
- Line 51, ”…… for Soil Aquifer Treatment (SAT), a MAR technique that uses treated wastewater as a source for recharge” is a misleading interpretation of SAT and MAR. The reason is the same as in the above comment.
- Line 51–55， Treated wastewater is not a typical recharge source for MAR in most regions, and SAT is not widely recommended as a water quality improvement technology for MAR systems. This is primarily because pollution prevention and contaminant attenuation are paramount considerations for MAR, and SAT may not consistently meet the stringent water quality requirements for aquifer recharge without additional validation.
- Line 114–115， The statement "following the recommendations of a soil scientist" does not constitute a rigorous scientific basis for the described methodology. To justify the approach’s rationality, the authors must cite relevant peer-reviewed literature or standardized protocols that support the adopted procedures.
- Line 144–145， Two critical concerns arise regarding clogging-related claims:
- Logical Inconsistency: Clogging is defined as the phenomenon of reduced hydraulic conductivity. The absence of operational shutdowns for maintenance does not equate to the absence of clogging—mild to moderate clogging can occur without necessitating downtime. Notably, the statement in Line 224 ("we have observed the development of a water layer due to the reduction of infiltration capacity") directly indicates clogging-induced reduced permeability, creating a contradiction that requires resolution.
- Insufficient Evidence: The claim regarding vegetation’s role in clogging control lacks empirical support. The authors must provide verified references demonstrating vegetation’s efficacy in mitigating clogging in SAT/MAR systems, or supplement with in-situ data (e.g., vegetation root distribution, clogging layer composition).
- Line 165， Formatting error: "250 10⁻³" should be corrected to "250×10⁻³".
- Line 206， Terminology Ambiguity: The phrase "aquifer head (depth to water)" is imprecise.
- Line 393–395, The opinions in this section lack support from validated peer-reviewed references. To enhance credibility, the authors must supplement with relevant literature that corroborates the proposed mechanisms or conclusions.
- Line 442-448, I think the conclusion that ERT allows visualizing and monitoring variations in water retention and biofilm growth is a verified fact in the past relevant research. So, the authors should keep up with the latest developments.
- A critical gap in the manuscript is the lack of discussion on how the study’s conclusions translate to MAR applications. Specifically: What actionable principles or technical guidelines can be derived to inform MAR system design, operation, or performance assessment? How does the ERT-based monitoring approach address key challenges in MAR (e.g., recharge rate optimization, contamination risk mitigation)? The authors should supplement this section with concrete implications for MAR practice to enhance the study’s applied value.
Citation: https://doi.org/10.5194/egusphere-2025-3994-CC2
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Publisher’s note: this comment is a copy of RC1 and its content was therefore removed on 2 October 2025.