Reply on RC2

Reply to general comment 1: We agree with the reviewer’s comment that dissolved oxygen (DO) measurements would provide further information regarding the removal rates and redox conditions in the SAT subsurface. DO sensors were installed at the site, but due to technical problems and limitations, no reliable dissolved oxygen data could be obtained. Our understanding and personal experience is that this low reliability of DO sensors in variably saturated and dynamic field conditions is common.

Reply to general comment 1: We agree with the reviewer's comment that dissolved oxygen (DO) measurements would provide further information regarding the removal rates and redox conditions in the SAT subsurface. DO sensors were installed at the site, but due to technical problems and limitations, no reliable dissolved oxygen data could be obtained. Our understanding and personal experience is that this low reliability of DO sensors in variably saturated and dynamic field conditions is common.
We claim that the E h is a useful practical tool to define the optimal duration, from a geochemical perspective, of a wetting and a drying cycle in SAT. Note that the good correlation between DO and E h measurements deviates once the DO is depleted, since the E h conditions continue to decrease as other oxidized species presence in the pore-water solution (e.g., nitrate). Brettar et al. (2002) indicated that the end of the denitrification process at topsoil occurs at the value of 0 mV and for deeper soils at a value of 100 mV. These conclusions were explained by the higher carbon availability at topsoil compared to deeper parts of the soil. Ultimately, the E h observations at the SAT vadose zone enable the prevention of establishment of anoxic conditions (100mV >), where no O 2 or NO 3 are available. This discussion is now provided in the manuscript (lines 413-423).
General comment 2: I find the analysis and interpretation of the drying cycle data a bit short/insufficient. The authors conclude that a 36-hr drying period is optimal for both the summer and winter season, irrespective of the starting conditions in redox potential at the beginning of the drying cycle. Yet the analysis does not fully explore or explain why Eh is recovering so much faster in the summer than in winter.
Reply to general comment 2: We suggest that the E h recovery is dominated by the rates of gaseous O 2 intrusion to the soil. During winter, the gaseous O 2 concentrations show moderate increase with time, while during summer all the observed O 2 curves show a steep recovery that ceases after about 20 hours. The recovery of the E h conditions reflect comparable trends. During summer the E h conditions increase rapidly and during winter the E h recovery shows moderate increase with time. The revised Fig. 9 displays the differences between oxygen intrusion during winter and summer and an additional discussion was provided (lines 489-511).

Specific comments:
Comment 1: Line 114: Suggest rephrasing to "…was to examine the temporal variability in redox potential…" Reply to Comment 1: The text has been revised (lines 115-116).

Comment 2:
Line 116: Seasonal changes in climate such as rainfall and temperature are likely to also influence the wetting/drying stages and not just operation of the SAT system. I would suggest mentioning climate or season in line 116.

Reply to Comment 2:
The text has been revised accordingly (lines 117).
Comment 3: Line 135: You state that the surface of the spreading basins is plowed on a regular basis to prevent clogging. In the past 10-15 years operators of infiltration basins have moved away from this practice because they observed that the plowing allowed fine particles to move deeper into the vadose zone (e.g. 1-3 m), where they would accumulate and form a flow impeding layer. Have you run any geophysical scans of the vadose zone underneath the infiltration basins whether percolation has been impacted by the plowing? Reply to Comment 3: During the establishment of the monitoring stations, two boreholes were drilled to 2 m depth and soil samples were collected at 25 cm intervals (8 × 2 soil samples in total). The soil samples were analyzed for particle size distribution (PSD). According to the PSD analysis in the figure below, there is no indication for accumulation of fine particles or establishment of a layer with low permeability. The figure below is provided in supporting information (Fig. S3).

Comment 4: Line 145: were the suction cups installed at the same depth as they sensors?
Reply to Comment 4: The suction cups were installed at similar depths to the other sensors. The text has been revised to clarify this point (lines 146).
Comment 5: Line 195: Could you please clarify if the data on the long and short cycles are averages over the stated periods (e.g. Nov-April) or what is the time frame for these? If so, please also add a column stating the number of event (N). In addition, please clarify if the duration of these stages was set by the operator of the basin or whether there was a systematic operation scheme that was tested in this study. It is not quite clear who defined the long and short cycles.
Reply to Comment 5: We agree with the reviewer's comment that there might be a confusion concerning the different data sources. The data presented in Table 1 was provided by the Shafdan operators. Note that the number of recorded cycles is included in Table 1. Our monitoring systems collected data every 20 minutes as mentioned in the Method section (lines 146 -154). The text has been revised accordingly (lines 204-216).
Comment 6: Line 217: Please clarify what you mean by "high saturation values" -field capacity, 80% pore space filled?
Reply to Comment 6: We removed the word high and stated that the soil remained at similar level of saturation throughout each wetting stage according to θ observations (line 235).

Comment 7: Line 225: Add "parameters" after Ks and β.
Reply to Comment 7: The text has been revised accordingly (lines 243).

Comment 8: Line 234: How is the soil drainage process defined in the operation? Is there a minimum water content or redox value that needs to be achieved? If so, please state it?
Reply to Comment 8: The water levels at the soil surface of the Shafdan ponds are monitored with water level sensors. As can be observed in Figure 2, the drying process starts when no water is present at the soil surface (black line). Nevertheless, the water level is measured only at a single point (close to the pond inlet). Therefore, some variations in water heads, at the order of a few cm, may exist due to microtopography and the distance between the inlet and the far parts of the pond. This may lead to some delays in water arrival or recession from the stations. We added the definitions to the text (lines 208 -212).

Comment 9: Line 246: How do you explain the higher VWC value during the winter?
Reply to Comment 9: We claim, by using a hydrological modeling approach, that the soil physical properties alter during winter under long wetting stages (lines 261 -279). The decrease in biodegradation of organic matter (OM) during winter causes accumulation at the topsoil of the SAT. Increases of OM in the soil is reflected in the increase of the VWC. This is in addition to changes in viscosity and density that further change the retention and conductivity of the soil.
Reply to Comment 10: The text has been revised accordingly (lines 263).

Comment 11: Line 254: Please elaborate on how the winter cycles affect infiltration capabilities.
Reply to Comment 11: Due to the lower biodegradation during winter, there is an accumulation of organic matter in the vadose zone of the SAT. The low specific surface area of sandy soils (~0.0077 g m -2 ) compared to clay (~900 g m -2 ) (Doerr et al., 2000) implies that a limited amount of organic matter is required in order to coat the sand grains and to develop soil water repellency. Arye et al. (2011) showed that soil hydrophobicity is attributed to the reduction of the liquid surface tension and increasing of the contact angle. These changes in soil properties are related to the reduction of the soil permeability. This explanation is now provided in the text (lines 269 -277).
Comment 12: Line 294: What is the Ksat at the site? It is a bit surprising that Eh is recovering so quickly. It is hard to determine based on Figure 4, hence I would recommend stating average recovery times in days or hours for Eh to return to positive values. Is this typical for these infiltration basins? What is the retention time of the wastewater in the unsaturated zone?
Reply to Comment 12: The subsection 'The length of the drying stage using E h and gaseous O 2 measurements' (lines 478 -511) elaborate the issues regarding the E h recovery time. The estimated Ks for the site is presented in Table 2. In a recent study by our group we found similar Ks values for a different infiltration basin of the Shafdan (Grinshpan et al., 2022). According to our estimations, the average pore-water velocity (q/θs) is between 2.8 and 2.2 cm/h. For a vadose zone thickness of 35 m, the retention time should be between 52 and 70 days. These estimations are in accordance to previous residence times suggested by Elkayam et al. (2015). Figure 6 is indicating that the infiltration basin has an inverted water table below the basin bottom which is maybe 50-80 cm thick. Below this inverted water table oxygen content and Eh seem to be higher potentially indicating unsaturated conditions. Most denitrification is therefore occurring withing the saturated zone (or inverted water table) below the basin bottom, which varies in thickness depending on texture. Gorski et al. 2019 (ES&T) recently summarized some of these dynamics in a nice conceptual way, which could be helpful for this study. Have you checked whether the saturated thickness is changing with the duration of the wetting and drying cycles as well as season?

Comment 13:
Reply to Comment 13: According to the gaseous O 2 measurements, unsaturated conditions prevail at 150 cm during most of the monitoring period. It takes a longer time for the gaseous O 2 to disappear at 25, 50 and 75 cm depths during winter (see the new Fig. 8). However, we are not sure if the absence of gaseous O 2 indicate saturated conditions. As can be seen in the Reply to Comment 3, above, there is no evidence for substantial soil texture variability. Therefore, we do not anticipate considerable changes in the saturated thickness. The conceptual model by Gorski et al. (2019) is indeed interesting and relevant and was discussed earlier in the manuscript (lines 316-325). In addition, to accurately measure the saturated thickness both the volumetric water content (θ) and soil matric pressure (ψ) should be measured simultaneously at multiple depths. Unfortunately, since it was not in the scope of the current study, we did not install pressure sensors.
Comment 14: Line 388: Changes in Eh are usually lagging behind changes in oxygen content as highlight in Figure 4. Unfortunately this study does not show DO data for the pore water at 25, 50 and 100 cm depth, therefore it is difficult to estimate whether Eh is indeed a useful proxy for predicting DO depletion.

Reply to Comment 14:
Please see reply to general comment 1 above. Figure 8: It would have been helpful to overlay the recovery of the volumetric water content since start of the drying cycle on top of the Eh recovery. The steep increase in Eh around 18 hrs since start of the drying cycle could be supported by the higher ET during summer. I am also wondering if the timing of the operation plays a role (e.g. operator stops flooding at the end of a workday (e.g. evening), hence ET is highest the next day around noon or 18 hours later)?

Comment 15:
Reply to Comment 15: We plotted O 2 data together with the E h observations during the drying cycle in the revised Fig. 9. The timing of operation does not play a role since the effluent supply to the infiltration basin is according to requirement and it is not affected by the operator's availability.