Delineation of Groundwater Potential Zones (GWPZs) in a Semi-Arid Basin through Remote Sensing, GIS, and AHP Approaches

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This study is very similar to the work done by Dhar et al. 2015 [Dhar, A., Sahoo, S., and Sahoo, M. Identification of groundwater potential zones considering water quality aspect, Environmental Earth Sciences, 2015, Springer, 74(7), 5663-5675, doi:10.1007/s12665-015-4580-7.] This work has not been cited anywhere within the manuscript.

Thank you so much for your observation. We added this work.

“… For example, [27] identified groundwater potential zones, integrating groundwater potential index and water quality index based on AHP…”

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Explicitly mention the necessity of this study and what have you concluded from it?

Thank you so much for your question.

San Luis Potosí (SLP) aquifer is an overexploited aquifer located into semiarid basin of SLP. Previous studies have reported a large depression cone ubicated within SLP city, where is located a highest density well. Nowadays, it has withdrawn, an approximate annual volume of 153.42 million m3, while the annual recharge volume estimated is 78 million m3. This is explained by rapid population growth rate that have greatly impacted in the land use, affecting recharge and water extraction. Previous studies expected and demonstrated that changes from agricultural to industrial and urban land use in the study area could have a great impact on groundwater availability. On the other hand, researches have demonstrated that impermeable rocks strata in SLP aquifer constitute vertical and side barriers for recharge. Therefore, scarce water renewal on the deep aquifer has been reported. This was confirmed, due to elevated concentrations of fluoride and arsenic found in drinking water. In addition, high temperatures of 30 ºC have been identified in some wells. So, it has been recommended water extraction of new zones. However, is necessary carry out suitable water resources management in the study area. The identification and delineation of freshwater potential zones based on remote sensing (RS), GIS and Analytic Hierarchy Process (AHP) have not been determined in the SLP basin. This study could contribute to suitable groundwater management.

We have concluded that more than 50% of the basin's surface area showed low groundwater potential zones situated in the central part, where SLP city is situated. This was explained by the changes from agricultural to urban land use. This area was considered a recharge zone toward shallow aquifer but was altered by land use change. This has led to overextraction of water from the deep aquifer. Currently, a large drawdown cone is located. We found very low potential zones were located in the northwest and southeast parts of the basin. This was attributed to volcanic rocks formations impermeable from porosity that confine the recharge. We identified that high (1.95%) and moderate (26.3%) potential zones were located in the flank areas and within the SA. This was explained by reef and platform origin rocks formations allocated in these zones are suitable for water storage. On the other hand, we found some moderate potential zones situated in the central part of the SLPB, this was explained by agricultural activities that increase groundwater occurrence.

What is the importance of the age of groundwater on the potential zoning of an area?

Thank you so much for your question.

The identification of freshwater potential areas should consider quantity and quality aspects. In semi-arid environments the over-extraction of groundwater could cause mining groundwater. Therefore, fossil water could be found. For example, in SLP aquifer the groundwater level is located at 170 m depth. Studies have reported water of approximately 6,300 years old. This water is characterized of low quality, due to exhibit high levels fluoride and arsenic that could affect population health. Also, this reveals very slow groundwater recharge, so, identify and delineate freshwater potential zones is significant in the SLP basin. The spatial location of very low, low, moderate and high potential zones could contribute to develop strategies to mitigate and prevent groundwater contamination and suitable groundwater resources management.

What is the hydrographic framework?

Thank you so much for your question. The hydrographic framework can be defined as structured information of surface water features such as streams, rivers, canals, lakes, lagoons and ponds in a basin. The data is spatially represented with a linear and polygons system.

A rewritten paragraph is presented next:

The SLPB belongs to the hydrologic region El Salado and is classified as an endorheic basin, lacking any perennial runoff due to the semi-arid conditions [36–38]. The runoffs originated from the SSM and SA feed several intermittent rivers such as Españita, Paisanos, El Potosino, La Parada, Mexquitic and the main collector of the basin: the Santiago River [36,37,39]. To capture runoffs, the San José and El Peaje dams were built, which supply water to the urban areas in the SLPV; meanwhile, El Potosino and Cañada del Lobo dams were built for flood control [40].

On the other hand, the runoffs also feed the principal streams: Grande, La Virgen, Calabacitas, San Antonio, Paraíso and Portezuelo. These last two form the Santa Rita and Laguna Seca lagoons in the rainy season, respectively [40,41].

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Minor typographical corrections are required in the manuscript.

Thank you so much for your observations. We addressed the typographical corrections in all the manuscript.

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In the introduction, it is necessary to formulate the purpose of the study more precisely. What potential groundwater zones do you plan to identify, freshwater zones for drinking water supply of the population or zones of any waters that can be used for technical needs, balneotherapy, etc.

Thank you very much for your comments.

The aim of this study is to identify and delineate freshwater zones for drinking, industrial and agricultural water supply of the population in the San Luis Potosí Basin in order to collaborate to the sustainable water resources management in the basin. The methodology applies a combination of remote sensing (RS) and GIS with Analytic Hierarchy Process (AHP). Multiple thematic layers such as geology, lineament density, land use and land cover, topographic wetness index (TWI), rainfall, drainage density and slope were generated. After the AHP procedure and rank assignment, the thematic layers were integrated using the raster calculator to obtain the Groundwater Potential Zones (GWPZ’s) map. Additionally, the resulted GWPZ´s map was cross validated with the water residence time of 15 wells and the receiver operating characteristic curve (ROC).

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The bibliography (28-45) testifies to the good knowledge of the hydrogeological conditions in the San Luis Potosi basin. Therefore, it is necessary to provide information on the results of previous studies using traditional groundwater research methods based on geophysics, geology and hydrogeology.

Thank you very much for your comments. We added information on the results of previous studies.

For example, in the San Luis Potosí Valley (SLPV) [17], realized a hydrogeological study and proposed a new model conceptual of SLP aquifer based on vertical electrical sounding and magnetic surveys. Additionally, analyzed and integrated the land use changes. They reported that natural and anthropogenic sources have mined the aqui-fer system. Scarce groundwater recharge was identified, attributed to impermeable rocks strata in the SLPV. This was explained by water extraction with water residence time between 3300 and 3600 years, indicating low water renewal. On the other hand, [18], applied a numeric model of flow simulation on San Luis Potosí aquifer. The vari-ations in potentiometric levels over the past 30 years were studied and future water level decline related to increasing water extraction was modelled. They found a signif-icant drawdown in groundwater levels at the east of the urban zone and proposed a decreasing of 30% the water extraction of wells. Mean-while to compensate this reduc-tion four new wells nearby of the Sierra de San Miguelito (SSM) was suggested. This due to SSM was considered groundwater recharge zone. Meanwhile, in other study re-alized by [19], analyzed the groundwater flow system in the San Luis Potosí Valley (SLPV) to examine the effect of increasing water and land use changes on the deep aq-uifer. They implemented a transient groundwater flow model. They reported that the change from agricultural to urban land use affected the natural recharge areas de-creasing recharge shallow aquifer. Moreover, the groundwater extraction of new zones was recommended. Additionally, [20] presented a methodology of exploration and groundwater prospecting. They applied geophysical methods such as: aeromagnetic surveys, ground magnetic surveys and vertical electrical sounding. Subsurface zones with high probability of being fractured were located and related to permeable areas with groundwater potential. However, more studies were suggested in order to con-firm the methodology.

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What aquifers are currently used for water supply? By María Inés Navarro-Hernández, the San Luis aquifer system consists of two aquifers separated by a layer of thin, poorly permeable clay. The upper aquifer is composed of granular material from Quaternary deposits with a maximum thickness of 250 m. The deep aquifer with a maximum thickness of 350 m (?) is composed of Quaternary deposits, as well as highly fractured volcanic rocks. Cretaceous sedimentary rocks are impermeable, therefore they represent the lower boundary of the aquifer. Today 84% of the water demand in the valley is covered by groundwater. Currently, the water table is at a depth of 170 m, there are almost 200 authorized wells, the annual intake of which is approximately 153.42 million m3 (420,000 m3/day), and the annual replenishment volume corresponds to 78 million m3

You write “The groundwater extraction carries out from the deep aquifer” (Page 4, Line 132). Why is the upper aquifer not being used?

Thanks a lot for your time in reviewing our article. We added another subsection (2.1.3) with information that clarified the groundwater extraction and water supply from the aquifer system.

2.1.3 Groundwater extraction and water supply

According to [45], by 1960, surface resources supplied 59% of water and the remainder was collected from the aquifer system. However, nowadays 84% of the water used to supply public-urban, agricultural, industrial and mining activities comes from the groundwater and only 16% from surface water [43,45]. The groundwater extraction is carried out through wells and deep wells, with an active number of 282 and 370, respectively [40,46]. The shallow aquifer has been the most exploited and is almost depleted, so pumping wells with depths up to 1000 m have been used to obtain groundwater from the deep aquifer [19,38]. At present, 96% of the total groundwater volume is contributed by the deep aquifer, and only 4% comes from the shallow one [40]. On the other hand, there is a groundwater withdraw of approximate 153.42 Mm³ , with an annual recharge volume of 78 Mm³, considering the aquifer system as overexploited ^[40,45]. In addition, the highest density of extraction wells is concentrated in the urban area, which has produced a large depression cone [17,19].

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For water supply, water of regulated quality is used, so the hydrochemical conditions must also be characterized, especially since the depth of occurrence of exploited aquifers is very large.

Thank you very much for your comments. Regarding the water quality of the aquifers, in the shallow aquifer the best quality is found on the southern portion of the city, whereas the lowest quality, unfit for human consumption, is observed at the central portion of the urban area, industrial zone and agricultural fields (Almanza-Tovar et al., 2020).

The groundwater pollution in the shallow aquifer has been associated principally to anthropogenic origins due to the land use changes in the SLPV (López-Álvarez et al. 2013; López-Álvarez et al. 2014). Derived from this source of contamination, considerable levels of NO₃^-1, SO₄^-2, Cl^-, As, F^-, K⁺, Ba, Sr, Cd, Pb, Ag, Hg, Zn, Al, Mn, Co, Cu, Fe, EC, TDS and total coliforms are present (López-Álvarez et al., 2013; Camargo-Castro, 2020; Almanza-Tovar et al. 2020). In the case of the deep aquifer, according to Almanza-Tovar et al. (2020), the water quality ranges from excellent to slightly contaminated. In this aquifer concentration of F^-, As and Li have been reported and associated with the volcanic rocks that form the aquifer (López-Álvarez et al. 2013; Camargo-Castro, 2020).

Are other aquifers known besides the alluvial aquifer in the San Luis Potosi Basin? If they exist, information should be given on their characteristics and their use in water supply.

Thank you so much for your question. In the subsection 2.1.3 we added information about the characteristics and water supply of the aquifer system in SLPB.

The main point is the depth to which you conduct your research. Considering a set of your choice of seven thematic layers, it should be limited to the depth of fresh groundwater, unless there are any additional justifications. Therefore, you need to present a conceptual cross-section indicating the boundary of freshwater location.

Thanks a lot for your comments. We made the respective changes and gave specific information of the aquifer system through a conceptual cross-section model.

A Quaternary granular sequence (alluvial, sands, silts, gravels and clay) was deposited on top of the volcanic units as basin-fill material [45,48]. This sequence includes a compact, fine-grained clay-sand layer of low hydraulic conductivity (10^-9 m/s) under most of the flat area (except at the edges). It allows the presence of two nor-interconnected aquifers: 1) a shallow aquifer and 2) a deep aquifer [37,39,48,49]. Figure 2 shows the conceptual cross-section model of the aquifer system in the SLPB.

Figure 1. Conceptual cross-section model of the SLPB aquifer system. Adapted from Hernández-Constantino (2020) [43]

The shallow aquifer has a maximum thickness of 250 m and presents textural variations, towards the SSM conglomerates in a clayey matrix predominate, whereas towards the SA silts and sands [45,50]. This aquifer is sensitive to seasonal effects with the presence of contaminants and with a very dynamic behavior [17,38]. In contrast, the deep aquifer is constituted by strongly fractured volcanic rock and it has an irregular distribution due to a system of pillars and trenches in the valley. This confined type aquifer is bordered by the SSM and the SA and has an approximate maximum thickness of 300 to 350 m [38,43,50].

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You write “Groundwater likelihood is inversely proportional to drainage density; a high drainage density leads to lower infiltration and minors GWPZ’s whereas a low density leads to major GWPZ’s [56,71,72]”.

However, coastal aquifers located near watercourses and water bodies are characterized by the maximum groundwater resources due recharge from these watercourses and water bodies. In your case, a water recharge of the alluvial aquifer from the Santiago River may be possible. Surface water from the intermittent rivers and streams in the rainy season may also be used to replenish the alluvial aquifer.

Thanks a lot for your time in reviewing our article. Regarding your comments, several studies have mentioned the inverse relationship of drainage density with groundwater occurrence, so we added more citation to sustain this argument.

[58,70–74]

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However, it is worthy to mention that drainage density depends on several variables such as slope gradient, absorption capacity of soils, rainfall, vegetation cover, climate, topography and the subsurface characteristics. So, it’s possible to find different patterns in other environments such as coastal aquifers.

In order to avoid generalization, we re-organize the original paragraph and the changes are presented below:

The drainage density is defined as the ratio between the total length of the watercourses in a basin and the surface area of the drained basin [51,68,69]. Typically, groundwater occurrence is inversely proportional to drainage density; a high drainage density leads to lower infiltration and minors GWPZ’s whereas a low density leads to major GWPZ’s [58,70–74]. However, the drainage system depends on several variables such as slope gradient, absorption capacity of soils, rainfall, vegetation cover, climate, topography and the subsurface characteristics [65,75,76]. The knowledge of this variable provides a suitable numerical measure and allows to understand and assess characteristics of runoff potential, relief, groundwater infiltration and permeability information [62,71,73].

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On the other hand, literature has reported that the shallow aquifer is recharged by the infiltration of rainwater and wastewater from agricultural irrigation, and leakage from sewers and water-supply pipelines (IPICYT, 2007; Martínez et al. 2013; López-Álvarez et al. 2013, Almanza-Tovar et al. 2020; Camargo-Castro, 2020). In addition, according to Flores-Marquez (2011), intermittent rivers do not contribute to the infiltration of the shallow aquifer.  

Thus, the possible recharge by streams and rivers (e.g., from Santiago River) need to be detailed studied. To our knowledge, no quantified infiltration rates from these watercourses have been previously reported. Further studies are required to better understand the drainage patterns of the basin and its controlling factors, since drainage density is one variable of a set in this research.

In section headings and in tables, you use two different wordings: LUCL= Land Use and Land Cover and Land Use/Land Cover (LULC). The second wording is misleading, as it represents the Land Use/Land Cover ratio (similar to the ratios km/km2, mm/year).

Thank you so much for your observation. We made the changes and homogenized the wordings in the whole manuscript.

Land Use and Land Cover

(Page 1, Line 23; Page 3, Line 122; Page 6, Line 232; Page 8, Line 318; Page 11, Table 5; Page 13, Table 6)

Figure 3. In this figure, it is difficult to determine the area of distribution of limestones, due to the similar color range with slates, etc.

Thank you very much for your observation. We changed the color range in order to make clearer the geological distribution in Figure 3.

Figure 2. Geology map of the San Luis Potosi Basin

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In conclusion, it is necessary to reflect what is the scientific novelty of the results of your research in comparison with the results of previous studies using conventional techniques for prospecting groundwater. Maybe you have practically completed a hydrogeological reconnaissance in areas where the coverage of detailed geological maps and field data is insufficient, and identified new promising aquifers? Or have you identified structural and lithological features that previously went undetected, and which will provide an improved understanding of the hydrogeological system which, for example, can lead to a more efficient and sustainable use of groundwater resources for water supply?

Thank you very much for your comments. We added the scientific novelty of the results.

“…The identification and delineation of freshwater potential zones based on remote sensing (RS), GIS and Analytic Hierarchy Process (AHP) have not been determined in the SLP basin. This study could contribute to suitable groundwater management. The spatial location of very low, low, moderate and high potential zones could help to develop strategies to mitigate and prevent groundwater contamination and suitable groundwater resources management…”

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Your formal constructions with the creation of the final map with the allocation of four areas (68.21, 26.30, 3.54 and 1.95%) must be filled with physical content. The phrase "Zones with moderate to high groundwater potential are concentrated predominantly towards the Sierra de Álvarez, whereas the Sierra de San Miguelito presents low to moderate potential" does not carry any scientific information. And the phrase "The central zone includes areas with very low and low potential, corresponding with the depression cone reported in the San Luis Potosí aquifer" is illogical, since the depression cone formed in the area of maximum groundwater use, and this area was selected for use because it has been identified as an area with large groundwater resources (high potential) (?). That is, the potential, apparently, is high due to the good reservoir properties of rocks, but artificial replenishment of reserves due to surface runoff is necessary.

Thank you very much for your comments. We have rewritten the conclusions.

In this study, freshwater potential zones in the SLPB were identified and analyzed with remote sensing, GIS and Analytic Hierarchy Process (AHP) using seven thematic layers such as geology, lineament density, LULC, TWI, rainfall, drainage density and slope.

The results accomplished from this methodology revealed that more than 50% of the basin's surface area showed low groundwater potential zones situated in the central part, where SLP city is situated. This is explained by the changes from agricultural to urban land use. This area was considered a recharge zone toward shallow aquifer but was altered by land use change. This has led to overextraction of water from the deep aquifer. Currently, a large drawdown cone is located. Very low potential zones (3.5%) are located in the northwest and southeast parts of the basin. This is atributed to volcanic rocks formations impermeable from porosity that confine the recharge. Meanwhile, high (1.95%) and moderate (26.3%) potential zones are located in the flank areas and within the SA. This can be explained by reef and platform origin rocks formations allocated in these zones are suitable for water storage. On the other hand, some moderate potential zones situated in the central parto f the SLPB, could be explained toagricultural activities increasing groundwater occurrence. The results were validated by water residence time of 15 wells ubicated within SLPB and ROC curve. It observed a correlation between low potential zones and the oldest water. This could indicate a scarce recharge or slow water renewal in zones. 60% of the wells were located in low groundwater potential zones and were related to water residence time ≥ 1000 years. Meanwhile, 40% of the wells were located in moderate potential areas and were associadas to water residence time < 55 years. This was confirmed with ROC curve, which showed a value of 0.677, indicating a prediction accuracy of 67% (satisfactory). The identification and delineation of freshwater potential zones based on remote sensing (RS), GIS and Analytic Hierarchy Process (AHP) have not been determined in the SLP basin. This study could contribute to suitable groundwater management. The spatial location of very low, low, moderate and high potential zones could help to develop strategies to mitigate and prevent groundwater contamination and suitable groundwater resources management.

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It's not clear how you relate the residence time of the water in the aquifer to the GWPZ. To do this, it is necessary to show a conceptual model of the flow from the recharge area to the discharge area. The youngest water is near the surface of the earth and is most susceptible to pollution. At depths of 50-200 m, the speed of water movement is about 1-3 m/year, so the age of 5-6 thousand years of water only indicates that the recharge area is located at a distance of 5-18 km and that the water is well protected from pollution.

Thank you very much for your comments. We show conceptual model of the aquifer system.