Assessment of Groundwater Quality Using the Pollution Index of Groundwater (PIG), Nitrate Pollution Index (NPI), Water Quality Index (WQI), Multivariate Statistical Analysis (MSA), and GIS Approaches: A Case Study of the Mnasra Region, Gharb Plain, Morocco

Round 1

Reviewer 1 Report

The study by Sanad et al. assesses Groundwater Quality, the Pollution Index of Groundwater (PIG), Nitrate Pollution Index (NPI), Water Quality Index (WQI), Multivariate Statistical Analysis (MSA), and GIS approaches for the Mnasra region, Gharb plain, Marocco.

Already from the information of the long title the reader gets the impression that a lot of aspects are attempted to be addressed. The manuscript is generally well written. However, this does not help the clarity of the manuscript. The reader is moved from one aspect to another and at the end wondering what is happening at the study site.

The manuscript is with 33 pages extremely long and appears to be more like a PhD thesis than a focused research publication. One of the issues is that the groundwater quality is compared against drinking water standards from the World Health Organisation (WHO) and at the same time assessed for irrigation water suitability using various Irrigation Water Quality parameters such SAR and MAR and different graphical methods such as Wilcox diagram. The reader is clearly informed in the introduction that the region is an important agricultural region, and that groundwater is being used for irrigation purposes. It is confusing to have both assessment for drinking water purposes and for irrigation purposes for the same wells. Are these wells used for drinking water purposes and for irrigation purposes ? If the wells are used only for irrigation purposes than an assessment for drinking water purposes would not be needed and all comparison to WHO standards and discussion as well as Table 1 should not be presented.

One aspect that does not receive enough attention in the manuscript in my opinion is the aspect of seawater intrusion from pumping relatively close to the sea, it is only more generally addressed in the manuscript. The manuscript is also mute with regards to the pumping rates of the wells. In this regard the manuscript does not offer much insight into why chloride is higher inland than close to the coast for example, and what could cause this spatial distribution. Studying the geological map and the spatial distribution of individual water constituents, there appears to be a close link with the geological formations. Aspects like this are not addressed but should be more discussed.

The Material and Methods section should clearly introduce the Material and Methods. However, details are lacking at a number of points. In the methods section it is stated that sampling was already done after 3 minutes after pumping, which indicates that the sampling was done too soon after pumping. The manuscript does not detail what pump e.g. was used. Normally sampling should be done after 3 well volumes have purged and key indicator parameters such as EC have stabilized. Giving that the majority of wells in the order of 50 m deep 3 minutes is likely not sufficient to purge 3 well volumes and achieve stable sampling conditions. Thus, it is likely that samples are not fully representative. This should be added as a limitation of the study.

The results from the multivariate statistical approaches utilized (PCA and HCA) do not provide much insight with regards to explanation of processes or causes for the current groundwater quality situation. Giving that limited samples (30) were analysed having clusters with only one sample does not really provide confidence into the significance of the different clusters from the analysis. The authors also do not provide explanation what each cluster could represent or stand for. For example, the authors do not provide explanation what each cluster or principal component could stand for or represent.

In addition, a number of statements in the Introduction are lacking references.

Additional comments are provided below:

The page numbering in the manuscript is not correct. Page Numbers run from 1 to 11 but then seem to start over again. This needs to be corrected.

L51: “According to, approximately 31% of Morocco's groundwater has been affected by anthropogenic pollution and natural degradation.” The sentence appears incomplete. Please provide reference.

L57: ‘However, it is noteworthy that irrigated agriculture in Morocco has 57 been associated with adverse effects on groundwater quality” It is not necessary to add the wording “it is noteworthy” here.

L68: “Moving towards the coastal region of Mnasra, it stands out as the most agriculturally productive zone within the Gharb Plain” This sentence does not link well with the previous sentence. Thus, the text should be either re-structured or rephrased here.

L75: “Previous research has demonstrated that agriculture is a significant contributor, accounting for 60% of global groundwater contamination [16].” This sentence is not fully clear. Do the authors want to say that agriculture is a significant contributor with regards to pesticide usage or a significant contributor to groundwater contamination? Please rephrase.

L83: “Various techniques, including binary plots, ionic ratios, Wilcox plots, Piper diagrams, Schoeller diagrams, USSL diagrams, and Gibbs diagram, have been employed to evaluate the hydrogeochemical behavior of groundwater.” As the authors mention the different techniques that have been used in the past it would be useful to add references here from hydrochemical studies assessing irrigation suitability for example. Azffri et al. 2022, 2023 for example is such a study.

L103: “. Past research endeavors have evaluated water quality for irrigation purposes by examining key parameters including the Sodium Adsorption Ratio (SAR), Residual Sodium Carbonate (RSC), Residual Sodium Bicarbonate (RSB), Sodium Percentage (Na%), Permeability Index (PI), Corrosivity Index, Magnesium Hazard (MH), Puri's Salt Index (PSI), Potential Salinity (PS), and Kelly's Ratio (KR).” Again, here it would be useful to add references from studies assessing water quality for irrigation purposes.

L112: “In recent years, GIS techniques, coupled with the Inverse Distance Weighting (IDW) interpolation method, have been employed for the assessment and monitoring of groundwater quality.” Also here references should be added.

L140: “potential evaporation surpasses 150 mm” Do the authors mean per month here ? It is not clear.

Figure 1: The scale bar is not correct. Please revise. How can the scale be only 0.006 km for such a region. Please revise. The font of legends of the two lower Figures is very small and should be increased for readability.

Is it meaningful to interpolate the depth of the wells ? What should this information convey ? It is likely that the depth of the wells does not have significance and were just drilled to that depth based on e.g. financial reasons. Please consider removing this figure.

Figure 2: This figure shows a freshwater saltwater interface, but this is very hypothetical that the interface would look like this. It may also give the wrong impression indicating that seawater intrusion is not an issue when in fact seawater intrusion could be a factor for the higher chloride concentrations observed. This should be clarified or corrected.

L162: “were meticulously collected” What do the authors mean by this statement ? Please explain.

L168: “..for more than three minutes” Why would prior pumping of 3 minutes be sufficient to receive representative samples ? Please explain. It is doubtful that this is sufficient. Thus, this should be added as a limitation for this study.

“These parameters were measured in situ and immediately filtered through a 45 μm filter.” Please rephrase, the parameters were not filtered….

L176: “The sampling procedures adhered strictly to the Standard Methods for the Examination of Water and Wastewater [23]” How were the samples taken exactly ? For example, what pump was used ?

L180: “maintaining a constant low temperature of 4 °C” Please explain how the temperature was kept constant in the portable cooler at 4 °C.

L188: “Nitrate (NO3-) and ammonium (NH4+) were analyzed through distillation using a distillation apparatus” Please provide the method and the equipment used.

L190: “were determined using the nephelometric method, were measured using a UV-visible colorimeter at 650 nm” Please provide more details on the equipment used.

L207: “..Subba Rao in 2012…” The authors should cite this study here at this location in the manuscript. I think it is reference #26.

L223: Table : Why are comparisons with WHO drinking water guidelines being made ? Are the wells being used for drinking water purposes ? If not, it is not necessary to make this comparison.

L322: “Elevated EC levels in groundwater may stem from increased mineral ion concentrations due to dissolved electrolytes, water-rock interactions, and the infiltration of saltwater or industrial discharges into aquifers, with possible seawater intrusion in coastal areas exacerbating EC levels” The authors provide different reasons for elevated EC concentrations in aquifers. However, what the readers really want to know is what is causing the elevated EC concentrations at the study site.

Figure 3: The font in the legends is very small and should be increased. Again, the scale is not correct.

L368 onwards: Why is the water quality repeatedly compared against drinking water guidelines? Are the wells used for drinking water purposes ? Where is the drinking water coming from in this region ? Is it coming from these shallow wells ?

L428: “Moreover, positive correlations were observed between EC and TDS with several ions including Na+, Cl-, Ca2+, Mg2+, CO32-, HCO3-, NO3-, NH4+, PO43-, SO42-, Fe, and Cu, underscoring their role in controlling groundwater chemistry” This sentence is very general and does not provide insight into the existing correlations.

L436: “Conversely, the association between SO42- and Fe (r > 0.7) may indicate anthropogenic con-tamination, possibly from agricultural or domestic sources [53].” Please explain how the correlation between iron and sulfate would indicate anthropogenic contamination. It is not clear.

L462 onwards: While Principal Components PC1 and PC3 are discussed PC2 is not mentioned.

L518: “hinting at the hypothesis of groundwater interaction with saline waters.” This is very vague. In addition, where was this hypothesis made in the manuscript before? The potential and possible indicators for seawater intrusion should be discussed in the manuscript.

Figure 8: The scale bar is not correct and the font in the legend of the figures is too small.

L635 onwards: If the elevated sodium content in the groundwater poses a significant threat to soil structure, permeability and crop productivity, why are almost 80 % of the samples excellent to good for irrigation purposes ? There seems to be a contradiction here.

L680: “Higher values in the southeast region may result from chloride presence in agricultural and seawater intrusion (Figure 10p).” This  sentence appears incomplete. What could be the main factor for elevated chloride concentrations ?

L690 onwards: “In the study area, groundwater contamination is predominantly linked to intensive 690 pesticide use and agricultural activities, compounded by the unregulated discharge of 691 polluted water from the Sebou River” While this sentence appears out of place here and should rather occur in the conclusion section, the authors have not provided any direct evidence to support this statement. The authors should consider 

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Reviewer 2 Report

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Round 2

Reviewer 1 Report

The authors have answered to all my comments. Congratulations and well done. The manuscript will be of interest to researchers and practitioners who are interested in water quality in this area and coastal regions in general in which water quality is threatened by anthropogenic impacts.