Spatiotemporal Variation of Groundwater Arsenic in Pampanga, Philippines
Round 1
Reviewer 1 Report
The authors investigated the arsenic concentrations in samples collected from wells in Guagua, Philippines. About 40% of the wells were found to be contaminated with arsenic. The dry season arsenic levels were higher than that in the wet season primarily due to dilution. Such surveillance study is important to be known and its data can be valuable. However, other than the locality of the data, there is almost nothing exciting. The tone of the manuscript is way too descriptive that shows very little depth or interest in the topic. Overall, the English is acceptable but the flow is sloppy and haphazard. Hence, I do not think the manuscript is suitable for publication at its current state.
Below are my comments/suggestions:
1) The authors mentioned that this is the 'first year-long characterization of groundwater As in the Philippines'. - Based on the literature database, there have been studies on arsenic contamination of groundwater in the Philippines. So what is the novelty of this work?
2) Arsenic contamination is unlikely to be a new problem in the country, it probably has been a long-term problem that needs to be fixed urgently. The authors should write with caution to avoid misleading the readers.
3) Only about 20% of the wells were used for drinking purposes. The authors used the local drinking water standards to apply to groundwater samples - is that appropriate? Is there a standard for groundwater?
4) In the materials and methods, the authors mentioned measuring conductivity, turbidity, DO, phosphate, sodium etc, but none were discussed. The analytical method for ORP is missing.
5) In Table 1, what is the MDL for As?
6) Line 180, at higher pH, oxyanions become less sorbed - Why?
7) Line 180-181, As is more mobile than other oxyanions - As oxyanions? compared with what oxyanions?
8) Line 186, high levels of Mn is typical of anoxic and reducing conditions - why? It's weird Mn measurement was not measured in the analytical methods and yet the authors can discuss about Mn levels.
9) The dilution effect of wet season resulting in higher As values should not be the only highlight of this work.
10) The authors suggested the installation of ECAR treatment plant to solve the problem. This clearly shows a commercial interest of the authors over the desire to protect the community.
Author Response
Reviewer 1.
- The authors mentioned that this is the 'first year-long characterization of groundwater As in the Philippines'. - Based on the literature database, there have been studies on arsenic contamination of groundwater in the Philippines. So what is the novelty of this work?
While several studies about groundwater As in the Philippines have already been published, there are no known studies that look at the variation in As levels for an extended period of time (i.e. at least a year). Most of the currently available studies have focused on As speciation and/or distribution for one sampling period. Unlike other regions in Asia, there are no known investigations on the effect of seasons in As level fluctuation in the Philippines. Another importance of our study is the possible contribution of ground subsidence in addition to volcanic influence due to the nature of the study area.
- Arsenic contamination is unlikely to be a new problem in the country, it probably has been a long-term problem that needs to be fixed urgently. The authors should write with caution to avoid misleading the readers.
We appreciate the suggestion. We have removed the parts in question but retained the statement that there is still much room for improvement and progress when it comes to As research and policy-making in the country.
- Only about 20% of the wells were used for drinking purposes. The authors used the local drinking water standards to apply to groundwater samples - is that appropriate? Is there a standard for groundwater?
Yes, we believe that the Philippine National Standard for Drinking Water is appropriate for our study. The guidelines written in the PNSDW 2017 follow the WHO and EPA guidelines for acceptable As in drinking water (10 ppb). Also, local authorities and drinking water providers use the PNSDW for their operations.
- In the materials and methods, the authors mentioned measuring conductivity, turbidity, DO, phosphate, sodium etc, but none were discussed. The analytical method for ORP is missing.
The authors have included some data for the conductivity, turbidity, DO, phosphate, and sodium in Figure 2. The DO, and ORP data were included in the text. Conductivity, total phosphate, total chlorides and total sodium levels were measured for the ECAR feasibility study and in addition, sodium was measured for the health of the ICP-OES torches. The ORP method will be added to the materials and methods section.
- In Table 1, what is the MDL for As?
The experimental MDL for As is 5.5 ppb at 95% confidence interval. The MDL has been included in the methods section.
- Line 180, at higher pH, oxyanions become less sorbed - Why?
At higher pH, the active sites of adsorbents, such as minerals in the aquifer, tend to be occupied by the OH molecule. Dipole-dipole repulsion occurs between the oxyanions and the OH molecule, hence, the oxyanions stay in aqueous phase. This has been added to the main text with reference.
- Line 180-181, As is more mobile than other oxyanions - As oxyanions? compared with what oxyanions?
Arsenic oxyanions were found to be more mobile than other oxyanions such selenite, Cr(VI) oxyanion, molybdate and several others, as presented by Stubbe et al.
- Line 186, high levels of Mn is typical of anoxic and reducing conditions - why? It's weird Mn measurement was not measured in the analytical methods and yet the authors can discuss about Mn levels.
Manganese and iron-containing minerals are abundant in clay layers of aquifers. The reduced form of manganese is more soluble than its oxidized form. The authors have included this in the discussion. The authors have changed the methods section to include total Mn and total Fe analysis. These elements were measured using the same ICP-OES used for As measurements.
- The dilution effect of wet season resulting in higher As values should not be the only highlight of this work.
The study was designed to evaluate the effect of precipitation on As level fluctuation, which is largely unstudied in the Philippines. In addition, other factors such as site elevation and ground subsidence were brought up.
- The authors suggested the installation of ECAR treatment plant to solve the problem. This clearly shows a commercial interest of the authors over the desire to protect the community.
Part of the authors’ bigger goal is the installation of an ECAR treatment plant in the area to provide safer water. We have not discussed it in this study, but the cost of the plant has been shouldered by our grant and the local government has also signified its support by providing space and manpower. However, significant financial investment is required of future ECAR plant adopters. A successful deployment of an electrochemical coagulation treatment technology in India was done via a community organized enterprise because financial sustainability is needed as suppose to just giving the residents the equipment. Grant-driven project normally fail in the Philippines because of a lack of buy-in from the community and not enough money to continue the operation without collecting tariffs. The arsenic treatment project is not targeted to be a commercial money-making scheme, but a way to make technology-based social enterprise work in our context.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
In attachment are comments and sugestions for Authors
Comments for author File: Comments.pdf
Author Response
Reviewer 2
- In the introduction part, please consider a section talking about the arsenic toxicity depending on the form of occurrence, e.g. AsH3, non-organic As3+ , organic As3+ , organic As5+, organic As5 + , arsonium compounds.
The authors appreciate the suggestion. We have included a line discussing As species toxicity in the Introduction section
- What is the influence of pH on the dissociation of arsenic acid (III) and arsenic acid (V)?
The solution pH has various effects on the dissociation of arsenous As (III) and arsenic acid As (V) as shown by the As species present in solution. The toxicity of water varies with the species of As in it. Arsenous acid dissociates to AsO33- in alkaline solution while it is suggested that As3+ is present in strongly acidic conditions (experimental evidence is lacking). Arsenic acid behaves like an oxidizing agent in acidic solution. The authors opted to limit the discussion on As (III) and As (V) since we used total As for quantifying groundwater in our study.
- What are the methods of removing arsenic from water intended for human consumption?
Several As removal technologies have been tested and implemented such as adsorption (iron fillings, lignite, manganese ore), electrocoagulation, and filtration (modified ultrafiltration and nanofiltration). This line has been included in the Introduction.
- There is no chapter Conclusions in the manuscript - the author should complete it.
The Conclusion section has been included in the manuscript.
- Have you investigated organic substances in groundwater (TOC, DOC, UV254)?
No, the authors have not measured TOC, DOC, UV254 in this study and their potential interactions with As were not explored. One of the reasons was that most of the wells with high As are deep aquifers that have low risk of TOC contamination. The authors acknowledge that the inclusion of these parameters could be helpful in future research especially in characterizing shallow wells.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors have only addressed some of the questions. Upon careful reading, the manuscript needs some serious editing before it can be published. Sometimes, less is more. The highlight of this manuscript is the relationship between As levels and rainfall. The novelty will be As contamination in drinking water supplies (in this case, groundwater) and its locality. The others were less important and many issues were brought up unnecessarily.
Below are my comments/suggestions:
1) Line 267-273 - the authors added factors of 'ground subsidence, agricultural practices and river flow rates' that may affect the arsenic levels - this is a sweeping sentence. If the authors have no intention to develop the discussion, it is best not to mention it. For example, how does ground subsidence affect As level? will it increase or decrease As level?
2) 'The authors have included some data for the conductivity, turbidity, DO, phosphate, and sodium in Figure 2.' - I do not see any of them; some were listed in Table 2. I will suggest the author move the data to the supplementary material. The data should be paired with specific samples. There is no need for all samples to have those values measured, but it is important to show which data belong to which sample. For example, the values of Mn shows 0.5 - 4 ppm, meaning all samples measured for Mn are above the regulatory standard of 500 ppb. If that's the case, I will say Mn contamination is more serious than As.
3) Total phosphate, sodium, chloride, silica and sulfate were all not discussed. Same suggestion in (2)
4) In figure 2, MDL for arsenic is 5.5, but there are separate columns for MDL and 1-10 ppb?
5) Line 130 - Qualitative assessment of water samples was performed in-situ - what measurement were performed in situ?
6) Line 137-141 - describes the pretreatment of samples for As measurement, how does this pretreatment affect Mn and Fe measurement?
7) Reference 21 in Line 85 is not used correctly.
8) The authors should not mention 'ECAR' as a treatment method to maintain neutrality of the issue.
Author Response
The authors have only addressed some of the questions. Upon careful reading, the manuscript needs some serious editing before it can be published. Sometimes, less is more. The highlight of this manuscript is the relationship between As levels and rainfall. The novelty will be As contamination in drinking water supplies (in this case, groundwater) and its locality. The others were less important and many issues were brought up unnecessarily.
Thank you. We have revised the manuscript following the reviewer’s suggestions.
Below are my comments/suggestions:
- Line 267-273 - the authors added factors of 'ground subsidence, agricultural practices and river flow rates' that may affect the arsenic levels - this is a sweeping sentence. If the authors have no intention to develop the discussion, it is best not to mention it. For example, how does ground subsidence affect As level? will it increase or decrease As level?
The authors appreciate the suggestion. We have removed any mention of agricultural practices and river flow, but we have retained ground subsidence as one of the factors that could influence As levels. Ground subsidence causes the increase in As in aquifers. There is enough information to describe the ground subsidence in the study area. There were also suggestions in other studies to investigate the effect of ground subsidence on water quality, among others.
- 'The authors have included some data for the conductivity, turbidity, DO, phosphate, and sodium in Figure 2.' - I do not see any of them; some were listed in Table 2. I will suggest the author move the data to the supplementary material. The data should be paired with specific samples. There is no need for all samples to have those values measured, but it is important to show which data belong to which sample. For example, the values of Mn shows 0.5 - 4 ppm, meaning all samples measured for Mn are above the regulatory standard of 500 ppb. If that's the case, I will say Mn contamination is more serious than As.
The authors appreciate the suggestion and we apologize for the confusion. It was supposed to be Table 2. We will put Table 2 in the supplementary information since we haven’t discussed much in terms of these parameters. The authors have considered including Mn and Fe, but the data gathered were significantly less than the As data collected.
- Total phosphate, sodium, chloride, silica and sulfate were all not discussed. Same suggestion in (2)
It was supposed to be Table 2. We will put Table 2 in the supplementary information since we haven’t discussed much in terms of these parameters.
- In figure 2, MDL for arsenic is 5.5, but there are separate columns for MDL and 1-10 ppb?
The authors have adjusted figure 2 to reflect the measurements taken below MDL and the value within 5.5 to 10 ppb.
- Line 130 - Qualitative assessment of water samples was performed in-situ - what measurement were performed in situ?
The qualitative assessment performed were for visual and odor tests. The data collected were used to suggest measures to the LGU and to confirm the reports by locals about their water quality.
- Line 137-141 - describes the pretreatment of samples for As measurement, how does this pretreatment affect Mn and Fe measurement?
The authors haven’t specified that hydride generation was solely used for As measurements. Other heavy metals were measured using the standard sample introduction system that came with the ICP-OES. All samples were still acidified as recommended by the instrument manufacture to improve signal-to-noise ratio. The distinction between As tests and other heavy metals analyses was included in the updated manuscript to avoid confusion.
- Reference 21 in Line 85 is not used correctly.
Upon review, the authors have removed that reference. A more appropriate study about electrocoagulation was cited in an earlier paragraph along with adsorption and filtration. Thank you.
8) The authors should not mention 'ECAR' as a treatment method to maintain neutrality of the issue.
The authors have omitted mention of the term ECAR in the manuscript.
Author Response File: Author Response.pdf
Round 3
Reviewer 1 Report
The authors have addressed my concerns to my satisfaction. The inclusion of supplementary information will be useful for future referencing. This paper is now suitable for publication.
