Microplastics in Groundwater: Pathways, Occurrence, and Monitoring Challenges

Round 1

Reviewer 1 Report

Review Report:

·       Journal: Water (ISSN 2073-4441)

·       Manuscript ID: Water-2941557

·       Type: Review

·       Title: Microplastics in groundwater: Pathways, occurrence and monitoring challenges.

·       Authors: Elvira Colmenarejo Calero * , Manca Kovač Viršek , Nina Mali

·       Topic: Microplastics Pollution

A)  General Comments:

The authors address a global and serious enironmental-as well as health-related, recent problem in their review paper: Microplastics (MPs) are plastic particles measuring less than 5 mm in size according to generally accepted definitions. They are considered as emerging pollutants deserving special attention. In water they affect and interact with ecological processes, disturb the water cycle and hence cause serious problems, that are not only environmentally threatening but also posing health damage to living organisms. In several countries, groundwater (GW) is the primary source of drinking water, hence, monitoring MPs in GW and studying their potential sources, properties, fate and reaction pathways is very important for their control and management. In this paper, the primary contamination pathways of MPs from surface water, seawater and soil into GW is extensively reviewed. This paper also examines the sources and behaviours of MPs in GW. The auhtors aim to identifiy the major challenges related with the quantitative and qualitative assessment / monitoring of MPs in GW. In this respect, the challenges/difficulties associated with sampling, sample preparation, sample protection, measuring MPs in GW, comparing these with results of other related work, and the need for implementing standardised techniques for their accurate sampling and detection is addressed. Based on their own experience and this literature review, the authors emphasize the importance of understanding the case-specific hydrogeological and hydrogeographic conditions, collecting representative samples, using sampling devices with comparable specifications and standardized laboratory techniques for MPs identification, as well as preventing contamination at all stages of the sampling and monitoring processes. Their aim is to present valuable insights and practical guidelines on how to improve the accuracy, reliability and comparability of results between studies monitoring MPs in GW.

This is a well-written, comprehensive review paper. The problem addressed is interesting and significant. However, some parts/sections of this review paper seem to be very general, they are not specifically indicating what specific precautions, measures, procedures and other information would be more useful and relevant for MPs analyses in GW. Hence, some improvements could be made to satisfy the main aim and scope of this review paper. Some specific comments with emphasis on the “Conclusions Section” follows below.

B)   Specific Comments:

·       Introduction: MPs (depending on their size and shape) also enter living organisms. For instance, in the present case, aquatic species living in the GW and being part of the GW’s ecosystem are under serious threat. MPs in living organisms means tha they accumulate/aggregate in their body tissue. This should be addressed in the Introduction Section. Alternatively, it can be briefly mentioned here and its effects/contribution to this subjects/review paper. This issue is only barely mentioned in the paper.

·       2.1. Transport mechanisms of MPs to GW from rivers, lakes, and seawater: Which river in Germany is reported here in Scherer et al (2020). Besides, a formal (format) question; when citing a paper, is the publication year and at the same time, the reference number given (both)-also marked in yellow below in an example-? Please check citation style in te manuscript (also elsewhere)...

·       2.2. Transport of MPs through the soil and the unsaturated zone to GW: Due to intensive pumping, aquifers closely located to saline water/seaside may be subjected to sewater (saline water) intrusion. How does this affect the MP concentration in the aquifer/well? Seawater intrusion is a serious problem in for example Mediterranean Countries where overpumping of GW can cause serious water quality problems.

·       3. Occurrence of MPs in GW: This is the main subject of the review paper. Here, the specific problems related to MPs sampling and sample analysis/monitoring are mentioned for the first time. It comes a bit too late, considering the length, aim and scope of this review paper. The authors may explain or consider some changes in their manuscript (organization, content, etc.).

- “Informal landfills” = are “open dumps” meant with “informal”?

Table 1:

-Panno et al., 2019 [25]-?;

-Also in the same table and same row: Craked = Cracked?

-H2O2 concentration mentioned in the same table: Is it concentration by  weight (w/w) or weight/volume- 30%?

·       4.1. Challenges associated with sampling methods: This is a general problem, in particular for solid /hazardous samples. Sampling from different locations in time and space, sampling devices, etc., all these are precisely specified only in a few cases; generally speaking there is no special and detailed procedure for different types of solid contaminants/hazardous wastes.

·       4.2. Challenges associated with laboratory analysis: No details are provided here about whether there are some specific precautions taken for sample preparation with digestion, filtration and extraction in terms of used chemials, specific treatment times and materials. Only general information is given and some references are cited.With details some specific procedures with quantitative measures is meant.

·       b. Limitations of methods for MP detection and quantification: When using FTIR/IR analysis, what size range is appropriate? Are there any specific limitations/specifications for using intrumental analysis methods to detect/identify, or qualtitatively/quantitatively measure MPs?

·       4.3. Challenges associated with quality assurance: In this section, again, the authors provide only very general infomation about risks associated with contamination during sampling and that special precautions should be taken during sampling. Moreover, it should be noted here that taking samples in replicates is recommended/mandatory for appropriate sampling and hence not a special/specific procedure for MPs. More specific raher than specific information would e inteesting here, since the authors are also expertized in MP ampling and monitoring.

·       Conclusions and Summary:

As the authors indicate, MPs are present in rivers, lakes, seas and soils and hence can enter drinking water sources such as GW via different pathways. The authors emphasize here that in the papers tht have studied MPs in GW show different results in terms of the quantity, shape and size of the MPs detected in GW. They also use different sampling and preparation methods This could be due to reasons such as the differences in the studied area (the population density, climatology, geology, land use, etc.) as well as differences in the sampling methods (and equipment) used in these studies. In the papers that the authors review it is agreed upon the fact that in the monitoring of MPs in GW, there is a lack of standardized techniques and procedures for the reliable, sensitive and accurate MP measurement which can lead to serious discrepancies between the results of individual work. They conclude that there is an urgent need to develop standard methodologies and procedures for more accurate results and safe comparison of MPs in GW. Monitoring MP particles is difficult due to their diversity and the hydrogeological conditions of the sampling sites, as well as the associated risks of contamination during sampling and monitoring. Based on the analysis of twelve (12)research articles on the presence of MPs in GW and personal experiences of the authors, a list of challenges being faced during sampling of  MPs in GW has been prepared, including experience, proper understanding and documentation of site conditions, appropriate and careful sampling and sampling devices/volumes, selection of the correct equipment for sampling, studying the water characteristics (its organic matter content), proper quality control during sampling and monitoring, and in-depth understanding of fate, transport  processes and mechanisms of MPs through the soil. All questions related to proper measurement of MPs in GW need to be clarified through a combination of sensitive experimental work, extensive field measurements and appropriate mathematical modelling. This review mainly focuses on identifying the challenges with the ultimate aim to develop reliable, simple, sensitive, standardised methods to enable comparison of different studies’ results and form a baseline for the implementation of legislative measures and management strategies of MPs control in water sources.

What is missing in the entire manuscript is the recommendations and suggestions of the auhtors they write about several problems and weaknesses in MPs analyses (for GW as the water matrix), but give not specific information about them. There is obviously a problem, ut how to deal with it, is only barely mentioned (only the situation is briefly described).

The Conclusion Section” of this manuscript is quite long; it is a brief summary of the review and also addresses lessons learned, not only the problems related to MP measurement but also some suggested precautions to be taken for reliable, standardized, sensitive analysis. Some details are given about filters and pumps used for sampling, which can be considered as a detail for this final section. The problems indicated in this review could be considered as general problems related to appropriate measurement and monitoring of emerging contaminants, micropollutants and other types/groups of problematic pollutants. Sampling, transportation, sample preparation and analytical/instrumental procedures need to be carefully considered for reliable results. The point here is that for microplastics relatively less information is available about how to sample, handle, measure or monitör microplastics present in different water sources and effluent samples. The authors may also emphasize that microplastics are also entering aquatic (living organisms, their organs and tissues) organisms and accumulate there causing serious health problems. This also needs to be considered when monitoring them in groundwater. Besides, the number of references used to review related work (study and compare previous related, published work) is rather low for a review paper.

Comments for author File: Comments.pdf

The English is fine, there are almost no (only minor) typos and language errors.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

This is a well written, comprehensive, and interesting review of what's known (and not) of microplastics in groundwater.   

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

REVIEW ON MANUSCRIPT ID: WATER 2941557

GENERAL REMARKS

·       The main issue addressed by the review is the assessment of a particular kind of environmental pollution by microplastics (MPs), emerging pollutant of groundwater environments.

·       The topics discussed in the manuscript is very interesting because it analyzes the main contamination pathways of MPs into groundwater.

·       The review is very innovative because it represents the first report to summarize the main findings of previous research about MPs pollution into groundwater.

·       It could be better to insert in the manuscript some figures to better explain the contents of the article.

·       The references are appropriate and well detailed.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have answered all comments in detail and considered most of them in the revised manuscript. The authors have addressed most of the question marks regarding issues related to analyses and sampling of MPs.

The publication can be accepted after some minor revision of typos, format, etc. (formal issues).

There is no problem with language, only minor issues.