the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Oct 2025
Microbial communities and isotopes as novel tracers for groundwater flow paths in the multi-layered aquifer system in Kurikka, western Finland
Abstract. Groundwater is a critical resource supplying nearly half of the world's drinking water. This study focuses on the Kurikka buried valley aquifer system in western Finland, characterized by complex hydrogeology dictated by the bedrock topography and sediment cover producing artesian conditions in deep aquifers. Using a multitracer approach, the study incorporates hydrogeochemical, isotopic (δ34S, 87Sr/86Sr) and microbial community analyses with residence time indicators (CFCs, SF6, 3H, 3H/3He). Groundwater samples collected from 10 sites revealed differences in residence times, microbial diversity and community compositions, as well as large variation in the strontium and sulfur isotopic compositions. The bedrock groundwater sample revealed a more evolved water type, consistent with longer residence time, strong water-mineral interactions and typical deep subsurface bacterial members. Groundwater from the superficial unconsolidated aquifers contained a modern water component (<60 years) whereas the deeper buried valley aquifers were characterized by older waters. The information provided by this study is crucial for groundwater management during extensive extraction for municipal water supply.
- Preprint
(2280 KB) - Metadata XML
-
Supplement
(329 KB) - BibTeX
- EndNote
Status: open (until 16 Dec 2025)
- CC1: 'Comment on egusphere-2025-4755', Thomas L. Kieft, 12 Nov 2025 reply
-
RC1: 'Comment on egusphere-2025-4755', Anonymous Referee #1, 19 Nov 2025
reply
The manuscript reports on a study that added microbial community analyses (amplicon sequencing of 16S rRNA genes for bacteria and archaea and ITS for fungi) to more conventional methods for dating and tracing groundwater (isotopes, CFCs, SF6). Samples included water from sediments of various depths (~6-76 m) and a deep (162 m) rock-hosted aquifer. The findings follow patterns in which shallower, younger waters have greater microbial diversity and older, deep waters have less diversity but contain microbial groups (e.g. sulfate reducers) typical of older water with longer rock-water interactions. This supports the idea of including microbial community analysis for groundwater dating and flow path analysis, but I would like to have seen more of a big-picture wrap-up in which flow paths are modeled, possibly circling back to figure 1.
Specific comments:
- Title: Microbial communities may be novel tracers, but isotopes are not. Also, did they actually determine flow paths?
- Lines 115-118: This is a long, but incomplete sentence. Maybe delete “which”?
- Lines 157, 159, and 167: I probably should know what “uc.” denotes with regard to these acids, but I don’t. Explain?
- Lines 235-236: If this chloride value is obviously wrong, then I suggest leaving it out altogether. Chloride concentrations are unlikely to change in stored samples. Could this be measured in the lab on a stored sample if one exists?
- Line 264: I may be misunderstanding here, but it seems to me that the Cl values in the text don’t match the values in the table above.
- Also in Table 2, two of the samples have very high dissolved oxygen values, supersaturated, in fact. Are these correct? If so, then what are the implications?
- Line 322: Insert “Nearly” at the beginning of this sentence.
- Lines 343-344, 373-373, and 385: Evidently three samples failed to yield bacterial amplicon sequences and four samples failed to yield archaeal or fungal amplicon sequences. I think this is not discussed anywhere. What if any explanation is there for this? Very low biomass? Interfering substances? What are the implications?
- Lines 409-413: This is repetition from the Introduction and can be skipped.
- Sections 4.2.1 and 4.2.2: I suggest being more specific about the chemical species of S and Fe, rather than simply saying “sulphur” or “iron”.
- Line 540: I think that “Omnitropha” should be “Omnitrophales.”
- Lines 570-594: It’s good to discuss the limitations of a study, but I would like to see some discussion of the strengths as well (unless the aim is to dissuade others from this approach, but I think this is not the intent here).
- Lines 611-612: Incomplete sentence.
Citation: https://doi.org/10.5194/egusphere-2025-4755-RC1 -
RC2: 'Comment on egusphere-2025-4755', Anonymous Referee #2, 27 Nov 2025
reply
Review available within the PDF.
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 155 | 57 | 18 | 230 | 23 | 10 | 13 |
- HTML: 155
- PDF: 57
- XML: 18
- Total: 230
- Supplement: 23
- BibTeX: 10
- EndNote: 13
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Publisher’s note: the content of this comment was removed on 20 November 2025 since the comment was posted by mistake.