1. The reviewers have not raised that question but the original manuscript contains many grammatical and stylistic errors. Please, read the text carefully and make sure that all your statements are clearly expressed and the language is corrected before the manuscript gets at the proof stage. The typical errors seem to be wrong endings of verbs (wrong person, active form instead of passive) and wrong or lack of prepositions. I am not sure if KMNO4 consumption can be "present" in water (page 8668, line 19-20), I would also recommend "filtered" instead of "filtrated".

2. The replies to reviewers' comments promise that the novelty of your research is highlited in the rewritten conclussions but the replies do not elaborate on that. I am very keen to see how you have handled this question.

The significance of your research for the scientific community is not evident. The abstract, introduction and conclusions place the study in the local context of the Finnish coastal aquifers. Meanwhile, rising sea levels and climatic and land use changes affect coastal areas worldwide. Answering several questions might help expose the importance of your findings.
How relevant are your results for coastal areas in Finland, Baltic Sea basin, worldwide?
Are there examples of studies similar in their aims, scope and methods and what distinguishes your study among them?
Is there any added value in the combination of the hydrogeological, hydrochemical, isotopic and statistical methods you applied?
An interesting notion is presented in the final paragraph of chapter 5.2.2 (page 17, lines 6-9) but not repeated in conclusions. Your results seem to have some importance in the context of groundwater vulnerability assessments. See:
Vulnerability of coastal aquifers to groundwater use and climate change
Ferguson, G; Gleeson, T
NATURE CLIMATE CHANGE 2(5), 342-345
DOI: 10.1038/NCLIMATE1413

I have also several minor comments.
1. There is no explanation on how CO2 and alkalinity were determined. It is only mentioned that CO2 was measured by titration. Provide more details. Was it potentiometric or colorimetric titration?
2. In chapter 3.3, page 8 line 33 – page 9 line you write that “the isotopic composition of the local mean annual precipitation closely follows that of local groundwater”. The opposite statement is more logical as local groundwater is almost entirely derived from precipitation.
3. I suggest removing Table 3. The range of variation in lake water isotopic composition can be seen in Fig. 5. Most wells revealed small variations and the exceptions are described in the text.
4. Subchapters 5.2.1 and 5.2.2 could be merged into one and Table 4 removed. The correlations between variables are reflected in results of the PCA. Providing the Pearson correlation coefficients does not add anything to the discussion.