Factors controlling dissolved <sup>137</sup>Cs activities in Matsukawa-ura lagoon, a semi-closed estuary, after the Fukushima accident

Niida, Takuya; Takata, Hyoe; Watanabe, Sho; Namura, Shinya; Wada, Toshihiro

doi:https://doi.org/10.5194/egusphere-2025-2436

Preprints

https://doi.org/10.5194/egusphere-2025-2436

Preprints

26 Jun 2025

| 26 Jun 2025

Status: this preprint is open for discussion and under review for Biogeosciences (BG).

Factors controlling dissolved ¹³⁷Cs activities in Matsukawa-ura lagoon, a semi-closed estuary, after the Fukushima accident

Takuya Niida, Hyoe Takata, Sho Watanabe, Shinya Namura, and Toshihiro Wada

Abstract. The spatial and seasonal dynamics of ¹³⁷Cs were investigated from 2021 to 2023 in Matsukawa-ura lagoon, a semi-closed estuarine area approximately 40 km north of the Fukushima Daiichi Nuclear Power Plant, Japan. Weighted mean dissolved ¹³⁷Cs concentrations in the lagoon waters ranged from 5.3 to 19 Bq m^-3, 2.4–8.6 times higher than those in the surrounding coastal seawater and inflowing river waters. Mass balance calculation suggests that the dissolution of ¹³⁷Cs from bottom sediments sustained the high dissolved ¹³⁷Cs concentrations in the estuarine water. Furthermore, dissolved ¹³⁷Cs concentrations in the lagoon were higher in summer (July) than in winter (February). Quantification of the source revealed that dissolution caused ¹³⁷Cs concentrations in bottom sediments to decrease by more than 2.4 % over 30 days in July, but only by <0.8 % over 30 days in February. This finding indicates that warmer waters during the summer accelerate the dissolution of ¹³⁷Cs from bottom sediments.

Received: 23 May 2025 – Discussion started: 26 Jun 2025

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

Download & links

Preprint (PDF, 1148 KB)

Supplement (312 KB)

Download & links

Takuya Niida, Hyoe Takata, Sho Watanabe, Shinya Namura, and Toshihiro Wada

Status: open (until 15 Oct 2025)

Post a comment Subscribe to comment alert

RC1:
'Comment on egusphere-2025-2436', Anonymous Referee #1, 12 Sep 2025 reply
The manuscript addresses an important topic with valuable data, but significant reorganization, clearer figures, and expanded discussion are needed to improve clarity and scientific impact. I therefore recommend major revisions. With the above changes, the paper would be much more accessible to a broader readership and would better highlight its contributions.
Major Comments
Figures and Visualization

Figure 1: The current layout is difficult to interpret: the legends are too small, and colors are used simultaneously to represent ¹³⁷Cs deposition and river catchments. I recommend separating these elements:

Create a first panel where colors represent only ¹³⁷Cs deposition, and identify river catchments with labels instead of colors.

Zoom in slightly so that Fukushima Dai-ichi Nuclear Power Plant is clearly visible.

Include an inset map showing the study area within Japan to orient readers unfamiliar with the region.

Describe all acronyms in the figure caption.

Contextual location: There is no figure linking the lagoon to its main ¹³⁷Cs source (Fukushima). A simple map indicating the relative position of the lagoon and nuclear plant would strengthen context for non-local readers.

Methods vs. Results

Some parameters discussed in Results and Discussion could be introduced in Methods:

K_d calculation: Please specify explicitly how K_d was obtained (e.g., Bq m⁻³ divided by Bq kg⁻¹).

Voronoi partitioning: Mention and briefly describe this procedure in Methods (currently introduced only in line 199).

In general, all analytical and data-processing steps used to represent or interpret results should be described in Methods before they appear in the Results section.

Representation of Results

Figure 2: The statistical plot combines all river catchment data, but much of the underlying data are not shown elsewhere (only partially in Table S1). Consider a clearer, more illustrative figure:

For example, a four-panel plot—one per river—showing monthly dissolved and particulate ¹³⁷Cs concentrations (Bq m⁻³) as column plots.

This would highlight the key findings: (a) Ume and Nikkeshi contribute most ¹³⁷Cs to the estuary; (b) fluxes peak in summer.

Remaining parameters (Bq kg⁻¹, g m⁻³, K_d) could either share a secondary axis or be kept in a table placed in the main text rather than in the Supplementary Information.

Tables:

Table 1 seems to contain background or supporting data rather than primary results. Consider moving it to the Supplementary Information and bringing Table S1 into the main text.

Organization of Results and Discussion

Section 3.2 (Lagoon results):

Subsection 3.2.1 is titled “Relationship between dissolved ¹³⁷Cs and salinity” but includes all parameters and does not first present the spatial distribution of ¹³⁷Cs itself. Begin with a clear presentation of particulate and dissolved ¹³⁷Cs distributions—perhaps using isosurface plots or panel figures similar to those for rivers (even a subset of months for clarity).

The investigation of relationships with salinity fits better in the Discussion rather than Results. Splitting Results and Discussion would improve flow and clarity.

Figure 3 (salinity relationships): The panels suggest little to no correlation. Consider exploring:

Subsets of data (individual rivers, specific lagoon stations near river mouths, or specific seasons).

If no relationship exists under any subset, move these plots to the Supplementary Information or remove them.

Section 3.2.2 and Figure 4: Similar concerns apply here—repeated salinity analysis without a clear signal. A deeper targeted analysis or removal is advised.

Figure 5: This clearly shows seasonal variability (higher in summer, lower in winter) and could be placed under a dedicated subsection focused on temporal variability rather than under temperature–salinity relationships.

Section 3.3 (Modeling)

Add a schematic diagram summarizing the model setup and assumptions for desorption/dissolution of Cs. Values adopted from the literature could be noted on the schematic for transparency.

Include final model results within the main text (e.g., from Figure S1). Panels a–c of Figure S1 are particularly informative, illustrating that most ¹³⁷Cs is discharged in August and that the particulate fraction dominates.

Broader Perspective and Implications

Quantify, if possible, the flux of ¹³⁷Cs from the lagoon to the Pacific Ocean, addressing one of the paper’s stated aims.

Place findings in a broader context:

How significant are lagoon discharges compared with other ¹³⁷Cs sources to the Pacific?

Are there similar studies (in Japan or elsewhere) that could provide comparison?

What are the implications of these results for understanding radionuclide transport or for monitoring strategies?

Suggest potential next steps or research directions based on the conclusions.

Reply
Citation: https://doi.org/10.5194/egusphere-2025-2436-RC1

Takuya Niida, Hyoe Takata, Sho Watanabe, Shinya Namura, and Toshihiro Wada

Supplement

https://doi.org/10.5194/egusphere-2025-2436-supplement

Takuya Niida, Hyoe Takata, Sho Watanabe, Shinya Namura, and Toshihiro Wada

Viewed

Total article views: 407 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	Supplement	BibTeX	EndNote
353	43	11	407	18	10	21

HTML: 353
PDF: 43
XML: 11
Total: 407
Supplement: 18
BibTeX: 10
EndNote: 21

Views and downloads (calculated since 26 Jun 2025)

Month	HTML	PDF	XML	Total
Jun 2025	48	7	5	60
Jul 2025	43	7	4	54
Aug 2025	114	25	2	141
Sep 2025	148	4	0	152

Cumulative views and downloads (calculated since 26 Jun 2025)

Month	HTML	PDF	XML	Total
Jun 2025	48	7	5	60
Jul 2025	43	7	4	54
Aug 2025	114	25	2	141
Sep 2025	148	4	0	152

Viewed (geographical distribution)

Total article views: 402 (including HTML, PDF, and XML) Thereof 402 with geography defined and 0 with unknown origin.

Country	#	Views	%

Latest update: 17 Sep 2025

Short summary

A study on ¹³⁷Cs was conducted in Matsukawa-ura lagoon, a semi-enclosed estuary 40 km north of Fukushima Dai-ichi Nuclear Power Plant. Few studies exist on ¹³⁷Cs in estuaries, so this research aimed to estimate its source using mass balance calculations. Results indicated that dissolved ¹³⁷Cs in the lagoon is influenced by water temperature, with bottom sediments contributing more significantly to ¹³⁷Cs levels than river sources.


Total:	0
HTML:	0
PDF:	0
XML:	0

Factors controlling dissolved 137Cs activities in Matsukawa-ura lagoon, a semi-closed estuary, after the Fukushima accident

Supplement

Viewed

Viewed (geographical distribution)

Factors controlling dissolved ¹³⁷Cs activities in Matsukawa-ura lagoon, a semi-closed estuary, after the Fukushima accident