the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Sep 2024
A numerical model of microplastic erosion, transport, and deposition for fluvial systems
Abstract. Rivers are the primary pathway of microplastic pollution from source to the eventual sink in the marine environment. However, like sediments, microplastic will become trapped within the fluvial system as it makes its way from source-to-sink. There is therefore the potential that rivers are an important reservoir of microplastic pollution globally. To explore the transport of microplastic through the fluvial system we develop a reduced complexity model of microplastic erosion, transport, and deposition that builds on methods developed for the transport of sediment. We apply this model to the river Têt, France, where there has been punctual monitoring of the flux of microplastic at the outlet. We find that the reduced complexity model captures the observed quantity of microplastic under reasonable assumptions of the relationship between microplastic sources and population density. The model that best matches observed fluxes of microplastic at the outlet of the Têt river requires between 1 and 10 ppm volume concentration of microplastic per 200x200 m in the top half a meter of soil. The microplastic of grain size 300 μm then travels within the river network with a settling velocity of the order of 10-4 m/sec. The model results imply that a large proportion of microplastic will become entrained within the sediments along the fluvial system. This model is a first step in assessing where to sample for microplastic pollution within fluvial systems and points to regions susceptible to microplastic pollution.
Status: open (until 12 Dec 2024)
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RC1: 'Comment on egusphere-2024-2788', Anonymous Referee #1, 31 Oct 2024
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Overview
Microplastics are hot topic in contemporary environmental research. This study adds to the rapidly developing field of microplastic pollution studies, by developing a new reduced complexity model for the simulation of microplastic mobility within a river catchment. The model is first tested and analysed in simplified 1D and 2D contexts, before application in a catchment-scale scenario.
Evaluation
The paper is well within the remit of ESurf. It is well structured, and presents an elegant and comprehensive analysis of the new model’s performance: first a sensitivity analysis in a 1D simulation, followed by a qualitative analysis in a simplified 2D simulation, and finally a quantitative verification against observed data in a real-world catchment. This study thus presents a useful contribution, and deserves to be published, after a number of corrections and clarifications – as outlined below. I consider these to be moderate revisions.
Specific Comments
Some model assumptions need to be better clarified or justified:
- Sediment transport is calculated from Wilcock and Crowe (2003), but microplastic transport is calculated from Meyer-Peter-Muller (ln 125). Why the difference? Why not use Willcock-Crowe for microplastics as well? Please explain. Also, why use Meyer-Peter-Muller specifically for the microplastics? Why not any particular other one?
- The model uses a single microplastics size (ln 130). As microplastics range from approximately 1 µm to 5000 µm (Weber et al., 2022, Sci Tot Env, 819), this covers a wide range of sizes. Is it realistic to represent this variation as one size in the model?
Also, in the microplastic size sensitivity analysis, the range of sizes cover essentially one order of magnitude (from 1000 µm to 1000 µm), whereas the full range spans three orders of magnitude (from 1 µm to 5000 µm). Sensitivity of fall velocities is explored over multiple orders of magnitude. So why not do the same for microplastic sizes? Is it possible that flux significantly increases as microplastics get up to two orders magnitude of smaller (cf. Figure 3b)?
- On what basis are the active layer thicknesses chosen? Active layer thicknesses vary significantly between different experiments. The active layer in the 1D model is assumed to be 1 m thick (ln 198). This seems to be an especially large value. Why such a large value? The active layer in the 2D CAESAR-Lisflood simulations is set to 0.1 m thickness (ln 250), which seems much more reasonable. The active layer thickness in the landscape-scale simulations (Têt catchment) is set at 0.5 m (ln 356), which again seems quite large.
The idea of the active layer is this is where the sediment exchange between river bed and water column occurs. So, setting this to a relative small value (e.g. 0.1 m) seems to make sense. If the microplastics are likely to occur to greater depth, maybe it would be possible to also include microplastics in the first stratum below the active layer – which presumably could be thicker as well.
- The model’s flow solver is adjusted to resolve fluxes at the cell interfaces rather than within the cells (ln 244). It is not explained why this is necessary and, more importantly, it is not explained how this is done. As this seems a rather fundamental change to one of the model’s core underlying algorithms, it would be good if the authors can elaborate on this change – maybe in Supplementary Material, as to not detract from the flow of the paper. Moreover, the authors note that their change to flow algorithm has an impact on the sensitivity to spatial resolution (ln 246), but do not clarify if their new flow model is more sensitive or less sensitive to spatial resolution than the original code.
- For the landscape scale scenarios, the simulations focus on the area downstream of a dam. Is CAESAR-Lisflood then run in reach mode, with a dam release discharge in conjunction with rainfall? Or is it run in catchment mode (i.e. rainfall only), thereby ignoring any dam release flow.
- The authors note that the CAESAR-Lisflood model is sensitive to two key parameters: the assumed quantity of rainfall that passes through the evapotranspiration into run-off, and the storage of water within the subsurface. Can the authors provide a reference for this statement? And, although the model might indeed be sensitive to these two parameters, can the authors confirm that it not sensitive to other parameters.
- In the landscape scale scenarios, microplastic concentration is related to population in a discretized manner (ln 345-348; Table 2). Why such a discretized approach? Why not proportionally scale microplastic concentration to population with a constant factor?
Some intriguing model results are not discussed or not sufficiently discussed:
- Figure 5b: A roughly 24-hour pulse of microplastics is observed between 38 and 62 hours into the simulation. This is briefly described in the main text (ln 262). However, the model also produces an increase in microplastic flux after 90 hours into the simulation. What is the cause for this rather unexpected increase in microplastic flux?
- Initial MP volume concentration for 2D test simulations is 100 ppm (Figure 6c,d). It then seems that erosion of microplastics from the active layer should have minimal impact on active layer thickness. Conversely, deposition of microplastics also does not seem to exceed 100pm (Figure 6d), so presumably should not significantly affect the active layer thickness. Yet the authors note that the unsteady sediment flux “is due to the deposition of microplastic impacting the active layer thickness” (ln 278). This raises two questions: 1) How much of the mobilized microplastic is deposited in the thalweg? And 2) How do the authors know that the it is the deposition of microplastics that is impacting the active layer thickness rather than the deposition of sediments?
- The observed microplastic concentration in the Tet river catchment is largely independent of the water flux (Figure 11). Why would that be?
Minor Comments
ln 38,39: I do not understand the contrast set-up in this statement “it has been observed that the quantity of microplastic that enters the rivers is related to the population density, yet the focus has been on estimates for the flux of microplastic as suspended load”. Why is the notion of estimating microplastic flux as suspended load problematic in the context of the microplastic amounts being related to population density? Moreover, if there is anything problematic about this notion, then that problem remains in your study as well, since your model also treats microplastic flux as suspended load.
ln 50: Add comma after “In effect”.
ln 77: “These models use the empirical transport equations for sediments developed by (Wilcock and Crowe, 2003) to link the water flux to sediment flux”. Surely not all reduced complexity models use the Wilcock and Crowe equations.
ln 79: Delete “CAESAR-Lisflood”. This mention is not applicable when still discussing reduced complexity models in general.
ln 89: Add comma after “That is”.
ln 171: “The finest grain size is treated as a suspended particle”. This is not necessarily true. In CAESAR-Lisflood, the finest sediment can be treated as suspended material, but does not need to be.
ln 213: “At steady state the water depth …”. Does the 1D model ever reach steady state? There could be a steady water flux (ln 217), but I presume the 1D model will never have a steady sediment or microplastics flux. As long as there is flow, there will be erosion, and with continued erosion, the slope would be ever reducing. Thus, true steady state would only occur if there is no further erosion (but then there also would not be any microplastic transport), or if there is a steady uplift to compensate for the erosion (but this is not mentioned).
Table 1: This Table is confusing. It is not clear that these parameters are varied independently. Initially, I interpreted the Table to indicate that each row indicates a set of linked parameters. But later it became clear that this is not the case. Please make it clearer that the table should be read as three separate tables, not as a series of 3-column rows.
Moreover, it is not clear what the default value is for each parameter when one of the other parameters is varied. Thus, what microplastic grain size and settling velocity is used as the median sediment grain size is varied? Or what are the median sediment size and microplastic grain size as the settling velocity is varied?Figures 2, 3: Why are water flux and microplastic flux in area per time, i.e. m2/hr and mm2/hr? Why not volume per time, i.e. m3/hr and mm3/hr? Presumably the 1D slope has unit width, but it would still be more intuitive to interpret the data as volume/time.
ln 222-226: When analysing the impact of the sediment size, which microplastic grain size and fall velocity values did you use for these simulations?
ln 227-232: When analysing the impact of the microplastic grain size, which median sediment size and which microplastic fall velocity did you use for these simulations?
ln 233-238: When analysing the impact of the microplastic fall velocity, which median sediment size and microplastic grain size did you use for these simulations?
ln 239: Correct typo in “Casear-Lisflood”.
ln 240: Replace “would suggest” with “suggests”.
Figure 4: In caption (c) and (d), replace “microplastic that remains in the active layer” with “microplastic in the active layer”. (for simplicity, but also because the active layer in the downstream thalweg may contain microplastics where there were none before – so microplastics were added rather than remaining).
ln 297: Replace “Institute nationale d’infromation géographique et forestiére", with “Institut national de l'information géographique et forestière”. (4 typos in one name, well done)
ln 301: Correct typo in “Casear-Lisflood”.
Figure 8: Correct typo in “hgher” in caption.
ln 320: Correct typo in “bast-fit model”.
ln 324: Replace “however” with “although”.
ln 324: Replace “related for” with “related to” or with “from”.
ln 333: Replace “however” with “but”.
ln 334: Replace “vary” with “varied”.
ln 339: Add reference for “Atmospheric falls could also act as a source of microplastic in soil and along catchment slopes.”
ln 349: Replace “Kedzierski et al. (2023)” with “Kedzierski et al., 2023”
ln 352: Add comma after “polluted soil”.
ln 369: Add “and” after “this model,”.
ln 369: Add comma after “200 m cells”.
Figure 12: Contours are set at uncommon values: 214 m, 414 m, 614 m, … Please drop the 14, and set contours at multiples of 200 m.
Figure 12a: Increase font size of legend label, i.e. “Thickness (m)”.
ln 401: Delete “really”.
ln 420: What is an “addition roughness”?
throughout: Replace “miss-management” with “mismanagement”.
throughout: Check consistency of capitalization of “CAESAR-Lisflood” vs “Caesar-Lisflood”.
Citation: https://doi.org/10.5194/egusphere-2024-2788-RC1
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