Spatial distribution and export of nutrients and metal elements in the subterranean estuary of Daya Bay
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Abstract: Subterranean estuaries (STE) are important seawater-groundwater mixing zones with complex biogeochemical processes, which play a vital role in the migration and transformation of dissolved materials. In this study, we first investigated the spatial distributions of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorous (DIP), dissolved inorganic silicon (DSi) and metal elements (As, Ba, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) in STE including upper intertidal, seepage face and subtidal zones. We then estimated submarine groundwater discharge (SGD) and associated nutrient and metal element fluxes. From the generalized Darcy’s law method, SGD was estimated to be 30.13 cm/d, which was about 7 times larger than the inflow (4.16 cm/d). The nutrient and metal fluxes from SGD were estimated to be (5.33 ± 4.99) mmol/(m2·d) for DIN, (0.22 ± 0.03) mmol/(m2·d) for DIP, (16.20 ± 2.05) mmol/(m2·d) for DSi, (1325.06 ± 99.10) μmol/(m2·d) for Fe, (143.41 ± 25.13) μmol/(m2·d) for Mn, (304.06 ± 81.07) μmol/(m2·d) for Zn, (140.21 ± 13.33) μmol/(m2·d) for Cu, (84.49 ± 2.94) μmol/(m2·d) for Pb, (37.38 ± 5.51) μmol/(m2·d) for Ba, (27.88 ± 3.89) μmol/(m2·d) for Cr, (10.10 ± 6.33) μmol/(m2·d) for Ni, and (6.25 ± 3.45) μmol/(m2·d) for As. The nutrient and metal fluxes from SGD were relatively higher than those from the inflow, suggesting that nearshore groundwater acted as the sources of nutrients and metal elements discharging into the sea. The environmental potential pollution of coastal seawater was evaluated by pollution factor index (Pi), comprehensive water quality index (CWQI), and ecological risk index (ERI). Pb mainly caused potential danger of nearshore environment with considerable contamination (Pi = 5.78 ± 0.19), heavy pollution (CWQI = 4.09) and high ecological risk (ERI = 18.00). This study contributed to better understanding the behavior of nutrients and metal elements and improving the sustainable management of STE under the pressure of anthropogenic activities and climate change.
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Key words:
- subterranean estuaries /
- submarine groundwater discharge /
- nutrients /
- metal elements /
- pollution assessment /
- Daya Bay
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Figure 1. Location information of the study site: a. Daya Bay; b. sampling area and the spatial distributions of monitoring wells, as well as the groundwater and seawater samplings; c. seepage face zone; d. spatial distributions of monitoring wells (rectangles) including LTC-Diver and sampling points (dots) along the transect from intertidal zone to subtidal zone.
Figure 2. Time series of groundwater levels and tide levels from May 19 to 30, 2022 at W1, W2, and W3.
Figure 3. Spatial distributions of physicochemical parameters and nutrients, including pH (a), salinity (b),
$ {\rm{NH}}_4^ + $ (c),${\rm{NO}}_{\rm{x}}^ -\ ({\rm{NO}}_{\rm{x}}^-={\rm{NO}}_{\rm{3}}^ - + $ $ {\rm{NO}}_{\rm{2}}^ - )$ (d), dissolved inorganic silicon (DSi) (e), and dissolved inorganic phosphorous (DIP) (f). The black dots mean the sampling points of groundwater.
Figure 4. Spatial distributions of metal elements, including As (a), Ba (b), Cr (c), Cu (d), Fe (e), Mn (f), Ni (g), Pb (h), and Zn (i). The black dots mean the sampling points of groundwater.
Figure 5. Principal component analysis (PCA) bi-plot for groundwater samples of upper intertidal (n = 9), seepage face (n = 9) and subtidal zones (n = 9). The samples of three zones have been divided by different color. n means the number of samples.
Figure 6. The Pearson correlations analysis for physicochemical parameters, nutrients, and metal elements of groundwater samples in three zones. Sal. means an abbreviation for salinity.
Figure 7. The nutrient fluxes from submarine groundwater discharge (SGD) and inflow (a); metal element fluxes derived by SGD and inflow (b).
Figure 8. Pollution factor index of metal elements (Cr, Ni, Zn, Cu, As, and Pb) in the groundwater among three zones.
Table 1. The nutrient fluxes derived by submarine groundwater discharge (SGD) in the Daya Bay in comparison with other studies
Case studies SGD/(cm·d−1) Fluxes of nutrients/(mmol·m−2·d−1) Reference $ {\rm{NO}}_{\rm{x}}^ - $ $ {\rm{NH}}_4^ + $ DIN DIP DSi STE in Daya Bay, China 30.13 5.09 0.24 5.33 0.22 16.20 this study Daya Bay, China 1.18±0.43 ND ND 0.285 0.015 2.66 Wang et al. (2018a) Daya Bay, China 5.3 ND ND 3.62 0.1 ND Gao et al. (2018) Daya Bay, China 28.2–30.9 ND 0.656 ND 0.0012 1.794 Wang et al. (2017) Jiaozhou Bay, China 2.12–5.59 ND ND 4.36 0.04 2.49 Zhang et al. (2020) Sanya Bay, China (4.3±2.1)–(7.8±4.1) ND ND 8.54 0.1 13.85 Wang et al. (2018b) Bohai Bay, China 7.3 ND ND 7.78 0.15 9.26 Wang et al. (2020) Kakinada Bay, India 17.93 ND ND 121.21 9.50 199.00 Rengarajan and Sarma (2015) Masan Bay, Korea 6.1–7.1 ND ND 1.16–6.01 0.028–0.140 0.74–3.84 Lee et al. (2009) Note: STE: subterranean estuaries; ND: no data; DIN: dissolved inorganic nitrogen; DIP: dissolved inorganic phosphorous; DSi: dissolved inorganic silicon. 
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Table 2. The metal fluxes derived by submarine groundwater discharge (SGD) in the Daya Bay in comparison with other studies
Case studies SGD/(cm·d−1) Fluxes of metal elements/(μmol·m−2·d−1) Reference Cr Mn Fe Ni Cu Zn As Ba Pb STE in Daya Bay, China 30.13 27.88 143.41 1325.06 10.10 140.21 304.06 6.25 37.38 84.49 this study Bangdu, Korea 24.38 ND 2.5 270 2.4 0.7 ND ND ND ND Jeong et al. (2012) Jiaozhou Bay, China 0.0036–7.6 11.92 ND ND ND 5.87 3.85 0.13 ND 1.59 Qu et al. (2020) Greater Bay , China 4.57 0.81 ND ND 0.48 0.7 10.46 2.2 ND 0.03 Luo et al. (2022) Hailing Island, China 5–10 1.37 ND 70.88 0.2 0.37 24.94 1.88 10.45 0.082 Li et al. (2022) Bohai Bay 2.00–4.81 1.69–4.63 750.0–1687.5 125–375 ND ND 18.75–51.25 ND ND ND Wang et al. (2019) Note: STE: subterranean estuaries; ND: no data.
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