Surface water-groundwater interactions and associated nutrient loading into the coastal waters: An integrated modeling study in the Guangdong-Hong Kong-Macao Greater Bay Area, China

River deltas typically have high population density and support a wide range of intensive and prosperous socioeconomic activities. The coastal waters near river deltas face significant challenges of eutrophication caused by excessive terrestrial nutrient loading carried by both river runoff and submarine groundwater discharge (SGD). The hydrological cycle in delta areas is complex under the joint influences of meteorological-driven forcings and ocean tidal forcing. Hydrological processes in the presence of highly dynamic river-aquifer-sea interactions have rarely been explored via integrated hydrological modeling approaches. In this study, a fully integrated numerical surface water-groundwater model was developed for the Guangdong-Hong Kong-Macao Greater Bay Area, which is the world’s largest urban area in terms of both size and population. The model’s accuracy was validated and cross-checked using observation data from gauging stations and independent remote-sensing products such as soil moisture, evapotranspiration (ET) and total water storage anomalies (TWSA).

Based on the 10-year simulation results (2004-2013), the major findings of this study are as follows: 1) it is necessary to include tidal forcing, in addition to conventional meteorological-driven forcing, to capture the characteristics of long-term hydrological fluxes and states while simulating short-term flow dynamics; 2) the flow-model-computed average SGD rate per unit length of the coastline for the Greater Bay Area is 3.01 m³/d/m, which is comparable with those derived from water budget approaches but 1-2 orders of magnitude lower than the total SGD estimated by isotopic tracing methods; 3) the controlling factor for SGD was tidal forcing on the hourly time scale, and terrestrial hydrological processes on the monthly and annual time scales, respectively; and 4) an integrated hydrological model can be used to identify distinct and large subsurface zones sensitive to tidal fluctuations, quantifying the pivotal role of ocean tides in shaping the coastal groundwater system. Based on the SGD flux simulated by the integrated flow model, the annual average nutrient loadings delivered by fresh SGD into the sea were estimated to be 9.14×10⁶ kg/a for DIN (dissolved inorganic nitrogen, referring to the sum of nitrate nitrogen and ammonium nitrogen in this study) and 0.95×10⁶ kg/a for DIP (dissolved inorganic phosphorus). The percentages of DIN and ammonium nitrogen loadings carried by fresh SGD to those by river runoff were about 1.6% and 11.6%, respectively. Although the proportion was relatively small, the absolute amount of nutrient loading carried by fresh SGD was quite considerable. This study represents a first step in using an integrated hydrological model simultaneously driven by meteorological and tidal forcing to explore regional hydrological processes impacted by complex river-aquifer-sea interactions in a large delta area.

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