Nutrient dynamics and discharge in a coastal sandy beach aquifer

It is of great significance to understand the behavior of nutrients in the groundwater seawater mixing zone (GSMZ) and quantify the input of terrestrial nutrients into the sea. This study focuses on the coastal sandy beach of Beijin Bay, Guangdong Province. Based on the stratified sampling and analysis of the hydrochemical composition of coastal groundwater, this study investigated the distribution characteristics, migration, and transformation of nutrients in coastal groundwater.The submarine groundwater discharge (SGD) and associated nutrient flux into the sea were also evaluated, exploring the potential environmental impacts on coastal water. The results show that compared with surface water, coastal groundwater had higher nutrient content. The concentrations of nitrate and nitrite ($ {\mathrm{N}\mathrm{O}}_{{x}}^{-} $), phosphate ($ {\mathrm{P}\mathrm{O}}_{4}^{3-} $) and silicate (Si) in groundwater gradually decreased from land to sea and from shallow layer to deep layer. Non-conservative removal of $ {\mathrm{N}\mathrm{O}}_{{x}}^{-} $ and $ {\mathrm{P}\mathrm{O}}_{4}^{3-} $ occurred after passing through the GSMZ. $ {\mathrm{N}\mathrm{O}}_{{x}}^{-} $ was mainly removed by denitrification reaction, with the concentration decreasing by 95.81% from land to sea, while $ {\mathrm{P}\mathrm{O}}_{4}^{3-} $ was mainly removed primarily by the adsorption to iron oxide/hydroxide end products. A hotspot of ammonia nitrogen ($ {\mathrm{N}\mathrm{H}}_{4}^{+} $) was generated in the middle of the aquifer, and non-conservative addition of $ {\mathrm{N}\mathrm{H}}_{4}^{+} $ occurred, mainly due to the decomposition and release of organic matter. The estimated SGD rate was 1.49×10⁶ m³/d, comparable to local river discharge. SGD-derived nutrients were estimated to be 983.0 kg/d for dissolved inorganic nitrogen (DIN), 37.00 kg/d for $ {\mathrm{P}\mathrm{O}}_{4}^{3-} $, and 4023 kg/d for Si, making SGD a a significant source of nutrients to coastal waters. In addition, groundwater had a high ratio of nitrogen to phosphorus (mean: 139.6) and ratio of silicon to phosphorus (mean: 274.1), while the ratios in seawater were 21.03 and 33.12, respectively. SGD with high ratio of nitrogen to phosphorus had important impacts on the nutrient structure of coastal seawater. Sandy beaches are widely distributed, and the findings of this study can provide scientific basis for the management of ecological environment in similar areas.