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 paper selected the coastal sandy beach of Beijin Bay, Guangdong Province as the study area. Based on the stratified sampling and analysis of the hydrochemical composition of coastal groundwater, this paper investigated the distribution characteristics, migration and transformation of nutrients in coastal groundwater, evaluated the submarine groundwater discharge (SGD) and associated nutrient flux into the sea, and explored the potential environmental impacts on coastal water. Results showed that compared with surface water, coastal groundwater had higher nutrient content. The concentrations of nitrate and nitrite ( \mathrmN\mathrmO_\mathrmx^- ), phosphate ( \mathrmP\mathrmO_4^3- ) and silicate (Si) in groundwater gradually decreased from land to sea, from shallow layer to deep layer, and non-conservative removal of \mathrmN\mathrmO_\mathrmx^- and \mathrmP\mathrmO_4^3- occurred after passing through the GSMZ. \mathrmN\mathrmO_\mathrmx^- was mainly removed by denitrification reaction, and the concentration of \mathrmN\mathrmO_\mathrmx^- from land to sea decreased by 95.81%. However, \mathrmP\mathrmO_4^3- was mainly removed by the adsorption of iron oxide/hydroxide end products. A hot area of ammonia nitrogen ( \mathrmN\mathrmH_4^+ ) was generated in groundwater in the middle of the aquifer, and non-conservative addition of \mathrmN\mathrmH_4^+ occurred, mainly due to the decomposition and release of organic matter. SGD rates with magnitude comparable to local river discharge were 1.49×10⁶ m³/d. SGD derived nutrients was estimated to be 983.0 kg/d for DIN, 37.00 kg/d for \mathrmP\mathrmO_4^3- and 4023 kg/d for Si, which is a key 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.