Numerical Simulation Of Coastal Nutrient Input Processes Using Transport-Reaction Numerical Model

Previous studies have shown that Submarine Groundwater Discharge（SGD）is an important input path for inland chemical substances to the ocean,such as nutrients,heavy metals.At the regional scale,the influx of various chemical substances carried by SGD into the sea is comparable to the influx of rivers and other surface water bodies.Therefore,the study of groundwater dynamics in the aquifer of the coastal zone and the input of SGD-associated nutrients are of vital importance to the study of biogeochemical processes in coastal areas,the protection of coastal ecosystems and the rational development of marine resources.The traditional method for calculating the nutrient flux carried by groundwater is relatively straightforward,e.g.,simply approximated by the value of SGD multiplied by the average concentration of nutrients in groundwater.Obviously,such approaches ignore the spatiotemporal changes of nutrient concentrations in the near-shore groundwater,and therefore have greater uncertainty.The transport-reaction model provides a new idea for the quantitative study of the input of nutrients into the ocean through coastal aquifers.It considers the interaction among groundwater,mixing processes of fresh and brackish water and biogeochemistry.Therefore it can quantify the transfer and conversion of nutrients in the aquifer.In this dissertation,a nutrients transport-reaction model based on the MARUN program was established,and the migration and transformation of nutrients and related components（₃^-、₄⁺、₄^-、₂、、0）²⁺)in a typical sandy beach profile aquifer were simulated.The model considers the reaction network of nutrients and related components,and considers the recharge of dissolved oxygen in the unsaturated zone of the aquifer,and compares the distribution of nutrients and related components in the aquifer under different conditions.Based on the simulation results,the following conclusions are shown:（1）In the aquifer of the coastal zone,the reactions of nutrients and related components in groundwater are mainly controlled by redox conditions.Seawater contains a large amount of dissolved oxygen,which is highly oxidizing,while inland groundwater is generally oxygen deficient and has a strong reduction.Therefore,the salt-freshwater interface zone has also become the interface zone of the redox environment.A large number of chemical reactions took place here,and the zoom became the most active areas for nutrients and related components;（2）Oxygen recharge in the unsaturated zone in previous studies was often overlooked.In this study,we found that the oxygen replenishment of groundwater by the unsaturated zone is considerable.When considering the reaction network between the components,the oxygen in the unsaturated zone changes the inherent redox environment in the aquifer.A large amount of oxygen input causes a large number of chemical reactions in the upper part of the aquifer,changing the original components distribution of the aquifer and the flux of nutrients into the sea.