Seawater-Groundwater Exchange And Nutrient Transport In A Muddy Tidal Flat At The West Coast Of Jiaozhou Bay,China

Mudflats play an important role in increasing the availability of land resources,coast protection and keeping biodiversity.Intertidal zone is area with strong groundwater-seawater interaction and an important carrier of nutrients and contaminants.Mechanism analysis of groundwater-seawater interaction is of great practical significance in the study of nutrient and pollutant transport and biological characteristics of muddy tidal flat.This paper reported our field measurements in a typical muddy intertidal flat with a cross-shore length of 3km and an average slope of 0.1% in the west coast of Hongshi Ya town,Jiaozhou Bay,China.Eight “pair-wells” were installed along the whole transect to monitor the groundwater level,electrical conductivity and temperature.The measurement frequency was once per hour and the monitoring period was about 20 days.Hydraulic conductivity measured by the in situ falling head method at the typical wells ranges from 4.37×10-8 to 1.12×10-6m/s.Salinity measured at the 8 pair-wells ranges from 26.23 to 31.52g/L.Along the entire transect,the averaged submarine groundwater discharge(SGD)and Inflow during the 100 hours was estimated using the generalized Darcy law to be 2.22 and 1.77 m2/d,respectively.Based on the field monitoring data and the actual situation,a two-dimensional vertical profile model was established.The numerical simulations were conducted by MARUN code to quantify the groundwater-seawater interaction in the tidal flat.The influences of hydraulic conductivity,seepage face and the tidal run-up on the groundwater flow were investigated.Sensitivity analysis of tidal run-up effect and hydraulic conductivity of surface and middle layers in the model were conducted.Our work was the first study to simulate observed data of both the hydraulic heads and water exchange quantity along the land-sea boundary.The simulation results indicate the following main conclusions:(1)The seepage face length changed periodically with tidal forcing and reached its maximum length of ~2266 m at low tide.(2)The simulated groundwater heads and groundwater flow rates at the 8 pair-wells are more sensitive to the hydraulic conductivity in the surface layer than that in the middle layer.Tidal run-up may significantly influences the simulated groundwater heads and groundwater flow near the land-sea interface in large-scale muddy tidal flat.(3)The slope break affected thegroundwater flow significantly,the maximum flow rates(Inflow: 17.08 mm/d,outflow: 5.50mm/d)occurred in the vicinity of the slope break,showing that the slope break was playing an important role in the groundwater recharge and discharge processes.Finally,the numerical simulation results of SGD and Inflow values averaged over the 100 hours are 2.16 and 1.60m2/d,respectively,which are in line with the results obtained by the pair-well measurements.The SGD from seepage face accounted for 85.6% of the total SGD.Analysis of the nutrient data has the following conclusions.Results of conventional ions indicated that the study area was influenced by seawater and groundwater.The concentrations of NO2-N and NO3-N in groundwater were much larger than those in the seawater,probably due to the combined effects of chemical plant emissions and denitrification.The DIN/DIP ratio of groundwater was much higher than the Redfield ratio,maybe due to emissions of sanitary sewage and industrial wastewater.