Seawater-Groundwater Exchange In A Silty Tidal Flat In The South Coast Of Laizhou Bay, China

Laizhou Bay is an important aquatic breeding base in Northern China. In recent years,following the development of marine economy, the environmental quality is decreased due to groundwater table depression, seawater quality decline and pollution. As an essential part of global water cycle, Submarine Groundwater Discharge is an important source of various marine chemical substances including pollutants. SGD is found to be not less than the amount of surface water input in some coastal areas, so is the marine chemicals including various pollutants carried by it. There were few researches about seawater-groundwater exchange in Laizhou Bay,especially in the low-permeability silty tidal flats with very gentle slopes in the south. Large scale of seepage face was formed there during low tides. This paper reports the monitoring data and preliminary analytical results on a typical transect in a silty tidal flat in the south coast of Laizhou Bay.We selected 14 locations along a typical transect in a silty tidal flat, which has a cross-shore length of about 3500 m and slope of 0.4‰, to install 14 pair-wells（named as W1, W2, …, and W14 sequentially seaward）. Every pair-wells was used to monitor the seawater/groundwater head, salinity and temperature at two different depths simultaneously once per hour from 20:00August 10 th to 23:00 September 12 th, 2014. The vertical hydraulic conductivity of intertidal sediments measured in situ by the multi-diameter falling-head method ranges from 5.4×10^-7 to1.1×10^-5 m s^-1. Based on the observed data and the “pair-well vertical hydraulic gradient method”, the submarine groundwater discharge and inflow along the entire transect were estimated to be 161.5 and 4.6 m³ d^-1 m^-1, respectively. It is found that 81.6% of the total SGD occurred between W6 and W10, probably because of a freshwater discharge tube nearby, and the high hydraulic conductivity there which is one or two orders of magnitude greater than that at other wells. And 63.7% of the total inflow occurred between W3 and W5, probably because of low groundwater level between W3 and W5 located in the high-tide zone caused by the pumping of salt field nearby.If estimated by the “single-well vertical hydraulic gradient method”, the SGD and inflowwere 213.5 m³ d^-1 m^-1 and 13.3 m³ d^-1 m^-1 respectively. The SGD between W3 and W5 decreased by 4.6 m³ d^-1 m^-1, while the inflow increased by 42.3 m³ d^-1 m^-1 when the nearby salt field pumped under brine to obtain the well salt. If the study area is assumed to be homogenous,taking the geometric average of 5.4×10-7 to 1.1×10-5 m s^-1(3.5×10^-6 m s^-1) as the hydraulic conductivity, then the SGD and Inflow value estimated by the “pair-well vertical hydraulic gradient method” were 60.5 and 4.4 m³ d^-1 m^-1. Neglecting the density effect may lead to an overestimation of the SGD by 27.6% and underestimation of the inflow by 37.9%. In addition, a comparison is made between "the pair-well vertical hydraulic gradient method" and "single-well vertical hydraulic gradient method", and analyzed the reasons of the differences between the two methods.Based on this research, further understanding of the hydrogeological conditions and seawater-groundwater exchange mechanism was obtained about the south coast of Laizhou Bay after this research. This study also provides a theoretical basis for the environmental pollution prevention in the coastal region of Laizhou Bay.