Seawater-groundwater Exchange In The Intertidal Zone At Haimiaohou In Laizhou Bay

Studying the effects of seawater-groundwater exchange interaction in coastal aquifers, including submarine groundwater discharge, groundwater dynamics and solute transport, has an important theoretical and practical significance to evaluate the groundwater resources in coastal areas and to correctly distinguish the sources of nutrients in coastal ocean, as well as to improve the marine aquaculture model.In the past, the study of the seawater-groundwater exchange in the coastal zone is mainly concentrated in the sandy and gravel beaches. There were few studies about seawater-groundwater exchange in mudflat which is widely distributed in coastal areas of China and other countries. This paper reports the field experimental results and their preliminary analyses on a typical transect in mudflat at the east coast of Laizhou Bay, China. Taking into account the effects of seepage face and density, the “pair-wells method”, which was an improvement of the single-well method used previously in this site, which was used to estimate the seawater-groundwater exchange rate. We selected 6 locations in a transect perpendicular to the coastal line in the intertidal zone to install pair-wells for monitoring the groundwater dynamics. The pair-wells are numbered W1, W2, W3, W4, W5 and W6 in the seaward direction. The mudflat transect has a slope of 1.8‰ and cross-shore length of 450 m. Each pair-well has two LTC-Divers installed at two different depths to monitor the groundwater head, temperature and electric conductivity simultaneously once per hour from 00:00 August 22 th to 00:00 September 21 th, 2013, a total of 720 hours. The vertical hydraulic conductivity of the sediments at each well was also measured by an in-situ multiple diameter falling-head method. The vertical hydraulic conductivity ranges from 4.06×10-6 ms-1 to 7.30×10-5 ms-1. The vertical hydraulic conductivity at W2 is the largest, and that at W1 is the smallest. The average salinity of the upper and lower Diver in each observation well ranges from 14.2 g L-1 to 24.5 gL-1. The average salinity is the lowest in the lower Diver at W1, and that is the highest in the lower Diver at W5. Due to the larger soil disturbance after the installation of W2, the groundwater level at W2 is below the elevation of the upper Diver during the low tides. As a result, the upper Diver changes from the saturated to unsaturated status periodically. The pressure measured by the upper Diver becomes negative pressure periodically. Based on the generalized Darcy’s Law, we calculated the groundwater exchange rate, the submarine groundwater discharge(SGD) and the inflow along the entire transect were estimated to be 11.1 m2 d-1 and 21.0 m2 d-1, respectively. The inflow accounts for 42.6% of the total inflow of the entire transect occurred between W4 and W6, and the discharge(SGD) accounts for 43.9% of the total SGD occurred between W4 and W6. The submarine groundwater discharge(SGD) in this study was 1.3 times of SGD obtained by the previous studies, and the inflow was 1.4 times of the inflow obtained by the previous studies.