A Study Of The Fresh-salt Water Interface And Groundwater Regimes In Coastal Zones

The study area lies in the northern coast of the Beibuwan and near Beihai, Guangxi. In the1980s, seawater intrusion began to happen in Beihai. The shape and motion of the fresh-saltwater interface, the dynamic characteristics of groundwater, the fluctuation period of tide andcoastal groundwater, the time of coastal groundwater lag tide are described in the thesis. Theycan help prevent the seawater intrusion and protect the freshwater resources in Beihai.This paper studies the stability and the shape characters of fresh-salt water interface underthe effect of the difference of density and head. The software SEAWAT is used to build atwo-dimensional model to make a numerical simulation based on the experiment. The resultsshow that the head differences of fresh-salt water are4cm,2.5cm and2cm. The less the headdifference is, the interface moves further towards the freshwater and has a convex shape. Thegreater the head difference is, the interface moves shorter towards the saltwater and has aconcave shape. The density differences also have influence on the interface. The greater thedensity difference is, the interface is relatively more stable and has a flat long shape with a slowslope. The less the density difference is, the interface is more unstable and has a flat short shapewith a steep slope.In this paper, four groups of time series observation data are used to study the dynamiccharacteristics of groundwater levels in coastal areas. The period of tide and the coastalgroundwater is calculated using the method of the spectrum analysis. Data shows that the tideand the coastal groundwater levels have a short period of half a day and one day and have a longperiod of about15days.Groundwater levels of observation wells in the same location have significant differencesat different depths. The maximum groundwater level amplitude is about1.35m in the wellSK1-3which has the deepest depth. The minimum groundwater level amplitude is about0.33min the well SK1-4which is the shallowest. The amplitude reflects the change in groundwaterlevels influenced by the tides in coastal areas. It shows that the fluctuation of groundwater levelsin unconfined aquifer is less than that that in confined aquifer.In the same aquifer, groundwater elevations are also different in different positions. If theobservation well is close to the seaside, the water elevation is higher, on the contrary, if the observation well is far from the seaside, the water elevation is relatively lower. The coastalgroundwater appears the periodic change under the influence of the periodic change of the tide.But the two changes don’t happen at the same time, the fluctuation of costal groundwater levelshave time lags behind the tide. The distances between the six observation wells and the seasideare710m，780m，1150m and1366m, and the time lag of coastal groundwater lelvels are2.089h,2.089h,3.133h,3.655h,4.177h and4.569h. Linear relationships exist between the time lagand the distance increase along with the seashore distance.