PUSH-PULL TEST FOR SALTWATER / FRESHWATER INTERFACE SURVEY AT A COASTAL AREA

Spatial distribution of saltwater / freshwater interface is more complicated in the deeper aquifer than in the shallow one. Diffusion is one of major force to form saltwater / freshwater interface in a deep aquifer where groundwater flows very slowly. Because of the very low hydraulic gradient, old freshwater recharged at the last marine regression may remain in the aquifer without infiltrating seawater from the sea bottom. Therefore, it is important to understand the behavior of saltwater / freshwater interface with the sea level change by the repetition of marine regression and transgression, especially in a deep aquifer. In this study, push-pull test was planned to imitate saltwater intrusion and freshwater washout artificially. Test site is located about 300m close to the coastline, at the northern part of Hokkaido, the northernmost island of Japan. The 100m depth borehole was used with the screen 91-100m. The corresponding sediments are rich in sand and poor in clay, and look well-infiltrated. The hydraulic conductivity was given as 6.8 x 10^-6 m/s from the previous laboratory test. At the saltwater test, artificial saltwater manufactured from several chemicals which are available on the market, were injected as tracer water. At the freshwater test, deionized water was made from filtering system in the field and injected. The approximate volume of injection and pumping were 8 m³ and 25 m³, respectively, in the both tests. Groundwater sampling was executed continuously to draw a breakthrough curve (BTC) during pumping. The BTC of saltwater test showed different decrease trends between chloride and sodium ions. The chloride concentration dropped rapidly during pumping, but the sodium concentration kept stable longer time than chloride. On the other hand, the BTC of freshwater test showed almost same time series of concentration with chloride and sodium. Therefore, sodium ion was retarded only at the saltwater test, and it might be caused by adsorption in the aquifer. It indicates sodium ion may retard when the salt concentration of groundwater is decreasing, which is the same condition with marine regression.