Impact of Groundwater Level Decrease and Sea Level Fluctuations on Potential Saltwater Intrusion in the Subsurface Coastal Area of West Hokkaido, Japan

International Symposium on "Dawn of a New Natural History - Integration of Geoscience and Biodiversity Studies". 5-6 March 2004. Sapporo, Japan.
In order to limit possible inland seawater intrusion, important freshwater resources contained in coastal aquifers should be exploited carefully. Salinization of coastal waters is often due to inflow of dense saline water from the sea or deep inland geological layers, during heavy groundwater withdrawals, drought or long-term sea level rise. A continuous trend in the decrease of groundwater level and land subsidence, resulting from freshwater over-pumping, has been noticed in the subsurface of Hokkaido's coastal area facing the Sea of Japan, since the 1960's. This could lead to the decline of groundwater quality. This decline may be further amplified by seawater level increase. Past sea level records along this coast have shown continuous oscillations over various timescales of years and decades. A particularly high rate of sea level increase (3.2 mm/year) compared to the rise of the world ocean level, has been observed towards the northern edge of this coast. To avoid inland saltwater encroachment causing salt contamination of coastal aquifers, it's vital to determine the position of the seawater-freshwater interface and control its movement. For this purpose, water chemical analyses from drilled wells as well as analytical and numerical simulations are often employed to approximate the location of this boundary and understand the relevant processes that cause saltwater intrusion in coastal aquifers. For the present study, two modeling solutions are used to determine the shape and position of the interface between the landward potential seawater intrusion and subsurface freshwater outflow to the coast. Then, investigations are conducted to examine the impact of groundwater level decrease and sea level fluctuations, on the extent of this saltwater interference in the dynamics of the coastal flow system of the area.