Abstract
In general, chlorine content in groundwater on the coastal area are said to be relatively high, due to subsurface salt water invasion or seepage of salt water which intruded in to river water. Wind driven salt particles falling into surface water bodies may cause to increase chlorine content in groundwater. While, fossil sea waters are sometimes found in areas far apart from the present-time coastline.
The origin of chlorine in groundwater, therefore, should be investigated if it has been supplied under present hydrologic conditions.
The authors investigated the distribution of chlorine content in groundwater in the alluvial plain of the lower reaches of the Tone River. After careful consideration of the origin of chlorine there, we have arrived at the following results.
1) In the alluvial plain of the lower reaches of the Tone River, chlorine contents in ground-water is very high.
2) Chlorine contents are different for areas of the right-hand and the left-hand side of the Tone River. On the right-hand side (Omigawa and Sawara districts), there are several points whose chlorine contents are over 1000 mgcl-/L. These points are distributing forming band.
3) In the Omigawa district, the band area coincides with a valley shown in Figure 4.
4) The directions of the band areas have no correlation with present landform, but coincide with the directions of fossil valley of the top of diluvium, i.e. the base of alluvial deposits.
5) Chlorine contents of these groundwater are of the same order as that of confined water tapped in the river bed of the Tone River.
6) Based on the facts above-mentioned and the land history of the alluvial period in the investigated areas, it is considered that chlorine of the band areas is originated from fossil sea water.
7) Judging from the age of deposition of the main unconfined aquifer, the absolute age of fossil sea water can be considered to be 10 to 15 thousands years.
8) The pH value of fossil sea water is 7.0 (neutral) to 6.5.
