ABSTRACT
In the Kanto plain, the largest Quaternary groundwater basin in Japan, water samples were obtained from 130 water supply boreholes to characterize the groundwater system. The spatial variation in chloride (Cl−) concentration indicates that groundwater with high ClCl− concentrations of more than 100 mg/l (up to 216 mg/l) occurs at the centre of the plain and at depths between 200 m and 450 m below ground surface. The area with the ClCl− -rich groundwater, spreading from the northwest to southeast, corresponds with the so-called Motoarakawa tectonic zone (8 km wide by 35 km long) bounded by the faults on its longer sides. We have found the Cl−-rich groundwater is also characterized by low δD, low δ 18O, small d-parameter, and low 14C values. Both the chemical and isotopic evidence strongly suggests that the Motoarakawa tectonic zone divides the groundwater system in the Kanto plain into three distinct hydrologic sub-areas. Two faults, which delineate the Motoarakawa tectonic zone, act as barriers to the southward and eastward regional flow of groundwater in the Kanto plain. As a result, the Motoarakawa tectonic zone has been under an isolated hydrologic environment for a long period of time, resulting in the occurrence of groundwater with anomalous hydrochemistry. With regard to the origin of isotopically-depleted groundwater in the tectonic zone, a potential source is assumed to be precipitation in a cooler climate than the present. Admixture of interstitial water (residual sea water) squeezed out of the adjacent aquitards and/or upward movement of the deep-seated Na-Cl groundwater along the faults is likely to account for the elevated ClCl− concentrations of groundwater in the tectonic zone.
