The land subsidence due to groundwater pumping in Senshu area, southern part of Osaka pref., began in 1969 and the highest value amounted to 40 cm for five years. The principal aguifers in this area are the sand and gravel strata in the Plio-Pleistocene Osaka Group, The water quality of 53 wells was examined during 1977 to 1980, and the chemical charactreistics were investigated. Among the main constieuents in water the alkalinity appeared to be related with the amount of subsidence. Using another data of 50 wells examined in 1972 besides the above data, the spatial distribution of alkalinity was checked more closely. The area pumping the water of high alkalinity coincides with the severe subsidence area. The alkainity also related with both of the concentrations of ammonium nitrogen and phosphate phosphorus. It is said, accordingy, that the groundwater in this subsidence area is characterized by high concentrations of C, N and P compounds.
The reasons for this relationship are that (1) the younger marine clayey strata which serve as a con fining beds are rich in organic matter, (2) in an anaerobic condition the organic matter produces bicarbonate, ammonium and phosphate, and (3) heavy pumping leads to lowering piezometric head which results in squeezing out the pore water with high concentrations of C, N and P compounds through the confining beds.