Abstract
This paper deals with the water quality characteristics of groundwater in Nakajima Island, which is located in the western part of Seto Inland Sea. Nakajima Island is mountainous with steep sloping granitic soil and the greater part of land is used as orange farm. Nutrient salts (T-N, NO_3-N, NO_2-N, NH_4-N, T-P, PO_4-P), pH, and EC of groundwater at selected 50 wells were measured once a month from December 1998 to November 1999. At the same time, the amount of fertilization, applied to the orange farms, was investigated to reveal its effects on groundwater qualities. The major results are as follows ; 1) Both the concentrations of nitrogen and hosphorus of groundwater are higher than those in rivers and reservoirs throughout the year. 2) Both the concentrations of nitrogen and phosphorus of groundwater in the southeast area of Nakajima Island are high, compared to those in the northwest area. 3) A perceptible positive correlation exists between the amounts of fertilization and the nitrogen concentration of groundwater. 4) Due to intensive fertilizations in spring and autumn, the nitrogen concetrations in most of the study area are over 10 mg/l in May and October.
