Abstract
Okinoerabu Island is a typical Island located in the subtropics and covered with coral limestone which is characterized by a high water permeability. Intensive floriculture has been actively promoted in recent years on this island. In this report the concentrations of nitrate and some other chemical components in the groundwater mainly from the area with intensive cultivation on this island were determined. The relations between the nitrate concentration and the geographical characteristics or land use was reviewed. Furthermore, the nitrate concentration in the water of several wells for municipal water supply that had been recorded over a long period of time was examined in relation to land use in the surrounding areas.
1. The distribution of the nitrate concentration of groundwater in this area was not related to the geographical characteristics of the land. Instead, it showed an apparent increase from the central area of the village toward the cultivated fields.
2. In the study area, the proportion of cultivated land was very high and a relatively large part of the farms was used for intensive floriculture. The nitrate concentration of groundwater in this area was generally high. The concentration of Seven out of 17 samples exceeded 10 ppm.
3. Among the wells for municipal water supply in Wadomaritown on this island, the water of some wells which were surrounded by farms showed a continuous increase in nitrate concentration over a long period of time. On the other hand, the concentration of the water of the wells located near a preserved forest was low.
4. These results indicate that agricultural activity on the ground exerts a significant influence on the underground water environment in this island. The strategy for land use or agricultural development must be planned in considering the need for the preservation of the quality of drinking groundwater.
