Characterizing the regional pattern and temporal change of groundwater levels by analyses of a well log data set

摘要

Preservation of the amount and quality of groundwater resources is an important issue around the world. Changes in groundwater levels need to be monitored in efforts to preserve groundwater. This study investigates suitable methods to characterize changes in the groundwater level and determine the factors involved. The area of Kumamoto, a city in central Kyushu, southwest Japan, was selected to demonstrate the usefulness of the methods because this area is one of the richest in Japan in terms of groundwater resources and takes all its water from groundwater. Data of the groundwater level recorded at 69 wells from 1979 to 2007 were used in geostatistical and correlogram analyses. First, strong correlation between the topography and groundwater level was identified. Incorporating this correlation into spatial modeling of the groundwater level, co-kriging was demonstrated to be more accurate than ordinary kriging. The co-kriging results clarified the hydraulic characteristics of the Kumamoto area; the patterns of shallow and deep groundwater levels were agreeable generally, and the general trends of their annual average levels were similar regardless of precipitation. Another important feature was that the correlograms for the precipitation amount and groundwater level had a constant shape and changed smoothly with a change in lag time regardless of the precipitation only in the area of Togawa lava. These characteristics are probably due to the connections between shallow and deep aquifers and the high permeability of Togawa lava.