Field investigations for sustainable groundwater utilization in the Konan basin

The Konan groundwater basin of Kochi Prefecture, Japan, is experiencing fre shwater shortages for the past few years, particularly during the winter se ason. The present study aimed at analyzing the problem and exploring increa sed groundwater withdrawals to meet burgeoning freshwater demands in variou s sectors. The trend analysis of water consumption indicates that the groun dwater demand would increase by 43 and 52% by the years 2010 and 2025, resp ectively, compared to 32 100 m(3) d(-1) at present. The streamflow of the M onobe River reduces to a minimum in winter and it shows a decreasing trend in recent years. The geologic investigation results indicate that unconfine d (phreatic) aquifers comprising alluvial sand and gravel and/or diluvial s ilty sand and gravel are dominant, and that the northwestern and western po rtions of the basin have greater potential for groundwater. Groundwater flo ws essentially from north to south into the ocean. Groundwater level fluctu ations over the basin vary appreciably in space and time, indicating a wide variation of natural recharge. Field observations confirm the hypothesis t hat stream-aquifer interaction also significantly contributes to groundwate r recharge, besides the rainfall infiltration. Further, the pumping tests r esults demonstrate that though the aquifers are low-yielding, enhanced grou ndwater withdrawals could be possible. The aquifer hydraulic conductivity i s characterized as high and ranges from 65 to 804 m d(-1) (mean = 229 m d(- 1)), thereby suggesting great aquifer heterogeneity. Finally, it is conclud ed that the long-term sustainability of the scarce groundwater resource of the Konan basin is doubtful, and that a comprehensive analysis of the groun dwater system is essential prior to formulating plans for the future ground water development and management.