Hydro-geochemical evolution of groundwater in an alluvial aquifer: Case of Kurokawa aquifer, Tochigi prefecture, Japan

摘要

The northern Kanuma alluvium region in Japan is known for intensive agricultural activities. Groundwater is the main water resource used for drinking and irrigation purposes in the region. As groundwater quality assessment is an essential component for its safer use. For this study, a field survey was carried out on February 23rd, 24th and 25th, 2005. Thirty-six groundwater and surface water samples were taken from the uniformly distributed water wells and representative river points. The measurements on various water quality parameters were conducted on surface water and groundwater samples taken from various sites covering an area of approximately 9 km~2. Influence of land use on hydrochemistry of groundwater was investigated through analyzing the water chemistry at selected points in the region. Water quality classification was derived through a multitude of diagram methods and equations. Results of this study indicate that the groundwater in the region is mainly of Ca-HCO3 and shallow meteoric water percolation type. Furthermore, water chemistry seems to be influenced by the topography of land. Thus, samples were classified into three distinct groups according to their geochemical characteristics: upland, floodplain and stream groups. The examination of water chemistry variation along the flow line indicates that the chemical fertilizers and organic matter originate from high concentrations of nitrate and bicarbonate in uplands. However, in floodplain, denitrification process and dissolution of dolomite can explain the groundwater chemical characteristics. In downstream region, freshening phenomena and dilution processes seems to be dominant.