Hydrogeology and geochemical characterization of groundwater in a typical small-scale agricultural area of Japan

Tsukuba city is located in the center of the Kanto Plain, the largest basin in Japan. Its dramatic increase in population and agricultural production make it imperative to quantify the groundwater system to ensure the long-term sustainability of the resource. In response to this need, the hydrogeology and chemistry of groundwater in a typical agricultural catchment of the region were characterized on a detailed scale in order to serve as a scientific tool for the local management authorities. The uplands include two aquifers separated by confining clays. In the lowlands, these aquifers behave as a single unit. Infiltration from precipitation recharges the groundwater, which moves laterally through the sands and discharges mainly into an artificial drainage. The age of groundwater increases with distance from the drain, with a maximum of 63 years for particles that originated at the catchment divide. The chemistry of shallow groundwater reflects a fertilizer's source, with approximately 75% of the samples collected within the croplands exceeding the Japan's drinking water standards for NO3−. Biochemical processes and the absence of agricultural practices reduce the salts concentrations throughout the rest of the area. Better water quality was found at depth, where groundwater of Ca–HCO3 type prevails.