Abstract
Hadano basin, located at the centre of K a nagawa Prefecture, is composed of high permeability alluvial fan where lots of springs are distributed. High precipitation condition in this basin is favourable to examine the relationship between groundwater chemistry and surfacewater chemistry. Nine springwater samples, and three riverwater samples collected every month from August 1998to October 1999 in Hadano basin were measured.
The seasonal variation of chemical composition of most constituents in groundwater was observed in this area. The concentrations of most constituents increased in wet season, while decreased in arid season. This result indicates the increasing of most constituent concentrations in groundwater is due to the penetration of surfacewater (soilwater) with high concentration of constituents to the groundwater. Surfacewater-groundwater mixing model was applied to explain the seasonal variation of NO3- and Cl-. It was found by this calculation that maximum mixing ratio of soilwater and riverwater to groundwater is 0.5 and 0.05, respectively. The cation exchange model and plagioclase dissolution model in soil showed that high Na concentration in shallow soilwater (30cm) depth and high Ca concentration in deep soilwater (100cm) depth is likely due to cation exchange and plagioclase dissolution, respectively. The compositional change with depth from Na dominance to Ca dominance was actually appeared in the soilwater. The surfacewater-groundwater mixing model was also applied to explain Na and Ca concentration variations with depth. This model indicates that approximately 60% of Na and 35% of Ca in groundwater came from surface soil.
