Effects of bedrock groundwater on hydrological and biogeochemical processes within riparian zones of two adjacent granite headwaters

Hydrochemical processes in headwaters are important for predicting streamwater quantity and quality. Riparian zones play an important role in the processes, and hillslope-riparian-stream linkage may be a key subject for the understanding of the role. This linkage is controlled by groundwater dynamics within catchments, but differences in hydrochemical processes within riparian zones among catchments with different runoff mechanisms are still unclear. To clarify the mechanisms causing differences in the processes within riparian zones among catchments, we observed groundwater and streamwater chemistry as well as runoff discharges and groundwater levels in two adjacent granite headwater subcatchments in Kiryu Experimental Watershed (KEW), Japan. Catchment H (0.4 ha) is a zero-order catchment, with a stonemasonry dam and a spring outflow point below it. In contrast, Catchment R (1.75 ha) is a first-order catchment, with a stream channel of 125m length. In Catchment H, annual bedrock infiltration is the larger and the mean residence time of streamwater is the shorter than those in Catchment R (Katsuyama et al., 2010). Saturated zones within the riparian zone were transiently present during summer rainy season in Catchment H, although they were perennially present in Catchment R. At the edge of perennially saturated zones in Catchment R, transient saturated zones were also present like in Catchment H. Considering the relationship between SiO₂ and Na⁺ concentrations, both of them were almost zero in throughfall and increased along with the water movement by the chemical weathering of minerals. They increased in the same ratio from the throughfall to the streamwater in Catchment H via the transient groundwater. However, the ratio of SiO₂ to Na⁺ decreased during the water movement from the transient groundwater to the streamwater in Catchment R. The SiO₂-Na⁺ relationships of the perennial groundwater and the streamwater in Catchment R were plotted on the line between the transient groundwater and the groundwater within the bedrock, which had low SiO₂-Na⁺ ratio. This fact means that the bedrock groundwater constantly recharges the riparian zone in Catchment R, although it may less contribute in Catchment H. Besides, in Catchment R, the hydrograph responded more quickly to rainfall than in Catchment H corresponding to the quick response of groundwater level in the perennially saturated zone. Moreover, the perennial groundwater in Catchment R had low concentrations of NO₃^- and SO₄^2-, and high concentrations of Mn and DOC in summer rainy season. These facts imply that the presence of perennially saturated zones contributes to the quick response to rainfall and the formation of a reductive environment. In conclusion, dynamics of bedrock groundwater affects the existence of perennially saturated zones, which causes various differences in hydrological and biogeochemical processes within riparian zones among granite headwater subcatchments.