Base cation concentrations in subsurface flow from a forested hillslope: The role of flushing frequency

A 20-m-wide trench was excavated to bedrock on a hillslope at the Panola Mo untain Research Watershed in the Piedmont region of Georgia to determine th e effect of upslope drainage area from the soil and bedrock surfaces on the geochemical evolution of base cation concentrations ir: subsurface flow. S amples were collected from ten 2-m sections and five natural soil pipes dur ing three winter rainstorms in 1996. Base cation concentrations in hillslop e subsurface flow were generally highest early and late in the storm respon se when flow rates were low, but during peak flow, concentrations varied li ttle. Base cation concentrations in matrix flow from the 10 trench sections were unrelated to the soil surface drainage area and weakly inversely rela ted to the bedrock surface drainage area. Base cation concentrations in pip e flow were lower than those in matrix flow and were also consistent with t he inverse relation to bedrock surface drainage area found in matrix flow. The left side of the trench, which has the highest bedrock surface drainage area, had consistently lower mean base cation concentrations than the righ t side of the trench, which has the lowest bedrock surface drainage area. D uring moderate size rain events of about 20-40 mm, subsurface flow occurred only on the left side of the trench. The greater volume of water that has flowed through the left side of the trench appears to have resulted in grea ter leaching of base cations from soils and therefore lower base cation con centrations in subsurface flow than in flow from the right side of the tren ch. Alternatively, a greater proportion of flow that bypasses the soil matr ix may have occurred through the hillslope on the left side of the trench t han on the right side. Flushing frequency links spatial hillslope water flu x with the evolution of groundwater and soil chemistry.