Tracing solute mobility at the Panola Mountain Research Watershed, Georgia, USA: variations in Na+, Cl-, and H4SiO4 concentrations

Concentration variations of sodium (Na+), chloride (Cl-, and dissolved sili ca (H4SiO4) in rainfall, throughfall, soil water, groundwater and streamwat er were evaluated at the Panola Mountain Research Watershed near Atlanta, G eorgia, to determine how variations in concentrations of these solutes prov ide greater understanding of water quality evolution at the hillslope and c atchment scales. Stormwater moves rapidly to depth along preferred pathways in a deciduous forest hillslope, but the composition of the mobile unsatur ated zone water in the hillslope is not reflected in compositional variatio ns of streamwater during rainstorms. The Na+, Cl-, and H4SiO4 concentration s behave similarly in streamwater, decreasing with increasing discharge and increasing with water residence time. Consequently, the lowest flows are a ssociated with the highest concentrations and the oldest water. Streamwater composition is most similar to groundwater and stormflow variations reflec t a dilution of groundwater. Subtle differences in the relations among solu te concentrations and discharge reflect different sources, especially for C l-. For example, the residence time of groundwater, as inferred from landsc ape position, generally is positively related to Na+ and H4SiO4 concentrati ons, but not to Cl- concentrations. The Na+ and H4SiO4 are derived from min eral weathering and are continuously supplied along hydrological pathways. In contrast, Cl- is derived from atmospheric deposition and is affected onl y by evapotranspiration (ET) and transport. ET increases Cl- concentrations in matrix soil waters, which are subsequently transported to the saturated zone where Cl- is effectively isolated from further evaporative concentrat ion.