Tracing Hydrologic Pathways Using Chloride at the Panola Mountain Research Watershed,Georgia, USA

An analysis of chloride (Cl-) concentrations and fluxes at the 41 ha Panola Mountain Research Watershed indicates that Cl- may be used effectively to differentiate "new" and "old" water flow through the hillslope and their respective contributions to streamwater. Rainfall and throughfall, the "new" water inputs, are marked by low Cl- concentrations (<15 µeq L-1). Stormwater moves rapidly to depth along preferred pathways in a deciduous forest hillslope, as evidenced by low Cl- concentrations (<20 µeq L-1) in mobile soil water from zero-tension stainless-steel pan lysimeters. "Old" waters, matrix soil waters and groundwater, typically have high Cl- concentrations (>30 µeq L-1). Timing of soil water transport is not sufficiently rapid to suggest that soil water from this hillslope site (20 m from the stream) contributes to streamwater during individual rainstorms. The source of streamflow, therefore, must be a combination of channel interception, overland flow and soil water from near-channel areas, and runoff from a 3 ha bedrock outcrop in the headwaters. Groundwater contribution to streamflow was estimated using Cl- concentrations of throughfall and groundwater as the two end members for a two-component hydrograph separation. For the study period, groundwater contributed 79% of the streamflow and from 1985 to 1995, contributed 75% of the streamflow. Rainfall was the source of 45% of the Cl- flux from the watershed in the long term; the remaining Cl- is hypothesized to be derived from dry deposition, consistent with the enrichment noted for throughfall. At peak flow during individual rainstorms, "new" water can contribute 95% of the runoff.