Flowpath contributions of weathering products to stream fluxes at the Panola Mountain Research Watershed, Georgia

Short-term weathering rates (chemical denudation) of primary weathering products were derived from an analysis of fluxes in precipitation and streamwater. Rainfall, streamflow (runoff), and related water quality have been monitored at the Panola Mountain Research Watershed (PMRW) since 1985. Regression relations of stream solute concentration of major ions including weathering products [sodium (Na), magnesium (Mg), calcium (Ca) and silica (H4SiO4)] were derived from weekly and storm-based sampling from October 1986 through September 1998; runoff, seasonality, and hydrologic state were the primary independent variables. The regression relations explained from 74 to 90 percent of the variations in solute concentration. Chloride (Cl) fluxes for the study period were used to estimate dry atmospheric deposition (DAD) by subtracting the precipitation flux from the stream flux; net Cl flux varied from years of net retention during dry years to >3 times more exported during wet years. On average, DAD was 56 percent of the total atmospheric deposition (also assumed for the other solutes); average annual net cation and H4SiO4 fluxes were 50.6 and 85.9 mmol m-2, respectively. The annual cumulative density functions of solute flux as a function of runoff were evaluated and compared among solutes to evaluate relative changes in solute sources during stormflows. Stream flux of weathering solutes is primarily associated with groundwater discharge. During stormflow, Ca and Mg contributions increase relative to Na and H4SiO4, particularly during wet years when the contribution is 10 percent of the annual flux. The higher Ca and Mg contributions to the stream during stormflow are consistent with increased contribution from shallow soil horizons where these solutes dominate.