VARIATIONS IN AQUEOUS SULFATE CONCENTRATIONS AT PANOLA MOUNTAIN, GEORGIA

Aqueous sulfate concentrations were measured in incident precipitation , canopy throughfall, stemflow, soil water, groundwater, and streamwat er at three locations in a 41 ha forested watershed at Panola Mountain State Park in the Georgia Piedmont. To evaluate the variations in sul fate concentrations, sampling intensity was increased during storms by automated collection of surface water and by incremental subsampling of rainfall, throughfall, and soil solution. Canopy throughfall, stemf low, and runoff from a bedrock outcrop in the watershed headwaters wer e enriched in sulfate relative to incident precipitation due to washof f of dry deposition that accumulated between storms. Soil waters colle cted from zero-tension lysimeters at 15 cm and 50 cm below land surfac e also were enriched in sulfate relative to precipitation, groundwater and streamwater. Sulfate concentrations in groundwater and in streamw ater at base flow varied in an annual sinusoidal pattern with winter m axima and summer minima. Stream discharge and groundwater levels varie d in a similar annual pattern in phase with the sulfate concentrations . The temporal variability of sulfate concentrations at most groundwat er sites was small relative to the spatial variability among groundwat er sites. Streamwater sulfate concentrations during base flow were con trolled by low-sulfate groundwater discharge. As flow increased, an in creasing proportion of shallow, high-sulfate groundwater and soil wate r contributed to streamflow. The dominant control on stream sulfate co ncentration shifted from sulfate retention by adsorption in the minera l soil at base flow to mobilization of sulfate from the upper, organic -rich horizons of the soil at high flow.