Examining Sulfate Behavior along a North-South Gradient of Unglaciated Eastern U.S. Catchments

Karen C. Rice1, James R. Webb2, Frank A. Deviney, Jr.2 and James P. Schaberl3

The U.S. EPA’s Direct/Delayed Response Project (DDRP) concluded that stream recovery from acidification would vary with the ability of the receiving catchment soils to store sulfur by sulfate (SO42-) adsorption. In the northeastern U.S., where Wisconsinan glaciation scraped away soils and gouged into bedrock, soils generally are young, thin, rocky, and largely unable to retain SO42-. In the unglaciated southeast, soils are older, thicker, have higher clay contents, and thus are more likely to retain SO42-. As such, there is a regional difference in SO42- retention, with higher retention in southeastern catchments. This suggests that northeastern streams should recover more quickly than southeastern streams from reduced sulfur dioxide (SO2) emissions resulting from the Clean Air Act Amendments of 1990 and subsequent reduced atmospheric deposition of SO42-. As SO42- deposition subsides, therefore, northeastern streams should experience an almost immediate decrease in SO42- export, whereas southeastern streams should continue to “leak” (desorb) SO42- stored on adsorption sites in the thicker soils. The DDRP indicated that streams in the Mid Appalachian Region range from being at steady state (input = output) with respect to SO42- to having greater than 90% net sulfur retention. Long-term monitoring in multiple forested catchments in the southeastern U.S. reveals that effects of the decreased SO2 emissions in stream waters have been mixed. There are decreases in SO42- concentrations in some streams but little change at others, indicating varying degrees of retention.

We tested the DDRP hypothesis by examining sulfate behavior along a north-south gradient of unglaciated catchments in the eastern U.S. By analyzing the cumulative net flux (input – output) of SO42- at the catchment scale with the longest-term data available, we can determine if the catchment is: 1) at steady state; 2) exporting SO42- (desorbing from soil sorption sites or there may be an internal source of SO42-); or 3) retaining SO42- (more SO42- being deposited by atmospheric deposition than is exported by stream water) and examine why nearby catchments respond differently. For example, at Catoctin Mountain, Maryland, we observe three catchments, each one demonstrating the three different SO42- dynamics described. Likewise, at Shenandoah National Park, Virginia, we observe a range of SO42- behavior. We offer ideas that may help explain SO42- retention variability across a large spatial gradient, and we conclude that the DDRP was on target with the prediction that Mid Appalachian Region streams would be diverse and slow to respond to decreased atmospheric SO2 emissions.

1 U.S. Geological Survey, 900 Natural Resources Dr., Charlottesville, Va., 22903; ; 434-297-0106
2 University of Virginia, Charlottesville, Va.;
3 Shenandoah National Park, Luray, Va.