DEVELOPING CRITICAL LOADS OF NITRATE AND SULFATE DEPOSITION TO WATERSHEDS OF GREAT SMOKY MOUNTAINS NATIONAL PARK, UNITED STATES

Zhou, Qing; Driscoll, Charles; Moore, Stephen; Kulp, Matt; Renfro, James; Schwartz, John; Cai, Meijun

Report describing critical load (CL) modeling effort using PnET-BGC model on 12 GRSM streams of various size and elevation at various time steps and reduction scenarios. Long-term impacts of acidic deposition on Great Smoky Mountains National Park include elevated inputs of sulfate and nitrate, the depletion of available base cations from soil, acidification of high elevation streams and extirpation of trout. Critical Loads and dynamic critical loads (CLs/DCLs) are useful tools to evaluate ecosystem response to controls on acidic deposition and help guide future air quality management. We evaluate the application of CLs/DCLs of nitrate and sulfate deposition for 12 watersheds in Great Smoky Mountains National Park (GRSM), USA using the hydrochemical model, PnET-BGC. Twelve sites were chosen for model application based on a block design to represent characteristics of the entire Park. Two of the streams studied are listed by the state of Tennessee as impaired due to low stream pH, and have acid neutralizing capacity (ANC) ranging from -14 μeq/L to 60 μeq/L. We reconstructed historical meteorological, atmospheric deposition and land disturbance data for study watersheds for the period 1850 to present for model hindcasts. As future emissions are expected to decline, the model was run under a range of future scenarios from 2008 to 2200 of decreases in sulfate, nitrate and ammonium, and combinations of sulfate and nitrate deposition to estimate CLs and DCLs to evaluate how watersheds might respond to emission control strategies. Model simulations of stream chemistry generally agreed with long-term (>10 yr.) observations. Results of model simulations also compare favorably with biogeochemical data from the long-term study watershed, Noland Divide Watershed. Model simulations suggest that stream response to historic atmospheric sulfate deposition is in part controlled by soil sulfate adsorption and mobilization of base cations from soil exchange pools. Retention of atmospheric nitrogen deposition is limited in some watersheds resulting in elevated leaching losses of nitrate. Model hindcasts indicate that watersheds in GRSM are inherently sensitive to acidic deposition. Simulated mean stream ANC of 71 µeq/L (range 32 µeq/L to 107 µeq/L) prior to industrial development (~1850) decreases in response historical acidic deposition to 33 µeq/L (-13 µeq/L to 88 µeq/L) in 2007. Historical acidification of GRSM exhibited a similar long-term temporal pattern across the watersheds. Future model projections for GRSM show that decreases in sulfate deposition result in smaller increases in stream ANC compared with equivalent decreases in nitrate deposition. Although there are no programs in the U.S. to control ammonia emissions, model simulations suggest that decreases in ammonium deposition could also help mitigate acidification to a comparable of greater extent than equivalent controls on nitrate deposition. The timescale of watershed recovery to decreases in acidic deposition is multiple decades to centuries. Increases in soil pH associated with decreases in atmospheric nitrate deposition results in desorption of sulfate from soil to drainage water, delaying watershed recovery for decades from acidic deposition. Simulations suggest that simultaneous reductions of nitrate and sulfate deposition are essential to limit ongoing acidification to GRSM watersheds and they are more effective than individual reductions of nitrate or sulfate.