Long-Term Annual and Seasonal Patterns of Acidic Deposition and Stream Water Quality in a Great Smoky Mountains High-Elevation Watershed

Abstract

The recovery potential of stream acidification from years of acidic deposition is dependent on biogeochemical processes and varies among different acidsensitive regions. Studies that investigate long-term trends and seasonal variability of stream chemistry in the context of atmospheric deposition and watershed setting provide crucial assessments on governing biogeochemical processes. In this study, water chemistries were investigated in Noland Divide watershed (NDW), a high-elevation watershed in the Great Smoky Mountains National Park (GRSM) of the southern Appalachian region. Monitoring data from 1991 to 2007 for deposition and stream water chemistries were statistically analyzed for long-term trends and seasonal patterns by using Seasonal Kendall Tau tests. Precipitation declined over this study period, where throughfall (TF) declined significantly by 5.76 cm year −1 . Precipitation patterns play a key role in the fate and transport of acid pollutants. On a monthly volume-weighted basis, pH of TF and wet deposition, and stream water did not significantly change over time remaining around 4.3, 4.7, and 5.8, respectively. Per NDW area, TF SO 4 2-flux declined 356.16 eq year −1 and SO 4 2-concentrations did not change significantly over time. Stream SO 4 2-remained about 30 μeq L −1 exhibiting no long-term trends or seasonal patterns. SO 4 2-retention was generally greater during drier months. TF monthly volume-weighted NH 4 + and NO 3 -concentrations significantly increased by 0.80 μeq L −1 year −1 and 1.24 μeq L −1 year −1 , respectively. TF NH 4 + fluxes increased by 95.76 eq year −1 . Most of NH 4 + was retained in the watershed, and NO 3 -retention was much lower than NH 4 + . Stream monthly volumeweighted NO 3 -concentrations and fluxes significantly declined by 0.56 μeq L −1 year −1 and 139.56 eq year −1 , respectively. Overall, in NDW, inorganic nitrogen was exported before 1999 and retained since then, presumably from forest regrowth after Frazer fir die-off in the 1970s from balsam wooly adelgid infestation. Stream export of NO 3 -was greater during winter than summer months. During the period from 1999 to 2007, stream base cations did not exhibit significant changes, apparently regulated by soil supply. Statistical models predicting stream pH, ANC, SO 4 2-, and NO 3 -concentrations were largely correlated with stream discharge and number of dry days between precipitation events and SO 4 2-deposition. Dependent on precipitation, governing biogeochemical processes in NDW appear to be SO 4 2-adsorption, nitrification, and NO 3 -forest uptake. This study provided essential information to aid the GRSM management for developing predictive models of the future water quality and potential impacts from climate change.

Developing site-specific nutrient criteria from empirical models

“…The relationship that is the least interpretable was the positive association between TN and atmospheric SO 4 22 deposition. This relationship is similar to the relationship seen by Cai et al (2011) deposition could have a direct effect on stream TN by suppressing plant growth and, hence, N uptake, but we think it more likely that SO 4 22 deposition is a surrogate for another process or N source, such as dry deposition.…”

Acidification Impacts and Goals for Gauging Recovery of Brook Trout Populations and Fish Communities in Streams of the Western Adirondack Mountains, New York, USA

“…Although fish assemblages were not explicitly studied, Warren et al (2008) attributed the replacement of Slimy Sculpin Cottus cognatus, Blacknose Dace Rhinichthys atratulus, and Brook Trout communities during the 1960s with Brook Trout monocultures in most streams of the Hubbard Brook watershed during recent (2005)(2006)(2007) fish surveys to acidification during the 1970s-1990s. Changes in monthly volume-weighted pH, sulfate (SO 2À 4 ), and total base cations were insignificant, yet nitrate (NO À 3 ) concentrations declined significantly in streams of the Noland Divide in the Great Smoky Mountains between 1991 and 2007 (Cai et al 2011). Concentrations of SO 2À 4 decreased and ANC increased significantly in seven limed and control streams of northern Sweden between 1982 and 2010 (Borg and Sundbom 2014).…”

“…Changes in monthly volume‐weighted pH, sulfate (), and total base cations were insignificant, yet nitrate () concentrations declined significantly in streams of the Noland Divide in the Great Smoky Mountains between 1991 and 2007 (Cai et al. ). Concentrations of decreased and ANC increased significantly in seven limed and control streams of northern Sweden between 1982 and 2010 (Borg and Sundbom ).…”

Influence of basin characteristics on baseflow and stormflow chemistry in the Great Smoky Mountains National Park, USA

“…Two known stream acidification sources in GRSM surface waters include atmospheric acid deposition and disturbed Anakeesta phyllite (Huckabee et al ., ; Cook et al ., ; Deyton et al ., ; Cai et al ., 2011c). The GRSM receives elevated rates of acid deposition in the form of sulfur and nitrogen compounds, which contribute to the acidification of poorly buffered surface waters (Baumgardner et al ., ; Weathers et al ., ; NADP, ).…”

“…Stormflow water is generally more acidic than baseflow because the majority of the source water is routed rapidly through the upper layers of the soil, which supply protons from acid deposition and organic processes for transport (Wigington et al ., ; Lawrence, ). Long‐term acid deposition may cause a loss of exchangeable base cations, resulting in base cation dilution during stormflows (Fernandez et al ., ; Laudon and Norton, ; Cai et al ., 2011c). Nitrification is a dominant process in the upper soil horizons, in which ammonia is converted to nitrate, generating two protons and thus acidifying soil water (Cai et al ., ).…”