Observed changes in chronic and episodic acidification in Virginia mountain streams in response to the Clean Air Act and amendments
Introduction
The U.S. Environmental Protection Agency's Long Term Monitoring (LTM) program tracks chemical changes that have occurred within a diverse set of lakes, rivers, and streams in the eastern United States (Lynch et al., in review). Among these freshwater bodies, streams within the Blue Ridge and Valley and Ridge physiographic provinces of Virginia serve a unique role in documenting such change in response to the 1990 Amendments to the Clean Air Act (CAA). Unlike their counterparts in New York and New England, which have exhibited a relatively rapid response to decreases in acidic deposition, signs of improvement in Virginia streams have been more mixed (Robison et al., 2013), adding complexity to the overall narrative of aquatic recovery. Here, we highlight some of the changes that have occurred in Virginia mountain streams over the last three decades and discuss plausible mechanisms associated with these observed changes.
The deep, clay-rich soils in the southeastern U.S. play a key role in shaping the stream chemical response in this region. Located south of the extent of the most recent glacial maximum, which terminated in northern Pennsylvania, these well-weathered soils have a high sulfate adsorption capacity. The influence of these soil properties on the timescales of recovery from acidic deposition, particularly with regard to delayed decreases in stream sulfate concentrations, was predicted decades ago (e.g. Galloway et al., 1983; Cosby et al., 1985a; 1986; Rochelle et al., 1987) and has been reported more recently (Rice et al., 2014; Eng and Scanlon, in review). Documenting aquatic response to changes in atmospheric deposition requires observational timescales that are commensurate with such long-term dynamics.
The Shenandoah Watershed Study (SWAS) at the University of Virginia was established in 1979 and has maintained a weekly record of stream water chemistry within Shenandoah National Park (SHEN) to present. In 1987, the Virginia Trout Stream Sensitivity Study (VTSSS) expanded the geographical footprint of the monitoring to include over 60 additional streams within SHEN and the surrounding national forests, sampled on a quarterly basis. Event-based sampling, augmented by weekly grab sampling, was added at three SHEN streams in 1992. Over the years, this monitoring infrastructure has been used to investigate biogeochemical, hydrological, and ecological processes within these watersheds, including the impacts of insect defoliation (e.g., Webb et al., 1995; Eshleman et al., 1998, 2004; Riscassi et al., 2009), the effects of acid-base status on fish populations (e.g., Dennis et al., 1995; Bulger et al., 2000; Roghair et al., 2002; Harmon et al., in press), and the transport of mercury from soil to stream (e.g. Riscassi et al., 2011; Riscassi and Scanlon, 2011, 2013; Jensen et al., 2017) among other topics (a full bibliography is available on https://swas.evsc.virginia.edu). The research legacy of the SWAS-VTSSS program includes the development of the MAGIC model (Cosby et al., 1985b), which has been used on a widespread basis throughout North America and Europe to simulate the effects of acidification on watershed soil and stream chemistry.
Prior research in these Virginia headwater streams has noted the strong influence of bedrock geology on chemical composition (Webb et al., 1989; Herlihy et al., 1993; Hyer et al., 1995; Sullivan et al., 2007) and temporal trends (Robison et al., 2013; Riscassi et al., 2019). Indeed, the dominant influence of bedrock on stream chemistry precludes any sweeping assessment of the acid-base status in this region, since the results are so contingent upon the underlying bedrock type. Therefore, in undertaking our analysis we considered the three major bedrock classes: siliciclastic (base-poor, silica-rich sedimentary bedrock; examples include sandstone, shale, and quartzite), felsic (igneous bedrock with intermediate levels of base saturation; examples include granite and gneiss), and mafic (base-rich, highly weatherable igneous bedrock; examples include basalt and anorthosite). Doing so provides a more complete and contextualized depiction of the observed trends in this region.
The 1990 Amendments to the Clean Air Act have been resoundingly successful in terms of reducing acidic atmospheric deposition (Eng and Scanlon, in review; Figure A1), yet Virginia streams have been notable for their lack of positive response (Kahl et al., 2004; Burns et al., 2011; Robison et al., 2013). Three decades after the enactment of this legislation, it is worthwhile to update the status of these streams. Specifically, our objectives are to: (1) determine long-term trends in the stream acid/base condition, specific to bedrock type, (2) investigate in the influence of flow on stream chemistry, thereby distinguishing between chronic versus episodic acidification, and (3) infer the biogeochemical processes responsible for the observed trends. The acid/base status of these streams has been shown to have direct implications for fish species diversity in this regional setting (Harmon et al., in press).
Section snippets
Methods
The current analysis focuses on 63 headwater streams within the Blue Ridge and Valley and Ridge physiographic provinces of Virginia (Fig. 1), located within forested watersheds in Shenandoah National Park, Thomas Jefferson National Forest, and George Washington National Forest. These sites were chosen because they are considered to be largely free from direct human disturbance. Quarterly sampling commenced at a majority of these streams (n = 56) in late October 1987, with sampling beginning in
Results
The influence of underlying bedrock on stream chemistry is evident by the effective stratification of pH and ANC by majority bedrock type (Fig. 2a), with siliciclastic bedrock having the lowest pH and ANC, felsic having intermediate values, and mafic having the highest pH and ANC. Also shown are the trajectories of pH and ANC for each site, with arrows originating from the 1988–1992 mean values (around the time of the 1990 CAA amendments) and extending to the values representing the more recent
Trends in ANC and pH
Pre-industrial ANC levels in Virginia streams underlain by siliciclastic bedrock have been estimated at between 60 and 90 μeq L−1, based on application of the MAGIC model (Sullivan et al., 2008). By the time of the passage of the 1990 CAA Amendments, ANC in many of these streams hovered around the 0 μeq L−1 and 20 μeq L−1 thresholds associated with chronic and episodic damage to native brook trout populations, respectively (Bulger et al., 2000) (Fig. 2a). Critical load modeling estimated that
Synthesis
How have Virginia streams responded to decades of decreased acidic deposition following the passage of 1990 Amendments to the CAA? This basic question has no simple answer, since it depends upon bedrock type and the specific timescales of acidification (i.e. chronic versus episodic). In general, sites with the lowest ANC and pH in 1990 were less likely to exhibit improvements over subsequent decades. The recovery of ANC in the most acid-sensitive streams has not occurred on a widespread basis
CRediT authorship contribution statement
Todd M. Scanlon: Conceptualization, Methodology, Software, Formal analysis, Writing – original draft, Writing – review & editing, Funding acquisition. Ami L. Riscassi: Investigation, Data curation, Writing – review & editing, Funding acquisition. James N. Galloway: Conceptualization, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank Susie Maben, the laboratory manager for the SWAS-VTSSS program, for her unwavering dedication toward maintaining a high-quality stream chemistry record. We also thank those who contributed to the original design and long-term operation of the monitoring network, including George Hornberger, Roger Pielke, Jack Cosby, Rick Webb, Frank Deviney, and the numerous volunteers who have collected water samples at the quarterly sites. The U.S. Environmental Protection Agency and the National
References (51)
- et al.
Trends in the hydrochemistry of acid-sensitive surface waters in the UK 1988–2008
Ecol. Indicat.
(2014) - et al.
Factors controlling dissolved and particulate mercury export from three forested, upland Appalachian catchments
J. Hydrol.
(2013) - et al.
Regional scale evidence for improvements in surface water chemistry 1990–2001
Environ. Pollut.
(2005) - et al.
Long-term recovery of lakes in the Adirondack region of New York to decreases in acidic deposition
Atmos. Environ.
(2012) - et al.
Implications of sodium mass balance for interpreting the calcium cycle of a forested ecosystem
Ecology
(2003) - et al.
Current, reconstructed past, and projected future status of brook trout (Salvelinus fontinalis) streams in Virginia
Can. J. Fish. Aquat. Sci.
(2000) A Non-parametric Monotonic Trend Test Computing Mann–Kendall Tau, Tau-B, and Sen's Slope Written in Mathworks-MATLAB Implemented Using Matrix Rotations
(2006)- et al.
National Acid Precipitation Assessment Program Report to Congress 2011: an Integrated Assessment
(2011) - et al.
Time scales of catchment acidification
Environ. Sci. Technol.
(1985) - et al.
Modeling the effects of acid deposition: assessment of a lumped parameter model of soil water and stream chemistry
Water Resour. Res.
(1985)
Modeling the effects of acid deposition: control of long-term sulfate dynamics by soil sulfate adsorption
Water Resour. Res.
An Investigation and Quantification of Nitrogen Sources and Sinks in Shenandoah National Park, Virginia, M.S. Thesis
The association of water chemistry variables and fish condition in streams of Shenandoah National Park (USA),
Water Air Soil Pollut.
The Biogeochemical Response of Eastern U.S. Watersheds to Declines in Acid Deposition, M.S. Thesis
Temporal patterns of nitrogen leakage from mid-Appalachian forested watersheds: role of insect disturbances
Water Resour. Res.
Regionalization of disturbance-induced nitrogen leakage from mid-Appalachian forests using a linear systems model
Hydrol. Process.
Proton and aluminum binding properties of organic acids in surface waters of the Northeastern U.S
Environ. Sci. Technol.
Freshwater acidification from atmospheric deposition of sulfuric acid: a conceptual model
Environ. Sci. Technol.
The effects of acidic deposition on streams in the Appalachian mountain and Piedmont region of the mid-Atlantic United States,
Water Resour. Res.
A nonparametric trend test for seasonal data with serial dependence,
Water Resour. Res.
Role of organic solutes in the chemistry of acid-impacted bog waters of the Western Czech Republic
Water Resour. Res.
Episodic acidification of three streams in Shenandoah national park, Virginia, USA
Water Air Soil Pollut.
The effect of wildfire on streamwater mercury and organic carbon in a forested watershed in the southeastern United States,
Environ. Sci.: Processes & Impacts
Cited by (4)
Regional patterns and drivers of total nitrogen trends in the Chesapeake Bay watershed: Insights from machine learning approaches and management implications
2022, Water ResearchCitation Excerpt :Another more speculative explanation is mobilization of N from internal pools, which is associated with deacidification of forest soils previously impacted by acid deposition (Groffman et al., 2018; Newcomer et al., 2021). Recent work suggests that decomposition in some forests is increasing as soils recover from acid deposition (Lawrence et al., 2020; Sabo et al., 2020; Scanlon et al., 2021), which can result in increased stream TN loads despite increased vegetative uptake (Marinos et al., 2018; Rosi-Marshall et al., 2016). A strength of the optimal RF models is the ability to predict clusters of TN trends for the entire Chesapeake Bay watershed at the finer spatial scale of river segments.
Strong variation in Brook Trout trends across geology, elevation, and stream size in Shenandoah National Park
2024, Transactions of the American Fisheries SocietyThroughfall Deposition Chemistry in the Great Smoky Mountains National Park: Landscape and Seasonal Effects
2022, Water, Air, and Soil Pollution