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Atmospheric Environment 46 (2012) 56e64

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Atmospheric Environment

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Long-term recovery of lakes in the Adirondack region of New York to decreasesin acidic deposition

Kristin Waller a,*, Charles Driscoll a, Jason Lynch b, Dani Newcomb b, Karen Roy c

a Syracuse University, Department of Civil and Environmental Engineering, 151 Link Hall, Syracuse, NY 13244, USAbU.S. Environmental Protection Agency, Clean Air Markets Division, 1200 Pennsylvania Ave. NW, Washington, DC 20460, USAcNew York State Department of Environmental Conservation, P.O. Box 296, Route 86, Ray Brook, NY 12977, USA

a r t i c l e i n f o

Article history:Received 29 April 2011Received in revised form13 October 2011Accepted 15 October 2011

Keywords:Acid neutralizing capacityAcid rain programAdirondacksAtmospheric depositionSulfateTIME

* Corresponding author. Tel.: þ1 315 443 2311 (offiE-mail addresses: kawaller@syr.edu, kristin.anne.w

ctdrisco@syr.edu (C. Driscoll), Lynch.jason@epa.gov (Jgov (D. Newcomb), kmroy@gw.dec.state.ny.us (K. Roy

1352-2310/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.atmosenv.2011.10.031

a b s t r a c t

After years of adverse impacts to the acid-sensitive ecosystems of the eastern United States, the Acid RainProgram and Nitrogen Budget Program were developed to control sulfur dioxide (SO2) and nitrogenoxide (NOx) emissions through market-based cap and trade systems. We used data from the NationalAtmospheric Deposition Program’s National Trends Network (NTN) and the U.S. EPA Temporally Inte-grated Monitoring of Ecosystems (TIME) program to evaluate the response of lake-watersheds in theAdirondack region of New York to changes in emissions of sulfur dioxide and nitrogen oxides resultingfrom the Acid Rain Program and the Nitrogen Budget Program. TIME is a long-term monitoring programdesigned to sample statistically selected subpopulations of lakes and streams across the eastern U.S. toquantify regional trends in surface water chemistry due to changes in atmospheric deposition. Decreasesin wet sulfate deposition for the TIME lake-watersheds from 1991 to 2007 (�1.04 meq m�2-yr) generallycorresponded with decreases in estimated lake sulfate flux (�1.46 � 0.72 meq m�2-yr), suggestingdeclines in lake sulfate were largely driven by decreases in atmospheric deposition. Decreases in lakesulfate and to a lesser extent nitrate have generally coincided with increases in acid neutralizing capacity(ANC) resulting in shifts in lakes among ANC sensitivity classes. The percentage of acidic Adirondacklakes (ANC <0 meq L�1) decreased from 15.5% (284 lakes) to 8.3% (152 lakes) since the implementation ofthe Acid Rain Program and the Nitrogen Budget Program. Two measures of ANC were considered in ouranalysis: ANC determined directly by Gran plot analysis (ANCG) and ANC calculated by major ionchemistry (ANCcalc ¼ CB � CA). While these two metrics should theoretically show similar responses,ANCcalc (þ2.03 meq L�1-yr) increased at more than twice the rate as ANCG (þ0.76 meq L�1-yr). Thisdiscrepancy has important implications for assessments of lake recovery and appears to be due tocompensatory increases in concentrations of naturally occurring organic acids coincident with decreasesin lake concentrations of strong acid anions, as evidenced by increases in concentrations of dissolvedorganic carbon.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Title IV of the 1990 Amendments of the Clean Air Act (CAAA)established the Acid Rain Program, which implemented decreasesin sulfur dioxide and nitrogen oxide emissions from electricitygenerating units in the U.S. using a market-based cap and tradeprogram for sulfur dioxide (USEPA, 2009a). This legislation was

ce); fax: þ1 315 443 1243.aller@gmail.com (K. Waller),. Lynch), newcomb.dani@epa.).

All rights reserved.

followed in 2003 by the Nitrogen Budget Trading Program tocontrol nitrogen oxide emissions and ground-level ozone in theeastern U.S., also using a cap and trade approach. Among manygoals, Congress and the U.S. Environmental Protection Agency(EPA) intended that these programs would decrease acidic depo-sition (e.g., sulfate, nitrate) to the sensitive lands and surface watersin the eastern U.S. (USEPA, 2009a), eventually reversing theanthropogenic acidification of ecosystems (Stoddard et al., 1998b).Indeed from 1990 to 2009 there has been a 63.7% decrease inemissions of sulfur dioxide (from 15.7 to 5.7 MT) and a 70.1%decrease in nitrogen oxides (from 6.7 to 2.0 MT) from CAAA andNitrogen Budget Program sources (USEPA, 2009a). While theseinitiatives offer the potential for ecosystem recovery, the EPA is

K. Waller et al. / Atmospheric Environment 46 (2012) 56e64 57

required to provide oversight on the effectiveness of air qualitymanagement in the U.S., including air pollution impacts onecosystems.

In the past, the EPA has relied on the National AtmosphericDeposition Program (NADP) National Trends Network (NTN), aswell as the Clean Air Status and Trends Network (CASTNET) to helptrack the success that emission control and trading programs havehad in decreasing acid deposition. While the NTN and CASTNEThave allowed the EPA to confirm decreases in sulfate and nitrateloadings to ecosystems (Lehmann et al., 2005), it is also critical toevaluate whether controls in emissions have facilitated therecovery of ecosystems from effects of acidic deposition, and toquantify the extent and rate of this recovery. Adverse ecologicaleffects of acidic deposition in the U.S. are best documented forsurface waters in the southern and central Appalachian Mountainregion, the Catskill and Adirondack regions of New York and NewEngland (USEPA, 2009b). The EPA developed the Long-TermMonitoring (LTM) program in the early 1980s to evaluate seasonaland annual trends in the water quality of individual surface watersin sensitive regions of the East in response to changes in acidicdeposition (Driscoll et al., 2007; Stoddard et al., 2003). As there isa critical need for understanding the extent towhich populations ofsurface waters have responded to air pollution control programs,the EPA also developed Temporally Integrated Monitoring ofEcosystems (TIME) to provide quantitative regional-scale infor-mation on changes in the acidebase status of surface waters in theNortheast and Mid-Atlantic states (Stoddard et al., 1998b).

Acid neutralizing capacity (ANC) is commonly used to assess theacidebase status of surface waters. The EPA suggested five ANCclasses coinciding with levels of concern for surface waters withrespect to effects of acidic deposition that are linked to effects onaquatic biota: acute (ANC < 0 meq L�1), representing chronicallyacidic conditions; severe (ANC0e20meqL�1),waters that experienceacidic conditions andacidic episodes; elevated (ANC20e50 meq L�1),waters that exhibit episodic acidification to low ANC values;moderate (ANC 50e100 meq L�1), waters that are moderately sensi-tive to episodic acidification; and low (ANC > 100 meq L�1), waterswith little sensitivity to acidic deposition (USEPA, 2009c).

The Adirondacks is arguably the region in the U.S. most highlyimpacted by acidic deposition (Driscoll et al., 1991). The AdirondackPark encompasses 2.4 million ha in northern New York, with about1 million ha of publicly owned state lands and 1.4 ha of privatelands. The region is a unique landscape of forested uplands andwetlands, and includes approximately 2800 lakes (>0.2 ha). Therehave been several surveys conducted to quantify the acidebasestatus of Adirondack lakes. In 1984, the probabilistic Eastern LakesSurvey determined that 10% of the Adirondack lakes (>4 ha) had pHvalues< 5.0 and 13.9% of the lakes had ANC values<0 meq L�1. Notethat the geographic boundary of the Eastern Lakes Survey for theAdirondack region was not consistent with the Adirondack Park orthe Adirondack Ecological Zone (Driscoll et al., 1991). In 1984e1987the Adirondack Lakes Survey determined that of 1469 lakes out ofa population of 2759 lakes (<0.2 ha), 26% had pH <5.0, 26% hadANC values <0 meq L�1 and 50% had ANC values <50 meq L�1

(Kretser et al., 1989). Since these early surveys, studies using theAdirondack LTM (ALTM) data have shown relative uniformdecreases in sulfate concentrations across virtually all study lakesand decreases in nitrate concentrations in many of the study lakesthat are consistent with long-term decreases in atmosphericdeposition of sulfate and nitrate (Driscoll et al., 2007). Thesechanges have resulted in increases in pH and ANC in many ALTMlakes. Although ALTM enables time-series analysis on individuallakes and is informative on seasonal patterns, it fails to provide thenumber of lakes impacted or “population-level” informationwhichis essential for a quantitative assessment of recovery for the region.

The objective of this study was to utilize NADP NTN and TIMEdata for the Adirondack region of New York to: 1) characterizechanges in precipitation and lake chemistry in response to the AcidRain Program and Nitrogen Budget Program; 2) quantify shifts inthe populations of lakes in acid sensitivity classes; and 3) evaluatethe factors responsible for changes in the acidebase status ofAdirondack lakes. To accomplish these objectives we conductedtime-series analysis on precipitation and lake chemistry data. Weused lake population weighting factors to extrapolate TIME data tothe larger population of lakes that the TIME Adirondack studyrepresented. The role of major ion chemistry and watershed land-scape attributes to changes in lake acidebase chemistry wereassessed. We also compared chemical trends estimated with TIMEwith previously published values from the ALTM program.

2. Methods

We utilized the NADP NTN and the EPA TIME to assess recoveryof Adirondack lakes from acidification by atmospheric deposition.In addition, we interpreted lake chemical trends in the context ofland cover and landscape characteristics for the TIME lake-watersheds using the National Hydrography Dataset Plus(NHDPlus) developed by the EPA and the U.S. Geological Survey(USGS).

Wet deposition data were extrapolated from a topographically-enhanced model (Grimm and Lynch, 2004). This model utilizesdeposition data reported by the NADP NTN and precipitationquantity data from eastern U.S. National Oceanographic andAtmospheric Administration sites. Only data from years(1991e2007) which coincide with the duration of TIME and sites inthe Adirondacks (Huntington Forest, NY20 and Whiteface Moun-tain NY98) were utilized. Annual volume-weighted concentrationsof major ions were utilized (Grimm and Lynch, 2004) to determinewet deposition using the ERSI ARCGIS 9.3 to provide lake-watershed specific estimates of precipitation and wet deposition.

TIME is a long-term monitoring program developed in 1990 tosample statistically selected subpopulations of lakes and streams inregions across the eastern U.S. to obtain quantifiable representa-tions of the effects of acidification (Stoddard et al., 1998b). Watersamples are collected annually from the surface of each lake insummer or fall for the determination of major solutes, as well as pHand Gran ANC (Paulsen et al., 1991; Stoddard et al., 1998b; USEPA,2009b). Forty-three lakes were originally selected from the Envi-ronmental Monitoring and Assessment Program (EMAP) pop-ulation to represent the Adirondack lakes. Forty-two of these lakeshave been sampled for the duration of the program except in 1996and 1998. The most recent years of TIME (1999e2007) are char-acterized by complete sampling of all 42 lakes.

Individual long-term annual trends of deposition and surfacewater data were calculated using MAKESENS Mann-Kendall Testand Sen’s Slope Estimates for the Trend of Annual Data (Computersoftware. Vers. 1.0 Freeware). MAKESENS is a non-parametricManneKendall test for the presence of monotonic increasing ordecreasing trends, which uses the non-parametric Sen’s method todetermine linear rates of trends. Trendswere considered significantat p < 0.05. To evaluate changes in the acidebase status of the 42Adirondack lakes, we examined trends in the number of lakes inANC classes, using the framework for lake sensitivity classesbased on biological effects of acidic deposition described above.For this analysis we considered two measures of ANC: ANCGdirectly measured by Gran plot analysis and ANCCalc whichis calculated based on major ion concentrations(ANCCalc ¼ Ca2þ þ Mg2þKþ þ Naþ � SO4

2� � NO3� � Cl�; where

concentrations are in meq L�1). In addition to the analysis oftemporal trends for individual lakes, the TIME design also allows

K. Waller et al. / Atmospheric Environment 46 (2012) 56e6458

for the extrapolation of chemical surface water changes for the fullpopulation of Adirondack Lakes using statistically determined lakeweighting factors (Stoddard et al., 1998b; Young and Stoddard,1996). The TIME lakes were separated into subpopulations basedon similar geologic characteristics and lake ANC. The subpopula-tions are related to the entire Adirondack population (w1840 lakes)through assigned weighting factors that represent the fraction ofthe subpopulation in the regional population.

To compare wet sulfate deposition (meq m�2-yr) with sulfate inindividual TIME lakes, we estimated lake sulfate fluxes (meq m�2-yr). Regional USGS discharge data were used with precipitationrecords to estimate the annual areal discharge for each TIME lakesite for each year of study. Precipitation quantity was estimatedusing NOAA data as used for wet deposition (Grimm and Lynch,2004). Measured annual lake sulfate concentrations at each TIMElake was multiplied by the estimate of average yearly discharge forthe lake sites to estimate annual sulfate flux. In order to furtheranalyze factors that influence long-term trends in lake chemistry,characteristics of TIMEwatersheds were determined fromNHDplususing ERSI ArcGis 9.3, including mean watershed elevation,watershed area, lake hydrologic residence time and the basesaturation of soils. Soils data for the TIME watersheds were takenfrom Sullivan et al. (2006).

3. Results

3.1. Trends in wet deposition and precipitation

For each of the 42 TIME lake-watershed sites, ManneKendallestimates of wet sulfate deposition showed decreasing trends, withall but one significant (p < 0.05) (�1.04 � 0.37 meq m�2-yr(mean � std.dev.)). The average annual wet sulfate deposition at alllake-watersheds decreased 32.9% over the study period(1991e2007), from 44.5 meq m�2-yr to 29.9 meq m�2-yr (Fig. 1).The average annual wet nitrate deposition for the lake sites showeda similar percent decrease (32.2%). Note, that the rate of decrease inwet nitrate deposition was considerably less than that for sulfate(�0.69� 0.37 meq m�2-yr vs.�1.04 � 0.37 meq m�2-yr). The long-term patterns of sulfate and nitrate decline were different as well.Sulfate showed a continuous decrease over the study period, whilenitrate was characterized by higher year-to-year variability until

Fig. 1. Long-term trends in average annual wet deposition of major solutes estimated forapproach of Grimm and Lynch (2004).

2003when amarked decline in deposition occurred. The number oflake-watersheds showing significant decreases in wet nitratedeposition (21) was much less than for sulfate (41).

Our analysis indicated wet deposition of base cations was rela-tively low, with few trends. Wet ammonium deposition wassomewhat less than nitrate but invariant over the long-term(0.057 � 0.147 meq m�2-yr; 2 significant trends). The lack oftrends for ammonium coupledwith the decrease in nitrate suggeststhe increasing importance in ammonia emissions in controllingatmospheric nitrogen inputs to Adirondack lake-watersheds. Incontrast, proton deposition showed strong decreases in the region(�1.35 � 0.52 meq m�2-yr, 38 significant trends). There was noregional trend in precipitation quantity (0.004 � 0.008 m yr�1),with only two sites showing significant trends.

3.2. Changes in annual lake concentrations

Since 1991, the average annual sulfate concentration for thepopulation of Adirondack TIME lakes (42 lakes) has decreasedsignificantly at a rate of �1.74 meq L�1-yr (Fig. 2), representinga 23.47% decline over the study period. Forty-one of the TIME lakeshad decreasing trends, with a mean trend for the 30 lakes withsignificant decreases of �2.21 � 0.83 meq L�1-yr (Table 1). Averageannual rates of change in lake sulfate concentrations grouped bythe five ANC sensitivity class showed relatively uniform decreasingtrends ranging from �1.58 � 0.59 meq L�1-yrto �2.86 � 0.66 meq L�1-yr across all classes. Similarly, the esti-mated fluxes of sulfate for TIME lakes largely showed decreasingtrends (mean flux �1.46 � 0.72 meq m�2-yr; 41 decreasing trends,30 significant). There was only one positive lake sulfate flux trend,which was not significant.

The average annual lake nitrate concentration for the populationof TIME lakes showed a decreasing but not significant trend(�0.15 meq L�1-yr; Fig. 2). Note the early years of monitoring(1994e1995) which were characterized by lower pH and ANCGvalues coincided with particularly high concentrations of nitrate(Fig. 2). The annual average hydrogen ion concentration alsoshowed a non-significant decreasing trend of �0.11 meq L�1-yr(Fig. 2). Of the 42 lakes, 34 had decreasing Hþ trends (8 significant),while 8 were increasing (1 significant; Table 1). The average annualpH values of the population of TIME lakes showed an increasing

TIME lake-watersheds. Spatial patterns of wet deposition were estimated using the

Fig. 2. Annual average concentrations and standard deviation of sulfate, nitrate, hydrogen ion, pH, acid neutralizing capacity (ANCG) and dissolved organic carbon (DOC) of the 42Adirondack TIME lakes.

Table 1Comparison of TIME and ALTM trends for Adirondack lakes. ALTM trendswere taken fromDriscoll et al. (2007). There are 42 TIME lakes and 48 ALTM lakes. s indicates standarddeviation of themean trend, # indicates the total number of trends and * indicates the number of significant trends. Units for most trends are meq L�1-yr, except for pHwhich ispH units yr�1 and DOC which is mg C L�1-yr.

TIME ALTM

Total trends Increasing Decreasing Total trends Increasing Decreasing

Mean s * # * # * Mean s * # * # *

SO42� �1.92 0.95 30 1 0 41 30 �2.16 0.81 47 0 0 47 47

NO3� �0.14 0.31 7 6 1 22 6 �0.53 0.34 22 8 1 40 21

CA �2.09 1.37 32 3 0 39 32 �2.53 1.06 45 0 0 48 45CB �0.12 1.97 8 15 0 27 8 �1.81 0.70 14 5 0 43 14Hþ �0.11 0.25 9 8 1 34 8 �0.33 0.34 27 6 1 42 26pH 0.02 0.02 9 34 8 8 1 0.022 0.014 27 41 26 7 1ANCG 0.76 1.01 10 34 9 8 1 1.20 0.46 30 46 30 2 0ANCCalc 2.03 1.44 26 41 26 1 0 N/ADOC 0.03 0.12 4 26 4 13 0 7.42 6.91 10 34 9 14 1

K. Waller et al. / Atmospheric Environment 46 (2012) 56e64 59

Fig. 3. Percentage of populations of Adirondack lakes in ANC sensitivity classes overdifferent time periods. Acute: ANC <0 meq L�1; severe: ANC 0e20 meq L�1; elevated:20e50 meq L�1; moderate: 50e100 meq L�1 and low: >100 meq L�1.

Fig. 4. Patterns in ANCG and ANCCalc of individual TIME lakes as a function of sum ofindividual base cations less than the sum of strong acid anion trends for individuallakes. Note that the extent of increase for ANCG is considerably less than ANCCalc.

K. Waller et al. / Atmospheric Environment 46 (2012) 56e6460

trend (þ0.024 pH yr�1; Fig. 2; 9 significant trends). Thirty-four ofthe TIME lakes had increasing pH trends (8 significant) and 8exhibited decreasing trends (1 significant, Table 1). Both ANCG(þ0.76� 1.01 meq L�1-yr; 10 significant trends; Table 1) and ANCCalc(þ2.03 � 1.44 meq L�1-yr; 26 significant trends; Table 1) had posi-tive trends. The range of individual lake trends for ANCG(�1.59 meq L�1-yr toþ3.45 meq L�1-yr) and ANCCalc (�0.68 meq L�1-yr to þ7.43 meq L�1-yr) was large and included a few lakes withnegative trends (i.e., acidifying), but the percentage of TIME lakesthat are experiencing positive ANCCalc and ANCG trends was 97.6%and 80.1% respectively.

In general, chloride concentrations in TIME lakes were low (41 of42 lakes have concentrations below 30 meq L�1; mean 12 meq L�1),indicating limited impact from human development. Twenty-sixlakes had a downward chloride trend, with only 4 significant(�0.51 � 0.29 meq L�1-yr). Eight TIME lakes had increasingchloride concentrations and 8 lakes had no trend. The overalltrend in lake chloride concentrations was slightly increasing(0.029 � 0.89 meq L�1-yr), suggesting that changes in chloride didnot substantially influence the acidebase patterns of TIME lakes.

The mean trend in the sum of base cation concentrations (CB) inTIME lakes was �0.12 � 1.97 meq L�1-yr (9 significant trends;Table 1). Of the individual lake CB trends, 27 were decreasing (8significant) and 15 were increasing (1 significant; Table 1). Mono-meric aluminum concentrations were only measured in the firstfew years of the TIME project, so a full time-series analysis couldnot be completed for this parameter. However, concentrations oftotal filtered aluminum were sampled through the duration of theprogram. The mean trend of total filtered aluminum for the TIMElakes was negative (�0.018 � 0.15 mmol L�1-yr); however only onereported trend was significant. The mean trend of DOC for TIMElakes was þ0.03 � 0 .12 mg C L�1-yr (Table 1), with 69.0% showingincreasing DOC with 4 significant. The trends of DOC rangedfrom �0.25 mg C L�1-yr to þ0.34 mg C L�1-yr. The averageannual DOC concentrations peaked in 2004, with valuesdecreasing thereafter (Fig. 2). For the first 13 years of the study(1991e2004), there were stronger increasing trends in DOC(þ0.105 � 0.135 mg C L�1-yr), a larger percentage of lakes withincreasing DOC trends (81.0%), as well as a larger number of lakesshowing significant trends (7).

3.3. Regional recovery: how has the number of acidic (acuteconcern) Adirondack lakes changed?

In the years prior to the implementation of the Acid RainProgram (1995) and at the start of the TIME program (1991e1994),we estimate 15.5% of Adirondack lakes were acidic(ANCG < 0 meq L�1, Acute ANC class; 284 lakes; Fig. 3). During theperiod immediately following Phase I of the Acid Rain Program(1997e1999), the percentage of acidic lakes for the regiondecreased to 10.5%. Most recently (2006e2007), the percentage ofacidic lakes has continued to decrease to 8.3% (152 lakes; Fig. 3).There has been a 46% decrease in the number of acidic lakes overthe TIME study period (Fig. 3), corresponding to 132 Adirondacklakes that were acidic prior to the Acid Rain Program but today areno longer acidic. The increases in lake ANCG were manifested inshifts from lower ANC classes to higher classes, with the exceptionof the Low Concern class that remained relative constant (Fig. 3).

3.4. What drives ANC trends?

We found a weak relationship between trends in wet sulfatedeposition at individual TIME lake-watersheds with lake sulfatefluxes (r2 ¼ 0.0074). Still, the percentage of lakes showingdecreasing trends in lake sulfate fluxes (97.6%), was similar to the

percentage of lakes that experienced decreasing wet sulfatedeposition (100%). In addition, the mean trend of wet sulfatedeposition (�1.04 meq m�2-yr) slightly exceeded the mean lakesulfate flux trend (�1.46 meq m�2-yr) suggesting that changes inatmospheric deposition are largely responsible for the observeddecreases in sulfate concentrations and fluxes in Adirondack TIMElakes.

Comparisons of trends in sulfate concentrations for individualTIME lakes with trends in both ANCG and ANCcalc also showed weakrelationships (r2 ¼ 0.002 and 0.0004, respectively). When trends inANC were compared to trends in CB for individual TIME lakes,stronger relationships were evident; change in lake CB with changein ANCG (slope 0.29 eq eq�1; r2 ¼ 0.33, P< 0.05) and with change inANCCalc (slope 0.48 eq eq�1; r2¼ 0.44, P< 0.05). These relationshipssuggest that lake-watershed variation in CB supply in response todecreases in atmospheric deposition is an important controller ofthe rate of lake ANC recovery.

Not surprising, the strongest relationships were found incomparison of ANC trends with the difference in the long-termtrends of base cation and strong acid anion concentrations(DCB � DCA) calculated for each TIME lake (Fig. 4). Note that

K. Waller et al. / Atmospheric Environment 46 (2012) 56e64 61

DCB� DCAwas calculated as the long-term trend in CA for a lake sitesubtracted from the long-term CB trend. The slope of the trendscomparing DCB � DCA with ANCG was 0.63 eq eq�1 for individuallakes (r2 ¼ 0.70, P < 0.05). A comparison of DCB � DCA with ANCcalcfor individual TIME lakes had a slope of 1 eq eq�1, as anticipated(r2 ¼ 0.86, P < 0.05).

3.5. Do watershed characteristics drive DCB � DCA?

We investigated if ANC trends in TIME lakes were correlatedwith lake-watershed landscape attributes, examining meanwatershed elevation, soil base saturation, hydrologic residencetime and watershed area. We found no significant relationshipswith any of these lake-watershed attributes and changes in lakeANC.

4. Discussion

4.1. Trickledown effect: do decreases in emissions lead to decreasesin wet deposition and lake sulfate and nitrate concentrations?

Overall, decreasing acid loadings to the TIME lakes allowed forrecovery from surface water acidification. The total tonnage ofsulfur dioxide emissions decreased 43.3%, from 15.7 million tons in1990 to 8.9 million tons in 2007 and nitrogen oxide emissionsdecreased 50.7%, from 6.7 million tons in 1990 to 3.3 million tons in2007 (USEPA, 2009a). Generally consistent with these decreases inemissions, annual wet sulfate deposition decreased 32.9% in theAdirondacks since 1991 with 41 of 42 TIME lake-watershedsshowing significant decreases (�1.04 � 0.37 meq m�2-yr) andwet nitrate deposition decreased 32.1%, but with only 21 of the 42sites having significant decreases (0.69 � 0.37 meq m�2-yr) (Fig. 1).Decreases in sulfur dioxide emissions associated with the Acid RainProgram were a continuation of control programs for electricitygenerating units, the dominant source of sulfur dioxide, imple-mented since the early 1970s under the Clean Air Act. Substantialnitrogen oxide emissions originate from both point and mobilesources (Driscoll et al., 2003). Controls on nitrogen oxide emissionfrom electricity generating units were modest for the Acid RainProgram. The Nitrogen Budget Program, implemented in 2003,focused on additional controls on nitrogen dioxide emissions in theeastern U.S. to decrease precursors of ground-level ozone.Decreases in wet sulfate deposition in the Adirondacks werecontinuous over the study period. In contrast, marked decreases inwet nitrate deposition were evident following the implementationof the Nitrogen Budget Program (Fig. 1; 0.69 � 0.37 meq m�2-yr).This difference may be attributable to characteristics of the emis-sion control programs. It is not surprising that the magnitude ofdecreases in national emissions of sulfur dioxide and nitrogenoxides are somewhat greater than the magnitude of decreases inwet sulfate and nitrate deposition, respectively, in the Adirondacksa region remote from major emission sources.

As expected with decreases in sulfur dioxide emissions and wetsulfate deposition, almost all of the TIME lakes (41) showeddecreasing trends in sulfate concentrations over the study period,with many significant (30). The decreasing trend of the annualaverage sulfate concentration in the population of Adirondack TIMElakes (�1.74 meq L�1-yr, Fig. 2) was similar to other studies for theAdirondack region using the ALTM data (�2.10 meq L�1-yr, Stoddardet al., 2003), �2.16 meq L�1-yr (Driscoll et al., 2007; Table 1), anda resampling of the ELS (�1.82 meq L�1-yr; Warby et al., 2005).

There are several reasons for the weak relationship between thechange in wet sulfate deposition and the change in estimates oflake sulfate flux. First, the analysis did not consider dry deposition,which could represent between a third to half of total deposition

(Johnson and Lindberg, 1992). Site specific runoff estimates arelimited due to relatively few stream gauging stations in the Adir-ondacks. The nature of our estimate of runoff extrapolated fromregional USGS gauging stations may contribute to the lack ofa relationship for individual sites between decreases in atmo-spheric sulfate deposition and decreases in lake sulfate flux. Finally,estimates of wet deposition of sulfate and changes in deposition arenot highly variable across the TIME lake-watersheds (Ito et al.,2002). Our estimates of wet deposition undoubtedly did notcapture the variability across complex mountainous terrain. Thisrelatively uniform decrease in sulfate deposition across the regionmakes it difficult to observe patterns given year-to-year variabilityin climate, which influences deposition and hydrology, andwatershed characteristics such as soil, wetland and lake sulfateretention which influence the transport of sulfate across the land-scape. Note, the mean decrease in sulfate flux in TIME lakes(�1.46 meqm�2-yr) somewhat exceeded mean trend inwet sulfatedeposition (�1.04 meq m�2-yr). This pattern is consistent withresults from other watershed analysis for the Adirondacks (Chenand Driscoll, 2004) and may suggest that declines in dry sulfurdeposition exceed rates of decline in wet sulfate deposition for theregion.

Similar to lake sulfate, the annual average nitrate concentrationin TIME lakes has decreased since 1991, but at concentrations muchlower than sulfate (Fig. 2). This difference between nitrate andsulfate response is not unexpected, as decreases in nitrate deposi-tion are not as strong as observed for sulfate. In addition, ammo-nium deposition has remained constant or increased over the studyperiod. Ammonium represents about a third of the wet depositionof inorganic nitrogen to the Adirondacks. Lastly, the annualsummer time sampling of TIME lakes does not depict an annualnitrate concentration (Driscoll and van Dreason, 1993). Lake-watershed nitrogen dynamics are inherently complex. Nitrogen isa limiting nutrient for forest watersheds and nitrate concentrationsin surface waters fluctuate with season and spatially acrossecosystems (Aber et al., 2003).

4.2. Shifts in regional ANC

Corresponding with decreases in lake sulfate concentrations,many TIME lakes also experienced increases in lake ANC(ANCG ¼ þ0.76 meq L�1-yr; ANCCalc ¼ þ2.03 meq L�1-yr; Table 1) inthe years following the Acid Rain Program and the Nitrogen BudgetProgram (Table 1). This pattern suggests widespread recovery ofsurface waters in the region from acidic deposition for nearly alllakes sampled. Only seven lakes show no change in ANCG with onestill acidifying (i.e., ANCG decreasing), likely due to increases in DOCconcentrations (see below).

The resulting improvement in ANC denotes important changesin the acidebase status of Adirondack lakes. There has been a cleardecrease in the number of chronically acidic lakes. Stoddard et al.(2003) estimated in 2002, the percentage of acidic lakes in theAdirondacks (Acute ANC <0 meq L�1) had decreased 38%, from13.0% (238 lakes, 1991e1994) to 8.1% (149, 2002) since the early1990s. We estimate that in 2007, the percentage of acidic lakes haddecreased 46% (132 lakes, Fig. 3) since the Acid Rain Program andNitrogen Budget Program. The shift toward increases in the numberof lakes that are not chronically acidic but susceptible to episodicacidification (20e100 meq L�1) is noteworthy. However, these lakesmay still experience episodic acidification when the maximumchange in ANC in Adirondack lakes during storm events or snow-melt is on the order of 50 meq L�1 (Wigington et al., 1996). Never-theless, lakes in the Moderate ANC sensitivity class are less likely toexperience acidic episodes (minimum ANC < 0 meq L�1) than those

K. Waller et al. / Atmospheric Environment 46 (2012) 56e6462

in more sensitive lakes classes and exhibit less severe biologicalimpacts.

We observed awide range of rate of change in ANC in TIME lakesover the study period. There were no clear patterns in the rate ofANC increase across different ANC lake classes. This lack of simi-larity within ANC class highlights the complexity of surface waterecosystems. Even though the lakes may have qualities that groupthem in terms of ANC class, individual geospatial and chemicalattributes affect the response pattern to changes in atmosphericdeposition. While there is variation in individual lake response, theAdirondack lakes as a region have exhibited clear increases in ANCwith respect to decreases in acidic deposition. As this overall esti-mation is the strength of the TIME program, this seems fitting forthis analysis and points to the need for comparison to individualsite studies (i.e., ALTM) for further analysis of these discrepancies(see below Section 4.4).

4.3. Factors driving ANC trends

ANC values should increase in response to decreases in strongacid anion concentrations in lake water. Indeed, recent increases inANC in ALTM lakes were attributable to a marked decrease insulfate and nitrate concentrations; however lakes showing thegreatest decreases in strong acid anion concentrations tended toalso show significant decreases in CB concentrations, which limitedANC recovery (Driscoll et al., 2007). This compensatory leaching ofCB in response to changes in inputs and leaching of mobile anions(sulfate, nitrate) has been called the “salt effect” (Reuss andJohnson, 1986). Stoddard et al. (1998a) indicated that regionaldecreases in CA concentrations in lakes in the Adirondacks were‘matched or exceeded’ by decreases in CB concentrations, limitingrecovery. The influence of changes in CB on ANC trends was alsoevident in our study, as stronger relationships were apparent whenchanges in CB concentrations of individual lakes were compared toANCG and ANCcalc trends (slope 0.29 eq eq�1, r2 ¼ 0.33 and slope0.48 eq eq�1, r2 ¼ 0.44, respectively).

One of the most interesting aspects of our analysis is the markeddifference between ANCcalc and ANCG increases in response todecreases in acidic deposition (Fig. 4). We believe the factor mostlikely driving the discrepancy between the trends in ANCG andANCcalc is naturally occurring organic acids, although changes inhydroxyl aluminum species could also contribute. Naturallyoccurring organic acids have strong acidic as well as weakly acidicfunctional groups (Driscoll et al., 1994). So increases in DOC (anestimate of naturally occurring organic acid concentrations) willdecrease values of ANCG without altering ANCcalc.

Although researchers have hypothesized a compensatoryincrease in DOC in response to decreases in acidic deposition (Krugand Frink, 1983) and monitoring data are suggestive (Driscoll et al.,2007; Monteith et al., 2007), trends are far from conclusive (Clarket al., 2010). The data from our analysis of TIME lakes are typicalof response of DOC to changes in the acidebase chemistry ofsurface waters; many lakes show increases in DOC with a fewsignificant trends but there is a wide range of responses acrossregional lake-watersheds. Increases in DOC may be linked tochanges in acidebase chemistry such as increases in the solubilityof DOC with increasing pH or decreasing aluminum, or decreases inthe sorption of DOC by soil with increases in pH (Ussiri andJohnson, 2004). Alternatively observed increases in DOC havebeen hypothesized due to long-term changes in temperature orhydrology (Lawrence et al., 2004; Clark et al., 2010). Regardless ofthe mechanism of DOC mobilization/immobilization, changes inthe quantity and/or quality of naturally occurring organic acids willalter the ANCG of low ANC lakes. Therefore, understanding andquantifying a compensatory DOC would seem to be critical to

developing and predicting targets of ANCG response to decreases inacidic deposition and the ultimate extent of recovery of acidifiedsurface waters.

4.4. The role of TIME in assessments of effectiveness of air qualitymanagement programs

In the Adirondacks the ALTM and TIME programs have beenestablished to track trends in lake chemistry. The goals of theprograms are somewhat different. The ALTM program is a monthlysampling to examine seasonal and long-term trends in waterchemistry (Driscoll and van Dreason, 1993; Driscoll et al., 2003;Civerolo et al., 2011). The ALTM monitoring sites representdifferent lake-watershed classes based on hydrology and surficialgeology; the sites are not representative of the region as a whole.The ALTM also involves a biological monitoring component. TheTIME program is more modest in scope, involving an annualsummer sampling of lake chemistry. TIME has the distinct advan-tage that the lakes were statistically sampled and the results can beextrapolated to lake population estimates for the region as a whole.Generally recent trends from the two programs have shown similarresults (Table 1), particularly for those analytes like sulfate and CBthat generally don’t exhibit large seasonal changes. For analytesthat exhibit marked seasonal variation, like nitrate, hydrogen ionand DOC, more lakes showed significant trends with the monthlysampling of the ALTM program.

Recently, Civerolo et al. (2011) compared chemical trends in sixlakes that are common to both TIME and ALTM programs. Theirobservations were similar to this comparison. They found that thetrends for sulfate and CB were consistent for the lakes common toboth programs. Trends in nitrate, hydrogen ion, and DOC weredirectionally consistent for most of the six lakes, but fewer signif-icant trends were detected with the annual TIME data than themonthly ALTM program. Statistically significant increases in ANCGwere observed at each of the six lakes using ALTM data, whereasonly two lakes exhibited statistically significant trends with theTIME observations. Not surprising their analysis suggests that ANCGtrends for acidification recovery are weaker when based on thesingle annual observation of the TIME program, than the monthlyALTM program.

The TIME and ALTM programs complement one another. TheALTM program provides a fairly detailed understanding of howdifferent lake-watershed types respond to changes in atmosphericdeposition and other disturbances. Because sampling is conductedyear-around, seasonal and climatic variations in water chemistrycan be assessed. The ALTM also provides useful data to test andevaluate watershed biogeochemical models (e.g., Chen et al., 2004;Zhai et al., 2008). The TIME program is invaluable because it can beused to provide quantitative population-level of water chemistryand changes in chemistry over time.

These two programs are essential in ongoing assessments of theimpacts of air quality management programs on ecosystems, suchas the Acid Rain Program and Nitrogen Budget Program. Moreovermonitoring programs help guide the need for additional emissioncontrols if impacts persist or recovery is delayed. TIME is particu-larly relevant to new initiatives in the U.S. on the use of criticalloads to protect ecosystems from air pollution (Burns et al., 2008).Critical loads are the inputs of an air pollutant below which thereare no adverse effects to ecosystems. The EPA has considered usingcritical loads as the framework for Secondary National Ambient AirQuality Standards for Oxides of Nitrogen and Oxides of Sulfur(USEPA, 2009c). In addition several states are evaluating the role ofacidic deposition in the impairment of surface water qualitythrough the development of Total Maximum Daily Loads (TMDLs)under the Clean Water Act (e.g., Tennessee, New Hampshire,

K. Waller et al. / Atmospheric Environment 46 (2012) 56e64 63

Vermont, Mississippi). An ongoing TIME program will be essentialto track the recovery of air pollution impacted regions like theAdirondacks and help guide planned future air quality manage-ment programs.

5. Conclusions

Time-series analysis of the NADP NTN wet deposition and TIMElake water chemistry show that acid-impacted lake-watersheds inthe Adirondack region of New York have responded to decreases inemissions of sulfur dioxide from the Acid Rain Program withdecreases inwet sulfate deposition and similar estimated decreasesin lake sulfate fluxes. This correspondence suggests that the AcidRain Program emission control strategy together with the ongoingcontrols associated with the Clean Air Act is largely responsible forlong-term decreases in sulfate concentrations since the early 1990s.We also observed decreases in wet deposition of nitrate whichappear to largely coincide with decreases in emissions of nitrogenoxides from the Nitrogen Budget Program in the early 2000s. It isdifficult to attribute decreases in lake nitrate to coincidentdecreases in wet nitrate deposition because of the complexity ofwatershed nitrogen processes and the fact that the summersampling of lake water is not indicative of overall annual condi-tions. Decreases in emissions and wet deposition and lakeconcentrations of sulfate and nitrate coincidedwith increases in theANC across many TIME lakes. Region-wide increases in ANCresulted in shifts in the distribution of populations of lakes in acidsensitivity classes since the early 1990s, with numbers of chroni-cally acidic lakes (ANC < 0 meq L�1) decreasing markedly anda redistribution of these lakes to less sensitive lake classes. Weconsidered two metrics of the acidebase status of Adirondacklakes: ANCG and ANCcalc. Changes in ANCcalc of TIME lakes weremuch greater than directly measured ANCG. This large discrepancysuggests that a compensatory increase in the quantity and/orquality of naturally occurring organic acids is occurring in Adir-ondack lakes, which partially offsets the effects of decreases inacidic deposition or increases in ANC. Moreover, this differencebetween ANCcalc and ANCG was qualitatively consistent with trendsof increases in DOC in TIME lakes. Assessments of surface waterrecovery from acidic deposition need to understand these differ-ences in metrics of ANC. As TIME allows for population-basedtrends in water quality, it provides quantitative information toongoing assessments of the effectiveness of U.S. air qualitymanagement programs in mitigating air pollution impacts toecosystems.

Acknowledgments

This work was supported by the U.S. Environmental ProtectionAgency, the New York State Energy Research and DevelopmentAuthority and the New York Department of EnvironmentalConservation. In particular KW was supported by an EPA NCERfellowship, and is currently working as a Staff Engineer at O’Brienand Gere Engineers.

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