impacts of simulated drought on pore water chemistry of peatlands

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Impacts of simulated drought on pore water chemistry of peatlands Myra Juckers, Shaun A. Watmough * Environmental and Resource Studies Department, Trent University,1600 West Bank Drive, Peterborough, ON K9J 7B8, Canada article info Article history: Received 24 May 2013 Received in revised form 6 August 2013 Accepted 20 August 2013 Keywords: Drought Peatland acidication Nitrate Metals Liming abstract Northern peatlands are increasingly threatened by climate change and industrial activities. This study examined the impact of simulated droughts on pore water chemistry at six peatlands in Sudbury, Ontario, that differ in copper (Cu), nickel (Ni) and cobalt (Co) contamination, including a site that had been previously limed. All sites responded similarly to simulated drought: pore water pH declined signicantly following the 30 day drought and the decline was greater following the 60 day drought treatment. The decline in pore water pH was due to increasing sulphate concentrations, whereas nitrate increased more in the 60 day drought treatment. Decreases in pH were accompanied by large increases in Ni and Co that greatly exceeded provincial water quality guidelines. In contrast, dissolved organic carbon (DOC) concentrations decreased signicantly following drought, along with concentrations of Cu and Al, which are strongly complexed by organic acids. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Peatlands are becoming increasingly threatened by climate change and industrial activities that can lead to increased levels of acid and metal deposition (Addison and Puckett, 1980; Gorham et al., 1984; Price and Waddington, 2000). Extreme weather events, including prolonged droughts, are anticipated to increase because of alterations in temperature and precipitation due to climate change (Trenberth, 2011). Specically in northern Ontario, Canada, mean annual temperature is projected to increase between 2 and 4 C by 2050 across the entire province while summer (Junee August) temperature is projected to increase by 1e4 C(Bourdages and Huard, 2010). In contrast, summer precipitation is projected to decrease in southern Ontario by as much as 25e40% by 2050 (Bourdages and Huard, 2010). The combined effect of warming temperatures and decreased precipitation will lead to increased incidence and severity of summer droughts (Trenberth, 2011). An adverse effect of this risk is the temporary acidication of peatlands (Tipping et al., 2003), which is of particular concern for peatlands that act as storehouses of metals from atmospheric deposition. Organic matter in peatlands is primarily composed of lignins, humic acid, and cellulose, which contain polar functional groups, enabling them to partake in chemical bonding (Chaney and Hundemann, 1979). As such, large quantities of atmospherically deposited metals are adsorbed by peat, making them biologically unavailable to plants, soil biota, and surface waters (Chaney and Hundemann, 1979). Indeed, peatlands (ombrotrophic bogs) throughout the world have been used as archives of metal contamination (Shotyk, 1995) and constructed wetlands are increasingly used to improve water quality (Marchand et al., 2010). The capacity of peat to bind metals is largely inuenced by pH as well as ionic strength (Brown et al., 2000). At low pH levels, high H þ concentrations compete with metal ions for binding sites on peat (Coupal and Lalancette, 1976), which increases concentrations of free metal ions in pore water (Sader et al., 2011). Similarly, increases in ionic strength reduce metal binding by peat because ions in solution compete against each other for available binding sites (Brown et al., 2000). Droughts have been a frequent occurrence in Central Ontario over the past eight decades (Eimers et al., 2007). During a severe drought, stored sulphur in peatlands is oxidized to form sulphate (SO 4 ) as a result of water table draw down (Dillon et al., 1997) and in response, pH is reduced and metal concentrations in the pore water can signicantly increase (Tipping et al., 2003; Adkinson et al., 2008). With a reduction in acid (S) and metal deposition in many industrialized countries, Tipping et al. (2003) predicted the severity of pH declines will gradually decrease as peatland S pools decrease due to droughts, which will decrease the magni- tude of metal pulses from peatlands. However, pH depressions may be sustained due to continued enhanced N deposition and increased nitrication. For example, Watmough et al. (2004) found NO 3 concentrations greatly increased immediately following pro- longed summer droughts. * Corresponding author. E-mail addresses: [email protected] (M. Juckers), [email protected] (S.A. Watmough). Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol 0269-7491/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2013.08.011 Environmental Pollution 184 (2014) 73e80

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Page 1: Impacts of simulated drought on pore water chemistry of peatlands

lable at ScienceDirect

Environmental Pollution 184 (2014) 73e80

Contents lists avai

Environmental Pollution

journal homepage: www.elsevier .com/locate/envpol

Impacts of simulated drought on pore water chemistry of peatlands

Myra Juckers, Shaun A. Watmough*

Environmental and Resource Studies Department, Trent University, 1600 West Bank Drive, Peterborough, ON K9J 7B8, Canada

a r t i c l e i n f o

Article history:Received 24 May 2013Received in revised form6 August 2013Accepted 20 August 2013

Keywords:DroughtPeatland acidificationNitrateMetalsLiming

* Corresponding author.E-mail addresses: [email protected] (M. Juc

(S.A. Watmough).

0269-7491/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.envpol.2013.08.011

a b s t r a c t

Northern peatlands are increasingly threatened by climate change and industrial activities. This studyexamined the impact of simulated droughts on pore water chemistry at six peatlands in Sudbury,Ontario, that differ in copper (Cu), nickel (Ni) and cobalt (Co) contamination, including a site that hadbeen previously limed. All sites responded similarly to simulated drought: pore water pH declinedsignificantly following the 30 day drought and the decline was greater following the 60 day droughttreatment. The decline in pore water pH was due to increasing sulphate concentrations, whereas nitrateincreased more in the 60 day drought treatment. Decreases in pH were accompanied by large increasesin Ni and Co that greatly exceeded provincial water quality guidelines. In contrast, dissolved organiccarbon (DOC) concentrations decreased significantly following drought, along with concentrations of Cuand Al, which are strongly complexed by organic acids.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Peatlands are becoming increasingly threatened by climatechange and industrial activities that can lead to increased levels ofacid and metal deposition (Addison and Puckett, 1980; Gorhamet al., 1984; Price and Waddington, 2000). Extreme weatherevents, including prolonged droughts, are anticipated to increasebecause of alterations in temperature and precipitation due toclimate change (Trenberth, 2011). Specifically in northern Ontario,Canada, mean annual temperature is projected to increase between2 and 4 �C by 2050 across the entire provincewhile summer (JuneeAugust) temperature is projected to increase by 1e4 �C (Bourdagesand Huard, 2010). In contrast, summer precipitation is projected todecrease in southern Ontario by as much as 25e40% by 2050(Bourdages and Huard, 2010). The combined effect of warmingtemperatures and decreased precipitation will lead to increasedincidence and severity of summer droughts (Trenberth, 2011). Anadverse effect of this risk is the temporary acidification of peatlands(Tipping et al., 2003), which is of particular concern for peatlandsthat act as storehouses of metals from atmospheric deposition.

Organic matter in peatlands is primarily composed of lignins,humic acid, and cellulose, which contain polar functional groups,enabling them to partake in chemical bonding (Chaney andHundemann, 1979). As such, large quantities of atmospherically

kers), [email protected]

All rights reserved.

deposited metals are adsorbed by peat, making them biologicallyunavailable to plants, soil biota, and surface waters (Chaneyand Hundemann, 1979). Indeed, peatlands (ombrotrophic bogs)throughout the world have been used as archives of metalcontamination (Shotyk, 1995) and constructed wetlands areincreasingly used to improve water quality (Marchand et al., 2010).The capacity of peat to bind metals is largely influenced by pH aswell as ionic strength (Brown et al., 2000). At low pH levels, high Hþ

concentrations compete with metal ions for binding sites on peat(Coupal and Lalancette, 1976), which increases concentrations offreemetal ions in porewater (Sader et al., 2011). Similarly, increasesin ionic strength reduce metal binding by peat because ions insolution compete against each other for available binding sites(Brown et al., 2000).

Droughts have been a frequent occurrence in Central Ontarioover the past eight decades (Eimers et al., 2007). During a severedrought, stored sulphur in peatlands is oxidized to form sulphate(SO4) as a result of water table draw down (Dillon et al., 1997) andin response, pH is reduced and metal concentrations in the porewater can significantly increase (Tipping et al., 2003; Adkinsonet al., 2008). With a reduction in acid (S) and metal depositionin many industrialized countries, Tipping et al. (2003) predictedthe severity of pH declines will gradually decrease as peatland Spools decrease due to droughts, which will decrease the magni-tude of metal pulses from peatlands. However, pH depressionsmay be sustained due to continued enhanced N deposition andincreased nitrification. For example, Watmough et al. (2004) foundNO3 concentrations greatly increased immediately following pro-longed summer droughts.

Page 2: Impacts of simulated drought on pore water chemistry of peatlands

Fig. 1. Location of sampling sites in the Greater Sudbury Area, Ontario. Based on proximity to the smelters (black squares), grey circles indicate high disturbance sites, black circlesindicate low disturbance sites, and the white circle indicates a limed site (treated in 1994 and 1995).

M. Juckers, S.A. Watmough / Environmental Pollution 184 (2014) 73e8074

The Greater Sudbury Area has experienced more than a centuryof mining and smelting activities resulting in a substantial depo-sition of sulphur dioxide (SO2) and heavy metal particulates, whichhave accumulated in the soils of the region, including wetland soils(Taylor and Crowder, 1983). Nickel and Cu, and to a lesser extent Co,were the primary ores smelted and have consequently contributedthe greatest to soil contamination (Hutchinson and Whitby, 1974).Nickel and Cu concentrations in peatlands closest to the smelterswere reported to be as high as 9372 and 6912 mg g�1, respectively(Taylor and Crowder, 1983).

Based on climate data from Environment Canada since 1956, theSudbury Region has been experiencing a 0.03 �C increase in meanannual temperature (p ¼ <0.01). Temperatures have beenincreasing in all seasons, except for autumn (p ¼ 0.922). At present,little is known about the potential impact of summer drought onmetal release from peatlands in the Greater Sudbury Area or thepossible remedial options to mitigate metal release, such as liming.Information gained from this area may ultimately be used to pre-dict potential impacts of climate change and industrial activities inother northern peatlands.

The objectives of the present study were to determine the im-pacts of simulated droughts of different duration (30 and 60 days)on the pore water chemistry of peatlands along a distance gradientaway from the primary smelters in the Greater Sudbury Area. Thirty

and 60 day drought durations were chosen because over the periodof 1980e2000, droughts lasted on average 34 consecutive days peryear and SO4 is exported on a net basis when droughts occur forlonger than 54 consecutive days (Eimers et al., 2007). A secondobjective of the study was to assess the effect of historical dolomiticlimestone application (Gunn et al., 2001) on drought impact. It washypothesized that metal concentrations in peat and metal pulses inSO4, metals and pH depressions would be greatest in peatlandswith higher metal content and that the impact of drought would belessened in a historically limed peatland.

2. Materials and methods

2.1. Study sites

Six peatlands that drain into lakes within the Greater Sudbury Area wereselected. Three peatlands, locatedwithin the Laurentian (LU), Daisy (D4), and Broder(BR) lake catchmentswere considered tobe in an area of generally “high disturbance”because of their proximity to the smelters (Fig. 1). Two of the peatlands, locatedwithin theRockcut (RCK) andAshigami (ASH) lake catchments,were considered tobein an area of generally “lowdisturbance” because theywere locatedmore than 25 kmfrom the smelters (Fig. 1). The final peatland selected was also located in the DaisyLake (DL) catchment. However, itwasunique because in 1994 the sub-catchmentwastreatedwith coarse (95%> 0.5mm) dolomitic limestone (53.9% CaCO3, 44.8%MgCO3)at approximately 10 t ha�1 and in 1995 an additional 54 t of highly soluble pelletizedfine (60% < 0.1 mm) dolomitic limestone (54.5% CaCO3, 45% MgCO3) was applied(Gunn et al., 2001). All of the peatlands are classified as poor fens (Gignac andBeckett,

Page 3: Impacts of simulated drought on pore water chemistry of peatlands

M. Juckers, S.A. Watmough / Environmental Pollution 184 (2014) 73e80 75

1986). The peatlands at ASH, RCK, BR, and LU are dominated by Carex magellanica,Myrica gale, Juncus canadensis, and Chamaedaphne calyculata respectively, while D4and DL are dominated by Muhlenbergia uniflor.

2.2. Sampling

All peatlands were sampled in early May, 2012. At each site, five peat sampleswere collected using a trowel from the peat surface (0e15 cm) near the centre ofeach peatland. Samples were placed into 3.8L Ziploc� bags and stored at 4 �C in thedark until analysis. A bulk density sample was also collected from each peatland.Fresh peat samples were classified using the von Post humification scale (Von Post,1937). All peat samples were thoroughly mixed prior to analysis.

2.3. Baseline conditions

The moisture content of each sample was measured gravimetrically by dryingapproximately 5 g of wet peat at 105 �C for 24 h. Site moisture content was calcu-lated by averaging the moisture content of each sample per site. Pore water forchemical analysis was extracted from subsamples of approximately 60 mL bycentrifuging at 4500 rpm for 20 min. Pore water was first analysed for pH using anOAKTON pH 510 Series meter and then filtered using a 0.45 mm nylon syringe filterprior to chemical analysis. Dissolved organic carbon (DOC) concentrations weredetermined using a Shimadzu TOC-V instrument and inductively coupled plasmaoptical emission spectrometry (ICP-OES) was used to measure concentrations ofdissolved metals (Ni, Cu, Co, Al) and base cations (Ca, Mg). Concentrations of NO3

and SO4 were measured using ion chromatography.Subsamples of peat were dried, pulverized, and acid digested to measure metal

(Ni, Cu, Co, Al) and base cation (Ca, Mg) concentrations using ICP-OES. Peat samples(0.2 g) were cold-digested in 2.5 mL of HNO3 overnight and then digested underreflux at 100 �C for eight hours. Following digestion, samples were filtered usingWhatman No. 42 filter paper and diluted using B-pure water to 25 mL. Precision ofresults was confirmed by analysis of NIST-1515-SRM apple leaves. Total C, N, and Scontent of the peat was measured using an Elementar CNS Analyzer and precision ofresults was confirmed using blanks and sulfadiazine for CNS recalibration and QAstandards (NIST-1515-SRM).

2.4. Experimental design

Peat samples were left to stabilize in the dark at 4 �C for a week. Following stabi-lization, four separate treatmentswere established: a 30 day drought, a 60 day drought,and two continually wet (i.e. no drought) treatments (30 day and 60 day). For eachtreatment, approximately 100 goffieldmoist homogenizedpeat (five sites/samplesperwetland) was added to a 120 cm3 plastic sample container with a screw top lid andplaced intoan incubator (ThermoScientificPrecisionEconomy Incubator3522) at26 �C.Lidswere left ajar in thedrought and ‘wet’ treatments to allowairexchange.Themass ofeachsamplewas recordedat thebeginningof the experiment andmonitoredevery fourdays tomaintainfieldmoist conditions of the ‘wet’ treatments. Following the 30 and 60day droughts, the mass of water lost from the samples was added using B-pure waterand the sampleswere homogenized using a glass stirring rod and left for one day. Porewaterwas subsequentlymeasured for pH, and dissolvedmetals (Ni, Cu, Co, Al) and basecation (Ca,Mg) concentrations and the peatwasmeasured formetal (Ni, Cu, Co, Al) andbase cation (Ca, Mg) concentrations and total C, N, and S content, as described above.

2.5. Statistical analysis

A one-way ANOVAwas conducted to assess differences in ambient peat and porewater chemistry across sites. Two-way ANOVAs were conducted to determine the

Table 1Von post, bulk density, and mean (�S.D.) carbon, nitrogen, sulphur, base cation and meOntario. Peatland distance (km) from Copper Cliff smelter and the nearest smelter are inchemistry among sites. Sites with different letters are significantly (p < 0.05) different u

ASH (39, 16) RCK (30, 19) BR (11, 14)

Von Post H2 H2 H4Bulk Density (g cm�3) 0.08 0.19 0.19C (g kg�1) 386 � 30.1a 391 � 26.4a 353 � 12.0N (g kg�1) 12.7 � 0.9a 17.0 � 4.0bc 21.2 � 1.2dS (g kg�1) 5.4 � 0.4ab 6.1 � 2.2ab 7.6 � 0.6aCa (g kg�1) 4.5 � 1.0a 1.1 � 0.2b 2.1 � 0.9cMg (g kg�1) 1.1 � 0.5a 0.3 � 0.0b 0.5 � 0.1cNi (mg kg�1) 176 � 20.4a 153 � 38.4a 845 � 33.4Co (mg kg�1) 9.8 � 1.6a 5.1 � 1.0b 18.0 � 1.4cCu (mg kg�1) 147.4 � 25.0a 174.2 � 66.5a 870 � 189Al (g kg�1) 4.1 � 1.2a 6.5 � 1.8b 10.8 � 1.6c

*** < 0.001.** < 0.01.* < 0.05

effects of site and drought treatment and interactions of the main factors on porewater chemistry. Prior to analysis, the data were log transformed to meet the as-sumptions of normality and equal variances. When significant F ratios were found,the individual means were compared by Tukey’s test (p < 0.05). Statistical analyseswere performed using “R” (R Core Team, 2012).

3. Results

3.1. Chemical characteristics of the peatlands

Peat chemistry varied considerably among sites, but differencesin surface peat chemistry were not always ascribed to distance fromthe smelters (Table 1). As expected, concentrations of Ni, Co, and Cuwere highest in peatlands closest to the smelters, ranging from 153to 1304 mg kg�1, 5.1 to 28.2 mg kg�1, and 147 to 1553 mg kg�1

respectively (Table 1). Differences in other chemical properties ofthe peat reflected inherent peatland differences as sites rangedfrom those with a relatively high mineral content (e.g. D4, 23.3% C),to sites that were primarily organic (e.g. LU, 46.2% C; Table 1). At thehistorically limed site (DL), concentrations of Ca and Mg were high,but in the case of Ca, concentrations were not outside the naturalrange of inter-site variability (Table 1). There was no evidence ofhigher N and S concentrations in peatlands closest to the smeltersbut the degree of humification of the peat ranged from H2 to H5,with less humified peat at sites farther from the smelters (i.e. ASH,RCK; Table 1).

Similar to peat chemistry, there was considerable variability inbaseline pore water chemistry among sites and only NO3 concen-trations did not significantly differ among sites (Table 2). Porewaterchemistry was influenced by a combination of inherent site dif-ferences and emissions from the smelters. For example, DOC con-centrations ranged from 9.7 to 65.6 mg L�1 (Table 2) and tended tobe higher in the more organic sites (e.g. LU, RCK, ASH; Table 1) andSO4, Ni, Co, and Cu concentrations tended to be greater in porewater at sites closest to the smelters (e.g. LU, D4; Table 2). The pH ofthe pore water in the six peatlands ranged from 4.3 to 5.6, with DL(limed site) exhibiting a significantly higher pH (Table 2). Magne-sium concentrations were highest in the pore water of the limedsite (DL; Table 2).

3.2. Effect of drought on pore water chemistry of the peatlands

The initial moisture content of the peat ranged from 81.9 to93.0% (Table 3). In the continually wet treatments (no drought),concentrations of DOC, Al and Cu increased on average by 2, 3, and5 times respectively relative to baseline conditions, whereas SO4and NO3 slightly declined, but pH and other metal values were

tal concentrations of peat from six peatlands located in the Greater Sudbury Area,dicated following site name. p Values indicate results of one way ANOVA comparingsing Tukey’s post hoc test.

D4 (14, 4) DL (14, 3) LU (8, 10) p

H5 H5 H40.12 0.08 0.07

a 233 � 38.1b 124 � 9.5c 462 � 7.5d ***15.1 � 1.5ab 7.6 � 0.9e 20.0 � 1.3cd ***

c 4.8 � 0.5b 2.3 � 0.3d 9.3 � 1.6c ***1.7 � 0.3bc 5.0 � 1.2a 6.6 � 1.5a ***2.1 � 0.8d 3.0 � 0.5d 0.4 � 0.0bc ***

b 723 � 123c 537 � 138c 1304 � 112d ***24.7 � 3.5de 28.2 � 4.0d 20.0 � 1.9ce ***

.7b 888 � 130.0c 915 � 76.2c 1553 � 584.2c ***24.1 � 4.2d 18.9 � 1.9d 4.2 � 0.6a ***

Page 4: Impacts of simulated drought on pore water chemistry of peatlands

Table 2Mean (�S.D.) pH, DOC, sulphate, nitrate, base cation andmetal concentrations of porewater from six peatlands with varying distances from the smelters located in the GreaterSudbury Area, Ontario. Peatland distance (km) from Copper Cliff smelter and the nearest smelter are indicated following site name. p Values indicate results of oneway ANOVAcomparing chemistry among sites. Sites with different letters are significantly (p < 0.05) different using Tukey’s post hoc test.

ASH (39, 16) RCK (30, 19) BR (11, 14) D4 (14, 4) DL (14, 3) LU (8, 10) p

pH 5.0 � 0.2a 4.4 � 0.1b 5.0 � 0.1a 4.9 � 0.5a 5.6 � 0.1c 4.3 � 0.1b ***DOC (mg L�1) 38.5 � 16.3a 57.7 � 22.6a 39.1 � 14.9a 9.7 � 3.8b 33.2 � 14.1a 65.6 � 15.8a ***NO3eN (mg L-1) 37.3 � 27.8a 23.8 � 4.2a 54.2 � 17.3a 98.8 � 111a 52.0 � 25.2a 46.4 � 61.0a 0.666SO4 (mg L�1) 7.8 � 3.4a 3.4 � 0.7b 6.4 � 0.8ab 11.6 � 2.9ac 13.8 � 5.0ac 22.6 � 14.6c ***Ca (mg L�1) 2.1 � 0.5a 0.5 � 0.1c 1.6 � 0.3a 1.5 � 0.3a 4.7 � 1.8b 6.7 � 3.6b ***Mg (mg L�1) 0.5 � 0.1ac 0.2 � 0.1b 0.4 � 0.1ab 0.7 � 0.2ac 3.5 � 1.2d 0.9 � 0.5c ***Ni (mg L-1) 32.9 � 3.1a 39.0 � 12.4a 122 � 17.2b 127 � 78.4b 94.2 � 20.2b 412 � 143c ***Co (mg L�1) 4.9 � 0.6ab 5.2 � 0.9ab 4.9 � 1.3a 10.4 � 1.9ab 9.5 � 1.6ab 15.8 � 2.9b **Cu (mg L�1) 11.9 � 3.5a 55.0 � 26.7ab 123.2 � 24.5bcd 61.1 � 6.0bc 275 � 75.9cd 355 � 73.5d ***Al (mg L�1) 0.2 � 0.0a 0.8 � 0.3b 0.7 � 0.3b 0.3 � 0.1a 0.9 � 0.6b 0.4 � 0.1b ***

*** < 0.001.** < 0.01.* < 0.05.

M. Juckers, S.A. Watmough / Environmental Pollution 184 (2014) 73e8076

comparable to baseline levels. Following the 30 day drought,percent moisture content decreased by an average of 4% andfollowing the 60 day drought, percentmoisture content was furtherreduced by an average of 7% (Table 3). Pore water chemistry (allparameters) significantly differed among treatments and site, butthere was no interaction for all variables (Fig. 2.), indicating thatwhile absolute concentrations of pore water chemical variablesdiffered among sites, they all responded similarly to the droughttreatment. In addition, impacts of drought on pore water chemistrywere not necessarily highest at peatlands closest to the smelters.

Across all sites, including the limed site (DL), the pH of thedrought treatments was significantly lower than the wet treat-ments and a 60 day drought resulted in a significantly lower pHthan a 30 day drought (Fig. 2; Tukey test, p < 0.001). However, thepH of the pore water at the DL site remained significantly higherthan the other sites as it had a higher initial pH compared with theother sites (Fig. 2; Tukey test, p < 0.001). Concentrations of DOCwere significantly lower in drought treatments compared with wettreatments and the decrease was greatest in the 60 day drought.

Sulphate concentrations in the pore water were significantlyhigher in the drought treatments compared with wet treatmentsand the SO4 concentrations increased with drought severity suchthat following the 60 day drought, SO4 concentrations were be-tween 8 and 40 times higher than the wet treatments (Fig. 3;Tukey’s test, p < 0.001). A similar response of NO3 to drought wasobserved, although the difference between the 30 day and 60 daydrought was much greater than for SO4 (Fig. 3; Tukey’s test,p < 0.001) as concentrations in pore water were 256e777 timesgreater than thewet treatments. Even though themagnitude of SO4and NO3 response to drought varied among sites, the magnitude ofresponse was not evidently greater in sites closest to the smelters(Fig. 3).

The response of pore water Ca, Mg, Ni and Co concentrations todrought was similar at all sites, but the magnitude of responsediffered among sites (Fig. 3). Pore water concentrations weresignificantly higher in drought treatments compared with wettreatments and concentrations were highest in the 60 day drought(Fig. 3; Tukey’s test, p < 0.001). The release of Mg into the pore

Table 3Mean (�S.D.) initial percent moisture content and percent moisture content following asmelters located in the Greater Sudbury Area, Ontario.

ASH RCK BR

Initial 91.9 � 1.2 93.0 � 1.7 91.8330 day drought 88.6 � 2.1 90.4 � 2.3 89.060 day drought 81.6 � 3.6 86.0 � 3.5 85.6

water was significantly greater at the limed site (DL) than the othersites following the 60 day drought (Fig. 3; p < 0.001). Sites furthestfrom the smelters (ASH, RCK) with lower peat metal contentexhibited similar increases in Ni and Co in drought treatments tosites closer to the smelter with higher metal concentrations in peat(Fig. 3).

Concentrations of Cu and Al in porewater responded similarly toDOC in the 30 day drought treatment, with concentrations of Cuand Al significantly lower thanwet treatments in pore water (Fig. 4;Tukey’s test, p < 0.001). However, in contrast to DOC following the60 day drought, Cu concentrations were significantly higher thanconcentrations following the 30 day drought at all sites excludingD4 and DL (Tukey’s test, p < 0.001), but concentrations remainedbelow levels of the wet treatments (Fig. 4). Aluminium concen-trations following the 60 day drought were also significantlygreater than concentrations following the 30 day drought (Fig. 4)and concentrations did not significantly differ from the wet treat-ments (Tukey’s test, p < 0.001).

4. Discussion

Simulated drought results in the acidification of pore water ofthe peatlands. However, impacts are not greater at sites closer tothe smelters with higher peat metal concentrations than thosemore distant from the smelters with lower Ni, Cu, and Co con-centrations due to inherent site differences in pH and organicmatter content. These trends were observed across all sites,including the historically limed site.

4.1. Baseline conditions

Concentrations of Ni, Cu, and Co were higher in peat and porewater samples in peatlands closer to the Sudbury smelters and SO4concentrations within the pore water were also elevated. Previousstudies have shown metal concentrations in peat are elevated atsites close to the Sudbury smelters (e.g., Gignac and Beckett, 1986;Taylor and Crowder, 1983). However, there is no spatial gradient inpore water pH and base cations and peat S, C, and N content, likely

30 and 60 day drought of peat from six peatlands with varying distances from the

D4 DL LU

� 0.3 87.7 � 1.5 81.9 � 2.9 90.0 � 1.2� 0.3 83.2 � 1.9 74.9 � 5.9 86.6 � 1.7� 2.5 77.7 � 3.1 62.9 � 6.3 79.2 � 4.6

Page 5: Impacts of simulated drought on pore water chemistry of peatlands

Fig. 2. Mean (�S.D.) pH and DOC concentrations of pore water extracted from mi-crocosms with peat collected from six peatlands with varying distances from thesmelters located in the Greater Sudbury Area, Ontario, and subjected to 4 treatments:30 day drought (black circle) and wet (open circle) treatment, and 60 day drought(black triangle) and wet (open triangle) treatment. Statistical significance: ***,p < 0.001; n.s., no significance.

M. Juckers, S.A. Watmough / Environmental Pollution 184 (2014) 73e80 77

due to inherent site differences in groundwater inputs, mineralcontent in the peat, and the fact that only a relatively small amountof S emissions from the stacks are deposited close to the smelters(Freedman and Hutchinson, 1980). In addition, although the his-torically limed site had higher pH and Mg concentrations in thepore water relative to the other sites, Ca concentrations did notsignificantly differ from other sites likely due to inherent sitedifferences.

4.2. Wet conditions

Pore water chemistry in treatments maintained under wetconditions for 30 or 60 days was different from baseline porewater chemistry sampled in May. In particular, DOC concentra-tions in pore water samples measured after 30 and 60 days at26 �C were approximately double the values recorded in spring-time. The higher DOC is likely a result of increased C minerali-zation of peat at the higher temperatures which is typicallyobserved in peatlands during the summer period (Blodau, 2002).Higher DOC concentrations in the pore water also resulted inhigher Al and Cu because of the strong affinity of these metals toDOC (LaZerte, 1991) whereas concentrations of base cations andNi and Co in the pore water were similar to base line pore waterchemistry.

4.3. Peatland acidification: SO4 and NO3

Droughts significantly increased SO4 and NO3 concentrations inthe porewater as a result of the oxidation of reduced S and N, whichis consistent with other studies (Fenner and Freeman, 2011;Watmough et al., 2004; Eimers et al., 2007; Freeman et al., 1993).The increase in SO4 and NO3 leads to a reduction in pH. Sulphateand NO3 are the thermodynamically stable forms of S and N (vanBreemen et al., 1983). Oxidation of reduced S and N compoundswill lead to the creation of SO4 and NO3 which, upon rewetting,releases equivalent amounts of Hþ, leading to peatland acidification(van Breemen et al., 1983). In the present study, S and N contentgreatly varied among peatlands, ranging from 2.3 to 9.3 g kg�1 and7.6 to 21.2 g kg�1 respectively (Table 1). Despite the variation, eachof the peatlands responded similarly to drought as SO4 and NO3concentrations were significantly greater in the pore waterfollowing a drought event and concentrations continued to in-crease as drought duration increased (Fig. 3), enhancing acidifica-tion. As well, the release of SO4 and NO3 did not appear to relate tothe quantity of S and N present in the peatlands as sites with higherS and N content did not necessarily release the largest quantity ofSO4 or NO3. This suggests acidification of poor fens due to releasesof SO4 and NO3 in response to drought is a risk for poor fens ingeneral, not just those in the Sudbury region which have beenaffected by mining and smelting activities (e.g. Adkinson et al.,2008; Eimers et al., 2007).

The increase in NO3 was much more pronounced than SO4

following the 60 day drought, which is consistent with otherstudies. For example, Freeman et al., 1993 observed a significantincrease in SO4 after 4 weeks of the drought simulation whereasNO3 concentrations largely increased after 6 weeks. Further, NO3reached a maximum 340-fold increase whereas SO4 reached a 29-fold maximum increase (Freeman et al., 1993).

Droughts inducing zero discharge from streams have occurredon average over 34 consecutive days from 1980 to 2000 in CentralOntario (Eimers et al., 2007). However, projections of significantreductions in precipitation during the summer months andincreasing temperatures will likely increase the severity ofdroughts. Tipping et al. (2003) predicted the severity of pH declinesfromombrotrophic bogs will decrease in response to declines in theS pool as a result of drought. However, on an equivalence basis, theincrease in NO3 following the 60 day drought in this study wassimilar to SO4 following the 30 day drought. This suggests the riskof increased severity of droughts may sustain declines in pHbecause of substantial increases in nitrification despite potentialreductions in the S pool. At peatland sites with lower N contentthan observed in the present study (e.g. bogs), this may not be thecase. Furthermore, historic application of limestone does not bufferdeclines in pH induced by drought as observations from the limedsite (DL) indicate similar trends in pH declines as those observed atthe other sites (Fig. 2). However, the severity of decline was not assubstantial as pH remained above 4.5 due to higher baselineconditions.

4.4. Metal release

Concentrations of base cations (Ca, Mg) and Ni, and Co greatlyincreased in pore water following drought at each of the sites(Fig. 3). The release of these metals corresponds with drought-induced acidification because competition by Hþ ions for bind-ing sites on organic molecules mobilizes metals in the peat(Brown et al., 2000). As peatland acidification intensifies inresponse to increasing drought, the release of Ca, Mg, Ni, and Cointo the pore water becomes more profound. The release of basecations from these poor fens in response to competition for

Page 6: Impacts of simulated drought on pore water chemistry of peatlands

Fig. 3. Mean (�S.D.) SO4, NO3eN, Ca, Mg, Ni, and Co concentrations of pore water extracted frommicrocosms with peat collected from six peatlands with varying distances from thesmelters located in the Greater Sudbury Area, Ontario, and subjected to 4 treatments: 30 day drought (black circle) and wet (open circle) treatment, and 60 day drought (blacktriangle) and wet (open triangle) treatment. Statistical significance: ***, p < 0.001; n.s., no significance.

M. Juckers, S.A. Watmough / Environmental Pollution 184 (2014) 73e8078

exchange sites from incoming Hþ ions is not great enough toprevent acidification. Further, the loss of base cations can lead tothe saturation of cation exchange sites with protons, which canincrease the natural acidification process of the peatlands(McLaughlin and Webster, 2010). The release of Ni and Co is ofconcern because of substantial exceedances in provincial waterquality guidelines (OMEE, 1999). The baseline concentrations of Ni(32.9e412 mg L�1) and Co (4.9e15.8 mg L�1) in the pore wateracross all sites exceed provincial water quality guidelines by 2e16and 12e39 times respectively. However, following the 30 daydrought, concentrations exceeded guidelines by 3e39 and 25e102 times respectively and following the 60 day drought, con-centrations exceeded guidelines by 12e105 and 91e271 timesrespectively. These large spikes in Ni and Co concentrations abovewater quality guidelines pose threats to aquatic biota in watersdraining these peatlands. Further, the release of these metals fromthe stored metal pools in the peatlands may be sustained forseveral years.

The Ni and Co pools among the peatlands ranges from1.65 gm�2 to 14.04 gm�2 and 0.06 gm�2 to 0.30 gm�2 respectively.Annual runoff in the Greater Sudbury Area is approximately200 mm. Assuming 1/4 of the runoff occurs following a droughtevent and metal deposition and terrestrial inputs to wetlands areexcluded from calculations, 0.14e0.45% of Ni and 0.25e1.18% of Co

could be exported from stored Ni and Co pools following a 30 daydrought event and 0.45e2.34% of Ni and 0.80e5.13% of Co could beexported following a 60 day drought event, with greater relativeexport observed from sites farther from the smelters due to lowermetal concentrations in peat. Based on these estimates, climatechange poses the risk of reversing environmental improvements inthe Greater Sudbury Area over a long term period because of thesustained release of metals resulting from drought-induced peat-land acidification. Obviously, if runoff following droughts is higherthen metals will be washed out of wetlands at a greater rate,whereas the recovery time will diminish if runoff is less thanassumed in these calculations and deposition and terrestrial inputsto wetlands are included in the calculations.

This study also shows that the risk is widespread because ofextensive metal contamination as metal pools of Ni and Co at thelow impact sites (ASH: 1.90 and 0.11 g m�2 respectively; RCK: 1.65and 0.06 g m�2 respectively) far exceed Ni and Co pools in the fibriclayer of other peatlands in forested catchments in central Ontario(e.g. Landre et al., 2010; Ni: 0.016 g m�2, Co: 0.007 g m�2). Thehistoric application of dolomitic limestone did not reduce themagnitude of metals released as the release of metals from thehistorically limed site (DL) followed similar trends and pore waterconcentrations were within similar ranges as those from the otherhighly disturbed sites (Fig. 3).

Page 7: Impacts of simulated drought on pore water chemistry of peatlands

Fig. 4. Mean (�S.D.) Cu and Al concentrations of pore water extracted from micro-cosms with peat collected from six peatlands with varying distances from the smelterslocated in the Greater Sudbury Area, Ontario and subjected to 4 treatments: 30 daydrought (black circle) and wet (open circle) treatment, and 60 day drought (blacktriangle) and wet (open triangle) treatment. Statistical significance: ***, p < 0.001; n.s.,no significance.

M. Juckers, S.A. Watmough / Environmental Pollution 184 (2014) 73e80 79

4.5. Dissolved organic carbon, copper and aluminium

Dissolved organic carbon concentrations significantly declinedin the pore water in response to the simulated drought, which isconsistent with findings from other studies (e.g. Clark et al., 2005;Clark et al., 2012; Freeman et al., 1993). Two primary hypotheseshave been proposed in the literature to explain the reduction inDOC during acid pulses that follow a summer drought, whichinclude: (1) increased biological activity and microbial consump-tion of DOC, leading to improved carbon metabolism and thus theproduction of carbon dioxide rather than DOC (Clark et al., 2012;Freeman et al., 1993); or (2) reduced solubility of DOC in responseto increased acidity and ionic strength (Clark et al., 2012). The riskof increased incidence and severity of droughts in response toclimate change has the potential of increasing the variability of DOCexport from peatlands which in turn can alter DOC concentrationsin drainage waters (Clark et al., 2005).

Copper and Al tend to follow the same pattern as DOC as bothmetals have a higher affinity for organic molecules than metalssuch as Ca, Mg, Ni, and Co, resulting in the complexation of Cu andAl in an organic form (LaZerte, 1991; Tipping et al., 2003). The highaffinity for organic molecules enables Cu and Al to outcompete Hþ

ions in response to declining pH (Tipping et al., 2003). Followingthe 60 day drought however, concentrations of Cu and Al in thepore water at several sites were significantly greater than concen-trations following the 30 day drought. As pH continues to decline as

a result of increasing drought duration, the release of Cu and Al isfavoured because of the greater presence of Hþ ions outcompetingCu and Al for binding sites (Nierop et al., 2002).

5. Conclusions

Simulated drought results in the acidification of pore water ofSudbury peatlands, although impacts on peatlands closer to thesmelters with higher peat metal concentrations are not greaterthan sites more distant from the smelters with lower Cu, Ni and Codue to inherent site differences in pH and organic matter content.This indicates that the potential impact of climate change onnorthern peatlands may be widespread and potentially exacer-bated by increased industrial activity. As drought continues, acidi-fication becomes increasingly severe and NO3 plays an increasingrole in pore water acidification. Historical liming reduces theseverity of pH decline; however, metals are still mobilized and theproblem intensifies as drought duration increases.

Acknowledgements

The authors wish to thank Caroline Sadlier, Andrea Hatton, andPaul Pennington for field assistance; Michael Preston for assistancein initiating the drought experiments; Liana Orlovskaya for labo-ratory assistance; and Erik Szkokan-Emilson for assistancethroughout the study. This study was funded by NSERC (NaturalSciences and Engineering Research Council of Canada), Vale INCO,Xtrata, and the City of Greater Sudbury.

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