the episodic acidification of a stream with elevated concentrations of dissolved organic carbon

HYDROLOGICAL PROCESSESHydrol. Process. 18, 2663–2680 (2004)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.5574

The episodic acidification of a stream with elevatedconcentrations of dissolved organic carbon

Brian I. Wellington* and Charles T. DriscollDepartment of Civil and Environmental Engineering, Syracuse University, 220 Hinds Hall, Syracuse, NY 13244, USA

Abstract:

Organic acids are generally thought to play a minor role in the episodic acidification of streams in the USA. Inthis study, we investigated the episodic acidification of a stream at the Hubbard Brook Experimental Forest in NewHampshire with high concentrations of dissolved organic carbon and naturally occurring organic acids. We studiedthree events in 2001: spring snowmelt, which occurred from 6 April to 14 May and resulted in two distinct meltevents; and two rain events, one on 17 June and the other on 17 July. During snowmelt events organic acids were aminor contributor to the short-term acidification of stream water, with increases in NO3

� and dilution of base cationsbeing the dominant mechanisms. During summer rainfall events, however, increases in inputs of organic acids werethe dominant mechanism of episodic acidification when soil water was the dominant contributor to stream discharge(59 to 66% of peak stream discharge). We also found that precipitation events occurring after relatively wet antecedentconditions (17 July event) resulted in more severe acid episodes than events that followed drier antecedent conditions(17 June event). The minimum acid neutralizing capacity (ANC) was only �19 µeq l�1 for the 17 June event, whereasthe minimum ANC for the 17 July event was much lower (�62 µeq l�1) although the total rainfall amount was similarfor the two events. Copyright 2004 John Wiley & Sons, Ltd.

KEY WORDS episodic acidification; dissolved organic carbon; organic acids; stream chemistry; flowpaths; HubbardBrook

INTRODUCTION

The focus of research on the acidification of surface waters has been on the long-term or chronic effects ofacidic deposition. However, the short-term acidification of surface waters or episodic acidification may alsosignificantly alter the acid–base chemistry of surface waters and result in the mortality of aquatic organisms(Barker et al., 1990). Episodic acidification refers to the short-term decrease in acid neutralizing capacity(ANC) that occurs during high flow associated with rainstorms and snowmelt (Wigington et al., 1990). Acidicepisodes can be defined to occur when ANC values decrease below 0 µeq l�1 and may occur over a periodof a few hours to several weeks.

Episodic acidification may be brought about by natural processes such as dilution of base cations relative toincreases in inorganic acid anions and the flushing of organic acids from soils (Galloway et al., 1987; Petersand Driscoll, 1987; Turner et al., 1990; Heath et al., 1992; Kahl et al., 1992). Atmospheric deposition of stronginorganic acids may also contribute to episodic acidification (Wigington et al., 1990). Variations in hydrologicflowpaths may control the extent to which these factors contribute to episodic acidification (Shanley andPeters, 1988; Wigington et al., 1996b; Sueker et al., 2000). During base flow, hydrologic flowpaths largelyoccur through lower mineral soils and groundwater storage zones that produce water with higher values ofANC. During hydrologic events, more water maybe routed through upper soil horizons, which are more acidicbecause of natural soil development processes or acidic deposition (Chen et al., 1984; Wigington et al., 1992).

* Correspondence to: Brian I. Wellington, Newfields, Two Midtown Plaza, 1349 West Peachtree St, Suite 2000, Atlanta GA 30309, USA.E-mail: brian [email protected]

Received 1 April 2002Copyright 2004 John Wiley & Sons, Ltd. Accepted 1 September 2003

2664 B. I. WELLINGTON AND C. T. DRISCOLL

The main control on episodic acidification in virtually all regions of the USA has been shown to be thedilution of base cations (Wigington et al., 1990). However, in watersheds with surface water characterizedby acidic or low ANC values, the extent of dilution of base cation concentrations during hydrologic eventsis small (Schaefer et al., 1990). NO3

� pulses are strongly associated with episodes in catchments of thenortheastern USA (Wigington et al., 1990), including the Adirondack (Schaefer et al., 1990; Galloway et al.,1987), and Catskill regions of New York (Murdoch and Stoddard, 1992), and Maine (Kahl et al., 1992). Pulsesof SO4

2� have been shown to contribute to ANC depressions during episodes in some streams in Pennsylvania(Lynch and Corbett, 1989; Barker and Witt, 1990; DeWalle et al., 1991) and dominate episodic acidificationin some streams in the US mid-Atlantic region (O’Brien et al., 1993). In the coastal areas of Maine, sea-salt effects dominate short-term acidification (Kahl et al., 1985; Haines et al., 1990). In many watersheds,particularly in the northeastern USA, slight increases in concentrations of organic acids have been observedduring hydrologic events (Wigington et al., 1990). Few studies have reported increases in organic acids inaffecting episodic acidification.

The limited contribution of organic acids to episodic acidification in the USA may simply reflect the factthat most studies have been conducted in streams with low concentrations of dissolved organic carbon (DOC),a surrogate for naturally occurring organic acids. For example, in the US Environmental Protection AgencyEpisodic Response Project (Wigington et al., 1996a), the median concentration of DOC in 92% of the streamsat base flow ranged from 100 to 400 µmol l�1. In New Hampshire and Vermont the median DOC concentrationof surface waters was 330 µmol l�1 (Driscoll, unpublished data). However, 20% of these surface waters haveDOC concentrations greater than 500 µmol l�1, further justifying the need to study surface waters containingrelatively high DOC concentrations.

The objective of this study was to investigate the controls on episodic acidification in a stream that ischaracterized by high concentrations of DOC. We hypothesized that: (1) naturally occurring organic acidsderived from soil waters dominate episodic acidification during rain storm events, but the effect of organicacids is less significant during snowmelt; and (2) maximum acidification during rain events occurs duringperiods when hydrologic flow paths are predominantly through organic-rich soil horizons.

METHODS

Study site

The study was conducted at the Hubbard Brook Experimental Forest (HBEF). The HBEF is located inthe White Mountains of New Hampshire (45°560N, 71°450W). The HBEF is subdivided into experimentalwatersheds, with six on the south-facing slope and three on the north-facing slope. This study focused onWatershed 9 (W9) a north-facing slope with an area of 68Ð4 ha and elevation range of 685–910 m (Figure 1).

The vegetation of W9 is dominated by conifers, red spruce (Picea rubens) and balsam fir (Abies balsamea),with less than 5% hardwoods, American beech (Fagus grandifolis), yellow birch (Betula alleghaniensis) andsugar maple (Acer saccharum) (C.W. Martin, unpublished data, 1998). The stream draining W9, CascadeBrook, has been shown to have a lower pH (average 4Ð6) and a significantly higher DOC concentration(average at baseflow 500 µmol l�1) than streams draining the south-facing watersheds and even the othernorth-facing watersheds (McDowell, 1982).

Soils in the HBEF are predominantly well-drained Spodosols consisting of a well-developed organic layer,3–15 cm thick (Likens et al., 1977). The soils have an average depth of approximately 60 cm (Johnsonet al., 1991), with the depth becoming progressively shallower at higher elevations. An impervious schist (i.e.Littleton Formation) underlies soils.

The climate of the region is humid continental, with short cool summers and long cold winters (Likens andBormann, 1995). Mean annual precipitation is approximately 140 cm, with 25–33% of the total occurring assnow (Federer et al., 1990). The mean temperature ranges from 17 °C in July to �10 °C in January.

Copyright 2004 John Wiley & Sons, Ltd. Hydrol. Process. 18, 2663–2680 (2004)

ACIDIFICATION OF A HIGH DISSOLVED ORGANIC CARBON STREAM 2665

Figure 1. Site map of w9 at the HBEF showing the locations of instrumentation

Sample collection

We collected samples of snowmelt water, throughfall, groundwater, soil solutions, and stream water duringspring snowmelt, 4 April to 10 May 2001, and two rain events, one on 17 June 2001 and the other on 17 July2001. The rain events represented different antecedent conditions for the watershed. One event was precededby dry antecedent conditions and the other was preceded by relatively wet antecedent conditions. We usedthe 7-day rainfall amounts prior to each event to estimate soil moisture conditions. The soils of W9 are welldrained, and rainfall events were studied during summer when evapotranspiration is high. These factors limitthe ability of soils to retain moisture for extended periods. Thus, we believe the 7-day rainfall amount is areasonable estimate of soil moisture conditions. We also used the total stream discharge 7 days prior to theevent to verify antecedent moisture conditions.

An automated water sampler located about 10 m upstream from the stream gauging station was used tocollect stream water samples. During storm events, the autosampler was activated by a small rise in streamstage. Stream samples were collected hourly during the first 12 h of collection and every 3 h for the last 36 h soas to obtain samples that represent the entire hydrograph for the event. During snowmelt, the autosampler wasprogrammed to collect stream water daily throughout the event. Weekly stream samples were also collectedto define base flow stream chemistry prior to events. Routine grab samples were also collected monthly.

Additional event samples were collected from three throughfall collectors (throughfall), three snowlysimeters (snowmelt), five zero tension lysimeters (sampling the Oa, Bh, and Bs soil horizons), and eightpiezometers constructed of 2 in PVC pipes installed at an average depth of 57 cm (shallow groundwater).The locations of the throughfall collectors, lysimeters and piezometers are shown in Figure 1. Throughfallcollectors and lysimeters were sampled after each rain event and weekly during snowmelt events. Piezometerswere sampled at the end of all events and when possible at around peak discharge. Additional measurementsincluded continuous stream discharge using a V-notch weir and stilling well at the outlet of the watershed.



Table I. Analytical methods for the determination of water chemistry used in this study

Ion Method Reference

SO42�, NO3

�, Cl� Ion chromatography Small et al. (1975)NH4

C Phenate colorimetry; autoanalyser US Environmental Protection Agency (1983)Ca2C, Mg2C, KC, NaC Atomic absorption spectrophotometry Slavin (1968)ANC Strong acid titration with Gran plot analysis Gran (1952)DOC UV-enhanced persulphate oxidation,

detection of CO2 by IRDohrmann Envirotech Corporation (1984)

Dissolved silica Heteropoly blue complex colorimetry;autoanalyser

APHA (1985)

Total F Potentiometrically with ion-selectiveelectrode after TISAB addition

Orion (1976)

Monomeric Al Colorimetric, PVC, autoanalyser McAvoy et al. (1992)Nonlabile monomeric Al Ion-exchange fractionation, analysis for

monomeric AlDriscoll (1984)

All samples were collected in plastic bottles, which had been previously acid washed and soaked in distilleddeionized water for 24 h. Samples were stored at 4 °C prior to analysis, which occurred within a few weeks.All samples were analysed for all major anions and cations, including monomeric aluminium fractions, ANCand DOC using standard methods as described in Table I.

Estimation of organic acid anions

We estimated the concentration of total organic acid anions �A�� for each sample collected by using thediscrepancy in charge balance (Driscoll and Newton, 1985; Driscoll et al., 1989, 1994; Grip and Bishop,1990; Hruska et al., 1996) and calculated as follows:

[A�] D∑

CC �∑

CA � [HCO3�] C [HC] �1�

where ∑CC D 2[Ca2C] C 2[Mg2C] C [NaC] C [KC] C [NH4

C] C n[AlinC] �2�

and ∑CA D 2[SO4

2�] C [NO3�] C [Cl�] C [F�] �3�

This approach assumes that the discrepancy in charge balance is not the result of analytical errors and thecharge associated with inorganic monomeric aluminium �Ali� can be determined. We determined n[AlnC

i ]and [HCO3

�] from speciation calculations using the chemical equilibrium model MINEQL C version 4Ð0(Schecher and McAvoy, 1998). We also estimated the proportion of strong �AS� and weak �AW� organic acidanions in [A�] by estimating AS from

[AS] D∑

CC �∑

CA � ANCG �4�

and[AW] D [A�] � [AS] �5�

where ANCG is the experimentally determined Gran ANC.In calculating Gran ANC, we conducted strong acid titrations down to pH 3. At this pH the weak organic

acids would be largely protonated, uncharged and thus would not contribute to the anion deficit. Munson andGherini (1993), working in the Adirondacks showed that 90% of the deviation between

∑CC � ∑

CA and



ANCG was due to the presence of strong organic acids, thus justifying the use of Equations (4) and (5) toestimate strong and weak organic acid anions.

Contributions to change in ANC

We estimated the contribution of base cation �CB� dilution, pulse increases of acid anions (SO42�, NO3

�,and Cl�), organic acid anions, and aluminium to variations in ANC during hydrologic events. The ioniccontributions at minimum ANC for each event were determined using the following equations:

dCB D [CBp � CBe]/ANC �6�

dSO42� D [SO4 e

2� � SO4 p2�]/ANC �7�

dNO3� D [NO3 e

� � NO3 p�]/ANC �8�

dCl� D [Cle� � Clp

�]/ANC �9�

dA� D [Ae� � Ap

�]/ANC �10�

dAl D [Alp � Ale]/ANC �11�

whereANC D ANCp � ANCe �12�

the subscripts ‘p’ and ‘e’ represent the pre-event event values respectively at minimum ANC. Positive valuesof any of the ratios calculated from Equations (6)–(11) indicate contributions to ANC declines, and negativevalues suggest contributions to ANC increases (Molot et al., 1989; DeWalle and Swistock, 1994).

Sources of stream discharge

We hypothesized that three end members contributed to stream discharge during hydrologic events:precipitation (throughfall or snowmelt), soil water from the Oa horizon, and shallow groundwater. We assumedthat the concentration of each end member was constant in time and space. End members were defined byusing average values from all collectors sampled during each event.

The contribution of each end member to stream discharge was determined by a combination of principalcomponent analysis (PCA) and end-member mixing analysis (EMMA) as described by Christopherson andHooper (1992) and outlined in Burns et al. (2001). We conducted the analysis as follows:

ž For each precipitation event we used a dataset consisting of six solutes (Ca2C, Mg2C, NaC, H4SiO4, Cl�,and SO4

2�). These solutes behaved conservatively and showed the greatest variability between the threeend members, important requirements for EMMA analysis.

ž The data were standardized into a correlation matrix to account for the differences in variation between thedifferent solutes.

ž PCA was performed on the correlation matrix using all six solutes and all combinations of five and foursolutes. Based on the PCA results, we selected a model that accounted for the greatest variability in streamchemistry with two principal components, implying that at least three end members are required.

ž We standardized the concentration of end members, projected them into the U space defined by the streamPCA and examined the extent to which the end members bound the stream chemistry for each event.

ž The goodness-of-fit of solute concentrations predicted by the EMMA were compared with the concentrationsmeasured during the events using linear regression.



We then used the EMMA model to calculate the contribution of each end member to stream discharge foreach sample during each event by solving the following equations:

U1st D U1pfp C U1sfs C U1gfg �13�

U2st D U2pfp C U2sfs C U2gfg �14�

fp C fs C fg D 1 �15�

where U1 and U2 are the first and second principal components respectively, and the subscripts ‘p’, ‘s’, and‘g’ represent precipitation (snowmelt/throughfall), soil water, and groundwater respectively. The contributionsof each end member were then used to estimate a hydrograph separation of stream discharge for each event.

RESULTS

Hydrologic response to events

The spring snowmelt event of 2001 occurred from 6 April to 14 May and resulted in two distinct meltevents. The first was a rain-on-snow event from 7 to 18 April, during which a total of 43 mm of rain fell,resulting in a maximum stream discharge of 108 l s�1 (Figure 2a). The second event was a snowmelt eventfrom 20 April to 14 May, resulting in a maximum stream discharge of 196 l s�1. The observed multiple peaksin stream discharge were the result of repeated cycles of snowmelt followed by freezing events during themelt period. Throughout most of the melt events the stream channel was covered with snow, which allowedfor the direct input of melt water to the stream.

The rainstorm of 17 June resulted in a total of 23 mm of precipitation. This storm occurred after aperiod of dry antecedent conditions; a total of 7 mm of rain had occurred during the previous 7 days, withaverage (plus/minus standard deviation) stream discharge of 2Ð6 š 1Ð1 l s�1. The resulting storm hydrograph(Figure 3a) peaked 7Ð5 h after the start of rainfall, with a peak discharge of 11Ð1 l s�1. After peaking, thedischarge plateaued for 4Ð5 h before receding gradually to base flow.

A total of 24 mm of rain fell during the storm of 17 July. This storm followed a period of wet antecedentconditions; a total of 22 mm of rain had fallen during the previous 7 days, with average stream dischargeof 3Ð9 š 2Ð5 l s�1. The resulting storm hydrograph (Figure 4a) exhibited a peak discharge of 73 l s�1 at 6 h,followed by a quick recession to base flow.

Variations in stream chemistry

Two events of episodic acidification occurred during the spring snowmelt of 2001. The first, a rain-on-snow event, resulted in a depression of ANC from 3 µeq l�1 to �53 µeq l�1. The second, a snowmelt event,resulted in a depression of ANC from �26 µeq l�1 to �48 µeq l�1. The rain events of 17 June and 17 July alsoresulted in episodic acidification, with minimum ANC values of �19 µeq l�1 and �62 µeq l�1 respectively(Figures 2b, 3b, and 4b).

NO3� concentrations peaked during the snowmelt events. In contrast, concentrations of NO3

� were belowthe detection limit (0Ð6 µeq l�1) during summer rain events (Figures 2e, 3e, and 4e). The concentrationsof SO4

2� (Figures 2c, 3c, and 4c) and CB (Figures 2d, 3d, and 4d) increased with increasing dischargeand then decreased with decreasing discharge during snowmelt events. However, during the rain events,concentrations of SO4

2� increased during the high discharge period, and CB concentrations, though variable,generally increased.

The concentrations of monomeric aluminium (Alm) increased with increasing discharge for all events(Figure 5b). The increases in Alm were largely due to increases in the organic fraction (Alo), whereas theinorganic fraction (Ali) showed only slight increases. At peak discharge, we calculated that the proportion ofAlm occurring as Alo ranged between 70 and 86%.



Figure 2. Temporal variation in (a) stream discharge, (b) ANC, (c) SO42� concentrations, (d) the sum of base cation concentrations,

(e) NO3� concentrations, (f) total organic acid anion concentrations (A�) during spring snowmelt of 2001 at Cascade Brook at the HBEF

The increases in concentration of Alo were strongly correlated with increases in DOC concentrations forall events (r D 0Ð91 to 0Ð93 and p < 0Ð001).

During all events, concentrations of organic acid anions increased with increasing discharge. Strong organicacid anions were the dominant fraction, accounting for 80–90% (Figure 5c) of total organic anions.

Contribution of major ions to episodic acidification

We found that the dominant mechanism responsible for maximum ANC decreases during the rain-on-snowevent was the pulsed increase in NO3

�, followed by increases in SO42� and dilution of CB. Increases in A�

played a minor role (Table II).




(e) NO3� concentrations, (f) total organic acid anion concentrations (A�) during rain event of 17 June 2001 at Cascade Brook at the HBEF

During the snowmelt event starting on 21 April, dilution of CB was the dominant acidification mechanism,followed by increases in A� and NO3

�. The contributing mechanisms during both rain events were increases inA�, dilution of CB and increases in SO4

2�. During both rain events, the dominant mechanism for acidificationwas the increase in concentrations of A�. However, during the event preceded by drier conditions, increasesof SO4

2� contributed more to episodic acidification than dilution of CB, whereas for the event preceded bywetter conditions the dilution of CB was a more important mechanism of stream acidification than increasesin SO4

2�. During all events, aluminium increases contributed to increases in ANC; thus, the mobilization ofaluminium acted as a buffer mitigating episodic decreases in ANC.




(e) NO3� concentrations, (f) total organic acid anion concentrations (A�) during rain event of 17 July 2001 at Cascade Brook at the HBEF

Mixing model and hydrograph separation

Four solutes, NaC, Mg2C, H4SiO4, SO42�, were retained for use in the end-member mixing model for all

events because the first two principal components explained between 96 and 99% of the variability in thesedata.

The three end members chosen for the EMMA model encompassed most of the stream data (Figure 6).This observation suggests that, for all events, these end members, i.e. precipitation (snow/rain), soil water,and shallow ground water, adequately explained the variability in stream chemistry. Regression relationsbetween predicted and measured concentrations for each solute ranged from r D 0Ð92 to 0Ð98, indicating thatthe EMMA model was a good predictor of stream solute concentrations.



Figure 5. Temporal variation in (a) discharge, (b) speciation of aluminium concentrations and (b) speciation of organic acid anion concen-trations during the rain event of 17 July 2001. Alm: monomeric aluminium; Ali: inorganic monomeric aluminium; Alo: organic monomeric

aluminium; A�: total organic acid anions; AS: strongly acidic organic anions; AW: weakly acidic organic anions

Table II. Change in ion concentrations relative to change in ANC, at minimum stream ANC during episodes

Start Date Hydrologic stimulus Minimum ANC (µeq l�1) Ion (µeq l�1/µeq l�1)

dNO3� dSO4

2� dCB dA�

6 April 2001 Rain/snowmelt �53 0Ð42 0Ð35 0Ð18 0Ð0421 April 2001 Snowmelt �48 0Ð07 �0Ð51 1Ð09 0Ð6517 June 2001 Rain �19 �0Ð05 0Ð41 0Ð32 0Ð8117 July 2001 Rain �62 �0Ð01 0Ð08 0Ð38 0Ð62

At the time of minimum ANC concentration, soil water was the largest contributor to stream discharge forall events. Soil water accounted for 77%, 54%, 59% and 66% of stream discharge for the peak dischargeperiod during the rain-on-snow, snowmelt, rain of 17 June, and rain of 17 July events respectively (Figure 7).

The average contribution of throughfall to stream discharge during the two rain events was estimated as14% from the separated hydrographs. Throughfall contributions to stream discharge were comparable to thearea of the watershed occupied by stream channel and wetlands (10%), justifying the use of throughfall as anend member.



Figure 6. Mixing diagram showing event water evolution and end-member composition in U space for (a) spring snowmelt of 2001,(b) rainfall event 17 June 2001, and (c) rainfall event 17 July 2001 at Cascade Brook at the HBEF. STW: stream water; SM: snowmelt end

member; TF: throughfall end member; SW: soil water end member; GW: groundwater end member

DISCUSSION

Mechanisms contributing to episodic acidification

The dominant role of increases in A� concentrations in the acidification of stream water, during both rainevents, is a significant observation from this investigation. Few studies in the literature have reported thatincreases in A� concentrations contribute significantly to episodic acidification. This lack of observationsprobably does not reflect the unimportance of this phenomenon, but rather the selection of streams with low



Figure 7. Stream discharge hydrographs showing contributions of end members during (a) spring snowmelt of 2001, (b) rainfall event 17June 2001, and (c) rainfall event 17 July 2001 at Cascade Brook at the HBEF. QSNOW: discharge associated with melt water; QTF: discharge

associated with throughfall; QSOIL: discharge associated with soil water; QGW: discharge associated with groundwater

DOC concentrations as study sites for previous studies on episodic acidification. An exception to this wasobserved in some streams in the Adirondack region of New York, where increases in A� concentrationscontributed to episodic acidification, but only during major events (Wigington et al., 1996b). The increasesin A� concentration during rain events and the role of A� in episodic acidification was apparently a resultof flushing of organic material from organic-rich soil horizons. Results of the mixing analysis are consistentwith this mechanism, showing that soil water contributed 59% and 66% of the total stream flow at the timeof minimum ANC during the 17 June and 17 July rain events respectively.

For both the rain-on-snow and snowmelt events, we observed increases in concentrations of A� derivedfrom soil horizons. However, this was not the dominant mechanism of acidification, as the pulses of NO3

�and dilution of CB made larger contributions.



The dominant role of increases in NO3� concentrations in episodic acidification during snowmelt events

has been observed in other watersheds in the northeastern USA (Galloway et al., 1987; Rascher et al., 1987;Schaefer et al., 1990; Kahl et al., 1992; Murdoch and Stoddard, 1992; Wigington et al., 1992; DeWalle andSwistock, 1994). The high concentrations of NO3

� in stream water are derived from NO3� and NH4

C nitrifiedin the snowpack and the accumulation of NO3

� in upper soil horizons due to limited biological activity anda reduced demand for nitrogen by vegetation during winter (Seip et al., 1979; Davies et al., 1992). Duringthe rain-on-snow event, much of the NO3

� that accumulated in the upper soil horizons was likely deliveredto the stream water. The initial flushing of NO3

� during the early phases of snowmelt has been observed byother researchers (Davies et al., 1987; Gjessing and Johannessen, 1987). This is consistent with the mixinganalysis, which showed that soil water contributed 77% of the total stream flow at minimum stream ANC.

During the snowmelt event, the dominant mechanism contributing to decreases in ANC was a dilution ofCB coupled with increases in A� and minor contributions from increases in NO3

�. Virtually all studies ofepisodic acidification have identified dilution of CB as an important mechanism (Wigington et al., 1990). Thediminished role of NO3

� in acidification during the snowmelt event was likely due to the previous flushing ofNO3

� from upper soil horizons during the rain-on-snow event. The flushing of NO3� during the rain-on-snow

event was reflected in soil solution NO3� concentrations from the Oa horizon lysimeters, which decreased

from an average concentration of 94 š 10 µeq l�1 during the rain-on-snow event to an average concentrationof 23 š 3 µeq l�1 during the snowmelt event.

Strong and weak organic acid anions

Our results suggest that organic acid anions present in the stream draining W9 are largely strongly acidicin nature. This pattern is in contrast to the findings of past studies, where organic acids have been treatedas weak acids (Munson and Gherini, 1993). Driscoll et al. (1994), using data from the Adirondack LakeSurvey, concluded that about one-third of organic acid function groups were strongly acidic. Our findings areconsistent with those of Kramer et al. (1990). Based on data from the US Environmental Protection AgencyEastern Lakes Survey, Kramer et al. (1990) showed that organic acids in surface waters at high elevations(>530 m) in the northeastern USA were predominantly strongly acidic. Our data clearly show decreases inthe charge density of organic solutes (i.e. [A�]/DOC) with decreases in pH, demonstrating the weak acidcharacteristics of W9 stream water (Figure 8). It seems likely that the distribution of organic anions in W9stream water reflects the low pH values and high DOC concentrations. The mean pH of W9 for the periodof study was 4Ð6, with values decreasing as low as 4Ð1 during events. At this low pH the values weaklyacidic functional groups associated with organic solutes are largely protonated, therefore greatly decreasingthe concentrations of weak organic anions. The observed low charge density observed during the rain eventsis likely a result of large inputs of DOC from organic soil horizons and the wetland at the headwaters of thewatershed. This input of DOC was mainly in the form of hydrophobic acids, which have low charge density(Wellington and Driscoll, unpublished results). David and Vance (1991), in studying the origin of streamorganic acids in Maine, found that DOC from wetlands and organic soil horizons had similar characteristics,both having high hydrophobic acid content and low charge density.

We observed that losses in ANC during rain events were strongly correlated to increases in strong organicacid anion concentrations (r D 0Ð78 and 0Ð92 for the 17 June and 17 July rain events respectively). Therewas no relationship between ANC losses and increases in weak organic acid anion concentrations. This isconsistent with the concept that only inputs of strong acids (either inorganic or organic) contribute to decreasesin ANC (Driscoll et al., 1989, 1994; Schaefer et al., 1990).

Aluminium chemistry

The higher proportion of Alo concentrations during events is in contrast to most previous studies. Studyingthe response of aluminium to hydrologic events, McAvoy (1989) observed that increases in Alm during eventswas mainly due to increases in Ali concentration, even in a stream draining a wetland where Alo accounted



Figure 8. Relationship between charge density of organic solutes and pH, showing the weak acid characteristics of organic acids in W9stream water

for 90% of the Alm at base flow. He concluded that this pattern was due to decreases in pH during events.Concentrations of Ali generally increase with decreases in pH (Driscoll et al., 1980; Driscoll and Bisogni,1984; Driscoll and Postek, 1995).

We attribute the dominance of Alo in stream water during precipitation events to the large inputs of DOCflushed from organic-rich soils in the watershed. The large inputs of DOC and organic acids, coupled with theincreased mobility of aluminium under more acidic condition, provided increased opportunity for complexationof aluminium with organic ligands, resulting in increases in Alo concentrations (Driscoll and Postek, 1995).Clearly, the marked increase in complexation capacity associated with increases in DOC exceeded the increasein available aluminium, so increases in aluminium were largely associated with the Alo fraction of Alm.

The strong relationship observed between Alo and DOC during all events (r D 0Ð73 to 0Ð94) also supportsthis hypothesis. The observation that DOC was linearly related to Alo suggests that the availability of organicligands is regulating the concentration of Alm in W9 stream water. Other researchers (Driscoll et al., 1988;Bailey et al., 1995) have also observed a strong relationship between Alo and DOC.

In general, increases in aluminium concentration during events act to increase ANC, thus reducing the extentof episodic acidification. DeWalle and Swistock (1994) concluded that the mitigating effects of aluminiumare more pronounced in acidic streams than in less acidic streams. However, our results suggest that thisgeneralization may not be entirely accurate. In acidic streams with high DOC concentrations the mitigatingeffect of aluminium is less pronounced as most of the aluminium is organically bound, limiting its acidneutralizing potential.

Increases in aluminium concentrations in surface waters during episodic events is one of the main concernswith episodic acidification, as elevated aluminium concentrations are toxic to fish and other aquatic life(MacAvoy and Bulger, 1995). However, the toxicity of aluminium has been shown to be related to Ali ratherthan Alo (Driscoll et al., 1980; Barker and Schofield, 1982). Concentrations of Ali of 2 to 3 µmol l�1 havebeen shown to be detrimental to fish (MacAvoy and Bulger, 1995). The concentration of Alm in W9 increasedto a maximum value of 23 µmol l�1. Although this Alm was largely in the Alo fraction, concentrations of Ali(approximately 7 µmol l�1) were still high enough to have detrimental effects on fish.



Effects of antecedent moisture conditions

Although increases in A� concentrations dominated episodic acidification during the two rainfall events inW9, there were interesting differences between the two storms. During the 17 June event an increase in SO4

2�concentrations made a secondary contribution to the ANC depression with little acidification associated withthe dilution of CB. In contrast, during the 17 July event, dilution of CB contributed to the short-term decreasein ANC, whereas an increase in SO4

2� made a lesser contribution. In addition, although the total rainfallwas similar for the two events, the minimum ANC was only �19 µeq l�1 for the 17 June event, whereas theminimum ANC for the 17 July event was much lower (�62 µeq l�1).

We believe that these differences are largely the result of contrasting antecedent moisture conditions duringthe two storms: the rain event of 17 June was preceded by a drier period than the 17 July event. Duringdry conditions, SO4

2� accumulates in the canopy due to dry deposition and in upper soil horizons due tothe mineralization of soil organic sulphur, and it is subsequently leached during hydrologic events (Lynchand Corbett, 1989; Wigington et al., 1990). The accumulation of SO4

2� in the canopy during dry periodswas evident in throughfall chemistry. During the rain event preceded by dry conditions the average SO4

2�concentration in throughfall was 40 š 25 µeq l�1 whereas during the rain event preceded by wetter conditionsthe throughfall SO4

2� concentration was only 9 š 3 µeq l�1. Easthouse et al. (1992), working in Norway,and Lovett et al. (1996), working at HBEF, have observed increases in throughfall SO4

2� concentrationsafter extended dry periods. Thus, the larger increase in SO4

2� during the 17 June rain event was likely aconsequence of extended dry conditions.

Differences in antecedent moisture conditions and flow path contributions to stream flow are the principalreasons for the significant difference in minimum ANC values between the two precipitation events. Examiningseparated hydrographs of both events, we found that the contribution of soil water to stream flow was less forthe 17 June precipitation event. The lag time between peak discharge and peak soil water contribution was alsolonger for the 17 June precipitation event. These observations indicate that a larger portion of the precipitationfalling during dry soil conditions infiltrates and is retained in soil, resulting in limited soil water input to thestream. However, during wetter conditions, a larger portion of precipitation infiltrating the soil is deliveredto the stream as lateral flow through upper soil horizons. Swistock et al. (1989) observed similar responsesin stream discharge and soil water contribution under similar antecedent conditions in the mid-Appalachianregion.

The variable source-area concept for storm flow generation (Hewlett and Hibbert, 1967) adequately explainsthe observed response of stream water to hydrologic events. According to this concept, storm flow is generatedin zones of saturation, which occur near the stream channel or where groundwater discharge zones exist.During extended dry periods these saturated zones shrink and become hydrologically disconnected from thestream. During storm events the saturated zones expand outward from the stream channel and upward to thestream headwaters, resulting in more hydrologically connected areas within the catchment. As these zones ofsaturation expand, not only do they contribute to storm flow, but they also flush accumulated chemical solutesin upper soil horizons, including from nutrient-rich upslope areas (Boyer et al., 1995). Thus, we believe that alimited quantity of soil water contributed to stream flow during the 17 June precipitation event than during the17 July precipitation event, which resulted in a larger contribution of soil water with elevated concentrationsof organic acids to stream discharge and a depression in stream ANC.

CONCLUSIONS

Episodic acidification of stream water draining W9 occurred during both snowmelt and rain events. Thedominant ion contributing to minimum ANC during episodic acidification varied during the year. Duringthe initial snowmelt events, NO3

� was the dominant ion contributing to episodic acidification. This NO3�

was largely derived from upper soil layers and snowpack during the winter months. Subsequent acidificationduring snowmelt was largely due to the dilution of base cations, as much of the NO3

� in the snowpack and



upper soil had been depleted during the initial melt. Episodic acidification during rain events was dominatedby increases in strongly acidic organic acids derived from organic matter flushed from the upper soil horizons.Antecedent conditions before rain events were important in dictating the role of ions in episodic acidification.SO4

2� increases were important in episodic acidification following dry periods, whereas dilution of CB wasimportant for acidic episodes following wetter periods. The extent of acidification during rain events alsoseems to be dictated by antecedent conditions, with a higher degree of acidification occurring after previouslywet periods. Short-term increases in Alm during events were largely due to increases in Alo, due to increasesin the complexation capacity associated with increases in DOC in this high DOC stream.

ACKNOWLEDGEMENTS

This work was funded by the National Science Foundation. Thanks to Peter McCormick and Jonathan Winterfor help with fieldwork. We thank the staff of the United States Department of Agriculture (USDA) forprecipitation and stream flow data, especially Amey Bailey and Ian Halm. We particularly thank R. Hooper,M. Mitchell, and C. Johnson for their critical review and comments of this manuscript. This is a contributionof the Hubbard Brook Ecosystem Study. The HBEF is administered by the USDA Forest Service, NortheastForest Experimental Station, Radnor, Pennsylvania.

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the episodic acidification of a stream with elevated concentrations of dissolved organic carbon

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