Forest Hydrology and Watershed Management - HydrologieForestiere et Amenagement des Bassins Hydrologiques{Proceedings of the Vancouver Symposium, August 1987; Actesdu Colloque de Vancouver, Aout 1987):IAHS-AISHPubl.no.167,1987.

Sources of acidity during snowmelt at aforested site in the west.central AdirondackMountains, New York

NORMAN E. PETERS

US Geological Survey,USACHARLES T. DRISCOLL

Syracuse University,

Doraville, Ga. 30360,

Syracuse, NY 13210, USA

ABSTRACT The chemical quality of precipitation, snow-pack, meltwater, and forest floor leachate was evaluatedto determine the impact of these sources on stream acidi-fication. Solutes in the snowpack were preferentiallyreleased at the beginning of snowmelt causing elevatedconcentrations of acidity and strong acid anions, NO] ands01-, in the meltwater; these decreased rapidly within afew days as the melt continued. Stream s01- concentra-tion was relatively constant (140 ~eq 1-1) through win-ter. Ground-water discharge appears to be a primarycontrol of stream composition, because the stream s01-concentration was comparable to, or slightly lower thanthat of ground water or soil water, but was more thandouble that of meltwater or forest floor leachate.

Nitrification in the forest floor produced 2-3 timeshigher acidity (H+) and NO] concentrations in leachatesthan in meltwater. Stream H+ and NO] concentrationsincreased during late winter, and the increases areattributed to the flushing of these solutes from theforest floor through the soil to the stream.

Les sources d'acidite durant la fonte des neiges de 1984et de 1985 a un site boise du centre-ouest des HontsAdirondack, New YorkRESUME La composition chimique de la precipitation, ducouvert neigeux, des eaux des fonte, et de l'eau sous lalitiere a ete evaluee pour determiner l'impact de cessources sur l'acidification du cours d'eau. Les solutesdans la neige etaient relaches surtout en debut de fonte;les concentrations elevees chutaient en quelques jours defonte additionnelle. La concentration hivernale en s01-dans Ie cours d'eau etait relativement constante (140 ~eq1-1). L'ecoulement souterrain semble etre Ie controleprimaire de la composition du cours d'eau: la concentra-tion en s01- dans Ie cours d'eau etait egale oulegerement plus faible que celIe de l'eau de la nappe, etetait plu6 du double de celIe de6 eaux des fontes au deseaux sous la litiere. La nitrification dans la litiere

produisit des concentrations de H+ et NO] 2-3 fois plus99

100 Norman E. Peters & Charles T. Driscoll

fortes dans l'eau de percolation que dans l'eau de fonte.Les concentration de H+ et N03 du cours d'eauaugmentaient a la fin de l'hiver, et ces augmentationsetaient attribuees a l'ecoulement de solutes de la

litiere, a travers le sol, jusqu'au le cours d'eau.

INTRODUCTION

Processes controlling the acidification (pH < 5.0) of streams andlakes during snowmelt are not well understood. In the northeasternUnited States, acidification is accompanied by decreases in basecation concentrations, increases in N03 concentration, and constantSO~ concentration (Galloway et al., 1980; Driscoll et al., 1986).These variations have been explained by atmospheric deposition ofacids directly from precipitation during the snowmelt period, re-lease of acids from the snowpack which accumulated there during thewinter, and dilution (Johannessen et al., 1977; Johannessen &Henriksen, 1978; Galloway et al., 1980). However, contributions ofground water to streamflow and interaction of the meltwater with theupper acidic soil horizons also may be major factors controllingacidification (Krug & Frink, 1983; Peters & Murdoch, 1985). Forthis paper, concentrations of major solutes in precipitation, snow-pack, meltwater, forest floor leachate, and a stream from Januarythrough April 1984 and 1985 in the west-central Adirondack Moun-tains, New York, were evaluated to assess processes controllingepisodic stream acidification.

METHODS

Site characteristics

The study site was in a northern hardwood forest at the south end ofTwitchell Lake (74'54'W, 43'51'N) in the west-central AdirondackMountains, New York. The live canopy species within the area wasdominated by beech (Fagus grandifolia) - 43%, red maple (Acerrubrum) - 26%, and sugar maple (Acer saccharum) - 13%. The meanbasal area of the live stems was 25 m2ha-l and the mean stem dia-meter was 15 em. The soils in each area are spodosols developed ontill which is underlain by bedrock consisting of granite gneiss.

Snowmelt experiment

During October 1983, two 1.2-m by 2.4-m stainless-steel pan ly-simeters were installed. One lysimeter was installed on top of theforest floor so that meltwater from only the snowpack could besampled. For the other lysimeter, the forest floor was excavated tothe top of the mineral horizon (10-20 em), the lysimeter was theninstalled on top of the mineral horizon, and blocks of the forestfloor (0.5 m by 1.2 m) were placed in the lysimeter. The forestfloor leachate and meltwater were sampled periodically throughout

Sources of acidity during snowmelt 101

the winter from 20-1 stainless-steel pots in which water was col-lected through 5.1-cm drains. The pots were enclosed in insulatedplywood and earth shelters.

Snowpack samples were collected for analysis with an Adirondacksnow coring device which consisted of a lOS-em depth-graduatedfiberglass tube with a 6.7-cm diameter, stainless-steel cutting headand a spring scale calibrated for measuring the snowpack waterequivalents. The depth and water equivalent of the snowpack wererecorded weekly for a 10-station snow course. Snowpack depths alsowere determined every three days and more frequently during meltperiods from readings of 10 depth-graduated snow stakes.

A stream, Pancake-Hall Creek, was sampled monthly and more fre-quently during the late winter snowmelt of 1985. The stream drainsa 1.4 km2 forested area in similar terrain outside the drainage ofthe snow experiment site and is 3 km southeast of the study site.

Precipitation quantity and quality were monitored using a wet/drycollector at a EPRI/UAPSP network site 3 km south of the study site.The procedures used for site operation and methods used for theanalytical determinations are documented by Rockwell International(1983a, 1983b). Samples of streamwater, snowpack, meltwater, andforest floor leachate were analyzed on the same day for pH andspecific conductance. Samples generally were processed withindays for all major chemical solutes using the methods reportedRascher et al., (1987).

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RESULTS

Hydrology

The pattern of changes in the water equivalent and depth of thesnowpack was similar during both winters (Fig.l). From January toFebruary, the pack aggraded to the maximum depth of about 80 ern. Amajor melt followed in February after which the pack again aggraded.The maximum water equivalent of the snowpack occurred in mid-Marchin each year, and a snowmelt occurred later in mid- to late March,which was followed by a period of accumulation prior to a majorsnowmelt in April. Melting of the snowpack was complete by aboutthe third week in April of each year. The snowpack melted con-tinuously throughout the winter as evidenced by accumulations in thelysimeter collection pots, but the snowmelt periods singled outabove were associated with relatively large changes in the waterequivalent and depth of the snowpack (Fig.l), and with large yieldsof meltwater and leachate.

Each melt period produced high streamflow with the highest flowoccurring during the final melt in April. Each melt was coincidentwith rain or mixed precipitation and concurrent atmospheric warming.

Variations in solute concentrations

Most solute concentrations of snowpack, meltwater, forest floorleachate, and streamwater varied systematically during each winter.Solute concentrations of meltwater and leachate generally were thehighest at the onset of each snowmelt and generally decreased asmelting progressed, as shown for H+, NO}, and soi- in Figs 2-4,respectively. The acidity (H+ concentrations) of snow and watervaried temporally consistent with snowpack accumulation and melting.The average volume-weighted H+ concentration of precipitation was 40~eq 1-1 in 1984 and 63 ~eq 1-1 in 1985, and ranged from 4 to 250 ~eq1-1 for the individual storms. The H+ concentration of precipita-tion decreased with increasing volume; H+ greater than 100 ~eq 1-1was found in samples with precipitation quantities less than 0.3 ern.The snowpack H+ concentration was similar to, or lower than that ofprecipitation the largest differences occurred during late winterwhen most of the H+ had been eluted from the snowpack. Meanwhile,the H+ concentration of meltwater was higher than that of the snow-pack consistent with the elution of H+ from the snowpack (Fig.2).The highest H+ concentration (400 ~eq 1-1) was that of leachate atthe onset of the February 1984 snowmelt (Fig.2). The H+ concentra-tion of leachate and meltwater decreased as melting increased butthe H+ concentration of leachate was consistently higher than thatof meltwater. Streamwater H+ concentration generally was the lowestof all water sampled and the maximum concentration (25 ~eq 1-1)occurred during the late melt (Fig.5).

The strong-mineral-acid anion (NO} and Soi-) concentrations ofsnow and water varied similar to the H+ concentration with the

exception of soi- of forest floor leachate and streamwater. Therelative concentrations of NO} and 50;- were comparable in precipi-

Sources of acidity during snowmelt 103

tation and snowpack. In 1984, the volume-weighted average NO]concentration of precipitation was similar to that of 80;-. In 1985,NO] concentration of precipitation was higher than 80;- with snow-fall having the most pronounced enrichment which is typical for this

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region (Johannes et al., 1981). The NO] concentration of leachatewas 2-3 times that of meltwater, whereas 801- concentration ofleachate was more similar to that of meltwater (Figs 3 & 4).8treamwater 80;- concentrations remained relatively constant duringmost of the winter, whereas NO] concentrations increased at the sametime that H+ concentration increased during April, particularly in1985 (Fig.5). During April 1985, the stream NO] concentration wasless than that of leachate but was higher than that of meltwater.8nowpack 801- concentration was considerably less than that ofmeltwater, leachate, or streamwater except at the end of the winterwhen solute concentrations of snowpack were similar to those ofmeltwater (Figs 4 & 5). 8treamwater 801- concentration averaged 140~eq 1-1 which was mUQh higher than that of meltwater or leachate forall samples in each year except those collected at the onset ofsnowmelt periods in February and March (Figs 4 & 5).

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DISCUSSION

Processes affecting the acidity of the snowpack

Strong mineral acids were eluted from the snowpack during theinitial stages of snowmelt as H+, NO) and s01- concentrations ofmeltwater increased during the onset of snowmelt and generally werehigher than those of the snowpack (Figs 2-4). These results areconsistent with those of other studies (Johannessen et al., 1977;Johannessen & Henriksen, 1978; Johannes et al., 1980). The decreasein meltwater acidity during late winter reflects the loss of acidsfrom the snowpack during previous snowmelt periods.

Effect of the forest floor on meltwater composition

Meltwater composition changed dramatically as it infiltrated theforest floor. The H+ and NO) of forest floor leachate generallywere much higher than those of the snowpack or meltwater, whereasSO;- concentration was similar. The NO) concentration of leachatewas highly correlated (r2 = 0.99) with H+ concentration; the slope

of a linear regression (in equivalents) of H+ on N°i of leachate was1. Nitrification is a probable cause of the high H and NO) con-

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106 Norman E. Peters & Charles T. Driscoll

centrations as the soils remained unfrozen during each winter whichis typical for this region (Peters, 1984). In addition, the warmsoil probably caused the continuous melting of the snowpack.

Sources of acidity in the stream

The acids released directly to the stream from the snowpack orforest floor did not have a pronounced impact on stream chemistry.

The H+, NO), and SO;-, concentrations of meltwater were highestduring the major melt in February. Concentrations of these solutesin the stream were similar to those for other streams in the area

(Galloway et al., 1980; Driscoll et al., 1986), but were differentthan those of the meltwater or forest floor leachate. Variations in

solute concentrations may have occurred in the stream which weresimilar to those of meltwater but they were not identified possiblybecause of infrequent stream sampling. In each year, stream sam-pling bracketed the intense melt in February occurring prior to anddays after the major transport had occurred. At the end of March of1985, however, solute concentrations of meltwater were lower thanthose of concurrent and subsequent stream samples (compare H+, NO),& SO;- concentrations of meltwater in Figs 2-4, respectively, withthose of the stream in Fig.5). Also, SO;- concentrations ofleachate were less than 50 ~eq 1-1 whereas SO;- concentrationsaveraged 140 ~eq 1-1. The snowmelt site was on a northern slopecausing solutes in the snowpack to be released later than an averagefor the stream drainage which slopes to the south. The SO;- con-centration of the stream, therefore, could not have been caused bythe direct release of solutes from the snowpack or forest floor tothe stream.

The major control on stream composition is the discharge ofground water and soil water to the stream. Although some water wasobserved to travel over and through the forest floor (overlandflow), the sol- concentration of the stream was much higher thanthat of meltwater or leachate. Stream sol- concentration was com-parable to, or slightly lower than that of ground water or soilwater collected from nearby sites (Cronan, 1986; Peters et al.,1987). These results are consistent with those of several recentstudies that employed isotopes to address the sources of streamflowduring storms and snowmelt (Hooper & Shoemaker, 1986; Kennedy etal., 1986; Pearce et al., 1986; Sklash et al., 1986). The stream

NO) and H+ (acidity), in part, were probably derived from nitrifica-tion in the forest floor, from which the NO) and H+ were flushedthrough the soil to the stream.

ACKNOWLEDGMENTS We would like to thank T.Suleski, J.Neubeck, andC.Rascher for their assistance. This research was part of theRegionalization of the Integrated Lake-Watershed Acidification Study(RILWAS). Financial support was provided by the Electric PowerResearch Institute (EPRI) and the Empire State Electric EnergyCorporation (ESEERCQ); the U.S. Geological Survey in cooperationwith Cornell University; and the National Science Foundation, EPRI,

Sources of acidity during snowmelt 107

and ESEERCO through a Presidential Young-Investigator Award toC.T.Driscoll.

REFERENCES

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Galloway, J.N., Schofield, C.L., Hendrey, G.R., Peters, N.E., &JOhannes, A.H. (1980) Sources of acidity in three lakes acidifiedduring snowmelt. In: Ecological impact of acid precipitation.Proc. Internat. Conf. Sandefjord, Norway, 1980 (ed. by D.Drablos& A.Tollan), 264-265. SNSF Project, Oslo, Norway.

Hooper, R.P., & Shoemaker, C.A. (1986) A comparison of chemical andisotopic hydrograph separation. Wat. Resour. Res. 22, 1444-1454.

Johannes, A.H., Galloway, J.N., & Troutman, D.E. (1980) Snow packstorage and ion release. In: Ecological impact of acid precipi-tation, Proc. Internat. Conf. Sandefjord, Norway, 1980 (ed. byD.Drablos & A.Tollan), 260-261. SNSF Project, Oslo, Norway.

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Johannessen, M., Dale, T., Gjessing, E.T., Henriksen, A., & Wright,R.F. (1977) Acid precipitation in Norway: the regional distribu-tion of contaminants in snow and the chemical concentration

processes during snowmelt. In: Isotopes and Impurities in Snowand Ice Symposium (proc. Grenoble Symp., 1975) 116-120. IAHSPubl. no. 118.

Johannessen, M., & Henriksen, A. (1978) Chemistry of snow meltwater:changes in concentration during melting. Wat. Resour. Res. 14,615-619.

Kennedy, V.C., Kendall, C., Zellweger, G.W., Wyerman, T.A., &Avanzino, R.J. (1986) Determination of the components of storm-flow using water chemistry and environmental isotopes, MattoleRiver basin, California. J. Hydrol. 84, 107-140.

Krug, E.C., & Frink, C.R. (1983) Acid rain on acid soil: A newperspective. Science 221, 520-525.

Pearce, A.J., Stewart, M.K., & Sklash, M.G. (1986) Storm runoffgeneration in humid catchments, 1. Where does the water comefrom? Wat. Resour. Res. 22, 1263-1272.

Peters, N.E. (1984) Comparison of air and soil temperatures atforested sites in the West-Central Adirondack Mountains. North.Environ. Sci. 3, 67-72.

Peters, N.E. & Murdoch, P.S. (1985) Hydrogeologicacidic-lake basin with a neutral-lake basin inAdriondack Mountains, New York. Wat. Air Soil387-402.

Peters, N.E., Murdoch, P.S., & Dalton, F.N. (1987) Hydrologic dataof the Integrated Lake-Watershed Acidification Study (ILWAS), inthe west-central Adirondack Mountains, New York, from October

comparison of anthe west-central

Pollut. 26,

108 Norman E. Peters & Charles T. Driscoll

1977 through December 1981.Report 85-80.

Rascher, C.M., Driscoll, C.T., & Peters, N.E. (1987) Concentrationand flux of solutes from snow and forest floor during snowmelt inthe west-central Adirondack Region of New York. Biogeochemistry,vol. 3, 209-224.

Rockwell International (1983a) UAPSP laboratory standard operatingprocedures. UAPSP rept. 102. Rockwell International, NewburyPark, California, USA.

Rockwell International (1983b) The utility acid precipitation studyprogram: field operator instruction manual. UAPSP rept. 104.Rockwell International, Newbury Park, California, USA.

Sklash, M.G., Stewart, M.K., & Pearce, A.J. (1986) Storm runoffgeneration in humid headwater catchments, 2. A case study ofhillslope and low-order stream response. Wat. Resour. Res., vol.22, 1273-1282.

U.S. Geological Survey Open-File