relative effect of temperature and ph on diel cycling of dissolved trace elements in prickly pear...

RELATIVE EFFECT OF TEMPERATURE AND pH ON DIEL CYCLINGOF DISSOLVED TRACE ELEMENTS IN PRICKLY PEAR CREEK,

MONTANA

CLAIN A. JONES1∗, DAVID A. NIMICK2 and R. BLAINE MCCLESKEY3

1 Department of Land Resources and Environmental Sciences, Montana State University, Bozeman,MT, U.S.A.; 2 U.S. Geological Survey, Helena, MT, U.S.A.; 3 U.S. Geological Survey, Boulder, CO,

U.S.A.(∗ author for correspondence, e-mail: [email protected], Fax: (406) 994 3933)

(Received 7 February 2003; accepted 23 September 2003)

Abstract. Diel (24 hr) cycles in dissolved metal and As concentrations have been documented inmany northern Rocky Mountain streams in the U.S.A. The cause(s) of the cycles are unknown,although temperature- and pH-dependent sorption reactions have been cited as likely causes. Alight/dark experiment was conducted to isolate temperature and pH as variables affecting diel metalcycles in Prickly Pear Creek, Montana. Light and dark chambers containing sediment and a strand ofmacrophyte were placed in the stream to simulate instream temperature oscillations. Photosynthesis-induced pH changes were allowed to proceed in the light chambers while photosynthesis was preven-ted in the dark chambers. Water samples were collected periodically for 22 hr in late July 2001 fromall chambers and the stream. In the stream, dissolved Zn concentrations increased by 300% fromlate afternoon to early morning, while dissolved As concentrations exhibited the opposite pattern,increasing 33% between early morning and late afternoon. Zn and As concentrations in the lightchambers showed similar, though less pronounced, diel variations. Conversely, Zn and As concen-trations in the dark chambers had no obvious diel variation, indicating that light, or light-inducedreactions, caused the variation. Temperature oscillations were nearly identical between light and darkchambers, strongly suggesting that temperature was not controlling the diel variations. As expected,pH was negatively correlated (P < 0.01) with dissolved Zn concentrations and positively correlatedwith dissolved As concentrations in both the light and dark chambers. From these experiments,photosynthesis-induced pH changes were determined to be the major cause of the diel dissolvedZn and As cycles in Prickly Pear Creek. Further research is necessary in other streams to verify thatthis finding is consistent among streams having large differences in trace-element concentrations andmineralogy of channel substrate.

Keywords: arsenic, cycling, diel, diurnal, metals, Montana sediment, streams, trace elements, zinc

1. Introduction

Acid mine drainage has caused elevated concentrations of metals and As in manystreams in the northern Rocky Mountains (U.S.A.), including Montana. For ex-ample, dissolved Zn concentrations in parts of the Prickly Pear Creek watersheddownstream from historical mines are as much as 10 times higher than the Montanachronic aquatic-life criteria (Klein et al., 2001). Long-term trend analysis of water-quality data for these streams is critical to evaluation of the effectiveness of ongoing

Water, Air, and Soil Pollution 153: 95–113, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

96 C. A. JONES ET AL.

Figure 1. Diel variation in dissolved Zn concentration in four streams, Montana and Idaho. Data arefrom Nimick et al. (2003) and Brick and Moore (1996). Shaded areas indicate general nighttimehours.

remediation efforts. However, trend analysis is inherently difficult for stream water-quality data because constituent concentrations change temporally in response tostreamflow variations. In addition to seasonal variations, research in the last fifteenyears has shown that diel (24 hr) changes in metal and As concentrations contributeto the variability in water quality (Fuller and Davis, 1989; Brick and Moore, 1996;Nimick et al., 1998, 2003). For example, dissolved Zn concentrations increased by50 to 500% from late afternoon to early morning in a number of streams in Montanaand Idaho (Figure 1); these streams had a wide range of ambient Zn concentrationsand neutral to alkaline pH. The opposite diel concentration pattern was observedfor dissolved arsenic in these streams, and the oscillations were smaller (Nimick etal., 2003). The causes of the diel concentration cycles are not known, but the cyclesmight be caused by several mechanisms including daily changes in (1) upstreammetal loading, (2) streamflow, (3) biological uptake, (4) precipitation/dissolutionreactions, and (5) adsorption/desorption reactions (Fuller and Davis, 1989; Brickand Moore, 1996; Nimick et al., 2003). Although these mechanisms could beworking in concert, previous research has indicated that the most likely mechan-isms responsible for diel metal and As cycling in neutral to alkaline streams aretemperature- and pH-dependent sorption reactions (Nimick et al., 2003).

DIEL CYCLING OF DISSOLVED TRACE ELEMENTS 97

Temperature-dependent adsorption onto inorganic surfaces has been well docu-mented (Machesky, 1990; Barrow, 1992; Rodda et al., 1996; Angove et al., 1998;Trivedi and Axe, 2000; Scheckel and Sparks, 2001). Results of these laboratorystudies demonstrated that metal cation adsorption generally increased while anionadsorption generally decreased with increasing temperature, in agreement with thediel oscillations of dissolved metal and As concentrations occurring in streams(Fuller and Davis, 1989; Brick and Moore, 1996; Nimick et al., 1998, 2003). Metalcation adsorption is favored by increased temperature because mineral surfacesbecome more negatively charged at higher temperatures and because metal cationadsorption is endothermic. At equilibrium, anion adsorption would be expectedto decrease with increased temperature because the reaction is exothermic andmineral surfaces become more negatively charged at higher temperature (Barrow,1992). The observed effect of temperature on anion adsorption was not consistentamong studies largely because the adsorption reaction rate increases at higher tem-peratures and temporarily counteracts the effect of temperature on the adsorptionequilibrium (Barrow, 1992).

The magnitude of the temperature effect on adsorption is influenced by a num-ber of factors including the sorption substrate, pH, and the sorbed trace element.For example, at pH 5.5 in a kaolinite slurry, the Langmuir isotherm equilibriumconstant for Cd at 25 ◦C was approximately 2-fold higher than at 10 ◦C, whereasat pH 7.5, the Cd sorption coefficients at 25 and 10 ◦C were essentially the same(Angove et al., 1998). Solution concentrations of Zn that had been equilibratedwith a loamy sand in a sorption experiment were found to be approximately 25%higher at 5 than at 25 ◦C, although the result was not consistent at different Znconcentrations (Barrow, 1986). In contrast, dissolved Zn concentrations in PricklyPear Creek increased by 500% when the temperature decreased from approxim-ately 22 to 11 ◦C (Nimick et al., 2003). Therefore, temperature may explain someof the concentration change in observed diel metal and As cycles, but likely doesnot account for the entire change.

The effect of pH on metal cation and As adsorption has been well documented(Schindler et al., 1976; McKenzie, 1980; Benjamin and Leckie, 1981; Goldberg,1986). Stream pH increases of up to 0.8 pH unit from early morning to late af-ternoon in streams sampled in Montana and Idaho likely cause a portion of thediel variation in dissolved metal and As concentrations (Nimick et al., 2003). Inaddition, diel Zn cycles have been linked to diel pH variation in the Lot River insouthwestern France (Bourg and Bertin, 1996). Diel variation in pH is largely dueto photosynthesis- and respiration-induced changes in carbonate equilibria.

The streams that have the largest diel variations in metal concentrations gen-erally have the largest shifts in both temperature and pH, making it difficult todetermine the relative effects of each factor (Nimick et al., 2003). Currently, onlyone known set of controlled experiments has been conducted to assess the relativeeffects of temperature and pH on dissolved metal concentrations in streams (Xie,2002). Specifically, these experiments used water, biofilm, and sediment from High


Ore Creek near Butte, Montana, and demonstrated that dissolved Zn concentrationsvaried in response to changes in either temperature or pH. In an experiment thatseparately replicated the normal range of diel temperature and pH variation in HighOre Creek, pH caused about 67% of the diel Zn variation while temperature causedabout 33% (Xie, 2002).

The objective of this study was to determine the effects of both temperatureand pH changes on dissolved Zn and As concentrations in a stream with large dielconcentration cycles. The experiment was designed to isolate the individual effectsof temperature and pH by maintaining the natural stream-temperature cycles inthe chambers while biologically manipulating pH. Natural stream pH cycles werereproduced in the light experiment while photosynthesis-induced pH changes wereeliminated in the dark experiment.

2. Materials and Methods

2.1. SITE DESCRIPTION

Prickly Pear Creek, near Montana City, Montana (Figure 2), was selected for thisstudy because of the large diel variation (as much as 500%) in dissolved Zn con-centrations. The stream drains 497 km2 and has an annual mean flow of 1.3 m3 s−1

at the gaging station (Shields et al., 2002). Prickly Pear Creek is a typical head-water stream in the northern Rocky Mountains and has a streambed composed ofpebbles, cobbles, boulders, and coarse sand. Riparian vegetation consists of grassesand shrubs (such as willows), streambed vegetation is dominated by Ranunculusaquatilis, and periphyton exists in thin biofilms on cobbles and boulders. Dailymean streamflow during 2001 was less than the historical mean owing to prolongeddrought conditions. Land-use in the upstream basin includes historical placer andhard-rock mining, logging, cattle grazing, and residential development. Graniticrocks of the Boulder batholith underlie most of the basin (Klein et al., 2001).Quartz veins were mined for Cu, Pb, Zn, Ag, and Au during the late 1800s and early1900s (Roby et al., 1960). Some streams in the watershed have concentrations ofone or more metals such as Cd, Cu, and Zn that are elevated relative to aquatic-lifecriteria; dissolved metal concentrations at the diel sampling site are elevated butgenerally do not exceed criteria (Klein et al., 2001). Concentrations of metals instreambed sediment at the sampling site are greater than crustal abundance valuesowing to drainage from upstream areas of historical mining (Klein et al., 2001).The average stream depth at the experiment site was approximately 0.3 m duringthe experiment, and the channel substrate was largely sand, pebbles, and cobbles.


Figure 2. Study area and sampling sites.


2.2. STREAM MONITORING

The sampling location was approximately 150 m downstream from U.S. Geolo-gical Survey (USGS) gaging station 06061500. Stream samples were collected atleast hourly from 12:00 hr on 25 July until 11:00 hr on 27 July 2001, using anautomatic pumping sampler. All times reported herein are Mountain Daylight Time(MDT). Hoses and polyethylene collection bottles used with the sampler wereacid rinsed prior to the sampling episode. The intake for the sampler was posi-tioned approximately 15 cm above the streambed in a location where the streamvelocity was sufficient to flush expelled water away from the intake. Collectedwater was removed from the sampler immediately after pumping ceased, taken toan onsite field laboratory, and filtered through a mixed cellulose ester membrane(142 mm dm; 0.1 µm). Filtrates were preserved with concentrated HNO3 (0.4%v/v) for Mn and Zn analyses and with concentrated HCl (0.4% v/v) for As and Feanalyses. Concentrated HCl was used to preserve filtrates for As and Fe analysesbecause both elements were speciated as part of a concurrent diel sampling study,and HNO3 was not compatible with speciation methods. The type of acid addedshould not affect dissolved trace-element concentrations. Stream temperature, pH,dissolved oxygen, and specific conductance were measured with a multi-parameterinstrument (DataSonde, Hydrolab Corp., U.S.A.) submerged in an area of slow-moving water near the intake of the pumping sampler. (Any use of trade, product,or firm names is for descriptive purposes only and does not imply endorsement bythe U.S. Government.)

2.3. LICHT/DARK EXPERIMENT

On 23 July, equal amounts of streambed substrate collected near the USGS gagingstation and at a site located about 5 km upstream (Figure 2) were composited in19 L high-density-polyethylene buckets. Substrate from the upstream location wasincluded because dissolved metal and As concentrations at the gage site likelywould be affected by water-substrate reactions occurring near the gaging stationand in the upstream reach. The collected substrate was primarily sand. The threebuckets and six 16 L rectangular (19 × 30 cm) clear polypropylene chambers wereplaced in the stream at the gage site with the openings facing upstream to equilib-rate the chambers and the substrate material with ambient stream water. On 25 July,three of the polypropylene chambers were wrapped completely in black plastic andthe openings covered with a sheet of black plastic (dark chambers). The other threechambers were left unwrapped (light chambers) but their openings were coveredwith a clear plastic sheet. All covers were loosely fitted to allow gas exchange. Atsunset on 25 July, 0.5 L of the composited substrate was placed in the bottom oftwo light and two dark chambers, forming a 1.3 cm layer. Subsequently, 11.5 L ofstream water were added to each of the six chambers. In addition, a 0.6 m strandof R. aquatilis was added to each of the four chambers holding substrate. A pre-liminary experiment on 25 July found that the macrophyte was capable of raising


the pH in the chambers to the maximum stream pH (about 8.5), whereas biofilmon the substrate material was only able to raise the pH to about 8.1. Although R.aquatilis was not the only pH-modifier in the stream, it was able to simulate streampH changes, which was the goal of this study. One dark and one light chamber(dark and light controls) contained neither substrate nor macrophyte to verify thatthe chamber walls were not sorbing Zn or As. The chambers were placed uprightin the stream about 10 m downstream of the automatic sampler. To best attainambient stream temperature, the chambers were positioned to maintain their waterlevel near the stream water level.

Sampling started at 06:00 hr on 26 July and continued until 04:00 hr on 27 July.A second 0.6 m strand of macrophyte was added to each chamber at 12:30 hr on 26July when the pH in the light chambers was found to not be increasing as quickly asin the stream. Water samples were collected with the automatic sampler every 2 hrfrom the stream and with a 60 mL syringe at a depth of 13 cm above the bottom ofeach chamber. Samples were filtered immediately through polyethersulfone filters(25 mm dm; 0.45 µm) and acidified with concentrated HNO3 (0.4% v/v) for As,Fe, and Zn analyses. Filter pore-sizes were larger in this experiment than in thestream monitoring study because hand-held syringes were used in this experimentto minimize water volume extracted from each chamber, and it was not possibleto hand-filter through 0.1 µm filters. Conversely, stream monitoring (described inprevious section) used 0.1 µm filters to best estimate ‘dissolved’ concentrationsand match diel work performed to date in regional streams, yet these larger dia-meter filters (142 mm) require more water volume for rinsing and therefore, werenot suitable for the light/dark chamber experiment. All syringes and filter holders,which were dedicated to a particular chamber or stream, were rinsed twice with de-ionized water between samples. For this study, the term ‘dissolved’ is operationallydefined as the concentration in 0.1- or 0.45 µm filtrates.

Water temperature in each chamber and the stream was measured with a full-immersion glass thermometer. Similarly, pH was measured with an Orion 250Ameter and a Ross probe that was calibrated at least twice daily with pH 7 and pH10 buffers.

2.4. ANALYTICAL METHODS

Water samples were analyzed at the USGS laboratory in Boulder, Colorado. Fe,Mn, and Zn concentrations were determined by inductively coupled plasma-opticalemission spectrometry. Arsenic concentrations were determined using a flow-in-jection analysis system for the generation of arsine and detection using atomicabsorption spectrometry (McCleskey et al., 2003). Each water sample was ana-lyzed twice and the values were averaged. All reagents were of purity at least equalto the reagent-grade standards of the American Chemical Society. Double-distilledwater and re-distilled acids were used in all preparations. USGS standard referencewater samples T159 and T163 (Farrar, 1999; Conner et al., 2001) were analyzed


repeatedly to monitor accuracy. Mean recoveries of nine or ten measurements ofthese standards were 101% for As (only T159), 95–102% for Fe, 87–100% forMn, and 105–108% for Zn. Relative standard deviations for these analyses wereless than 3% for Fe and Mn, 5% for As, and 7% for Zn. Three field duplicates werecollected for As and Fe, two for Mn, and one for Zn. The relative percent differ-ences in concentrations for these sets of duplicate samples were 4% or less. Onestream sample was analyzed repeatedly to monitor precision during and betweenanalytical runs. The relative standard deviations for repeat analyses of this samplewere less than 3% for Fe, Mn, and Zn; this sample was not analyzed for As.

3. Results and Discussion

3.1. STREAM RESULTS

Diel data for water temperature, stream pH, and dissolved (0.1 µm filtration) con-centrations of Zn, Mn, and As for Prickly Pear Creek are shown in Figure 3 for the47 hr sampling episode conducted 25–27 July 2001. Dissolved Zn concentrationsincreased 300%, ranging from 13.1 µg L−1 at 17:00 hr on 25 July to 52.4 µg L−1 at08:00 hr on 26 July. Maximum Zn concentrations were approximately 25% lowerthan in late June 2000 (Figure 1), most likely because somewhat higher streamflowin 2001 (0.42–0.54 m3 s−1) than in 2000 (0.35–0.41 m3 s−1; Nimick et al., 2003)provided more dilution. Dissolved Mn concentrations showed a diel variation sim-ilar to Zn, although concentrations increased by only 60% from late afternoon untilearly morning. Variations in dissolved As concentrations were small (4.2–5.8 µgL−1) and exhibited the opposite pattern of Zn and Mn; concentrations increased33% with minimum concentrations occurring near 08:00 hr and the maximum con-centrations occurring near 20:00 hr. The opposite pattern of the diel cycles of As,which exists as an oxyanion in solution, and the cationic metals, strongly suggeststhat dilution from diel streamflow changes did not cause the diel concentrationvariations.

Water temperature ranged from 12.9 to 22.1 ◦C between 08:00 hr and 16:00 hron 26 July. The large change in water temperature is attributed to the large air-temperature variation (7.5 to 28.5 ◦C), clear skies, shallow stream depths, and thelack of midday stream shading because the stream valley is oriented north-south.

Stream pH ranged from 8.00 to 8.49 between 05:00 and 16:00 hr on 26 July.This was a somewhat smaller range than in June 2000, when the pH ranged from8.28 to 9.05 (Nimick et al., 2003). The higher pH values and larger pH rangeobserved in June 2000 probably resulted from more photosynthesis caused bythe longer day light period and more incident solar radiation, combined with lessdilution from lower streamflow. Maximum pH values (on both 25 and 26 July)lasted less than 2 hr, whereas minimum pH values were stable for as long as5 hr each night, suggesting CO2 (aq) concentrations became stable during the night.


Figure 3. Diel variation in dissolved (0.1 µm filtration) As, Mn, and Zn concentration (top) andpH and water temperature (bottom) in Prickly Pear Creek, 25–27 July 2001. Shaded area indicatesnighttime hours.

Geochemical speciation modeling using PHREEQC (Parkhurst and Appelo, 1999)indicated that the stream water was supersaturated with respect to calcite for ap-proximately 80% of the 2 day sampling period (data not shown); therefore, cal-cite equilibria reactions, and hence CO2 (aq) concentrations, were likely controllingstream pH. Temperature also may have affected CO2 (aq) concentrations, and hencesome of the pH diel variation, because CO2 (aq) solubility increases at lower tem-peratures.


The extreme values of the diel cycles in temperature, pH, and the concentrationof each trace element generally coincided, indicating a possible link between thesefactors. Peaks in both water temperature and pH coincided with daily minimumdissolved Zn concentrations, and minimum temperatures coincided exactly withthe maximum Zn concentration (Figure 3). The maximum Zn concentration on 26July occurred 3–5 hr after the minimum pH was recorded, possibly as a result ofslow desorption rates. The finding that low temperature and pH nearly coincidedwith high Zn concentrations agrees with previous work by Nimick et al. (2003).Dissolved Mn concentrations showed a similar temporal pattern as Zn but themagnitude of the diel cycle was not as pronounced. In contrast, minimum andmaximum concentrations of dissolved As were generally in phase with minimaand maxima temperature, lagging by only 1–2 hr. Maxima pH values also werein similar phase with As maxima, but As minima lagged pH minima by about4 hr. Despite minor temporal shifts, the nearly coincident diel phases of eithertemperature or pH could account for the instream diel variations in dissolved Zn,Mn, and As concentrations.

Dissolved metal and As concentrations in the stream were determined in both0.1 µm filtrates and 0.45 µm filtrates, whereas only 0.45 µm filtrates were analyzedin the light/dark experiment. Zn and As concentrations in the 0.1 µm filtrates fromPrickly Pear Creek generally were 10–20% lower than in the 0.45 µm filtrates,although the diel concentration patterns were almost identical (Figure 4). The lowerconcentrations in 0.1 µm filtrates indicate that some colloidal Zn and As werepresent in the stream, but that the colloidal fractions did not change substantiallyover the course of the day, nor did they affect the conclusions of this study. Col-loidal material in streams is typically composed primarily of hydrous oxides ofAl and Fe (Nordstrom and Alpers, 1999). Although not quantified, the amountof colloidal material in Prickly Pear Creek during the sampling period likely wasrelatively small because the concentrations of filtered (0.45 µm) Al and Fe wereless than 80 and 60 µg L−1, respectively (unpubl. data; Figure 4). These low con-centrations indicate that little hydrous oxide material was in suspension to adsorbmetals and As. The large differences in Fe concentrations between 0.1 and 0.45 µmfiltrates demonstrates that most of the Fe in the 0.45 µm filtrates was colloidal; thisresult is not surprising given the low solubility of Fe hydrous oxides in oxidizedconditions at high pH.

3.2. LIGHT/DARK EXPERIMENT RESULTS

In the light chambers, concentrations of Zn and As, temperature, and pH (Figure 5)exhibited diel cycles similar to the diel cycles in Prickly Pear Creek (Figure 3)but with distinct differences. Specifically, temperature and pH were not as syn-chronous through their peak phase of the diel cycle as in the stream. Temperaturepeaked approximately 4 hr before the pH maximum and dissolved Zn minimum.In this experiment, average diel Zn concentrations were highest at noon and de-


Figure 4. Diel variation in dissolved As, Fe, and Zn concentrations measured in 0.1 and 0.45 µmfiltrates, Prickly Pear Creek, 26–27 July 2001.

creased 49% by early evening. Average diel As concentrations were lowest in earlymorning and increased 35% by early evening, before decreasing slightly. Zn andAs concentrations in the controls showed no consistent diel cycle, demonstratingthat the chamber walls were neither adsorbing nor desorbing As or Zn during theexperiment.

Zn concentrations in the dark chambers were significantly different (P = 0.05)from concentrations in the light chambers for the final five sampling times and


Figure 5. Diel variation in dissolved Zn and As concentration (0.45 µm filtrates), water temperature,and pH in light and dark chambers with sediment and macrophytes (left), and light and dark controls(right). Error bars for Zn show two standard deviations except at 04:00 hr in the dark chamber, whenonly one sample result was usable.


did not exhibit diel variations (Figure 5). Zn concentrations gradually increased inthe dark chambers and were almost 2 fold higher than concentrations in the lightchambers at the end of the experiment. Dissolved As concentrations in the darkchambers were significantly different from concentrations in the light chambers forall but one sampling time after the experiment commenced. Arsenic concentrationsgenerally increased in the light chambers, whereas As concentrations in the darkchambers were relatively unchanged throughout the experiment.

Results of the light/dark experiment strongly indicate that light and the presenceof vegetation were, directly or indirectly, controlling dissolved Zn and As concen-trations in Prickly Pear Creek. Light could affect dissolved metal and As concen-trations through numerous mechanisms, including its indirect effect on sorptionreactions due to increased temperature (Barrow, 1992), photosynthetic-induced pHchanges (Fuller and Davis, 1989; Bourg and Bertin, 1996), photoreduction of Fe(McKnight et al., 1988), redox potential (Brick and Moore, 1996), or biotic uptake(Hill et al., 2000), each of which will be discussed below.

Temperature never varied by more than 1.0 ◦C between chambers, likely toosmall a difference to account for the large differences in dissolved Zn and As con-centrations between light and dark chambers based on previous research (Barrow,1992). Zn and As concentration variations in the dark chambers did not mirrortemperature oscillations, strongly suggesting temperature was not the major factorcontrolling dissolved trace-element concentrations. Nevertheless, because of theconcurrent timing of pH and temperature cycles, pH variations may have maskedsome effects of temperature changes on dissolved Zn and As concentrations.

Dissolved Zn concentrations in both the light and dark chambers are inverselyrelated to pH (Figure 5), in agreement with known pH effects on adsorption andprecipitation reactions (Stumm and Morgan, 1996). Whether Zn concentrationswere controlled by adsorption/desorption or precipitation/dissolution reactions isnot known. However, speciation modeling using PHREEQC (Parkhurst and Ap-pelo, 1999) suggested the stream water was undersaturated with respect to Zn solidphases likely to precipitate or dissolve on the time scale of the diel concentrationcycles (Table I). Willemite (Zn2SiO4) was the only Zn solid phase with saturationindices (SI) above 0 for any part of the 2 day sampling period, yet the streamwater was undersaturated with respect to willemite in 35 of the 38 samples forwhich SI values could be calculated. In addition, willemite is not expected toform on the time scale of hours at temperatures below 25 ◦C. The stream wa-ter was also undersaturated with respect to major arsenate solid phases, with SIvalues never exceeding –7.7, strongly suggesting that arsenate solid phases werenot controlling dissolved As concentrations. In summary, it is likely that Zn andAs concentrations were controlled by sorption/desorption reactions rather thanprecipitation/dissolution reactions.

Differentiating between temperature and pH as controls of the diel Zn and Asvariations was not possible using stream data because of the general concurrenttiming of temperature and pH maxima and minima in the stream (Figure 3). How-


TABLE I

Saturation indices (SI) for Ca, Zn, and As solid phases calculated from analysesof Prickly Pear Creek stream-water samples collected 25–27 July 2001

Solid phase Formula Min SI Max SI K source

Calcite CaCO3 –0.1 0.5 a

Willemite Zn2SiO4 –1.0 0.1 a

Zn(OH)2 –2.3 –1.8 a

Smithsonite ZnCO3 –2.4 –1.9 a

Zincosite ZnSO4 –14.4 –13.6 a

Goslarite ZnSO4·7H2O –9.2 –8.0 a

ZnSiO3 –1.9 –1.5 b

ZnO(am) –2.1 –1.6 a

Zincite ZnO –2.4 –1.7 a

AlAsO4·2H2Oc –11.2 –8.9 a

Ca3(AsO4)2·4H2O –13.2 –11.8 a

Cu3(AsO4)2·6H2O –18.3 –15.0 a

Mn3(AsO4)2·8H2O –15.8 –12.7 a

Scorodite FeAsO4·2H2O –9.6 –7.7 a

Zn3(AsO4)2·2.5H2O –16.4 –14.4 a

a Wateq4f (Parkhurst and Appelo, 1999).b Norvell and Lindsay (1970).c Concentrations of Al were less than the detection limit of 0.08 mg L−1 inall samples. For SI calculations, Al concentrations were assumed to equal thedetection limit.

ever, the maximum pH in the light chambers lagged 2 to 4 hr behind the maximumtemperature (Figure 5) likely due to slower reequilibration with atmospheric CO2

in the covered chambers than in the stream. The dissimilarity between pH and tem-perature curves in the light chambers helped to differentiate between the relativeeffects of both pH and temperature in the light/dark experiments. A significantnegative correlation (P = 0.01) was found between pH and dissolved Zn con-centrations in both light and dark chambers (Figure 6), strongly suggesting thatpH was a major cause of the diel variations. Conversely, no significant correlation(P = 0.05) was found between temperature and dissolved Zn concentrations foreither the entire data set or for the light chambers, although a significant correl-ation was found for the dark chambers. Unlike the consensus in the literatureon temperature-adsorption relationships, increased temperature was found to bepositively (rather than negatively) correlated to dissolved Zn concentrations in thedark chambers. This apparent correlation likely resulted more from decreased pHand a subsequent increase in Zn as temperature increased, rather than a directtemperature effect. Although the pH decrease was small in the dark chambers, pH


Figure 6. Relation of dissolved As and Zn concentrations (0.45 µm filtrates) to pH (left) and watertemperature (right). The regression relation between Zn and the entire temperature data set is notshown because the regression was not significant at P = 0.05.

would have needed to remain constant to make definitive conclusions regarding theeffect of temperature on dissolved Zn concentrations. The gradual pH decrease inthe dark chambers was likely a result of increased CO2 from biological respirationor organic acid production. Although the chambers were not sealed, the chambercovers probably retarded CO2 degassing. In addition, the pH in the light cham-bers did not return to the initial pH, likely because the chamber covers inhibitedreaeration and equilibration with atmospheric CO2.

Dissolved As concentrations were positively correlated with temperature (P <

0.05) and pH (P < 0.01) for the combined light and dark data set. Significantcorrelations between dissolved As concentrations and temperature or pH did notexist for data from the dark chambers (data not shown), most likely because Asconcentrations were relatively constant (Figure 5). Dissolved As concentrationsin the light chambers were much more strongly correlated with pH than withtemperature (Figure 6).

As previously stated, the positive correlations between temperature and dis-solved Zn concentrations in the light/dark chambers were weak and sometimesopposite the general inverse direction found for Prickly Pear Creek and adsorption-controlled reactions (Angove et al., 2000; Barrow, 1986, 1992). The only


Figure 7. Dissolved Fe concentrations (0.45 µm filtrates) in light and dark experiments. Error barsrepresent two standard deviations of two replicated treatments, except at 12:00 hr on 26 July, whenonly one sample result was usable.

known research that shows decreased Zn adsorption at increased temperature wasin a Fe-oxide system at pH 8.2 (Srivastava and Srivastava, 1990), which is withinthe range of pH values recorded in the chambers and stream during this study.Although solid phases in Prickly Pear Creek sediment were not identified, the sandhad a reddish hue, suggesting the presence of Fe hydroxides or oxides. Therefore,the positive correlation between increased temperature and increased dissolved Znconcentrations may be relatively unique to Prickly Pear Creek. For example, Xie(2002) found a negative correlation between temperature and dissolved Zn concen-trations in a laboratory experiment with sediment from High Ore Creek near Butte,Montana. The differences in results demonstrate a need for further study of dieltrace-element variations in other stream systems.

The light/dark experiment strongly indicated that light was causing diel vari-ations in trace-element concentrations in Prickly Pear Creek, and regression ana-lysis demonstrated that pH was strongly correlated with dissolved Zn and As con-centrations. However, as stated earlier, light may also affect trace-element concen-trations through photoreduction of Fe hydroxides, reductive dissolution of Fe andMn hydroxides, and biotic uptake of trace elements.

Photoreduction of Fe hydroxides has been documented in Rocky Mountainstreams (McKnight et al., 1988). Theoretically, dissolved concentrations of ad-sorbed or co-precipitated trace elements would increase as the Fe hydroxides dis-solve during the day. However, dissolved Zn concentrations in the light chambersand in Prickly Pear Creek decreased during the day. In addition, dissolved Feconcentrations were nearly identical in both light and dark chambers (Figure 7),


whereas photoreduction of Fe hydroxides causes increased dissolved Fe concen-trations (McKnight et al., 1988). Photosynthesis-induced increases in dissolved O2

concentrations in the water column, and subsequent diffusion into the sediment,could increase redox potential in the sediment, precipitating more Mn and Fehydrous oxides during the day. This would cause an increase in Zn adsorptionsites and subsequent removal of Zn from the water column, consistent with the dielvariation in both dissolved Mn and Zn concentrations (Figure 3). However, light-induced increases in redox potential should have decreased, rather than increased,dissolved As concentrations in Prickly Pear Creek during the day due to an in-crease in Fe and Mn hydrous oxides and reduction of arsenate to the more mobilearsenite. Uptake by phytoplankton or macrophytes could account for some of thelower dissolved Zn concentrations during the afternoon in the light experiment, yetuptake could not have caused the daytime increases in dissolved As concentrations.

4. Conclusion

Dissolved concentrations of Zn and Mn in Prickly Pear Creek in late July 2001were highest in the early morning and lowest in late afternoon, similar to resultspreviously reported for streams in Montana and Idaho (Nimick et al., 2003). Spe-cifically, dissolved Zn concentrations increased by 300% from late afternoon toearly morning. Smaller diel variations in dissolved As concentrations also wereobserved, and the As concentration pattern was opposite the Zn concentrationpattern.

Results from a light/dark chamber experiment in Prickly Pear Creek demon-strated that the diel variation in dissolved Zn and As concentrations was signi-ficantly related (P = 0.05) to the presence of light and was likely caused byphotosynthesis-induced changes in pH. Linear regression analysis suggested thattemperature had either no effect, or a slight positive effect, on dissolved Zn concen-trations in the chambers. This result was opposite the correlation expected basedon Prickly Pear Creek stream samples, previous literature (Nimick et al., 2003),and a laboratory experiment using sediment from a nearby stream (Xie, 2002).In addition, regression analysis strongly suggested that changes in pH, rather thantemperature, controlled dissolved As concentrations in Prickly Pear Creek. In sum-mary, photosynthesis and respiration-induced pH changes are the most likely causeof the observed Zn and As diel variations in both the stream and the light/darkchambers. Additional work in a wider range of streams is necessary to better un-derstand the specific processes causing diel trace-element concentration cycles andthe relative effect of temperature and pH.


Acknowledgements

We gratefully acknowledge funding from the U.S. Geological Survey Toxic Sub-stances Hydrology Program and the Montana Agricultural Experiment Station,Montana State University-Bozeman (Journal Series No. 2003-32). We thank A. N.Johnson and J. M. Kilpatrick for field assistance. Reviews of early drafts of thismanuscript by J. A. Ball, T. E. Cleasby, and J. H. Lambing were greatly appreci-ated.

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relative effect of temperature and ph on diel cycling of dissolved trace elements in prickly pear...

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