Environ. Sci. Technol. 1992, 26, 1388-1396
Sorption of Dissolved Organic Carbon by Hydrous Aluminum and Iron Oxides Occurring at the Confluence of Deer Creek with the Snake River, Summit County, Colorado Diane M. McKnight,*rt Kenneth E. Bencaia,$ Gary W. Zeiiweger,t George R. Alken,t Gerald L. Feder,* and Kevin A. Thorn? U S . Geological Survey, US4, 325 Broadway, Boulder, Colorado 80303-3328, U.S. Geological Survey, MS 496, 345 Middlefield Road, Menlo Park, California 94025, and US. Geological Survey, MS 432, 12201 Sunrise Valley Drive, Reston, Virginia 22092
rn Organic solute sorption by hydrous iron and aluminum oxides was studied in an acidic, metal-enriched stream (the Snake River) at its confluence with a pristine stream (Deer Creek). From 1979 to 1986, typically 40% of the dissolved organic carbon (DOC) was removed from solution by sorption onto aluminum and iron oxides, which precipitate as the two streamwaters mix. Upstream DOC concentrations, which increase during snowmelt, were identified as the most significant variables in a multiple regression for determining the DOC concentration below the confluence, and the extent of A1 and Fe precipitation was much less significant. On hourly timescales, removal of A1 and Fe varied erratically but DOC removal was steady, indicating that %orbable" organic solutes are sorbed either by precipitating oxides or by oxides on the streambed. Characterization of two reactive DOC fractions (fulvic and hydrophilic acids) showed that sorption results in chemical fractionation. Molecules with greater contents of aromatic moieties, carboxylic acid groups, and amino acid residues were preferentially sorbed, which is consistent with the ligand exchange-surface complexation model.
Introduction In aquatic environments,sorption of organic solutes may effect the chemistry and fate of dissolved organic material, hydrous metal oxides, and additional sorbed constituents such as phosphate and trace metals (1-4). Sorption of organic solutes by hydrous metal oxides has been directly demonstrated in various model systems in laboratory experiments (5-7). Ligand exchange-surface complexation has been proposed as a conceptual model for the sorption of organic solutes by hydrous metal oxides ( 5 , 6 ) . In this model, an organic ligand replaces a hydroxyl group, which is coordinated to the metal ion on the surface of the oxide, to form a surface complex. Reacting species in natural waters may be heterogeneous and poorly characterized compared to model systems, thus field studies complement laboratory results in developing an understanding of organic solute sorption. Amorphous phases of hydrous metal oxides, with poorly defined solubilities and surface characteristics,typically are abundant relative to more crystalline phases in natural waters. Dissolved organic material (DOM) is composed of many different organic solutes and is also difficult to characterize. Organic acids typically account for much more of the DOM than organic bases and neutrals, and acidic functional groups are very likely to act as ligands in ligand exchange reactions at the oxide surface. In most natural waters, fulvic acid is the most abundant class of dissolved organic acids and is characteristically heterogeneous with average chemical properties differing among environments. An+ US.Geological Survey,
Boulder.
* U.S. Geological Survey, Menlo Park. U.S. Geological Survey, Reston.
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other class of dissolved organic acids is hydrophilic acids, which are also heterogeneous but are more hydrophilic, having greater oxygen and carboxylic acid contents than fulvic acids (8). Because of their heterogeneity, each of these organic acid classes may contain molecules with varying affinities for oxide surfaces. The first objective of the study presented here is to quantify the extent and temporal characteristics of organic solute sorption in a stream system with abundant hydrous iron and aluminum oxides. Because of the importance of acidic functional groups in ligand exchange reactions on oxide surfaces, the second objective was to determine the extent to which sorption caused chemical fractionation of fulvic and hydrophilic acids. Figure 1presents a schematic diagram of the multiple processes occurring in the study site. The upper Snake River is acidic and has high concentrations of dissolved A1 and Fe. The pH in the mixing zone of the confluence with pristine Deer Creek (5.0-6.0) is greater than the pH in the Snake River (3.5-4.3). Hydrous aluminum and iron oxides precipitate and coat the streambed in the mixing zone. Organic solutes can sorb within the water column on freshly precipitating oxides as well as on oxides already on the streambed. The range of pH in the confluence corresponds to the maxima in the fulvic acid sorption in laboratory experiments (6). In the upper Snake River, A1 concentrations vary seasonally due to snowmelt dilution in the spring (9),and Fe concentrations vary throughout the daylnight cycle due to photoreduction of hydrous iron oxides and other processes (10). In both streams, dissolved organic carbon (DOC) concentrations vary seasonally with peak values preceding peak stream discharge during snowmelt (9).
Site Description The watershed area is 11.7 km2for the Snake River and 10.4 km2for Deer Creek (Figure 2) (11). At the confluence, the two streams have approximately equal flows. A culvert immediately below the discharge of Deer Creek into the Snake River enhances mixing of the streamwaters. At the sampling site downstream of the confluence, lateral concentration gradients are not observed (12). The annual discharge in the two streams is dominated by snowmelt from May to July, with a peak discharge occurring in mid-June. Streamflow is continuously recorded by the U.S. Geological Survey (USGS) about 10 km below the confluence. Following the terminology of Theobald et al. (II), the Snake River watershed is underlain by the granitic Idaho Springs Formation. Pyrite is disseminated in the host rock near Landslide Peak, and lead and zinc mines operate near the headwaters. A bog-iron ore deposit covers portions of the upper Snake River valley. The Deer Creek watershed is underlain by Swandyke hornblende gneiss. Although some veins containing lead, zinc, and silver minerals have been mined sporadically, no effects on water chemistry in Deer Creek are evident.
Not subject to U.S. Copyright. Published 1992 by the American Chemical Society
105’50’30
Figure 1. Transport and removal of dissolved organic material, oxides, and trace metals through the mixing zone at the confluence of Deer Creek with the Snake River. The cut-out box illustrates the four possible fates of DOM entering the confluence. DOM may continue in transport both as DOM and as DOM sorbed to suspended oxides; DOM may be removed within the confluence both by sorption to oxides already on the streambed and by settling of fresh oxides.
Both streams have rocky streambeds with cobble diameter of about 10 cm. The general location at which distinct metal oxide deposits occur in the stream has apparently remained unchanged since the study of Theobald et al. (11). Upstream in the Snake River, the rocks are coated with a hard, black iron and manganese deposit. In the confluence the oxides are red-brown near the banks of the Snake River and are beige in the center. Downstream, the precipitate covering the rocks is lighter. Above the confluence, periphyton in the Snake River are typical of those in acid mine drainage streams and much less abundant than in Deer Creek; periphyton are very sparse below the confluence (13).
Methods Sample Collection. Water samples were collected from three sites (Figure 2) above and below the confluence from September 1979 until November 1986. Approximately 50% of the DOC samples were collected during snowmelt (May through July). An additional 30% of the DOC samples were collected in late summer and fall. Sampling was infrequent during ice cover (November to April). Samples were collected by one person on each day, with varying time elapsed between collection at each site. This data set is referred to as the ‘multiyear’ set. On September 9-10, 1984, samples were collected at the three sites, usually within minutes of each other, at hourly intervals. This data set is referred to as the ‘diel’ set. pH was measured onsite with various Orion glass pH electrodes. The use of trade names is for identification purposes only and does not constitute endorsement by the U S . Geological Survey. DOC samples were filtered through 0.45-pm Selas silver membranes with a stainless steel Gelman filtration unit and stored at 4 OC in glass. Samples for analysis of anions and cations were filtered through 0.4-pm Nuclepore membranes with an Antlia pneumatic hand pump and collected in 250-mL acidwashed plastic bottles rinsed with filtered streamwater. Samples for cations were acidified with 0.5 mL of Ultrex
0
10 METERS
Figure 2. Location of the study area near Montezuma, CO. Sketch of stream above and below confluence shows sampling sites and indicates typical oxide precipitates on the streambed.
nitric acid. Samples were processed in the field or within 0.5 h. Fulvic, humic, and hydrophilic acids were isolated with columns of XAD-8 and XAD-4 in sequence (8,14). From May 25 to June 1,1985 (a period of snowmelt preceding peak discharge), a total of 1026-1140 L from each site were collected in stainless steel, 38-L, milkcans. Samples were returned to the USGS laboratory in Arvada, CO, and filtered immediately though l-pm and 0.3-pm Balston filters in sequence. The flocculent hydrous metal oxide collected on the l-pm filters at both Snake River sites was removed by dissolution in 0.1 N HC1 and by scrubbing the filters with a small brush. The extract was then diluted with distilled water to pH 1.8; insoluble material containing humic acid was removed by settling and centrifugation, and fulvic and hydrophilic acids were isolated from the supernatant by the XAD-8/XAD-4 method. Fulvic acid samples were H+-saturated and freeze-dried. Hydrophilic acids were initially K+-saturated to retain volatile acids, but some were subsequently H+-saturated and refreezedried to reduce the carbonate contribution in the 13CNMR spectra. Analyses. DOC was analyzed on either a Technicon carbon analyzer or a Dohrman carbon analyzer. Blank values were less than 0.2 mg of C/L. Dissolved anions were analyzed with a Dionex Model 2022si/sp ion chromatograph. Cations and trace metals were analyzed with a Model 975 Jarrel-Ash inductively coupled plasma emission spectrometer. Environ. Sci. Technoi., Voi. 26, No. 7, 1992
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The elemental content of the isolated fulvic acid samples was determined by various combustive methods (15),and molecular weight was determined by vapor pressure osmometry (16). Quantitative liquid-phase 13CNMR spectra of fulvic and hydrophilic acids were recorded on a Varian XL300 NMR spectrometer at a resonant frequency of 75.4 MHz. Samples of 85-200 mg were dissolved in 1.5-2.0 mL of aqueous solution, 25% D20,and the pH was adjusted to 7.0 with NaOH. Acquisition parameters included a 50000-Hz spectra window, 45O pulse angle, 0.3-s acquisition time, 10.0-s pulse delay, and inverse gated decoupling. A line broadening of 100 Hz was applied to the free induction decays. Dioxane (67.4 ppm) was used as an internal reference standard. Peak areas were measured by electronic integration for the fulvic acids and by cut and weigh for the hydrophilic acids. 13CNMR spectra for humic substances typically contain five broad peaks: the first aliphatic peak, 0-60 ppm, representing carbons bonded to other carbons (methyl, methylene, and methine) and some carbons bonded to N or S; the heteroaliphatic peak, 60-90 ppm, representing carbons bonded to oxygen (carbohydrates, alcohols, and ethers) and some carbons bonded to N; the aromatic peak, 90-160 ppm, representing sp2-hybridized carbons (aromatic carbons and other doublebonded carbons) and anomeric carbons of carbohydrates (90-110 ppm); the first carbonyl peak, 160-190 ppm, representing carboxylic carbons; the second carbonyl peak, 190-230 ppm, representing ketones and quinones. For amino acid analyses, duplicate fulvic acid samples were hydrolyzed and 1 2 amino acids were determined, using HPLC by G. Miller, Amino Acid Geochronology Laboratory, University of Colorado. The percentage distributions for duplicates agreed well, and the variation in total amino acid content was f5-10%. Quantification of Solute Loss within the Confluence. Evaluation (12) of the behavior of chemical constituents at the confluence of the Snake River with Deer Creek has shown that manganese and sulfate concentrations are conservative through the confluence. Calculation of the proportional contribution, CN,of the Snake River to the streamflow at the site below the confluence (SN3 in Figure 2) using both solutes provides data on relative streamflow without having gaged the streams at each timepoint. The equation for CNfor the assumed conservative tracer (Mn and/or Sod2-)is
CN =
(ccSN3
- c c D C 5 ) / ( c c S N 2 - ccDC5)
(l)
where CCsNZ is the concentration above the confluence in the Snake River; CcDc5 is the concentration above the confluence in Deer Creek; and ccsN3 is the concentration below the confluence. Using CN,'predicted' concentrations, Cp, were computed for DOC, Al, and Fe as if they also were conservatively transported: Cp =
cNcSN2
+ (1 - CN)CDC5
(2)
Measured concentrations were subtracted from Cp to determine the removal within the confluence for DOC (DOC-loss), A1 (Al-loss), and Fe (Fe-loss): DOC-loss = cp - C S N ~ (3) The validity of using eqs 1-3 is based on the downstream sample being taken from the mixed parcels of waters from which the two upstream samples were taken. If the hydrology and chemistry of the streams are at steady state, then the analysis is also valid. For the multiyear data set, the samples were collected by one person at intervals of about 1h. Hourly variations in Fe concentration do occur, 1390
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and the calculation of Fe-loss is subject to uncertainty. were For sampling dates on which both Mn and Sod2measured at all sites, CNvalues calculated for both solutes generally agreed with each other (f0.05). The range of CN was from 0.45 to 0.60. When fulvic and hydrophilic acids were isolated (May 25-June 1, 1985), CNvalues were between 0.58 and 0.64, and an average value of 0.61 was used for calculations. Statistical Analysis of Multiyear Data Set. The stepwise, multiple regression routine STEPWISE (17) was used to determine the relationship of DOC-loss to other measurable stream variables. Regressions were determined for DOCSN3(DOC at SN3) and DOC-loss as related (singularly and in multiple groupings) to Q (stream discharge), 1% 8,DOCSN2, DOCDC5, A S N 2 , AlSN3, FeSN2, FeSN3, N-loss, and Fe-loss. The stepwise procedure first identified the most effective variables for regression using an F-test value of 2 as the criteria. Following this identification, a simple regression (REGRESS) was run using only those variables. Results Analysis of Multiyear Data. Although the vegetation and terrain in both watersheds are similar, the DOC concentration was about 2-fold greater in Deer Creek than in the upstream Snake River (Figure 3). The lower DOC in the upstream Snake River could be caused by retention of organic material in acidic subsurface zones or by instream removal of dissolved organic material. These processes could include DOC sorption by precipitating iron oxides or oxidation associated with hydroxyl radical production resulting from the photoreduction of the iron oxides. In Figure 4, DOCp, the predicted conservative DOC computed by eq 2 for 68 sampling dates, is compared to the measured DOCSN3.Across the entire DOC range the predicted value is higher than the corresponding measured value. The difference (DOC-loss)is the organic carbon that has been removed from solution by sorption on the precipitated oxides suspended in the streamwater or deposited on the streambed. DOC-loss is expressed as a concentration in units of milligrams of C per liter, and the ratio of DOC-loss to DOCp averaged 0.40. The statistical analysis was carried out to identify variables related to DOC-loss. From the 68 dates for which DOCp was computed, 48 included all of the variables to be tested. The regression was then rerun as a simple regression using as many of the dates as contained all of the identified independent variables. AlDc5or F e ~ were c~ not available for a few dates; and because these concentrations are much less than the concentrations in the Snake River, average Deer Creek values were substituted. A few dates were excluded because a solute gain was calculated, indicating a probable error, and a few others were identified as outliers in the statistical calculation. In the regression exercise, there was a noticeable gap in the absolute t-ratio values from the most important to the least important coefficients. The t-ratio is a measure of the degree of difference from 0 in a particular coefficient. The results of the statistical regression of the multiyear data showed the following relationship to explain 80% of the variance: DOCR = 0.1 + 0.21DOC~c5+ 0.49DOCs~z- O.lG(Al-10ss) (4) where DOCRis the DOC concentration below the confluence as calculated by regression. The 61 dates used to obtain eq 4 included dates during snowmelt and late summer. The relationship among t-ratios for the coefficients was DOCsN2(7.6)> DOC~c5(5.3)>> Al-loss(l.9) >
A.
Discharge -
120
6
4 i
r2
i .E 0 g 6 C 1
u 0,
s 4
0
2 0 2,
1
I
t
I
I
I
0
I
I
I
Yon i
o
0 82
80
Year
84
86
Figure 3. Multiyear monitoring data for 1980-1986. Discharge data are from an established USGS gaged site several kilometers downstream of the confluence. Concentration data are from the sites above the confluence in the Snake River (SN2, 0)and in Deer Creek (DC5, +).
I
0
I
1
/
i
I
I
I
3 2 DOC, rnglL
4
5
I
Figure 4. DOC below the confluence (at site SN3) as predicted for conservative transport by eq 2 versus observed concentration.
constant(l.0). This relationship indicates that DOC below the confluence increases with increasing DOC concentration above the confluence. Increases in DOCsNzhave a 2-fold greater effect in increasing DOCR compared to increases in indicating that DOCsN2is not as readily sorbed, possibly because of sorption reactions in the upper
Snake River. A much weaker relationship holds with increased DOC below the confluence and decreased aluminum precipitation (Al-loss). A second regression identified a potential equation for DOC-loss that was much weaker than the regression equation for DOCR. The regression for DOC-loss showed a positive relationship with only DOCDC5and not DOCSN2 DOCDCb has probably not been effected by upstream sorption reactions as much as DOCSN2,and therefore the potential for removal of reactive fractions from DOCDc5 is greater. Analysis of Diel Data. The data set of the hourly samples collected during September 9-10, 1984, is presented in Figure 5. The SN2 Fe concentration ranged from 0.65 to 1.0 mg/L over the 24-h period. The concentration of dissolved Fe was greatest during midday, a result of photoreductive dissolution of iron oxides. The concentrations of A1 and DOC and the pH at the two upstream sites were stable. Large, erratic variations in dissolved AI and Fe occurred at site SN3 below the confluence. Al-loss and Fe-loss had wide ranges, although decreases in AI-loss were coincident with decreases in Fe-loss. The variation in Al-loss and Fe-loss in the diel data set encompasses much of the variation in the multiyear data set (0.2-1.9 mg/L for Al-loss and
