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Character and Chlorine Reactivity of Dissolved Organic Matter from a Mountain Pine Beetle Impacted Watershed Katherine M.H. Beggs* and R. Scott Summers Department of Civil, Environmental and Architectural Engineering, University of Colorado at Boulder, Boulder, Colorado, United States
bS Supporting Information ABSTRACT: Lodgepole pine needle leachates from trees killed by the mountain pine beetle epidemic in Colorado were evaluated for dissolved organic matter (DOM) character, biodegradation, treatability by coagulation and disinfection byproduct (DBP) formation. An average of 8.0 ((0.62) mgDOC/g-dry weight of litter was leached from three sets of needle samples representing different levels of forest floor degradation. Fluorescence analysis included collection of excitation and emission matrices, examination of peak intensities and development of a 4-component parallel factor (PARAFAC) analysis model. Peak intensity and PARAFAC analyses provided complementary results showing that fresh leachates were initially dominated by polyphenolic/protein-like components (60 70%) and humic-like fluorescence increased (40 70%) after biodegradation. Humic-like components were removed by coagulation (20 64%), while polyphenolic/protein-like components were not, which may create challenges for utilities required to meet OM removal regulations. DBP formation yields after 24 h chlorination were 20.5 26.4 μg/mg-DOC for trihalomethanes and 9.0 14.5 μg/mg-DOC for haloacetic acids for fresh leachates; increased after biodegradation to 19.2 64.2 and 7.1 30.9 μg/mg-DOC, respectively; and decreased after coagulation (fresh: 11.3 17.7;5.7 7.6 μg/mg-DOC, respectively; biodegraded: 12.0 27.3 and 2.9 7.2 μg/mg-DOC, respectively), reflective of changes in concentration of humic material. Humic-like PARAFAC components and peak intensities were positively correlated (R2 g 0.45) to DBP concentrations, while polyphenolic/protein-like components were not (R2 e 0.17).
’ INTRODUCTION Dissolved organic matter (DOM) which is ubiquitous in surface waters, originates from both allochthonous and autochthonous sources. DOM quantity and quality can be impacted by a multitude of environmental factors including erosion, wildfire, temperature, and land use.1 In natural systems, DOM serves as a nutrient source, attenuates light and influences metal speciation and transport.2 DOM in the aquatic column is also subject to transformation processes including humification and photochemical and microbial degradation. In drinking water treatment, DOM is important because it reacts with chlorine to form organic disinfection byproducts (DBPs), some of which are regulated because of associated negative health effects. Spring snowmelt runoff, the dominant hydrological process in mountainous watershed systems, such as those in northern central Colorado, mobilizes terrestrially derived organic matter into surface waters.3 Natural and anthropogenic events, such as removal of forest canopy through wildfire or logging, can increase runoff rates and water yields and may have a negative impact on watershed water quality.4 Widespread tree pest outbreaks may yield high tree mortality, an increase in litter depth in infested stands,5 and can result in increased runoff peak flows and water may yield.4 Increased litter depth can yield increased DOM concentrations in forest floor leachates, which can be mobilized into surface waters during spring runoff. r 2011 American Chemical Society
A range of compounds including polyphenolics, carbohydrates, and humic substances (humic and fulvic acids) have been shown to leach from lodgepole pine forests during spring runoff.6 At the beginning of runoff, soluble materials, such as proteins, nonlignified carbohydrates, and polyphenolics are the dominant species, though these compounds are rapidly lost during decay, transformation and leaching processes.6 Later in runoff, lignin enrichment contributes more aromatic and complex compounds including humic and fulvic acids to the DOM pool.6,7 Yavitt and Fahey6 found that the top layer of the forest floor, the O1 horizon, had lower concentrations of lignin, holocellulose, and protein material compared with the O2 horizon, indicating that the material on the top layer rapidly loses its more complex compounds to leaching and/or forest floor degradation processes. In addition to leaching organic compounds, litter leachates may also be a source of biologically available nitrogen and phosphorus to surface waters which can increase algal activity.8 Humic substances, which account for about half of the DOM in most natural surface water supplies, are thought to be the most common DBP precursors,9,10 though nonhumic fractions of Received: December 17, 2010 Accepted: May 6, 2011 Revised: March 28, 2011 Published: June 01, 2011 5717
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Environmental Science & Technology DOM have also been shown to serve as precursor material.11,12 Studies by Reckhow et al.10 and Chow et al.13 have examined watershed sources of DBP precursors and found that tree species is an important factor in determining the amount of chlorine reactive DOM leached. Reckhow et al.10 found that white pine litter leachates were less chlorine reactive than other leaf litter leachates such as red maple and white oak. Fluorescence spectroscopy, which measures humic-like and protein-like compounds, provides information on DOM character without requiring lengthy fractionation or other separation techniques. Recent studies have shown that tannins and lignin fluoresce in what has previously been defined as the protein region.14,15 Fluorescence data has been used to assess DOM origin, transformations, and redox state.16 20 Drinking water utilities that treat water from pine beetle infested watersheds are concerned about the impact that increased DOM from pine litter may have on their ability to meet DBP regulations: both DBP concentration limits and the total organic carbon (TOC) removal requirement. Most utilities utilize coagulation as the main treatment for removing DOM, prior to disinfection. The majority of published research on mountain pine beetle in lodgepole forests has focused on ecological, hydrological, and economic impacts, while the effect on drinking water quality has yet to be explored. Grand County, home of the Colorado River headwaters and the Colorado-Big Thompson Project, has been undergoing a widespread mountain pine beetle (Dendroctonus ponderosae) epidemic since the late 1990s. The landscape in Grand County is primarily upper montane and subalpine forests, with many pure stands of lodgepole pine trees.21 The Colorado-Big Thompson Project transports water from the west side of the Continental Divide to Colorado communities on the east slope including Fort Collins, Boulder and Greeley. The Colorado River serves consumers in Colorado, Arizona, Utah, Nevada, and California, such that events which alter water quality at the headwaters may have widespread impact. The ongoing mountain pine beetle epidemic in western North America has impacted nearly three million acres of lodgepole pine forest in Colorado.22 Klutsch et al.5 provide an excellent overview of the current mountain pine beetle epidemic. The objectives of this study are to characterize pine litter leachates through (1) spectral measures, (2) treatability, and (3) DBP formation.
’ MATERIALS AND METHODS Leaching, Biodegradation, and Coagulation Methods. Samples were collected in August 2009 from a forest stand near the West Portal of the Adams Tunnel in Grand Lake, CO, that has been impacted by pine beetle. Within the impacted stand, samples were collected from three locations to represent the different stages of forest floor degradation present. Needles were collected from (a) a standing dead tree, which represents needles which have not been degraded on the forest floor, (b) a plot which had been cleared of all debris and contained only recently fallen litter (over a 2 month period), which represents empty bed degradation, and (c) the top layer of undisturbed forest floor, which represents established bed degradation. These samples are termed tree, empty bed, and established bed, respectively. It is important to note that needles on the dead standing tree have experienced some degradation, as trees killed by pine beetle may retain their needles for up to three years.23
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Pine litter was leached using a procedure modified from Reckhow et al.24 Briefly, litter was collected in clean plastic bags, dried for 24 h at 50 C, then leached in 2.5 L or 1 G amber bottles at a concentration in 1.25 g-dry litter/L of granular activated carbon (GAC) treated Grand Lake water (dissolved organic carbon (DOC): 0.67 mg/L-DOC, pH: 7.2 bromide: below detection limits (0.05 μg/L); alkalinity 13 mg/L-CaCO3). Samples were leached in triplicate to determine variability (standard deviations of triplicate DOC and UV254 measurements were less 10% of the mean). A preliminary 8-day study, conducted to determine appropriate leaching time, showed 75 99% of the 8-day DOC total leached occurred within the first 48 h. Therefore, all samples were leached in direct sunlight for 48 h (elev. 5430 ft., August 2009) with daily temperature fluctuations. After leaching, triplicate samples from each litter type were combined, and then split into two aliquots for further analysis (Supporting Information SI-Figure 1). Half of the first aliquot was filtered and half was coagulated and filtered before chlorination. All fresh leachates were diluted to a DOC concentration of ∼5 mg/L with GAC treated Boulder Reservoir water (DOC: 0.5 mg/L-DOC, pH 7.8, alkalinity 65 mg/L-CaCO3) before chlorination. The second aliquot was held for a period of 61 days in shaken reactors sealed with glass wool to allow for aerobic biodegradation. Samples were collected after 12, 44, 55, and 61 days of biodegradation for DOM characterization. After 61 days, half of the first aliquot was filtered and half was coagulated and filtered before chlorination. All biodegraded leachates were undiluted and the pH average 7.25. An aluminum sulfate (Fischer, ACS) dose of 20 mg/L, similar to the average dose used by the City of Fort Collins (CO) treatment facility, was used for coagulation. After coagulation, the pH of the water decreased by 0.5 to 1.0 pH units. A 0.45 μm cartridge filter (Memtrex, prerinsed with DI) was used for filtration. Chlorination was run under uniform formation conditions (UFC) and held for 24 h.25 Analytical Methods. DOC measurements were run following Standard Method 5310 (APHA, 2005) utilizing a TOC Analyzer (Sievers 800) and Shimadzu TOC-Vcsh analyzer both with autosamplers. DOC results between instruments were shown to be reproducible within (0.2 mg/L. After 24 h of reaction, chlorine residual was measured (SM 4500-Cl F) before samples were quenched with ammonium chloride. Chlorine demand, defined as the difference between the applied chlorine dose and the measured chlorine residual, increases over time as chlorine reacts with oxidizable constituents in the water. For most natural waters the chlorine demand is attributed to the reaction with DOM, though high levels of reduced inorganics can also contribute. Chlorine reactivity is defined as chlorine demand divided by DOC. After quenching, samples were analyzed for (a) chloroform, bromodichloromethane, dibromochloromethane, and bromoform and their sum reported as total trihalomethanes (TTHM), (b) chloropicrin, 1,1-dichloro-2-propanone, 1,1,1-trichloro-2propanone, (c) trichloroacetonitrile, dichloroacetonitrile, bromochloroacetonitrile, and dibromoacetonitrile by U.S. Environmental Protection Agency (USEPA) Method 551.1, (1995) and (d) monochloroacetic acid, dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), monobromoacetic acid, and dibromoacetic acid and their sum reported as five haloacetic acids (HAA5) using USEPA Method 552.2 (2005). Both methods used gas chromatography (Agilent 6890), with an electron capture detector and a high resolution gas chromatography column (Restek Rtx-1701). Only TTHM and HAA5 data is 5718
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Environmental Science & Technology presented in this paper as all other DBPs were below detection limits. The average relative percent difference for duplicate samples was 2.2% for TTHM and 1.3% for HAA5. TTHM and HAA5 yields were defined as DBP concentration divided by DOC. Spectral Characterization. Filtered samples (0.45-μm cartridge, prerinsed with DI) were analyzed for ultraviolet (UV) absorbance and fluorescence. UV absorbance spectra from 200 to 600 nm were measured with a UV vis spectrophotometer (Agilent 8453) using a 1 cm path length cell. Fluorescence measurements were taken using a scanning fluorometer (Fluoromax-2, Jobin Yvon Horiba) in conjunction with data acquisition software (DataMax). Daily instrument checks including a lamp scan, Raman scan and cuvette contamination scan were run to ensure the instrument was properly calibrated. Excitation emission matrices (EEMs) were collected in S/R mode for excitation wavelengths from 240 to 450 nm in 10 nm steps and for emission wavelengths from 300 to 600 nm in 2 nm steps, with an integration time of 0.25s. DI water blanks were subtracted in order to remove Raman scattering, and all samples were corrected to account for instrument biases, the inner-filter effect,26 and normalized to the Raman area in order to account for lamp decay. Fluorescence intensities are reported in Raman units (R.U.). Duplicate fluorescence analysis by Coble16 showed peak positions did not vary substantially and maximum intensity showed less than 5% difference. Optical proxies from absorbance and fluorescence data were calculated to provide additional information on DOM character. The ratio of UV absorbance at 254 nm to DOC, termed specific ultraviolet absorbance (SUVA), has previously been correlated with aromaticity.27 The fluorescence index (FI), calculated as the ratio of emission intensities at 470 and 520 nm at an excitation wavelength of 370 nm, also has been related to DOM aromaticity and lignin carbon-normalized yields.17,20 The humification index (HIX), a ratio of fluorescence intensities, determines the extent of organic matter humic content by quantifying the red-shifting of fluorescence emission which occurs with increasing humification.26 The HIX for our study varied slightly from Ohno26 and was calculated by dividing the sum of fluorescence intensities of longer wavelengths (436 480 nm) by the sum of intensities at shorter wavelengths (300 346 nm) and longer wavelengths (436 480 nm) all at excitation 250 nm. Peak A and C intensities were extracted from the EEMs to examine humic-like fluorescence.16 In this study Peak A was defined as the maximum intensity between 240 270 nm excitation and 380 470 nm emission and Peak C was defined as the maximum intensity value between 300 340 nm excitation and 400 450 nm emission. Because of instrument limitations, Peak A is difficult to properly quantify, so most attention has been on Peak C. Peak C intensity has been associated with aromaticity and hydrophobicity28 and THM formation potential.29 Peak C intensity normalized to DOC has been used to differentiate sources, with fresher DOM having higher fluorescence intensity per gram carbon.30 Protein-like Peak B (tyrosine-like) and Peak T (tryptophanlike) intensities were also extracted from the EEMs.16 Peak B was defined as the maximum intensity between 260 290 nm excitation and 300 320 nm emission and Peak T as the maximum intensity between 260 290 nm excitation and 326 350 nm emission. Recent work has shown that some polyphenolic compounds, such as tannin and lignin also fluoresce in the Peak B and T regions.14,15 Therefore, these peaks will be hereby
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referred to as polyphenolic/protein-like. Specific peak intensities, normalized to DOC, were calculated to allow for a comparison among samples and provide information on the relative concentration of different types of fluorophores. PARAFAC Analysis. Parallel factor (PARAFAC) analysis was utilized to examine the leachate EEMs following the tutorial provided by Stedmon and Bro.31 PARAFAC analysis decomposes the overall fluorescence signal into individual or groups of fluorescent components which may provide more information about the character of DOM.32 The model was developed from the EEMs of 71 samples at all treatment levels: fresh, biodegraded, coagulated, and chlorinated. A four-component model explained 99.2% of the variability and was validated using splithalf analysis. The low signal of the residuals suggests the data was well modeled by the four components.
’ RESULTS Dissolved Organic Carbon. After 48 h of contact the leached litter yielded an average DOC concentration of 10.06 ((0.77) mg/L (Table 1), independent of litter source, which resulted in DOC yields of 8.0 ((0.66) mg-DOC/g-dry weight of litter. After the 2 month biodegradation period DOC concentrations decreased by 38 80%. Kinetics were fastest for the tree, followed by the empty bed, and slowest for the established bed (average: 0.13, 0.10, and 0.06 mg-DOC/day, respectively). In general, kinetics were faster at the beginning of the biodegradation period and slowed throughout the degradation process, with the exception of the established bed litter which did not degrade during the first 12 days. DOC removal by coagulation ranged from 0 23% for fresh leachates and increased to 20 64% for biodegraded leachates. Chromophoric Dissolved Organic Matter. To facilitate comparisons, chromophoric DOM measures are presented normalized to DOC concentrations. SUVA values for all fresh leachates, which ranged from 0.80 1.20 L/mg-m (Table 1), were low relative to SUVA values from local watershed waters, 1.8 2.9 L/mg-m33 and to surface waters in general, 1.2 3.2 L/mg-m.34 After biodegradation, SUVA values increased to 0.79 2.22 L/mg-m, similar to findings by Reckhow et al.10 The tree litter had the highest SUVA before and after biodegradation, while the established bed litter had the lowest SUVA. Coagulation decreased SUVA values for all samples, as previously reported by Owen et al.11 Chlorination increased SUVA values, with a few exceptions in which little change was observed (Supporting Information SI-Table 1). EEMs from fresh and biodegraded litter leachates clearly show differences between the sources of litter and the impact of biodegradation. An example of this is shown in Figure 1, for the empty bed litter leachate. Humic-like specific peak intensity values (Table 1) for fresh leachates ranged from 4.01 4.53 R. U.-L/g and 6.23 7.79 R.U.-L/g, for Peaks C and A, respectively. Intensities for Peak C were low compared to watershed values.33 Polyphenolic/protein-like specific peak intensity values ranged from 8.56 14.3 R.U.-L/g for Peak T and from 9.13 20.8 R. U.-L/g for Peak B (Table 1). Biodegradation increased the humic specific peak intensities and decreased the polyphenolic/proteinlike specific intensities, whereas coagulation had the opposite impact, as seen in Figure 1 and Table 1. Chlorination decreased all specific peak intensities, though polyphenolic/protein-like peaks were more impacted than humic-like peaks (average 5719
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fresh fresh fresh biodegraded biodegraded biodegraded
7.42 9.83 9.91 0.68 2.78 4.78
0.85 0.76 0.68 0.8 0.73 0.62
1.74 1.98 2.07 1.73 1.67 1.7
0.29 4.89 0.39 5.30 0.37 5.50 0.68 17.7 0.54 7.22 0.44 5.56
2.80 3.06 3.37 7.50 3.53 3.63
15.6 16.2 19.9 10.10 9.22 12.80
15.60 11.80 13.60 10.60 8.97 10.60
7 9 7 25 17 10
9 14 13 29 20 17
48 61 67 3 29 47
36 17 13 43 34 25
0.30 0.34 0.36 1.33 0.31 0.25
11.3 17.0 17.7 27.3 18.4 12.0
5.7 7.2 7.6 7.2 4.5 2.9
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coagulated tree empty bed established bed tree empty bed established Bed
14.5 9.0 9.0 30.9 12.4 7.1 26.4 20.5 22.0 64.2 30.1 19.2 0.53 0.38 1.07 1.01 0.39 0.37 32 18 14 29 30 21 27 53 63 0 20 38 21 16 14 25 24 26 20 13 8 45 26 15 8.56 11.50 14.30 7.04 8.72 8.78 9.13 14.70 20.8 5.75 8.53 10.60 4.53 4.12 4.01 9.15 5.93 5.58 0.58 7.79 0.49 6.70 0.40 6.23 0.81 19.60 0.65 10.60 0.56 8.09 raw
1.20 0.81 0.80 2.22 1.05 0.79 fresh 9.60 fresh 10.95 fresh 9.63 biodegraded 1.89 biodegraded 4.84 biodegraded 5.98 tree empty bed established bed tree empty bed established bed
1.64 1.81 1.85 1.46 1.52 1.71
HAA5 μg/mg-DOC TTHM μg/mg-DOC Com 4 Com 1 Com 2 Com 3 T B C Fresh or DOC SUVA Biodegraded mg/L L/mg-m Litter Source Treatment
Table 1. Leachate Water Quality Data
FI
HIX
A
polyphenolic/protein-like humic-like polyphenolic/protein-like humic-like
specific peak intensity (R.U.-L/g)
PARAFAC component distribution (%)
chlorine reactivity mg/mg
chlorination
yield
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decrease of 67% and 30%, respectively) (Supporting Information SI-Table 1). FI values for all fresh leachates were relatively high, 1.64 1.85 (Table 1), suggesting lower DOM aromaticity compared to the surface water DOM in this watershed in which the FI ranged from 1.32 to 1.41.33 After biodegradation, the FI values decreased on average by 12% to the range of 1.46 1.71. Coagulation increased the FI of all but one leachate, which showed no change. Chlorination of fresh leachates had little impact on FI values. For biodegraded leachates, chlorination increased FI values (Supporting Information SI-Table 1). The HIX values for the fresh leachates ranged from 0.40 to 0.58, with the established bed litter leachate having the lowest value and the tree litter leachate having the highest (Table 1). The fresh leachate values were similar to that of fresh corn leachates.26 For biodegraded leachates, HIX values increased on average by 37% to the range 0.56 0.81. The tree leachate showed the largest increase in HIX after biodegradation and had a value similar to that reported for soil DOM.26 Coagulation decreased the HIX of all leachates, while chlorination increased the HIX of almost all leachates (Supporting Information SITable 1). PARAFAC Analysis. EEMs of the PARAFAC components and locations of maxima can be found in Supporting Information (SIFigure 2 and SI-Table 2). Component 1 has similar spectra as tyrosine-like (Peak B) fluorescence, though fluorescence in this region may also be attributed to polyphenolic compounds, such as tannin and lignin.14,15 Components 2 and 3 have spectra similar to other humic-like components reported in the literature20,32 and appear to combine fluorescence from humic-like Peaks A and C. Component 4 has spectra similar to tryptophan-like fluorescence (Peak T). PARAFAC components were similar to others reported in the literature, suggesting that the oxidative effects of chlorine do not differ significantly from natural oxidative processes. Disinfection Byproduct Formation. Fresh leachates did not contain high amounts of DBP precursor material, shown by low TTHM and HAA5 yields; 20.5 26.4 μg/mg-DOC and 9.0 14.5 μg/mg-DOC, respectively. Biodegradation increased TTHM and HAA5 yields for tree and empty bed litter and slightly decreased that for the established bed litter (Table 1). Chlorine demand also increased after biodegradation (Table 1). Coagulation effectively removed DBP precursor material, resulting in lower DBP yields for both fresh and biodegraded leachates (Table 1). For the fresh and biodegraded leachates, chloroform was the primary TTHM species formed (average of 93 ( 1% and 96 ( 1%, respectively), as expected due to the low bromide concentration in Grand Lake water. For fresh leachates, the two main HAA5 species were DCAA, 50 ( 1%, and TCAA, 41 ( 2%. For biodegraded leachates, DCAA contribution increased to 59 ( 1%, while TCAA contribution did not change (39 ( 1%). Coagulation did not impact TTHM speciation, which remained dominated by chloroform. Coagulation had little impact on HAA5 speciation for fresh leachates with DCAA contributing 53 ( 3% and TCAA 35 ( 2%. However, for the biodegraded leachates, coagulation did impact HAA5 speciation, which was strongly dominated by DCAA (78 ( 6%), with low contribution from TCAA (22 ( 6%).
’ DISCUSSION Fresh Leachate DOM. Fresh leachate DOC yields fell within ranges reported by others for coniferous litters,7,13,24 though 5720
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Figure 1. Excitation and emission matrices for empty bed litter leachates. Note: intensity scale differs between fresh and biodegraded leachate samples for better resolution.
some studies have shown that newer litter leaches higher levels.24 Litter for this study was collected from the same stand, thus it is likely of similar age, as trees killed by pine beetle can retain their needles for up to three years.23 Biodegradation of the fresh leachate resulted in a substantial loss of DOC mass for all litter sources. The laboratory degradation process, which served as a basic proxy for watershed microbial processes, was likely accelerated by indigenous benthic organisms in the leachate solution and pine litter.24 Spectral measures, SUVA, FI, HIX, specific peak intensities, and PARAFAC component distribution show that all three fresh leachates had low contributions of aromatic humic DOM, relative to the polyphenolic/protein-like contribution (Table 1), and compared to watershed values,33 which is expected as plant residue material is typically considered the nonhumic precursor material for humification.26 This finding is in agreement with work by Reckhow et al.10 which found that pine needles released primarily nonhumic materials. The fresh tree litter leachate DOM had a more aromatic character than the ground fall (empty bed and established bed) litter leachates, likely because needles on the tree have not experienced the level of leaching and degradation as litter on the forest floor. The fresh ground fall litter leachates had higher specific polyphenolic/protein-like peak and component intensities than the fresh tree litter leachate. The ratio of contribution of protein-like and polyphenolic material to the polyphenolic/protein peaks likely varies with litter decomposition stage. For example, tree litter may contain a greater concentration of labile amino-acids and proteins, whereas ground fall litter, which has been weathered and lost much of its labile N, may contain a larger amount of recalcitrant polyphenolic and protein-like material, which have been shown to be less reactive in the environment.6 Biodegradation. Two months of biodegradation yielded a 80% loss in DOC and a 85% increase in SUVA for the tree leachate. The established bed sample was least impacted by biodegradation with a 38% DOC removal and no change in SUVA. The impact of biodegradation on the empty bed sample yielded results that were intermediate between those of the other samples. For all leachates, the specific humic peak intensities (Peaks A and C) and the HIX increased after biodegradation,
while specific polyphenolic and protein-like intensities (Peaks B and T) and FI decreased. After biodegradation, the intensities of all PARAFAC components decreased, though at different levels, resulting in compositional changes in the remaining DOM (Table 1). The composition of the biodegraded tree litter was dominated by humic-like components 2 and 3 (45% and 25%, respectively), with a contribution from tryptophan-like component 4 (25%) and no contribution from polyphenolic/protein-like component 1. The biodegraded empty bed litter leachate had a relatively even contribution of components (20 30%), while the established bed litter leachate remained dominated by polyphenolic/ protein-like component 1 (38%), with lesser contributions from the tryptophan-like component 4 and the humic-like components 2 and 3 (21%, 26%, and 15%, respectively). Litter source clearly impacts the degree of biotransformation, with tree litter leachate undergoing the most dramatic change in the humic indicators, and established bed litter leachate showing least change. Biodegradation had the largest impact on the polyphenolic/protein-like content of the established bed litter. These findings indicate that the residual DOM and/or DOM transformed during biodegradation for the tree and empty bed litter leachates was more aromatic and more humic than the fresh leachate. Similar findings were reported by Reckhow et al.,24 with SUVA values increasing 3-fold for pine litter leachates. The increase in aromaticity may be attributed to a combination of the more rapid degradation of biologically labile compounds such as carbohydrates, which do not absorb and fluoresce, and humification of plant residues.7,24 The established bed litter leachate was less impacted, suggesting that the most degraded litter leachate DOM is not composed primarily of humic precursors, but instead nonreactive recalcitrant material. This data supports the likelihood that the established bed litter had experienced higher levels of prior leaching and degradation relative to the litter from the other two sources, thus had already leached much of its reactive organic matter. Coagulation. Coagulation of the fresh leachates resulted in relatively low DOC removal (0 23% removal), compared with coagulation DOC removal for surface waters in this watershed, which ranged from 14 72% and averaged 37%,35 and other 5721
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Environmental Science & Technology surface waters which averaged about 30% removal.12,36 Coagulation DOC removal efficiency increased for the more humic-like biodegraded leachates and ranged from 20 64%. The tree litter leachates consistently showed the highest removal, while the established bed litter leachates had the lowest. Coagulation preferentially removed aromatic humic-like compounds, resulting in DOM with less aromaticity, as indicated by the decrease in SUVA and HIX values, which is in agreement with previous studies.11,29 The preferential coagulation of humic material for both fresh and biodegraded leachates is also evident in the decrease in specific Peak A and C intensities and the shift in the PARAFAC component distribution to the polyphenolic/ protein-like PARFAC components 1 and 4. If the material represented by components 1 and 4 was primarily protein-like material, coagulation would have been more effective, suggesting that these fluorophores may be derived from organic material which is difficult to remove via coagulation.36 Chlorination. Fresh leachates had chlorine reactivities in the range of 0.38 1.07 mg-Cl/mg-C (Table 1), which are well below reported values for surface waters; 1.6 2.5 mg-Cl/mg-C.37 Fresh leachates also did not contain substantial DBP precursors, reflected by low TTHM and HAA5 yields. These results are consistent with the low humic content of the fresh leachates and TTHM yields were similar to those reported by Chow et al.13 for foothill pines. Detrital pine material has been shown to have higher TTHM yields than pine litter.13 As this study focused only on needles and not detritus, the results may represent a conservative estimate of the potential DBP yields. Biodegradation of the tree and empty bed litter leachates increased chlorine reactivities and DBP yields, reflective of the increase in humic substance fraction. Biodegradation of the established litter leachate slightly decreased the DBP yields, but preferentially decreased the nonsubstitution, chlorine oxidizable fraction, indicating that non-DBP precursor compounds also exert a chlorine demand within the leachates. Coagulation of fresh leachates resulted in DOM with lower DBP yields (17 57% for TTHM, 16 61% for HAA5) which can be attributed to the removal of the more reactive aromatic fraction and is expected from previous studies in the literature.11 Coagulation of the more aromatic biodegraded leachates was found to be very effective for reducing the DBP yields (38 57% for TTHM, 59 77% for HAA5), and removal was especially improved for the ground fall litter leachates. These levels of DBP precursor removal are similar to that found with local surface waters33 and for surface waters in general.37 HAA5 precursors were more effectively removed relative to TTHM precursors for all fresh and biodegraded leachates. Liang and Singer35 found that the aromatic fraction of DOM, which is more amenable to removal by alum coagulation, contains more HAA5 precursors than TTHM precursors. For biodegraded leachates, TCAA precursors were more effectively removed than DCAA precursors, resulting in the increased contribution of DCAA to HAA5 concentrations. Certain aliphatic compounds, which tend to be more difficult to remove by alum coagulation compared with aromatic compounds, have been shown to preferentially form DCAA over TCAA.12,38 The DBP yields from all of the coagulated leachates were low relative to average yields reported by Summers et al.,25 under UFC for a range of 10 coagulated surface waters (average TTHM: 29 μg/mg-DOC; average HAA5 19 μg/mg). For all leachates, chlorination resulted in a greater decrease of the polyphenolic/protein-like peak intensity compared with
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the humic peaks (Figure 1). On average, specific intensities for humic-like Peaks A and C decreased by 30%, compared with a 67% decrease for specific Peak B and T intensities. PARAFAC component distributions reflected a similar change, with polyphenolic/protein-like component 1 showing the most dramatic decrease in contribution (Supporting Information SI-Table 1). This dramatic decrease suggests that component 1 and Peaks B and T represent nonhumic material which is more easily oxidized than the other components. This finding is also reflected in the increase in SUVA and HIX values and the decrease in the FI for nearly all samples (Supporting Information SI-Table 1). While the literature shows that chlorination tends to result in a decrease in SUVA,12 the increase in SUVA seen in the leachates may be caused by the oxidation of the nonhumic material present in the leachate. No correlation between DOC and DBP formation was found. DOM character measures indicative of aromaticity, including SUVA, specific humic-like peak intensities (Peaks A and C), and humic-like component 2 were positively correlated with TTHM and HAA5 yields (R2 g 0.70 and R2 g 0.50, respectively), while measures of polyphenolic/protein-like material, such as specific Peak B and T intensities and components 1 and 4, showed little correlation (R2 e 0.40). Component 2, representing the most aromatic fluorophores, had the strongest correlations with both TTHM and HAA5 yields (R2 = 0.89 and 0.69, respectively). For pine leachates, aromaticity was found to be a strong indicator of DBP formation (Supporting Information SI-Table 3). Implications. The results from this study show that the current pine beetle epidemic in the Rocky Mountain region may have a long lasting impact on water quality in the affected watersheds. For mountain utilities which treat direct surface runoff, the results of this study suggest that while fresh leached DOM has low DBP yields, there is potential for difficulties with the removal of the DOC by coagulation. For utilities further downstream from beetle impacted areas, influent waters may have more aromatic DOM and contain higher amounts of DBP precursors, though coagulation was shown to be effective in removing this material. Older ground fallen litter remains a potential source of DOM that is difficult to coagulate for both mountain and downstream utilities. The results of this study, show that polyphenolic/protein material, which was difficult to remove by coagulation, was greatly reduced by biodegradation and chlorination, such that biofiltration or preoxidation may help utilities remove some of this refractory fraction and meet TOC removal regulations.
’ ASSOCIATED CONTENT
bS
Supporting Information. Leaching and biodegradation schematic (Figure SI-1). Water quality data for chlorinated leachates (Table SI-1). Four component PARAFAC model (Figure SI-2). Location of maxima for PARAFAC components (Table SI-2). R2 values for DOM measures and DBP concentrations (Table SI-3). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 5722
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