THE GEOCHEMICAL IMPACT OF WILDFIRE AND MINING ON THE ...

KECK GEOLOGY CONSORTIUM

PROCEEDINGS OF THE TWENTY-FIFTH ANNUAL KECK RESEARCH SYMPOSIUM IN

GEOLOGY

April 2012 Amherst College, Amherst, MA

Dr. Robert J. Varga, Editor Director, Keck Geology Consortium

Pomona College

Dr. Tekla Harms

Symposium Convenor Amherst College

Carol Morgan Keck Geology Consortium Administrative Assistant

Diane Kadyk Symposium Proceedings Layout & Design

Department of Earth & Environment Franklin & Marshall College

Keck Geology Consortium Geology Department, Pomona College

185 E. 6th St., Claremont, CA 91711 (909) 607-0651, [email protected], keckgeology.org

ISSN# 1528-7491

The Consortium Colleges The National Science Foundation ExxonMobil Corporation

KECK GEOLOGY CONSORTIUM PROCEEDINGS OF THE TWENTY-FIFTH ANNUAL KECK RESEARCH

SYMPOSIUM IN GEOLOGY ISSN# 1528-7491

April 2012

Robert J. Varga

Editor and Keck Director Pomona College

Keck Geology Consortium Pomona College

185 E 6th St., Claremont, CA 91711

Diane Kadyk Proceedings Layout & Design Franklin & Marshall College

Keck Geology Consortium Member Institutions:

Amherst College, Beloit College, Carleton College, Colgate University, The College of Wooster, The Colorado College, Franklin & Marshall College, Macalester College, Mt Holyoke College,

Oberlin College, Pomona College, Smith College, Trinity University, Union College, Washington & Lee University, Wesleyan University, Whitman College, Williams College

2011-2012 PROJECTS

TECTONIC EVOLUTION OF THE CHUGACH-PRINCE WILLIAM TERRANE, SOUTH-CENTRAL ALASKA Faculty: JOHN GARVER, Union College, Cameron Davidson, Carleton College Students: EMILY JOHNSON, Whitman College, BENJAMIN CARLSON, Union College, LUCY MINER, Macalester College, STEVEN ESPINOSA, University of Texas-El Paso, HANNAH HILBERT-WOLF, Carleton College, SARAH OLIVAS, University of Texas-El Paso. ORIGINS OF SINUOUS AND BRAIDED CHANNELS ON ASCRAEUS MONS, MARS Faculty: ANDREW DE WET, Franklin & Marshall College, JAKE BLEACHER, NASA-GSFC, BRENT GARRY, Smithsonian Students: JULIA SIGNORELLA, Franklin & Marshall College, ANDREW COLLINS, The College of Wooster, ZACHARY SCHIERL, Whitman College. TROPICAL HOLOCENE CLIMATIC INSIGHTS FROM RECORDS OF VARIABILITY IN ANDEAN PALEOGLACIERS Faculty: DONALD RODBELL, Union College, NATHAN STANSELL, Byrd Polar Research Center Students: CHRISTOPHER SEDLAK, Ohio State University, SASHA ROTHENBERG, Union College, EMMA CORONADO, St. Lawrence University, JESSICA TREANTON, Colorado College. EOCENE TECTONIC EVOLUTION OF THE TETON-ABSAROKA RANGES, WYOMING Faculty: JOHN CRADDOCK. Macalester College, DAVE MALONE. Illinois State University Students: ANDREW KELLY, Amherst College, KATHRYN SCHROEDER, Illinois State University, MAREN MATHISEN, Augustana College, ALISON MACNAMEE, Colgate University, STUART KENDERES, Western Kentucky University, BEN KRASUSHAAR INTERDISCIPLINARY STUDIES IN THE CRITICAL ZONE, BOULDER CREEK CATCHMENT, FRONT RANGE, COLORADO Faculty: DAVID DETHIER, Williams College Students: JAMES WINKLER, University of Connecticut, SARAH BEGANSKAS, Amherst College, ALEXANDRA HORNE, Mt. Holyoke College

DEPTH-RELATED PATTERNS OF BIOEROSION: ST. JOHN, U.S. VIRGIN ISLANDS Faculty: DENNY HUBBARD and KARLA PARSONS-HUBBARD, Oberlin College Students: ELIZABETH WHITCHER, Oberlin College, JOHNATHAN ROGERS, University of Wisconsin-Oshkosh, WILLIAM BENSON, Washington & Lee University, CONOR NEAL, Franklin & Marshall College, CORNELIA CLARK, Pomona College, CLAIRE McELROY, Otterbein College. THE HRAFNFJORDUR CENTRAL VOLCANO, NORTHWESTERN ICELAND Faculty: BRENNAN JORDAN, University of South Dakota, MEAGEN POLLOCK, The College of Wooster Students: KATHRYN KUMAMOTO, Williams College, EMILY CARBONE, Smith College, ERICA WINELAND-THOMSON, Colorado College, THAD STODDARD, University of South Dakota, NINA WHITNEY, Carleton College, KATHARINE, SCHLEICH, The College of Wooster. SEDIMENT DYNAMICS OF THE LOWER CONNECTICUT RIVER Faculty: SUZANNE O’CONNELL and PETER PATTON, Wesleyan University Students: MICHAEL CUTTLER, Boston College, ELIZABETH GEORGE, Washington & Lee University, JONATHON SCHNEYER, University of Massaschusetts-Amherst, TIRZAH ABBOTT, Beloit College, DANIELLE MARTIN, Wesleyan University, HANNAH BLATCHFORD, Beloit College. ANATOMY OF A MID-CRUSTAL SUTURE: PETROLOGY OF THE CENTRAL METASEDIMENTARY BELT BOUNDARY THRUST ZONE, GRENVILLE PROVINCE, ONTARIO Faculty: WILLIAM PECK, Colgate University, STEVE DUNN, Mount Holyoke College, MICHELLE MARKLEY, Mount Holyoke College Students: KENJO AGUSTSSON, California Polytechnic State University, BO MONTANYE, Colgate University, NAOMI BARSHI, Smith College, CALLIE SENDEK, Pomona College, CALVIN MAKO, University of Maine, Orono, ABIGAIL MONREAL, University of Texas-El Paso, EDWARD MARSHALL, Earlham College, NEVA FOWLER-GERACE, Oberlin College, JACQUELYNE NESBIT, Princeton University.

Funding Provided by: Keck Geology Consortium Member Institutions

The National Science Foundation Grant NSF-REU 1005122 ExxonMobil Corporation

Keck Geology Consortium: Projects 2011-2012 Short Contributions— Front Range, CO Project

KECK COLORADO PROJECT: INTERDISCIPLINARY STUDIES IN THE CRITICAL ZONE, BOULDER CREEK CATCHMENT, FRONT RANGE, COLORADO Project Faculty: DAVID P. DETHIER, Williams College & WILL OUIMET, University of Connecticut THE GEOCHEMICAL IMPACT OF WILDFIRE AND MINING ON THE FOURMILE CREEK WATERSHED SARAH BEGANSKAS, Amherst College Research Advisor: Anna Martini QUANTIFYING PHYSICAL CHARACTERISTICS AND WEATHERING OF BEDROCK IN RELATION TO LANDSCAPE DEVELOPMENT IN THE COLORADO FRONT RANGE ALEXANDRA HORNE, Mt. Holyoke College Research Advisor: David Dethier THE HYDROLOGY AND GEOCHEMISTRY OF TWO SNOWMELT-DOMINATED, ALPINE STREAMS IN THE BOULDER CREEK CRITICAL ZONE OBSERVATORY, FRONT RANGE, COLORADO JAMES N. WINKLER, University of Connecticut Research Advisor: Will Ouimet

Keck Geology Consortium Pomona College

185 E. 6th St., Claremont, CA 91711 Keckgeology.org

INTRODUCTION

Both wildfire and mining profoundly impact stream catchments. Wildfires change short-term geomorphic, hydrologic, and chemical processes in a watershed. Mine drainage affects long-term stream chemistry by contributing acidity, heavy metals, precipitates, and, when erosion occurs, sediment and attached metals. In post-fire landscapes, small precipitation events frequently generate abnormally large volumes of run-off as a result of reduced soil infiltration and burned vegetation (DeBano 2000; Ice et al. 2004). Reduced vegetative cover increases the availability of erodible sediment (Swanson 1981), and sediment delivery to streams can increase by several orders of magnitude (Moody and Martin 2001). The magnitude and dura-tion of change depends on the timing and intensity of post-fire precipitation (Tompkins et al. 2007; Moody and Martin 2008). Increased streamwater alkalin-ity and concentrations of major cations and anions, nutrients, and trace metals have been observed after wildfire (Bayley and Schindler 1991; Rhoades et al. 2011). In particular, large post-fire increases in Ca2+, K+, Mg2+, and SO4

2–, and to a lesser extent Na+ and Cl–, are commonly reported (Chorover et al. 1994; Smith et al. 2011). Reduced plant nutrient uptake, in-creased nutrient mineralization, and a large supply of nutrient-enriched ash cause an increase in stream lev-els of some nutrients, particularly N and P (Minshall et al. 1989; Ice et al. 2004). Elevated streamwater concentrations of cations, anions, and nutrients have been reported to persist for several years after a fire (Bayley and Schindler 1991; Chorover et al. 1994). Abandoned mine sites impact watershed chemistry for decades after mining has ceased (Sullivan and Drever 2001). Tailings expose sulfide minerals com-mon in ore deposits to oxygen and water, driving oxidation reactions (Tripole et al. 2006). As a result, H+, SO4

2–, Fe2+, and other metals are released into

THE GEOCHEMICAL IMPACT OF WILDFIRE AND MINING ON THE FOURMILE CREEK WATERSHED

saRah begaNskas, Amherst CollegeResearch Advisor: Anna Martini

25th Annual Keck Symposium: 2012 Amherst College, Amherst, MA

155

water flowing through the waste (Blowes et al. 2005). The production of sulfuric acid causes increased streamwater acidity, which increases the solubility of metals (Bradley 2008). The susceptibility of water to acidification depends on the system buffering capacity (Tripole et al. 2006). Fourmile Creek is a major (62.73 km2) tributary of Boulder Creek (Fig. 1a). The catchment is character-ized by sparse human population, forested land cover, steep slopes, and Precambrian metamorphic and granitic bedrock (Murphy et al. 2000). The Colorado Mineral Belt, a series of late Cretaceous and Tertiary intrusions associated with heavily mined vein de-posits, runs through the watershed along the western border of the Boulder Creek Granodiorite (Lovering and Goddard 1950; Kellogg et al. 2004). The Gold Hill mining district, which is drained by Fourmile Creek and several of its tributaries, began producing gold, silver, lead, copper, and zinc in 1859 (Murphy et al. 2000). Wildfires are common in the Colorado Front Range, particularly in the summer and fall, due to high-speed winds and low humidity (Graham et al. 2011). Start-ing on September 6, 2010 and burning for 10 days, the Fourmile Fire burned approximately 26 km2 (Murphy and Writer 2011). The area burned encom-passes large areas of the Gold Hill mining district, and the fire exposed many mines and tailings piles. The first major post-fire precipitation event occurred on July 13, 2011, when a severe convective storm with a recurrence interval of 2–5 years produced a maximum rainfall intensity of 40 mm/hr over the burned area (S. Murphy, personal communication), causing local overland flow on slopes, debris flows on tributaries, and flooding along Fourmile Creek. These conditions provide a unique opportunity to study the geochemi-cal response to wildfire of a catchment affected by mining.


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ArcGIS analysis was used to delineate the watersheds of sampled tributaries. A differenced Normalized Burn Ratio (dNBR; U.S. Geological Survey 2011) dataset divides the burned area into four categories of intensity: unburned, low, moderate, and high (Fig. 1c). The total percentage area burned for each water-shed was used to represent fire intensity. To quantify the degree of mining disturbance in each watershed, a map of mines and tailings was produced using field observations, satellite imagery, and historic mining data (Fig. 1b). Mines were represented as polygons, and the percentage area covered by mines and tailings was determined for each watershed. From a bedrock geology dataset (Stoeser et al. 2005), the percentage of bedrock types was calculated for each watershed (Fig. 1d). Water samples were analyzed for major solutes us-

MeThODs

Samples were taken from twenty-two tributaries of and ten sites along Fourmile Creek (Fig. 1a). Sedi-ment was taken from all tributaries except Emerson West Gulch and Ingram Gulch, which ran alongside roads that had been heavily reworked in response to debris flows and flooding. Streamwater was sampled at all locations except Dry Gulch, Arkansas Gulch, and Potato Gulch, which were dry. Each location was sampled two times, nine days apart. I measured dis-charge for all tributaries; discharge measurements for Fourmile Creek are from USGS stream-gaging sta-tions. At all streamwater sampling sites, I measured conductivity. I also measured conductivity synopti-cally on three additional days at ~20 other points along Fourmile Creek (Fig. 2). Basin analysis and streamwater chemistry will be discussed here; sedi-ment chemical analysis is ongoing.

Figure 1. Maps showing attributes of the study area. a) Tributaries to Fourmile Creek (Fourmile watershed shown in green and sampling sites in white) and their watersheds. b) Mines and tailings represented as points in ArcGIS. c) Fire intensity, classified according to dNBR (U.S. Geological Survey 2011). d) Simplified bedrock map.


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ing alkalinity titration (HCO3–), IC (SO4

2–, NO3–, Cl–,

and F–) and ICP-OES (Ca2+, Mg2+, Na+, K+, Sr2+, and SiO2). Samples from seven tributaries and all Four-mile Creek samples were analyzed at USGS Boulder; the rest were analyzed at Amherst. Conductivity measurements were taken in the field using an Amber Science digital conductivity meter. Tributary streamwater concentrations, relative abun-dances, mass fluxes, and conductivity were statisti-cally analyzed according to the watershed’s calculated burn intensity and degree of mining disturbance. Mass flux was calculated by multiplying concentra-tion by discharge. Using 17 degrees of freedom and a significance level of 0.05, Pearson’s critical correla-tion coefficient is 0.456; this was the threshold used to distinguish significant correlations. A Welch two-sample t-test was used to group tributaries based on underlying bedrock with a confidence level of 0.99.

ResULTs

Table 1 summarizes results of ArcGIS watershed analysis of fire intensity, mining disturbance, and bedrock distribution. It also indicates the discharge of tributaries on each sampling date. Fourmile Creek water samples upstream of Gold Run and undisturbed tributary samples contained primar-ily bicarbonate and calcium. In burned and mined tributaries, sulfate was the predominant ion, and

downstream of Gold Run, sulfate and bicarbonate dominate water chemistry. The charge balance for samples analyzed at Amherst ranged from 0.35% to 16.5%, with an average of 4.75%. Samples analyzed at USGS had an average charge balance of 2.3% and ranged from 0% to 9.09%. Tributary conductivity correlated with watershed fire intensity but not with degree of mining disturbance (Fig. 2). One outlier, the tributary with the highest conductivity, was removed from this analysis because the stream runs through a more developed area, and the conductivity was likely heightened disproportion-ately by sewage from nearby houses. The conductiv-ity of Fourmile Creek increased downstream, show-ing a notable increase after input from Gold Run, its largest tributary (Fig. 2). The discharge of Fourmile Creek stayed relatively constant downstream, but dropped significantly between sampling dates.

Tributary samplesTributary concentrations of most solutes - Ca2+, F–, K+, Mg2+, Na+, NO3–, SiO2, and SO4

2- - positively cor-related with fire intensity. When converted to milli-equivalents, all except F– correlated with fire intensity, as did the sum total cations. The degree of mining disturbance correlated only with sulfate concentra-tions; mining did not correlate with an increase in total anions or cations in milliequivalents. Concen-trations of major cations, particularly Ca2+ and Mg2+, correlated with concentrations of SO4

2– but not with

7/25/2011 3/8/2011

Granite and granodiorite,

Early Proterozoic

Biotite gneiss and schist,

Early Proterozoic

Granodiorite, Tertiary /

Cretaceous

Granite and granodiorite,

Middle Proterozoic

1 Dry 1,786,148 0.00 0.00 100.0 0.0 0.0 0.0 0 0 0.00 99.90 0.03 0.07 0.00 0.102 Sand 1,930,101 0.24 0.30 100.0 0.0 0.0 0.0 2 133 0.01 32.27 9.26 53.82 4.66 67.733 Arkansas 814,537 0.00 0.00 100.0 0.0 0.0 0.0 0 0 0.00 100.00 0.00 0.00 0.00 0.004 Sunbeam 1,604,765 0.07 0.00 100.0 0.0 0.0 0.0 10 4523 0.28 86.96 8.53 3.72 0.79 13.045 Sweet Home 660,481 1.08 0.39 100.0 0.0 0.0 0.0 7 2871 0.43 7.15 10.09 64.32 18.44 92.856 Gold Run 7,277,078 26.09 21.47 78.9 21.1 0.0 0.0 82 25375 0.35 29.13 14.11 37.17 19.60 70.877 Ingram 1,187,072 6.83 7.74 100.0 0.0 0.0 0.0 17 2415 0.20 3.72 5.03 54.64 36.61 96.288 Black Hawk 780,076 2.14 1.64 100.0 0.0 0.0 0.0 17 12964 1.66 18.97 17.92 50.21 12.90 81.039 Melvina East 230,450 0.14 0.05 100.0 0.0 0.0 0.0 2 291 0.13 3.28 18.71 71.76 6.25 96.72

10 Melvina 546,078 0.78 0.30 100.0 0.0 0.0 0.0 0 0 0.00 2.44 3.02 62.96 31.58 97.5611 Melvina West 378,903 0.96 0.52 100.0 0.0 0.0 0.0 0 0 0.00 2.89 6.24 83.63 7.23 97.1112 Wall Street 28,176 0.09 0.11 100.0 0.0 0.0 0.0 0 0 0.00 100.00 0.00 0.00 0.00 0.0013 Schoolhouse 547,282 1.47 0.40 100.0 0.0 0.0 0.0 13 3119 0.57 0.61 1.08 84.23 14.07 99.3914 Emerson 1,043,384 1.30 0.24 7.9 92.1 0.0 0.0 19 5498 0.53 2.06 4.90 60.16 32.88 97.9415 Banana 262,793 1.37 0.68 100.0 0.0 0.0 0.0 0 0 0.00 13.66 6.49 23.70 56.16 86.3416 Emerson West 447,121 0.60 0.40 22.8 77.2 0.0 0.0 5 1250 0.28 0.07 5.12 85.14 9.67 99.9317 Sugarloaf 537,420 0.26 0.05 97.8 2.2 0.0 0.0 0 0 0.00 100.00 0.00 0.00 0.00 0.0018 Long 4,075,780 2.73 1.38 0.0 100.0 0.0 0.0 4 1158 0.03 75.16 3.71 18.28 2.85 24.8419 Bald 1,890,617 0.73 0.42 34.0 62.8 3.2 0.0 1 396 0.02 100.00 0.00 0.00 0.00 0.0020 Potato 1,731,138 0.00 0.00 0.0 99.8 0.2 0.0 0 0 0.00 100.00 0.00 0.00 0.00 0.0021 Bear 2,232,343 0.78 0.53 0.0 78.3 21.7 0.0 0 0 0.00 100.00 0.00 0.00 0.00 0.0022 Todd 2,215,583 0.20 0.16 0.0 49.7 48.6 1.7 0 0 0.00 100.00 0.00 0.00 0.00 0.00

#

% Area Burned at

Low Intensity

% Area Burned at Moderate Intensity

% Area Burned at

High Intensity

Total % Area BurnedTributary Name

Watershed Area (sq. m)

% Area by Bedrock

# Mines

Area Disturbed by Mining (sq.

m)

% Area Disturbed by

Mining% Area

Unburned

Discharge (L/sec)

Table 1. Results from ArcGIS tributary watershed analysis for bedrock, fire, and mining, as well as discharge measure-ments for each tributary.


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Figure 2. Top: Map of conductivity measurements taken along Fourmile Creek (blue bullets) and at the mouths of tribu-taries (red bullets). Fourmile Creek conductivity increases downstream, notably rising after input from Gold Run. Bot-tom: Plots of tributary conductivity measurements against percent area burned and percent area disturbed by mining. There is a significant correlation with area burned but not with area disturbed by mining.

Figure 3. Tributary concentrations of major cations, especially divalent cations, positively correlated with SO4

2– concentrations. No significant correla-tion existed between these cations and NO3

– or HCO3–.


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HCO3– or NO3

– (Fig. 3). The relative abundance of HCO3

– negatively corre-lated with both fire and mining intensity and that of sulfate showed positively correlated with both. The relative abundance of all other solutes and the mass flux of all solutes considered did not correlate with fire or mining intensity.

With a confidence level of 0.99, concentrations and relative abundances of Ca2+, Cl–, K+, and Sr2+ were greater in watersheds with predominantly granitic bedrock.

Fourmile Creek samplesFourmile Creek concentrations of all solutes except NO3

– increased downstream, while discharge re-mained constant. Patterns of increase were similar across solutes and sample dates, though concentra-tions were slightly higher on the second day. Alkalin-ity as HCO3

– was always the most abundant solute. Upstream of the burned area, SiO2 was the second most abundant, but within the burned area concentra-tions of SO4

2– and Ca2+ surpassed SiO2. Conductivity and concentrations of all solutes except NO3

– increase

dramatically after input from Gold Run (Fig. 4). Fourmile Creek data were also analyzed using the change in concentration between adjacent sampling sites. Sample sites were chosen to be upstream and downstream of larger tributaries so that tributary input could be analyzed. Changes in concentration between sites were compared to tributary discharge input and changes in Fourmile Creek discharge between the sites. There was a strong positive correlation between incoming tributary discharge and change in concen-trations for all solutes considered except NO3

–. There was no correlation between changes in Fourmile Creek discharge and changes in the solute concentra-tions, except for a negative correlation with SiO2. The difference in concentration of each solute was also individually compared with the mass flux of that solute from tributary input; all showed a strong posi-tive correlation (Fig. 5).

DIsCUssION

Distinguishing between the effects of fire and min-ing is difficult for tributary data, because a small number of tributaries were sampled and the most

Figure 4. Left: Concentrations of major solutes downstream along Fourmile Creek on the first sampling date (July 25, 2011). Red dashed lines indicate the burned area. All concentrations except NO3

– increase downstream, with a dramatic increase at site FCLM, just after input from Gold Run. Patterns were very similar for all solutes on August 3, though all concentrations except NO3

– were slightly higher. Right: Fourmile Creek concentrations of solutes on July 25 plotted as a ratio of sample concentration to FCCR concentration. The FCCR site was farthest upstream and represents an area undisturbed by mining or fire; comparing concentrations using this baseline demonstrates the magnitude of changes for individual solutes. Patterns were similar on the second sampling date, with the exception of NO3

–, which is plotted for both dates.

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intensely burned area corresponds with heavy mining disturbance. Differing bedrock types and waste from nearby houses are further complicating factors. All concentrations except Sr2+ and HCO3

– were higher in mined watersheds than unmined watersheds, though no correlation existed between mining disturbance and the concentration of solutes other than sulfate. Most mined watersheds also burned, and fire inten-sity correlated with increased concentrations of most solutes. This suggests that increased concentrations in mined watersheds were related to fire influence in these watersheds. The average sulfate concentration was 417% higher in mined watersheds than in un-mined watersheds. This value is within the limits of reported sulfate increases in response to acid mine drainage (McConnell 1995; Tripole et al. 2006). Burned watersheds had an aver-age sulfate concentration that was 621% greater than unburned watersheds. Reported post-fire increases in sulfate vary, but most are around 300% (Earl and Blinn 2003; Mast and Clow 2008), with a few in the vicinity of 600% (Smith et al. 2011). The presence of mining likely contributed to the relatively large sul-fate increase observed here. Sulfate concentration and relative abundance in tribu-taries correlated strongly with both fire and mining. Fire increases sulfate supply by oxidizing sulfur in organic matter, necessitating a corresponding increase

in cations (Smith et al. 2011). Burning also likely increased exposure and weathering of sulfide miner-als. When compared relatively, sulfate increases at a greater rate with increasing fire intensity than mining intensity. This may explain why concentrations of all major cations, especially divalent cations, correlated with fire intensity and not with mining disturbance. Bayley and Schindler (1991) also found that in post-fire streams, increased sulfate concentrations drove divalent cation concentrations to increase. Though the concentrations of all major cations increased, their relative abundances did not change. Most reported water chemistry impacts of fire and mining are additive; both tend to increase solute concentrations to varying degrees (McConnell 1995; Rhoades et al. 2011). However, mining consistently increases acidity and wildfire consistently increases alkalinity. Interestingly, neither of these well-docu-mented responses was observed. The only significant change in alkalinity was a decrease in relative abun-dance, but not concentration, of HCO3

– in burned and mined watersheds. This was due to the dramatically increased concentrations of sulfate and major cat-ions in these watersheds. In this area, mine drainage does not contribute much acidity, because carbonate in ore-bearing veins can buffer acid produced from the weathering of pyrite, producing Ca2+ and HCO3

– (Murphy et al. 2000). Thus, decreased alkalinity in mined watersheds would not necessarily be expected. The lack increased alkalinity in burned watersheds may be due to the opposing effect of acidity in mine drainage. It could also be that baseline alkalinity is high and any contribution from the fire could not make a significant difference. Tributary nitrate concentrations correlated with fire intensity but not with mining. This was expected; wildfire has been reported to increase nitrate values to varying degrees, dependent on many other factors, while mining has not been reported to have an impact on nitrate. In Fourmile Creek, nitrate was the only solute whose concentration did not correlate with volume from incoming tributaries; it did correlate when incoming mass flux was individually calculated for each solute. This could be because NO3

– concen-trations were very low, although K+, F–, and Sr2+ had similar concentrations and did correlate with tributary

Figure 5. The mass flux of tributary input between Four-mile Creek sampling sites for each major ion (the sum of the ion’s mass fluxes for all tributaries entering between sample sites) plotted against the change in concentration for each ion between sites. There is a strong positive cor-relation for every ion considered.

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discharge. Nitrate did not show a steady increase downstream, and unlike other solutes, the down-stream curves changed shape dramatically between the two sampling days (Fig. 4). Nitrate concentra-tions, more so than those of any other solute studied, respond to biological processes, which could explain these differences. Additionally, Cl– and NO3

– could be responding to sewage from nearby houses; both increase sharply after input from Gold Run. The patterns of increased solute concentrations down-stream along Fourmile Creek seem to be controlled by input from burned and mined tributaries. Down-stream concentration changes correlated strongly with the tributary discharge entering Fourmile Creek between sampling sites. When the mass flux of each ion was individually calculated, these values corre-lated almost perfectly with concentration changes in Fourmile Creek. Interestingly, there was no correla-tion between the intensity of fire or mining and the calculated mass flux of solutes in tributaries. Four-mile Creek losing or gaining discharge from other sources did not appear to have a significant impact on water chemistry, as there was no correlation between solute concentrations other than SiO2 and changes in discharge of Fourmile Creek. Changes in water chemistry are primarily due to tributary input. The relative abundances of ions in Fourmile Creek were also affected—upstream of the burned area, SiO2 was the most abundant solute after HCO3

–, but SO42– and

Ca2+ from burned and mined tributaries became more concentrated than SiO2 as Fourmile Creek moved through the burned area.

CONCLUsIONs

The 2010 Fourmile Fire and mining affected the water chemistry of Fourmile Creek and its tributar-ies. Streamwater sulfate concentrations dramatically increased in burned tributaries, driving concentrations of major cations to also increase. Changes in solute concentrations downstream along Fourmile Creek correlate strongly with input from burned and mined tributaries. These results represent a work in progress, and further work will include analysis of trace elements and oxygen isotopes in water samples. Additionally, an

analysis of tributary streambed sediment and flood de-posit chemistry will be compared with ArcGIS data.

aCkNOWLeDgMeNTs I sincerely thank David Dethier, Will Ouimet, Anna Martini, and Sheila Murphy for their help and guid-ance.

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