Use of cosmogenic 35S for comparing ages of water from three alpine–subalpine basins in the Colorado Front Range

Geomorphology 27 Ž1999. 61–74

Use of cosmogenic 35S for comparing ages of water from three alpine–subalpine basins in the Colorado Front Range Julie K. Sueker a

a,)

, John T. Turk

b,1

, Rober L. Michel

c,2

Exponent EnÕironmental Group, Boulder, CO, USA b US Geological SurÕey, Lakewood, CO, USA c US Geological SurÕey, Menlo Park, CA, USA

Received 14 April 1997; revised 17 February 1998; accepted 11 May 1998

Abstract High-elevation basins in Colorado are a major source of water for the central and western United States; however, acidic deposition may affect the quality of this water. Water that is retained in a basin for a longer period of time may be less impacted by acidic deposition. Sulfur-35 Ž35S., a short-lived isotope of sulfur Ž t 1r2 s 87 days., is useful for studying short-time scale hydrologic processes in basins where biological influences and waterrrock interactions are minimal. When sulfate response in a basin is conservative, the age of water may be assumed to be that of the dissolved sulfate in it. Three alpine–subalpine basins on granitic terrain in Colorado were investigated to determine the influence of basin morphology on the residence time of water in the basins. Fern and Spruce Creek basins are glaciated and accumulate deep snowpacks during the winter. These basins have hydrologic and chemical characteristics typical of systems with rapid hydrologic response times. The age of sulfate leaving these basins, determined from the activity of 35S, averages around 200 days. In contrast, Boulder Brook basin has broad, gentle slopes and an extensive cover of surficial debris. Its area above treeline, about one-half of the basin, is blown free of snow during the winter. Variations in flow and solute concentrations in Boulder Brook are quite small compared to Fern and Spruce Creeks. After peak snowmelt, sulfate in Boulder Brook is about 200 days older than sulfate in Fern and Spruce Creeks. This indicates a substantial source of older sulfate Žlacking 35S. that is probably provided from water stored in pore spaces of surficial debris in Boulder Brook basin. q 1999 Elsevier Science B.V. All rights reserved. Keywords:

35

S; residence time; hydrology; surficial debris; headwater basin

1. Introduction High-elevation basins in the Rocky Mountain region are a major source of surface water for residen)

Corresponding author Žformerly with US Geological Survey.. Fax: q1-303-444-7528; E-mail: [email protected] 1 Fax: q1-303-236-4912; E-mail: [email protected] 2 Fax: q1-415-329-4538; E-mail: [email protected]

tial, commercial, and agricultural consumption in the central and western United States. In mountainous regions of Colorado, approximately three-fourths of precipitation falls as snow and accumulates in snowpacks through the winter. Concentrations of acid 2y . anions ŽNOy in snowpacks sampled in 3 and SO4 the northern Colorado Rockies were twice the regional background level in 1991 and 1992 ŽTurk et al., 1992; Ingersoll, 1995..

0169-555Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X Ž 9 8 . 0 0 0 9 0 - 7

62

J.K. Sueker et al.r Geomorphology 27 (1999) 61–74

Snowmelt is the dominant hydrological event on an annual basis in high-elevation basins in the Rocky Mountain region. Elution of acid anions from the snowpack, along with flushing of organic acids from soils and dilution of base cations originating in subsurface matrices, causes episodes of decreased pH and lowered acid-neutralizing capacity in surface streams ŽDenning et al., 1991; Campbell et al., 1995; Sueker, 1996.. Residence times of subsurface water may be on the order of hours to days during peak snowmelt, allowing only fast reaction processes to occur, such as cation exchange or dissolution of amorphous alumino-silicates ŽCaine, 1989; Campbell et al., 1995.. Currently, atmospherically derived acidity is neutralized in the basins described in this study and in other basins in north-central Colorado ŽDenning et al., 1991; Sueker, 1996., but ecosystems may respond negatively if future increases in atmospherically derived acidic anions occur. Basins that retain water for longer periods of time may be better able to neutralize acidic atmospheric deposition than basins with shorter residence times. Thus, estimates of residence times may provide an indication of the sensitivity of a basin to increases in acidic atmospheric deposition. The morphology and hydrology of a basin are intimately linked. Morphology affects the flowpaths of water through, the residence times of water in, and the chemical composition of water draining from a basin ŽCleaves et al., 1970; Peters and Murdoch, 1985; Beven et al., 1988; Wolock et al., 1990.. Steep basins may retain water for shorter periods of time than less steep basins. Surficial debris, such as glacial till, provides pore spaces for water storage and may cause a delay in the release of water to surface channels. Such a delay can increase solute concentrations ŽPeters and Driscoll, 1987; Clow et al., 1996.. In mountainous environments, the morphology of a basin affects the accumulation of winter snowpack ŽOutcalt and MacPhail, 1980.. In turn, geochemical denudation may be greatest in the areas of a basin that accumulate the most snow ŽCaine and Thurman, 1990.. Environmental tracers have provided valuable information on hydrologic processes, such as flowpaths of snowmelt or storm runoff, direction of groundwater movement, and chemical reactions oc-

curring along flowpaths ŽFritz and Fontes, 1980.. Environmental tracers such as tritium, carbon-14, and chlorofluorocarbons are well established as tools for studying hydrologic processes that operate on time scales of years to thousands of years ŽDincer et al., 1970; Martinec, 1975; Scanlon, 1992; Szabo et al., 1996.. These tracers may be inadequate, however, for studying hydrologic processes on time scales of a year or less. Sulfur-35 Ž35 S., a short-lived isotope of sulfur Ž t 1r2 s 87 days., may be useful for studying short-time scale hydrologic processes ŽCooper et al., 1991; Michel and Turk, 1995.. Sulfur-35 is produced by cosmic ray spallation of argon in the atmosphere. After its production, 35 S is quickly oxidized to sulfur dioxide and ultimately deposited as sulfate on the surface of Earth, where it enters the biogeochemical cycle ŽMichel and Turk, 1995.. Owing to the low concentrations of 35 S in precipitation, uptake of sulfate by biota, and possible waterrrock interactions, this isotope has rarely been applied to hydrologic systems. Michel and Naftz Ž1995. and Michel and Turk Ž1995. found 35 S to respond reasonably conservatively; that is, input of 35 S was equal to output of 35 S in a given year, in alpine basins in the Flat Tops Wilderness of Colorado, and the Wind River Range of Wyoming, respectively. Biological influences and waterrrock interactions were considered by these authors to be insignificant or absent. The 35 S activity of sulfate in water provides an estimate of the residence time of atmospherically deposited sulfate, and assuming conservative response of sulfate along the hydrologic flowpath, 35 S activity can be used to approximate the residence time of water. In this paper, the effect of basin morphology on the release of snowmelt water to surface streams is explored. The activity of 35 S is used to compare the estimated ages of water exiting three small alpine–subalpine basins in north-central Colorado. 2. Site descriptions The three study basins are located within Rocky Mountain National Park ŽRMNP. in the northern Colorado Front Range, east of the Continental Divide ŽFig. 1.. The Colorado Front Range underwent extensive Quaternary glaciation, with the last glacial

J.K. Sueker et al.r Geomorphology 27 (1999) 61–74

X

X

X

Fig. 1. Site map. Stippled areas show locations of surficial debris. A–A , B–B , and C–C are locations of cross-sections in Fig. 2.

63

64

J.K. Sueker et al.r Geomorphology 27 (1999) 61–74

retreat completed by approximately 7000 years ago ŽMadole, 1976.. The Fern Creek and Spruce Creek basins exhibit classic steep-walled, U-shaped crosssections of glaciated valleys ŽFig. 2. and retain small glaciers and permanent snowfields in cirques and headwalls ŽFig. 1.. The average slope is 228 for both basins, and 29% of the total area of both basins has slopes of 308 or greater ŽTable 1.. In contrast to Fern and Spruce Creek basins, only the upper reaches of Boulder Brook basin were affected by the Quaternary glaciation. Most of the Boulder Brook basin is

not characterized by a glaciated U-shaped cross-section ŽFig. 2.. The average slope is 178, and only 2% of the Boulder Brook basin has a slope of 308 or greater ŽTable 1.. Basin areas of the three study basins range from 780 to 1320 ha ŽTable 1.. Bedrock consists of granite, schist, and gneiss ŽTable 1. ŽBraddock and Cole, 1990.. Surficial debris Ždefined as alluvium, colluvium, organic-rich sediments, talus, landslide deposits, till, and rock glaciers. occurs in discontinuous patches in the Fern Creek and Spruce Creek drainages

Fig. 2. Basin cross-sections. Horizontal and vertical scales are the same. Locations of cross-sections are shown on Fig. 1.

J.K. Sueker et al.r Geomorphology 27 (1999) 61–74

65

Table 1 Characteristics of the three study basins

General site information Area Žhectares. Elevation at gage Žmeters. Average elevation Žmeters. Maximum elevation Žmeters. Average basin slope Ždegrees. Average stream gradient Ždegrees. Geology Granitea Schist and gneissb Surficial debris c Water Snow CoÕer types from aerial photographs Rock, water and snow Alpine meadow Forest Other d

Boulder Brook

Fern Creek

Spruce Creek

990 2689 3325 4345 17 18

780 2575 3200 3768 22 16

1320 2585 3360 3939 22 17

31% 6% 63%

7% 63% 29% 1% - 1%

34% 34% 31% 1% - 1%

28% 34% 32% 6%

45% 16% 37% 2%

45% 30% 23% 2%

a

Includes mapped areas of Silver Plume Granite, garnet–sillimanite granite, intrusion breccia, granite of Hagues Peak, quartz diorite, and pegmatite. b Includes mapped areas of biotite schist, hornblende gneiss and amphibolite, and granitic gneiss. c Includes mapped areas of alluvium, colluvium, organic-rich sediments, talus, landslide deposits, till, and rock glaciers. d Includes mapped areas of wet meadow, aspen, riparian zone, and willow.

and covers about 30% of the basins. In contrast, a deep Ž) 3 m., continuous mantle of surficial debris covers 63% of the Boulder Brook basin ŽFigs. 1 and 2.. Forests of Engelmann spruce and subalpine fir cover between 23% and 37% of all three basins; 16% to 34% of basin area is alpine meadow; and between 28% and 45% of basin area is unvegetated ŽTable 1.. Soils in this region are thin and immature ŽBaron et al., 1992..

3. Methods Four sets of samples were collected from Boulder Brook, Fern Creek, and Spruce Creek in 1994 for analysis of 35 S activity. Collection periods coincided with the onset of snowmelt, peak snowmelt, the falling limb of the snowmelt hydrograph, and late autumn baseflow ŽFig. 3.. Because concentrations of SO42y in surface water are low, 20 l of water was processed for each sample. The water, collected in a 20-l collapsible water jug, was acidified to a pH of

3.5 using concentrated hydrochloric acid and then passed by gravity flow through an ion-exchange resin ŽAmberlite 400. to extract sulfate. The resin was returned to the USGS Isotope Laboratory in Menlo Park, CA for sulfate extraction and scintillation counting. Activities are reported in millibecquerel per milligram ŽmBqrmg. of SO42y to account for changes in streamwater SO42y concentrations because of dilution from snowmelt. In a snowmelt-dominated system, the age of water can be calculated by comparing the sulfate-35 S activity of stream water to an initial snowpack sulfate-35 S activity ŽMichel and Turk, 1995.. Sulfur-35 undergoes radioactive decay that can be described by the exponential decay equation C s Co eyl t

Ž 1. 35

where Co is the starting S activity Žassumed equal to snowpack activity., l s Ž t 1r2 r0.693.y 1 s 0.007947rday, t is the number of days from the start of decay, and C is the resulting 35 S activity. The age of the 35 S in sulfate from surface-water samples was

66

J.K. Sueker et al.r Geomorphology 27 (1999) 61–74

Fig. 3. Area-normalized daily average discharge and 35 S sampling periods for 1994.

estimated using the measured activity of the sample, assuming an initial snowpack value of 21 mBqrmg SO42y on April 20, as measured at Loch Vale, a nearby basin ŽFig. 1., and solving for t in Eq. Ž1.. It was assumed that initial 35 S activities of snowpacks in three basins were similar because of the geographic proximity of the basins. Because of the difficulty of travel in these basins during periods of snowcover, no snowpack samples were collected to verify this assumption. Streamwater-chemistry samples were collected in 250-ml HDPE bottles approximately weekly April through October, and monthly at other times in 1994. Samples were filtered through 0.45-mm polycarbonate membranes within 24 h of collection. Chemical analyses were performed at the USGS Laboratory in Boulder, CO. Sulfate concentrations were determined for filtered samples by using liquid ion chromatography ŽFishman and Friedman, 1989.. Error of analysis for replicate samples was "2.3%. Concentrations of the base cations were determined on filtered samples that were acidified to pH 2 using nitric acid. Calcium, magnesium, and sodium concentrations were determined by inductively coupled plasma mass spectrometry, and potassium concentra-

tions were determined by atomic absorption spectroscopy ŽFishman and Friedman, 1989.. Errors of analyses for replicate samples were less than "5%. Alpine–subalpine basins within RMNP receive an average of 50 to 100 cm of precipitation annually, depending on basin elevation, orientation, and morphology ŽColorado Climate Center, 1984.. Precipitation input Žrainfall and snow accumulation. was estimated for the three basins using precipitation data from two National Atmospheric Deposition ProgramrNational Trends Network ŽNADPrNTN. stations located in the park ŽNational Atmospheric Deposition ProgramrNational Trends Network, 1996.. A chloride balance technique was used to refine the NADPrNTN precipitation data and to estimate evapotranspiration losses for each basin ŽTable 2. ŽCleaves et al., 1970; Likens et al., 1977; Sueker, 1996.. A gaging station was located at each basin outlet. Stage was recorded at 15-min intervals during ice-free periods Žearly April through October for Fern and Spruce Creek, year round for Boulder Brook. using float and counterweight stage recorders atop 10-cm diameter stilling wells. Discharge was measured by current-meter and dye-dilution methods in Boulder

J.K. Sueker et al.r Geomorphology 27 (1999) 61–74

67

Table 2 Precipitation and evapotranspiration ŽET. estimates for 1994 based on chloride balance Drainage basins

Rainfall Žcm.

Snowfall Žcm.

Total precipitationa Žcm.

Total runoff b Žcm.

Runoff percentage of precipitation Ž%.

ET Žcm.

ET as percentage of precipitation Ž%.

Boulder Brook Fern Creek Spruce Creek

24.2 23.3 23.5

30.9 84.4 50.8

55.1 107.7 74.3

22.2 73.9 55.0

40 69 74

32.9 33.8 19.3

60 31 26

Precipitation and ET values are reported in centimeters Žcm. of water. Precipitation that accumulates during winter is assumed to remain until snowmelt the following spring. To account for the precipitation that causes runoff for the year, precipitation is summed from October 1, 1993 through September 30, 1994, and runoff is summed from April 1, 1994 through March 31, 1995. a Precipitation summed from October 1, 1993 through September 30, 1994. b Runoff summed from April 1, 1994 through March 31, 1995.

Brook and by dye dilution in Spruce Creek and Fern Creek. Stage-discharge relations were applied to the continuous stage record to develop discharge hydrographs ŽFig. 3. ŽSueker, 1996.. Discharge was estimated using recession coefficients for Fern and Spruce Creeks during ice-covered periods. Any error produced by extrapolating discharge past the period of record using hydrograph recession coefficients

was considered to be insignificant compared to the total annual discharge estimates for these streams ŽCampbell et al., 1995.. The recession segment of a hydrograph represents release of water from saturated storage after snowmelt or rainfall inflow to the channel has ceased or is greatly reduced with respect to the volume of flow in the channel. The physical process of releasing water

Fig. 4. Activity of sulfate-35 S in 1994 streamwater samples. No sulfate was recovered from the mid-June Boulder Brook sample. Sulfate-35 S activities in Fern and Spruce Creeks increased during the snowmelt period but were lowest at October baseflow. Sulfate-35 S activities in Boulder Brook declined from a high at snowmelt onset to a low at October baseflow.

J.K. Sueker et al.r Geomorphology 27 (1999) 61–74

68

Fig. 5. Estimated age of sulfate in 1994 streamwater samples. Sulfate ages in Fern and Spruce Creeks averaged about 200 days throughout the study period, whereas the age of sulfate in Boulder Brook increased from 140 days at the onset of snowmelt to almost 500 days in October.

from storage in the ground is a phenomenon that can be described by an exponential decay equation ŽChow, 1964; Kattelmann, 1989. Q t s Q o K rt

Ž 2.

where Q t is the daily average discharge after t days, Qo is the initial daily average discharge, K r is a recession constant that is less than 1, and t is time in days. The closer K r is to 1, the more slowly water is released from basin storage. Flow-recession coefficients were determined for each basin on the falling limb of the snowmelt hydrograph during a frost-free period. Periods of increased discharge because of large rain events were excluded from the calculations.

The volume of water stored in a basin in drainable surface or subsurface storage was estimated using daily average discharge, Q, and the flow-recession coefficient, K r . During periods of baseflow, the volume of flow over some time period d t is Q t and is equal to the decrease in storage ŽydS . over the same interval. Integrating the relation Qd t s ŽydS . allows calculation of the storage, St , remaining in the basin at day t using the recession coefficient and the daily average discharge Q t for that day ŽChow, 1964; Kattelmann, 1989.: Sa s yQ trln K r .

Ž 3.

Melt from late-season snowfields and permanent glaciers, and partial recharge of subsurface storage

Table 3 Recession coefficients and monthly average basin storage Boulder Brook Kr Storage September 1994 April 1995

0.995

Ž0.994 to 0.998.

17 cm 10 cm

Ž14 to 42 cm. Ž8 to 25 cm.

Spruce Creek

Fern Creek

0.984

Ž0.978 to 0.990.

0.975

Ž0.963 to 0.984.

7 cm 0.5 cm

Ž4.7 to 12 cm. Ž0.3 to 0.8 cm.

6 cm 0.05 cm

Ž4.3 to 12.5 cm. Ž0.04 to 0.1 cm.

Storage estimates do not include water stored in soils and unsaturated zones. Ranges of results are shown in parentheses.

J.K. Sueker et al.r Geomorphology 27 (1999) 61–74

from summer and autumn storms, may contribute to recession flow and may be included in the storage calculations for late summer and autumn months

69

ŽKattelmann, 1989.. Recharge is greatly reduced or eliminated during the winter because of low temperatures; therefore, storage estimates determined for

Fig. 6. Concentrations of alkalinity, sulfate, and the sum of base cations in streamwater samples collected from Boulder Brook, Fern Creek, and Spruce Creek in 1994. Concentrations of solutes in Boulder Brook were less variable than concentrations of solutes in Fern and Spruce Creeks.

70

J.K. Sueker et al.r Geomorphology 27 (1999) 61–74

winter months are likely conservative and represent only the residual filled storage that supplies streamflow through the winter.

4. Results The activities of sulfate-35 S were similar for each sampling period for samples collected from Fern and Spruce Creeks ŽFig. 4.. Activities were highest in the samples collected at the end of July Ž6.0 " 0.9 and 6.9 " 1.0 mBqrmg SO42y . and were lowest in the October samples Ž3.1 " 1.0 and 1.9 " 1.1 mBqrmg SO42y . for Fern and Spruce Creeks, respectively. The activity of sulfate-35 S in samples collected from Boulder Brook decreased from a high of 7.6 " 1.0 mBqrmg SO42y in late April to a low of 0.4 " 1.0 mBqrmg SO42y in late October. For reasons related to sample processing, no sulfate was recovered from the mid-June Boulder Brook sample. The estimated age of the sulfate in sample waters varied according to the activity of 35 S, with lower activities indicating older ages ŽFig. 5.. Sulfate ages in Fern Creek samples ranged from approximately 150 to 240 days, and averaged 190 days. Sulfate ages in Spruce Creek ranged from about 140 to 300 days and averaged 215 days. Sulfate ages in Boulder Brook samples ranged from approximately 130 to 500 days and averaged 325 days. The sulfate ages in Boulder Brook were about 200 days older than Fern and Spruce Creek sulfate ages in August and October, but were younger in April. Estimated snow accumulation for the winter of 1993r1994 varied significantly among the three basins ŽTable 2.. Estimated snowfall in Boulder Brook basin was much less Ž30.9 cm water equivalent. than in Spruce Creek Ž50.8 cm water equivalent. and Fern Creek Ž84.4 cm water equivalent.. Peak discharge during 1994 snowmelt in Fern and Spruce Creeks was substantially greater than peak discharge in Boulder Brook ŽFig. 3.. Peak snowmelt area-normalized discharge was 1.15 and 0.9 cmrday of water for Fern and Spruce Creeks, respectively, and 0.25 cmrday of water for Boulder Brook in 1994. Change in daily average discharge in Fern and Spruce Creeks from winter baseflow Ž- 0.01 cmr day. to peak snowmelt Ž1 cmrday of water. was approximately two orders of magnitude. The change

in flow from baseflow to peak snowmelt was only about one order of magnitude for Boulder Brook, which maintained a substantially higher wintertime baseflow than Fern and Spruce Creeks ŽFig. 3.. Calculated values of K r fluctuated during the falling limb of the hydrograph, revealing no clear trend. Therefore, a characteristic value was chosen for each basin to represent the overall K r for that period ŽTable 3.. In 1994, the representative K r was 0.995 for Boulder Brook, 0.984 for Spruce Creek, and 0.975 for Fern Creek. Values of K r in 1993 and 1995 were similar to 1994 values for the three basins ŽSueker, 1996; Sueker, unpublished data.. Estimates of monthly average storage for September 1994 and April 1995 showed that the Boulder Brook basin had considerably more water stored in saturated drainable storage than Fern or Spruce Creek basins ŽTable 3.. Less than 1 cm of storage remained in Fern and Spruce Creek basins in April 1995, whereas Boulder Brook had about 10 cm of storage remaining ŽTable 3.. Sulfate and bicarbonate Žassumed equivalent to alkalinity. are the dominant anions in water sampled from the three streams. Solute concentrations in 1994 were less variable in Boulder Brook than in Fern and Spruce Creek basins, where snowmelt dilution caused larger decreases in the solute concentrations of sulfate, alkalinity, and the sum of base cations in streamflow ŽFig. 6.. Sulfate concentrations in these two streams ranged from about 3 mgrl during baseflow and the onset of snowmelt to less than 1 mgrl in August. Sulfate concentrations in Boulder Brook ranged from 1.8 to 1.2 mgrl during this same time period. 5. Discussion When using 35 S to estimate the age of water in a basin, one must assume a conservative response of sulfate as it travels through the basin. Soils in the Loch Vale watershed ŽFig. 1. were found to have very low sulfate adsorption capacities Žapproximately 1 mmolrkg adsorbed SO42y for a solution concentration of 1 mM SO42y wBaron et al., 1992x.. Because of the proximity of Loch Vale to the other three basins, and the similar bedrock material, the soil in Boulder Brook, Fern Creek, and Spruce Creek basins was assumed to have low sulfate adsorption

J.K. Sueker et al.r Geomorphology 27 (1999) 61–74

capacity. Therefore, it was assumed that detention of sulfate as it is transported through these basins would be minimal. Sulfate efflux in six streams in RMNP exceeded atmospheric wet-deposition loading to the basins by an average of 14% ŽSueker, 1996.. The difference between sulfate loading and export may have resulted from dry deposition of sulfate, although a contribution from mineral weathering could not be ruled out ŽCampbell et al., 1995.. Grant and Lewis Ž1982. determined that dry deposition accounted for 20% of the bulk deposition of sulfate to a site in the Colorado Front Range. Turk and Spahr Ž1989. found that dry deposition contributions of sulfate to alpine basins in Colorado were low compared to wet-deposition contributions. It was assumed that the excess of exported sulfate, relative to the wet-deposition loading, was due to dry deposition and that output of sulfate was equal to the input of sulfate in a given year for the three study basins. These observations suggest that sulfate responds reasonably conservatively in this environment, supporting the validity of our comparison of 35 S activities and estimated ages across the three basins. We had intended to collect baseflow samples in late April; however, an early snowmelt event occurred during this collection period ŽFig. 3., so these samples may not represent true baseflow conditions. The late April sample for Boulder Brook exhibited what seemed to be anomalously high 35 S activity and young age ŽFigs. 4 and 5., because isotopic and chemical data indicated that streamflow consisted of about three-fourths baseflow and one-fourth snowmelt water for all three streams ŽSueker, 1996.. Sulfate activities were about 4 mBqrmg SO42y, and ages about 200 days, for Fern and Spruce Creeks ŽFigs. 4 and 5.. Sulfate activity and age were 7.5 mBqrmg SO4 and 140 days, respectively for Boulder Brook and 21 mBqrmg SO42y and 0 days for snowmelt. Baseflow sulfate concentrations in Fern Creek and Spruce Creek Ž3 mgrl. were twice as high as those in Boulder Brook Ž1.4 mgrl. ŽFig. 6.. Sulfate concentrations typically are about 1.4 mgrl in early snowmelt waters ŽDenning et al., 1991.. Because the baseflow sulfate concentrations in Boulder Brook were lower than in Fern Creek and Spruce Creek, Boulder Brook had a relatively higher proportion of younger snowmelt sulfate added to streamflow compared to Spruce and Fern Creeks. This

71

caused a higher 35 S activity and lower estimated age of water in Boulder Brook for the early snowmelt event. In late July and October, the 35 S activities of sulfate were much lower and ages much higher in stream samples from Boulder Brook than in samples from Fern or Spruce Creeks ŽFigs. 4 and 5.. Sulfate older than about 1 year has little or no 35 S activity. Isotopically depleted sulfate in water may be derived from precipitation that was stored in a basin for a year or more prior to release. Assuming the conservative response of sulfate, the low 35 S activities and older ages of sulfate indicate that Boulder Brook has a substantially greater proportion of streamflow derived from older, isotopically depleted water Žas flow released from subsurface storage. than the other two streams. Basin topography affects the capture of snowfall and snowpack accumulation in these three basins. Total estimated accumulation of snow in Boulder Brook basin is much less than in Fern and Spruce Creek basins ŽTable 2.. About one-half of the Boulder Brook basin is above treeline and consists of broad, exposed areas. Situated on the north face of Longs Peak, the highest elevation in RMNP Ž4350 m., this basin is subject to extremely strong winds. Wind gusts of 100 kmrh or more are common at high-elevations in this region during winter months ŽBarry, 1973.. Strong winds across the broad, exposed surfaces of Boulder Brook basin provide ample opportunity for snow loss from blowing snow or sublimation. Visual inspections of the basin from lower elevations showed that much of the ground above treeline was bare during the winter, whereas other basins were blanketed in snow. Fern Creek and Spruce Creek basins have deeply incised valleys with surrounding cliffs hundreds of meters high. Situated east of the Continental Divide, incised basins such as these tend to be deposition zones for snow blowing from the western side of the Continental Divide ŽOutcalt and MacPhail, 1980.. Fern and Spruce Creek basins have glaciers and permanent snowfields situated high on cirque walls ŽFig. 1.. These two basins efficiently capture blowing snow and accumulate substantially more snow than Boulder Brook basin ŽTable 2.. Differences in the chemistry and discharge hydrographs of these three basins are related, in part, to

72

J.K. Sueker et al.r Geomorphology 27 (1999) 61–74

differences in snow capture. Because less snow accumulates in the Boulder Brook basin, less water is available during snowmelt compared to Fern and Spruce Creek basins. Greater volumes of snowmelt cause solute dilution and an increase in the contribution of isotopically active water in Fern and Spruce Creeks relative to Boulder Brook. Solute concentrations and water discharge are also affected by surficial debris and basin slope. Surficial debris provides pore space for water storage and greater mineral surface area for increased mineral weathering reactions, and shallow basin slopes delay the release of water to surface streams relative to steep slopes. Boulder Brook basin has shallower basin slopes and substantially more surficial material than Fern and Spruce Creek basins. Concentrations of base cations and alkalinity Žproducts of mineral weathering. were higher and less variable in Boulder Brook than in Fern Creek and Spruce Creek ŽFig. 6.. Discharge from the Boulder Brook basin had smaller seasonal fluctuations and higher winter baseflow than Fern and Spruce Creeks ŽFig. 3.. A large volume of surficial debris provides for the storage of substantial amounts of water in the Boulder Brook basin, whereas meltwater passes rapidly through the thinner debris layers to bedrock surfaces in Fern and Spruce Creek basins and flows quickly to surface stream channels. Thus, less opportunity exists for water storage and chemical interaction with soil and other subsurface materials in Fern and Spruce Creek basins. Shallow basin slopes and surficial debris in Boulder Brook basin increase the residence time of water in the basin. Estimates of saturated drainable water storage in the basins support interpretations of a longer residence time for water in Boulder Brook basin compared to Fern and Spruce Creek basins ŽTable 3.. Recession coefficients indicate that water drains more slowly from Boulder Brook basin than from Fern and Spruce Creek basins. Based on storage estimates, less than 1% of the precipitation from 1993– 1994 was present in saturated storage in Fern and Spruce Creek basins at the start of snowmelt in 1995 ŽTable 3.. Almost 20% of the total 1993–1994 precipitation in Boulder Brook basin, however, was still available in saturated storage at the onset of the 1995 snowmelt ŽTables 2 and 3.. This stored water would be isotopically depleted of sulfate-35 S relative to

sulfate-35 S in the newer snowpack. Sufficient contributions of isotopically depleted, stored water to streamflow increases the average age of the sulfate and the streamwater leaving the Boulder Brook basins compared to the Fern and Spruce Creek basins.

6. Conclusions Recession coefficients indicate that water drains more slowly from saturated subsurface storage in Boulder Brook basin than from the two other basins. Calculations of water storage and estimated ages of water suggest that most of the precipitation that falls in the Fern and Spruce Creek basins during one snowmelt water year exits the basin prior to snowmelt of the following year. In Boulder Brook basin, a large amount of water is stored in subsurface reservoirs. Almost 20% of precipitation from one snowmelt water year remains in the basin and contributes to streamflow during the following snowmelt water year or years. Morphologic differences in the slope, efficiency of snow capture, and amount of surficial debris in these basins account for the differences in surfacewater chemistry and hydrology. Steep valley walls in Fern and Spruce Creek basins enhance snow capture, whereas broad, exposed surfaces in Boulder Brook allow wind scour of snow to occur. Deep surficial debris and shallow slopes delay the release of water to Boulder Brook, increasing concentrations of mineral-weathering solutes and decreasing the variance in solute concentrations and streamflow. Water samples collected from Boulder Brook in July and October 1994 showed substantially older ages for sulfate35 S compared to samples from Fern and Spruce Creeks. Assuming conservative response of sulfate in these systems, after peak snowmelt, the water flowing from Boulder Brook was approximately 200 days older than water flowing from Fern and Spruce Creeks. These results demonstrate that 35 S is useful in estimating and comparing the ages of water exiting small alpine–subalpine basins where the time scales of hydrologic processes are of the order of 1 year or less, and sulfate responds as a conservative tracer. This technique may be useful in developing an alpine sensitivity map by measuring the ages of sulfate

J.K. Sueker et al.r Geomorphology 27 (1999) 61–74

exiting basins at different times during the snowmelt period. Basins with a higher percentage of streamflow derived from older water sources may be less sensitive to atmospherically derived acidic deposition, because of the higher acid neutralization capacity that accompanies increased residence time.

Acknowledgements This research was supported by the USGS Office of Surface Water, the USGS Water, Energy, and Biogeochemical Budget Program, the University of Colorado, the American Geophysical Union Horton Research Grant, Colorado State University, and the National Park Service. Hearty thanks are extended to Don Thorstenson, David Naftz, and two anonymous reviewers for their insightful comments, and to Mary Kidd and Rick Nelson for their editorial reviews.

References Baron, J., Walthall, P.M., Mast, M.A., Arthur, M.A., 1992. Soils. In: Baron, J. ŽEd.., Biogeochemistry of a Subalpine Ecosystem: Loch Vale Watershed. Springer-Verlag, New York, NY, pp. 108–141. Barry, R.G., 1973. A climatological transect on the east slope of the Front Range, Colorado. Arctic and Alpine Research 5, 89–110. Beven, K.J., Wood, E.F., Sivapalan, M., 1988. On hydrological heterogeneity—catchment morphology and catchment response. Journal of Hydrology 100, 353–375. Braddock, W.A., Cole, J.C., 1990. Geologic Map of Rocky Mountain National Park and Vicinity, Colorado. US Geological Survey, MAP I-1973, scale 1:50,000. Caine, N., 1989. Hydrograph separation in a small alpine basin based on inorganic solute concentrations. Journal of Hydrology 112, 89–101. Caine, N., Thurman, E.M., 1990. Temporal and spatial variations in the solute content of an alpine stream, Colorado Front Range. Geomorphology 4, 55–72. Campbell, D.H., Clow, D.W., Ingersoll, G.P., Mast, M.A., Spahr, N.E., Turk, J.T., 1995. Processes controlling the chemistry of two snow-melt-dominated streams in the Rocky Mountains. Water Resources Research 31 Ž11., 2811–2821. Chow, V.T., 1964. Handbook of Applied Hydrology. McGrawHill, New York. Cleaves, E.T., Godfrey, A.E., Bricker, O.P., 1970. Geochemical balance of a small watershed and its geomorphic implications. Geological Society of America Bulletin 81, 3015–3032.

73

Clow, D.W., Mast, M.A., Campbell, D.H., 1996. Controls on surface water chemistry in the Upper Merced River basin, Yosemite National Park, California. Hydrological Processes 10, 727–746. Colorado Climate Center, 1984. Colorado Average Annual Precipitation 1951–1980, Ft. Collins, Colorado. Cooper, L.W., Olsen, C.R., Solomon, D.K., Larsen, I.L., Cook, R.B., Grebmeier, J.M., 1991. Stable isotopes of oxygen and natural fallout radionuclides used for tracing runoff during snowmelt in an Arctic watershed. Water Resources Research 27 Ž9., 2171–2179. Denning, S.A., Baron, J., Mast, M.A., Arthur, M.A., 1991. Hydrologic pathways and chemical composition of runoff during snowmelt in Loch Vale Watershed, Rocky Mountain National Park, Colorado, USA. Water, Air and Soil Pollution 59, 107–123. Dincer, T., Payne, B.R., Florkowski, T., Martinec, J., Tongiorgi, E., 1970. Snowmelt runoff from measurements of tritium and oxygen-18. Water Resources Research 6 Ž1., 110–124. Fishman, M.J., Friedman, L.C., 1989. Methods for Determination of Inorganic Substances in Water and Fluvial Sediments. Techniques of Water-Resources Investigations of the United States Geological Survey, Book 5, Chap. A1. US Geological Survey, Washington, DC. Fritz, P., Fontes, C.J., 1980. Handbook of Environmental Isotope Geochemistry, The Terrestrial Environment, Vol. 1, A. Elsevier, Amsterdam. Grant, M.C., Lewis, W.M. Jr., 1982. Chemical loading rates from precipitation in the Colorado Rockies. Tellus 34, 74–88. Ingersoll, G., 1995. Snowpack Chemistry at Selected Sites in Northwest Colorado during Spring 1995. Open-File Report 96-411. US Geological Survey, Denver, CO. Kattelmann, R., 1989. Groundwater contributions in an alpine basin in the Sierra Nevada. In: Woessner, W.W., Potts, D.F. ŽEds.., Proceeding of the Headwaters Hydrology Symposium. American Water Resources Association, pp. 361–369. Likens, G.E., Bormann, F.H., Pierce, R.S., Eaton, J.S., Johnson, N.M., 1977. Biogeochemistry of a Forested Ecosystem. Springer-Verlag, New York. Madole, R.F., 1976. Glacial geology of the Front Range, Colorado. In: Mahaney, W.C. ŽEd.., Quaternary Stratigraphy of North America. Dowden, Hutchinson & Ross, Stroudsburg, PA, pp. 297-398. Martinec, J., 1975. Subsurface flow from snowmelt traced by tritium. Water Resources Research 11 Ž3., 496–498. Michel, R.L., Naftz, D.L., 1995. Use of sulfur-35 and tritium to study runoff from an alpine glacier, Wind River Range, Wyoming. In: Tonnessen, K.A., Williams, M.W., Tranter, M. ŽEds.., Biogeochemistry of Seasonally Snow-Covered Catchments. International Association of Hydrological Sciences. Boulder, CO. Publication No. 228, pp. 441–443. Michel, R.L., Turk, J.T., 1995. Use of sulfur-35 to study rates of sulfur migration in the Flat Tops Wilderness Area, Colorado. Isotopes in Water Resources Management, Vol. 1. International Atomic Energy Agency, Vienna, pp. 293–301. National Atmospheric Deposition ProgramrNational Trends Network, 1996. Internet: http:rrnadp.nrel.colostate.edurNADPr.

74

J.K. Sueker et al.r Geomorphology 27 (1999) 61–74

Outcalt, S.I., MacPhail, D.D., 1980. A survey of neoglaciation in the Front Range of Colorado. In: Ives, J.D. ŽEd.., Geoecology of the Colorado Front Range. Westview Press, Boulder, CO, pp. 203–208. Peters, N.E., Driscoll, C.T., 1987. Hydrogeologic controls of surface-water chemistry in the Adirondack region of New York State. Biogeochemistry 3, 163–180. Peters, N.E., Murdoch, P.S., 1985. Hydrogeologic comparison of an acidic-lake basin with a neutral-lake basin in the west-central Adirondack Mountains, New York. Water, Air and Soil Pollution 26, 387–402. Scanlon, B.R., 1992. Evaluation of liquid and vapor water flow in desert soils based on chlorine 36 and tritium tracers and nonisothermal flow simulations. Water Resources Research 28 Ž1., 285–297. Sueker, J.K., 1996. Isotopic and chemical flowpath separation of streamflow during snowmelt and hydrogeologic controls of surface-water chemistry in six alpine–subalpine basins, Rocky Mountain National Park, Colorado. PhD Dissertation. Depart-

ment of Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, CO. Szabo, Z., Rice, D.E., Plummer, L.N., Busenberg, E., Drenkard, S., Schlosser, P., 1996. Age dating of shallow groundwater with chlorofluorocarbons, tritiumrhelium 3, and flow path analysis, southern New Jersey coastal plain. Water Resources Research 32 Ž4., 1023–1038. Turk, J.T., Spahr, N.E., 1989. Chemistry of Rocky Mountain lakes. In: Adriano, D.C., Havas, M. ŽEds.., Acid Precipitation, Case Studies, Vol. 1. Springer-Verlag, New York, NY, pp. 181–208. Turk, J.T., Campbell, D.H., Ingersoll, G.P., Clow, D.W., 1992. Initial findings of synoptic snowpack sampling in the Colorado Rocky Mountains. Open-File Report 92-645. US Geological Survey, Denver, CO. Wolock, D.M., Hornberger, G.M., Musgrove, T.J., 1990. Topographic effects of flow path and surface water chemistry of the Llyn Brianne catchments in Wales. Journal of Hydrology 115, 243–259.