field characterization of hydraulic conductivity in a heterogeneous alpine glacial till

7
Field characterization of hydraulic conductivity in a heterogeneous alpine glacial till Michael J. Ronayne a,, Tyler B. Houghton a,1 , John D. Stednick b a Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482, USA b Watershed Science Program, Colorado State University, Fort Collins, CO 80523-1472, USA article info Article history: Received 4 April 2012 Received in revised form 15 June 2012 Accepted 20 June 2012 Available online 27 June 2012 This manuscript was handled by Corrado Corradini, Editor-in-Chief, with the assistance of Fritz Stauffer, Associate Editor Keywords: Hydraulic conductivity Glacial till Heterogeneity Mountain watershed Infiltration tests summary Three different measurement techniques (a mini-disk infiltrometer, a double-ring infiltrometer, and a Guelph permeameter) were used to characterize the saturated hydraulic conductivity of an alpine glacial till in the Rocky Mountains of southern Wyoming, USA. Measurements from 32 locations reveal significant spatial heterogeneity. The hydraulic conductivity varies over two orders of magnitude from approximately 0.05–5 m/d. Along with natural variability throughout the study area, the results also indicate that the estimated hydraulic conductivity is dependent on measurement technique. Compared to the mini-disk infiltrometer, hydraulic conductivities are consistently higher for the double-ring infiltrometer and Guelph permeameter. By considering surface–subsurface hydrologic response during snowmelt, we dem- onstrate the importance of accurately characterizing the hydraulic conductivity. A model parameterized with a low hydraulic conductivity underestimates the rate of shallow groundwater flow, suggesting that the subsurface saturated zone may not be able to accommodate all of the snowmelt-derived recharge. Saturation-excess overland flow is predicted as a result. These findings have important implications for integrated hydrologic assessments focused on understanding water flows in glaciated alpine watersheds. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction The Pleistocene glaciations resulted in glacial deposits that play an important role in the hydrogeology of many areas worldwide (Stephenson, 1988). One widespread type of deposit, glacial till, consists of poorly sorted sediment laid down directly by retreating ice. A number of previous hydrogeologic studies throughout Europe and North America have focused on tills that were deposited by continental ice sheets (e.g., Hendry, 1988; Simpkins and Bradbury, 1992; Nilsson et al., 2001; Cuthbert et al., 2010). These tills are often compacted and clay-rich and therefore have very low hydraulic conductivity. The conductivity values reported for unfractured con- tinental till are generally less than 10 4 m/d (Hendry, 1982; Shaw and Hendry, 1998; Cuthbert et al., 2010). In many regions that experienced continental glaciation, the till is characterized as an aquitard unit (Haldorsen and Krüger, 1990; Ruland et al., 1991; Shaw and Hendry, 1998). Although continental glacial tills have been extensively studied, relatively little work has been done to investigate the hydrogeol- ogy of alpine tills. Deposited by smaller glaciers in mountain envi- ronments, alpine tills have characteristics that differ significantly from continental glacial tills (e.g., alpine tills tend to be less consol- idated). Several recent and ongoing studies have considered the importance of shallow groundwater to the hydrology of glaciated mountain watersheds (Sueker et al., 2000; Clow et al., 2003; Liu et al., 2004; Hood et al., 2006; Kahn et al., 2008). A better under- standing of the physical and hydraulic properties of alpine tills is needed to complement these research efforts. In this work, we investigate the hydraulic conductivity of an alpine glacial till in the Rocky Mountains of southern Wyoming. Field infiltration tests were performed at numerous spatial loca- tions throughout the study area to characterize variability in the till’s hydraulic conductivity. In addition, we used multiple testing techniques to evaluate the dependence of hydraulic conductivity on test conditions and measurement scale. By considering hydro- logic response behavior during the snowmelt period, we demon- strate the importance of accurately characterizing the hydraulic conductivity. The results of this study are applicable to many other mountain watersheds where till from alpine glaciations is present as surficial geologic material. The remainder of this paper is orga- nized as follows. In Section 2, we introduce the study site and briefly describe its geologic characteristics. Section 3 covers the multiple methods that we used to measure hydraulic conductivity. Results are presented in Section 4. In Section 5, we summarize our key findings and offer a few recommendations for future work. 2. Study area This study was conducted at the Glacier Lakes Ecosystem Exper- iments Site (GLEES), a research area managed by the U.S. Depart- 0022-1694/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2012.06.036 Corresponding author. Tel.: +1 9704915661; fax: +1 9704916307. E-mail addresses: [email protected] (M.J. Ronayne), thoughton@ norwestcorp.com (T.B. Houghton), [email protected] (J.D. Stednick). 1 Present address: Norwest Corporation, Denver, CO 80246, USA. Journal of Hydrology 458–459 (2012) 103–109 Contents lists available at SciVerse ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol

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Page 1: Field characterization of hydraulic conductivity in a heterogeneous alpine glacial till

Journal of Hydrology 458–459 (2012) 103–109

Contents lists available at SciVerse ScienceDirect

Journal of Hydrology

journal homepage: www.elsevier .com/ locate / jhydrol

Field characterization of hydraulic conductivity in a heterogeneous alpine glacial till

Michael J. Ronayne a,⇑, Tyler B. Houghton a,1, John D. Stednick b

a Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482, USAb Watershed Science Program, Colorado State University, Fort Collins, CO 80523-1472, USA

a r t i c l e i n f o

Article history:Received 4 April 2012Received in revised form 15 June 2012Accepted 20 June 2012Available online 27 June 2012This manuscript was handled by CorradoCorradini, Editor-in-Chief, with theassistance of Fritz Stauffer, Associate Editor

Keywords:Hydraulic conductivityGlacial tillHeterogeneityMountain watershedInfiltration tests

0022-1694/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jhydrol.2012.06.036

⇑ Corresponding author. Tel.: +1 9704915661; fax:E-mail addresses: [email protected]

norwestcorp.com (T.B. Houghton), John.Stednick@colo1 Present address: Norwest Corporation, Denver, CO

s u m m a r y

Three different measurement techniques (a mini-disk infiltrometer, a double-ring infiltrometer, and aGuelph permeameter) were used to characterize the saturated hydraulic conductivity of an alpine glacialtill in the Rocky Mountains of southern Wyoming, USA. Measurements from 32 locations reveal significantspatial heterogeneity. The hydraulic conductivity varies over two orders of magnitude from approximately0.05–5 m/d. Along with natural variability throughout the study area, the results also indicate that theestimated hydraulic conductivity is dependent on measurement technique. Compared to the mini-diskinfiltrometer, hydraulic conductivities are consistently higher for the double-ring infiltrometer andGuelph permeameter. By considering surface–subsurface hydrologic response during snowmelt, we dem-onstrate the importance of accurately characterizing the hydraulic conductivity. A model parameterizedwith a low hydraulic conductivity underestimates the rate of shallow groundwater flow, suggesting thatthe subsurface saturated zone may not be able to accommodate all of the snowmelt-derived recharge.Saturation-excess overland flow is predicted as a result. These findings have important implications forintegrated hydrologic assessments focused on understanding water flows in glaciated alpine watersheds.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

The Pleistocene glaciations resulted in glacial deposits that playan important role in the hydrogeology of many areas worldwide(Stephenson, 1988). One widespread type of deposit, glacial till,consists of poorly sorted sediment laid down directly by retreatingice. A number of previous hydrogeologic studies throughout Europeand North America have focused on tills that were deposited bycontinental ice sheets (e.g., Hendry, 1988; Simpkins and Bradbury,1992; Nilsson et al., 2001; Cuthbert et al., 2010). These tills are oftencompacted and clay-rich and therefore have very low hydraulicconductivity. The conductivity values reported for unfractured con-tinental till are generally less than 10�4 m/d (Hendry, 1982; Shawand Hendry, 1998; Cuthbert et al., 2010). In many regions thatexperienced continental glaciation, the till is characterized as anaquitard unit (Haldorsen and Krüger, 1990; Ruland et al., 1991;Shaw and Hendry, 1998).

Although continental glacial tills have been extensively studied,relatively little work has been done to investigate the hydrogeol-ogy of alpine tills. Deposited by smaller glaciers in mountain envi-ronments, alpine tills have characteristics that differ significantlyfrom continental glacial tills (e.g., alpine tills tend to be less consol-idated). Several recent and ongoing studies have considered the

ll rights reserved.

+1 9704916307.(M.J. Ronayne), [email protected] (J.D. Stednick).80246, USA.

importance of shallow groundwater to the hydrology of glaciatedmountain watersheds (Sueker et al., 2000; Clow et al., 2003; Liuet al., 2004; Hood et al., 2006; Kahn et al., 2008). A better under-standing of the physical and hydraulic properties of alpine tills isneeded to complement these research efforts.

In this work, we investigate the hydraulic conductivity of analpine glacial till in the Rocky Mountains of southern Wyoming.Field infiltration tests were performed at numerous spatial loca-tions throughout the study area to characterize variability in thetill’s hydraulic conductivity. In addition, we used multiple testingtechniques to evaluate the dependence of hydraulic conductivityon test conditions and measurement scale. By considering hydro-logic response behavior during the snowmelt period, we demon-strate the importance of accurately characterizing the hydraulicconductivity. The results of this study are applicable to many othermountain watersheds where till from alpine glaciations is presentas surficial geologic material. The remainder of this paper is orga-nized as follows. In Section 2, we introduce the study site andbriefly describe its geologic characteristics. Section 3 covers themultiple methods that we used to measure hydraulic conductivity.Results are presented in Section 4. In Section 5, we summarize ourkey findings and offer a few recommendations for future work.

2. Study area

This study was conducted at the Glacier Lakes Ecosystem Exper-iments Site (GLEES), a research area managed by the U.S. Depart-

Page 2: Field characterization of hydraulic conductivity in a heterogeneous alpine glacial till

104 M.J. Ronayne et al. / Journal of Hydrology 458–459 (2012) 103–109

ment of Agriculture (USDA) Forest Service. GLEES is located in theMedicine Bow National Forest in southern Wyoming (Fig. 1). Anoverview of the site is provided by Musselman (1994). With eleva-tions ranging from 3250 to 3400 m above mean sea level, GLEES in-cludes the boundary between alpine and subalpine zones. Meanannual precipitation is 122 cm, with 85% of that occurring as snow(Korfmacher and Hultstrand, 2006). Most prior research at GLEEShas focused on evaluating the variability in precipitation chemistryand atmospheric pollutant deposition (Zeller et al., 2000; Burns,2003; Rohrbough et al., 2003; Musselman and Slauson, 2004).

GLEES includes two glacial cirque lakes (Fig. 1), which are rem-nants of alpine glaciation during the Pleistocene. This glaciationalso deposited a significant amount of till throughout the area.Recent geophysical surveys using ground penetrating radar indi-cate that the glacial till thickness at GLEES ranges from 0 to 9 m,with maximum thickness around the lake margins (Page, 2011).The thickness of the till decreases toward the steep slopes that sur-round the site. The bedrock, which is exposed in areas where thetill is absent, is the Medicine Peak Quartzite described by Flurkey(1983). Bedrock fractures are common. Soils including Typic Cryo-boralfs, Dystric Cryochrepts, and Histic-Aeric Cryaquepts are pres-ent in some areas, particularly on lower slopes around the lakeoutlet streams. However, the soil development and coverage isminimal across the majority of the site (Musselman, 1994). Uncon-solidated glacial till sediment occurs at or near the ground surface.Thus, we used surficial techniques (infiltration tests) to estimatethe hydraulic conductivity of the till. Given access limitations for

Fig. 1. Glacier Lakes Ecosystems Experiment Site (GLEES) location and base map. Land s

this remote mountainous site and environmental constraints ondrilling, surficial measurement techniques are more practical thanwell-based hydraulic testing.

3. Methods

3.1. Measurement of hydraulic conductivity

In situ testing was performed to estimate the field-saturatedhydraulic conductivity (Ks) at 32 locations across the study area(Fig. 1). At each location, measurements were carried out usingthree different experimental devices: (i) a mini-disk infiltrometer;(ii) a double-ring infiltrometer; and (iii) a Guelph permeameter.We performed the measurements in this sequence to minimize sitedisturbance, which allowed us to obtain representative hydraulicconductivities.

3.1.1. Mini-disk infiltrometer (MDI)The mini-disk infiltrometer (Decagon Devices, Pullman WA)

used in this study is a compact, highly portable tension infiltrome-ter. This device has been used in a variety of recent studies to deter-mine the hydraulic conductivity of sediment and soils (e.g., Murrayet al., 2007; Moody et al., 2009; González-Pelayo et al., 2010). Themain tube (water reservoir) of the MDI has a diameter of 3.1 cm.A 4.5-cm diameter stainless steel disk makes contact with the sur-ficial material at the base of the tube. A bubble chamber at the top of

urface elevation contours (meters above sea level) are shown using a 2 m interval.

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M.J. Ronayne et al. / Journal of Hydrology 458–459 (2012) 103–109 105

the apparatus allows for tension control. Since this instrument hasreceived less attention compared to the other measurement tech-niques used in this study, we briefly cover the theory and requiredcalculations for the MDI. The experimental approach for this meth-od involves tracking cumulative infiltration through time, which isassumed to follow the model of Philip (1957):

I ¼ C1t1=2 þ C2t ð1Þ

where I is the cumulative infiltration rate (L), t is time (T), and C1 (L/T1/2) and C2 (L/T) are coefficients whose values can be obtained bycurve fitting. Using numerical simulations to evaluate a broad rangeof material types, Zhang (1997) developed the following relation-ship to estimate hydraulic conductivity from a disk infiltrometerdata set:

K ¼ C2=a2 ð2Þ

where

a2 ¼11:65ðN0:1 � 1Þ exp½7:5ðN � 1:9aw0Þ�

ðar0Þ0:91 ð3Þ

a2 is a dimensionless coefficient, r0 is the radius of the disk (L), andw0 is the applied pressure head at the disk (L). The N and a in Eq. (3)are the van Genuchten moisture retention parameters. We usevalues of 1.41 and 0.02 cm�1, respectively, for N and a, which arerepresentative values for silty material with a significant sand con-tent (Carsel and Parrish, 1988). These values were selected based onthe observed grain-size distribution for till sediment samples col-lected at GLEES (see Section 4). In our discussion of the results forthis measurement method, we consider the sensitivity of the esti-mated hydraulic conductivity to these moisture retention values.

At each measurement location, we placed the MDI at three dif-ferent spots on the ground surface and conducted three separateinfiltration tests. The three spots were less than 60 cm from oneanother and were located inside the area covered by the outer ringduring the subsequent double-ring infiltrometer measurement.Each test was run for 10–15 min. Having three separate measure-ments allowed us to account for variability that may occur at thescale tested by the MDI. For the analysis presented in Section 4,we average the results to obtain a single MDI hydraulic conductiv-ity value at each measurement site. In order to evaluate conditionsnear saturation, we set the tension (w0) to low values ranging from�0.5 cm to �2.0 cm.

3.1.2. Double-ring infiltrometer (DRI)Following data collection using the MDI device, a double-ring

infiltrometer experiment (ASTM International, 2003) was con-ducted at each measurement site. The DRI is designed to forceone-dimensional, downward vertical flow from the inner ring.Water added in the annular space between inner and outer ringsproduces saturation throughout the subsurface region beneaththe rings, thereby minimizing lateral flow in response to pres-sure-head gradients. The DRI experiment is carried out until thereis a constant rate of water loss from the inner ring. From this stea-dy-state flow rate, Qi, the field-saturated hydraulic conductivity isdetermined as follows: Ks ¼ Q i=pr2

i , where ri is the radius of the in-ner ring. This equation is arrived at by assuming one-dimensional,downward vertical flow with a unit hydraulic gradient (Stephens,1996).

We used inner and outer stainless steel rings with diameters of30.5 and 61.0 cm, respectively. Water addition to the inner ringwas measured using a graduated Mariotte tube. Given the minimalsoil presence and high coarse fraction of the till, for some sites itwas a challenge to drive the rings to a depth that provided an ade-quate water seal. Where needed, we sealed the contact usingclayey material brought to the site. At locations where a seal was

needed for the inner ring, we applied the clay to the outside ofthe ring to avoid any effect on the measured infiltration rate. Thetime required to reach a steady-state inflow rate was between 60and 150 min. Following completion of the DRI test, we waited atleast 4 h before removing the rings and setting up for the Guelphpermeameter (GP) experiment. This allowed for complete drainageof remaining water at the surface and prevented any unnecessarydisturbance to the till material.

3.1.3. Guelph permeameter (GP)The Guelph permeameter is a constant-head well permeameter

that has been used extensively to estimate the hydraulic conduc-tivity of soils and near-surface geologic material. Given that theGP requires excavation of a shallow borehole, we conducted thisexperiment last in our sequence of measurements at each test loca-tion. During the GP experiment, a constant water depth is main-tained in the borehole. Unlike the DRI, where the data analysisassumes one-dimensional vertical flow with a unit hydraulic gradi-ent, the model for the GP involves three-dimensional movement ofwater away from the borehole (Reynolds and Elrick, 1985). Thisdifference in underlying conceptual models, as well as the boreholerequired for the Guelph permeameter, has led several previousinvestigators to compare hydraulic conductivity values obtainedusing the DRI and GP (e.g., Gupta et al., 1993; Mohanty et al.,1994; Gupta, 2006). These previous studies were conducted inareas with well developed soils. To our knowledge, no such com-parison has been performed in an alpine or subalpine region char-acterized by coarse-grained surficial material.

At each measurement site, we augered a 5-cm diameter bore-hole within the footprint of the inner ring that was used duringthe DRI experiment. The borehole depths ranged from 10 to15 cm; shallower boreholes at a few measurement locations weredue to the presence of large rock fragments in the subsurface. Weran the GP experiment using two different borehole water depths,ranging from 3 cm to 10 cm, and recorded infiltration volumesuntil steady-state rates could be identified. For our measurementsites and applied water depths, steady infiltration rates wereobserved in less than 30 min. We analyzed our GP experimentaldata using the analytical solution presented by Reynolds and Elrick(1986), which relies on dimensionless shape factors that accountfor the ratio of water depth to borehole radius. This solution re-duces uncertainty in the estimated Ks value by simultaneously con-sidering data from both tests performed with different boreholewater depths.

3.2. Sediment grain size analysis

We conducted laboratory analysis to evaluate the grain-sizedistribution of the glacial till sediment. Samples were collectedfrom three of our field test locations (S-5, S-6, and S-14) followingcompletion of the hydraulic conductivity measurements. Thecollected sample volumes were relatively large (�4000 cm3),which allowed us to capture very coarse gravel-sized particlespresent in the till. Sieve analysis was performed to determine thedistribution of particles larger than 0.05 mm (the USDA thresholdseparating silt-sized from sand-sized particles). Eight sieves wereused to characterize the size range from fine sand to coarse gravel.To identify the silt and clay fraction (<0.05 mm) within each sam-ple, hydrometer measurements were performed (Gee and Bauder,1986).

4. Results and discussion

To illustrate the physical character of the till material, we firstprovide results of the grain-size analyses. Fig. 2 shows grain-size

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106 M.J. Ronayne et al. / Journal of Hydrology 458–459 (2012) 103–109

distribution curves for the three samples. Each curve shows abroad distribution of grain sizes, as would be expected for glacialtill. For this poorly sorted sediment material, there is no represen-tative grain-size diameter (i.e., the till is not dominated by any sin-gle grain-size category). As a consequence, empirical methods thatestimate Ks based on grain size may not give reliable results; thisissue has previously been raised by Jenssen (1990). It is importantto use in situ field characterization techniques when quantifyingglacial till hydraulic properties.

The measured hydraulic conductivities are summarized inTable 1 and Fig. 3. As noted earlier, we attempted to use all threemeasurement techniques at each of our 32 test sites. There werea limited number of sites where we were not able to obtain anaccurate estimate of Ks using the double-ring infiltrometer orGuelph permeameter. With the DRI (three sites), this was causedby site conditions that prevented establishment of an adequateseal at the base of the rings. For the GP, there were five sites wheresteady infiltration rates could not be identified during one of thetwo experimental runs; in these cases, the data set was not clearlyinterpretable and a reliable Ks value could not be obtained. Table 1lists summary statistics for each measurement method. The resultsindicate significant variability in the measured hydraulic conduc-tivities around the site (i.e., Ks is heterogeneous). For each method,the estimated Ks varies by more than an order of magnitude. Thedistribution for the GP has the largest range. Across all three meth-ods, the estimated hydraulic conductivities span more than twoorders of magnitude.

The hydraulic conductivity values estimated using the MDImethod were characteristically lower than those determined usingthe DRI and GP test methods (Fig. 3). The Ks distributions are similarfor the DRI and GP, with arithmetic means of 2.31 m/d and 2.22 m/d, respectively. All three distributions are positively skewed, asevidenced by the position of the median within the lower half ofthe inter-quartile range, the longer tail extending out to the maxi-mum Ks value, and the arithmetic mean that is greater than themedian. As a more reliable measure of central tendency for distri-butions that are approximately lognormal, we include the geomet-ric mean Ks values in Table 1.

As noted in Section 3.1.1, the hydraulic conductivities reportedfor the MDI are calculated using estimates of a and N from the lit-erature. Clearly there is some uncertainty in the values of thesemoisture retention parameters. Carsel and Parrish (1988) reportedstandard deviations of 0.012 cm�1 and 0.12, respectively, for the aand N parameters in silt loams. These statistics were used to calcu-late reasonable ranges for the material that we studied:a = 0.02 ± 0.012 cm�1 and N = 1.41 ± 0.12. We reanalyzed the MDIdata using endpoints of these ranges for the moisture retention

0.001 0.01 0.1 1 10 100

20

40

60

80

100S5S6S14

Perc

ent f

iner

by

wei

ght

Grain diameter (mm)

Fig. 2. Grain-size distribution curves for three glacial till sediment samples.

parameters. Table 2 shows the effect this would have on the esti-mated geometric mean hydraulic conductivity. The highest andlowest values in this table (0.37 m/d and 0.08 m/d) correspond toscenarios where one parameter has its minimum value and theother parameter has its maximum value. Given the known positivecorrelation between a and N, these scenarios may not be plausible,yet we include the calculated results for completeness. This analy-sis shows that the hydraulic conductivities estimated using theMDI are sensitive to the selected moisture retention parameters.However, this effect cannot account for the large consistent differ-ence between the MDI and GP/DRI data sets.

4.1. Influence of measurement technique

The boxplots illustrate the variability in hydraulic conductivityas determined by the three different methods (Fig. 3). As describedearlier, the methods differ in the applied boundary conditions andflow system that develops. The MDI and GP establish three-dimen-sional flow whereas the DRI induces flow that is primarily vertical.Typical of unconsolidated tills, the glacial sediment at GLEES doesnot have any observable layered structure, and therefore theanisotropy for hydraulic conductivity is minimal. As a result, theflow dimensionality shouldn’t have a significant impact on theestimated conductivity. This notion is supported by the similar Ks

distributions obtained using the GP and DRI. The physical setupof the small-scale MDI is a key consideration when comparingthe data sets. Given the size of the instrument and the need toplace it on a level surface away from the coarsest fragments, theMDI appears to preferentially sample the finer-grained (less per-meable) fraction of the till. It therefore makes sense that the result-ing hydraulic conductivities are low. The MDI test may alsoexclude macropores that could contribute to the hydraulic conduc-tivity under more saturated conditions. Although we applied verylow tensions and avoided vegetated areas and areas with a dis-turbed land surface, macropores could still be a factor.

Another potential explanation for the observed differences isrelated to measurement scale. The scale effect for hydraulic conduc-tivity has been documented in a variety of theoretical and fieldstudies (Rovey and Cherkauer, 1995; Schulze-Makuch et al., 1999;Calver, 2001; Neuman and Di Federico, 2003). Although the precisesupport volume for a measurement depends on the instrumentdimensions, boundary conditions established during the hydraulictest, and characteristics of the subsurface material, it is nonethelessuseful to compare the contact areas for the different measurements.For the MDI, the stainless steel disk that contacts the ground surfacehas an area of 15.9 cm2. This compares to an inner-ring area of730 cm2 for the DRI. Although the contact area for the GP is variable,depending on the water depth maintained in the borehole, it isbetween the corresponding areas for the MDI and DRI. Comparedto the MDI, a larger subsurface volume is affected (and thereforecontributes to the measurement) during the GP and DRI tests. Thislarger volume integrates a more varied distribution of grain sizes,including the till coarse fraction, which may partly explain thehigher effective hydraulic conductivity. This analysis providesmotivation for further study to investigate a potential scale effectin alpine glacial tills. Importantly though, our results reveal largedifferences in the estimated hydraulic conductivities at commonlyused scales for surficial measurements.

4.2. Implications for watershed hydrologic response and groundwaterrecharge

In this section we briefly consider the implications of our differ-ent Ks measurements for characterizing hydrologic responsebehavior during the snowmelt season. Depending on the hydraulicconductivity, episodes of precipitation or snowmelt may produce

Page 5: Field characterization of hydraulic conductivity in a heterogeneous alpine glacial till

Table 1Summary statistics of the measured hydraulic conductivities using each method.

n Min(m/d)

Max(m/d)

Ks(A)

(m/d)Ks(G)

(m/d)

Mini-disk infiltrometer 32 0.024 0.62 0.24 0.20Double-ring infiltrometer 29 0.10 5.76 2.31 1.70Guelph permeameter 27 0.045 11.2 2.22 1.35

n = number of samples; Ks(A) = arithmetic mean; and Ks(G) = geometric mean.

6.0

5.0

4.0

3.0

2.0

1.0

0

Ks (

m/d

)

GPMDI DRI

Fig. 3. Boxplots showing the distribution of Ks values obtained using a mini-diskinfiltrometer (MDI), double-ring infiltrometer (DRI), and Guelph permeameter (GP).The open circles identify the arithmetic mean. Whiskers are plotted at theminimum and maximum values for each distribution. The maximum Ks obtainedwith the GP (not shown) is 11.2 m/d.

Table 2Sensitivity of the MDI geometric mean hydraulic conductivity to moisture retentionparameter values.

N

1.29 1.41 1.53

a (cm�1) 0.008 0.13 m/d 0.10 m/d 0.08 m/d0.02 0.27 m/d 0.20 m/d 0.17 m/d0.032 0.37 m/d 0.29 m/d 0.25 m/d

0 50 100 150 200 250 300 3500

0.2

0.4

0.6

0.8

1

1.2 20112007Average

SWE

(m

)Day of water year

(a)

210 220 230 240 250 260 2700

0.05

0.1

0.15

0.2Q

GW2

Rat

e (m

3 /d)

Day of water year 2007

(b)

210 220 230 240 250 260 2700

0.05

0.1

0.15

0.2

QGW1

QGW2

Rat

e (m

3 /d)

Day of water year 2011

(c)

QGW1

Fig. 4. (a) Snow water equivalent (SWE) measured at the Brooklyn Lake SNOTELstation during water years 2007 and 2011. The historical average is based on a 28-year period of record (1983–2010). (b and c) Estimated daily snowmelt rates during2007 and 2011. In order to provide a comparison to groundwater discharge rates,the initial calculated snowmelt rates (m/d) are multiplied by a unit land-surfacearea (1 m2). QGW1 is the calculated groundwater discharge using the average small-scale hydraulic conductivity value. QGW2 is the calculated groundwater dischargeusing a representative larger scale value.

M.J. Ronayne et al. / Journal of Hydrology 458–459 (2012) 103–109 107

runoff (overland flow). Hortonian overland flow occurs when waterencounters the surface at a rate that exceeds the saturated hydrau-lic conductivity. Saturation-excess overland flow occurs when ashallow water table rises to the land surface (Loague, 2010). Animportant factor that influences the potential for saturation-excessoverland flow is the groundwater discharge rate. If the groundwa-ter discharge is high relative to the rate of enhanced recharge pro-vided by precipitation or snowmelt, then the subsurface saturatedzone can act as a drain, effectively carrying away the rechargingwater (Healy and Cook, 2002). When the groundwater dischargeis not significantly higher than the recharge rate, then there isgreater potential for a rapidly rising water table that may producesaturation-excess overland flow.

Fig. 4a shows the snowpack accumulation/depletion curves forthe Brooklyn Lake SNOTEL station (SNOwpack TELemetry stationoperated by the U.S Natural Resources Conservation Service). Thisstation is roughly 3 km southeast of GLEES at an elevation of3121 m. For the purposes of this analysis, we assume that thesedata approximate the snowmelt dynamics that occur at GLEES.The measured data illustrate the snowpack through time as snowwater equivalent (SWE). Daily SWE values are provided for the2007 and 2011 water years. The 2011 water year was noteworthydue to the near doubling of the mean SWE as well as the late snow-melt. The 2007 data show conditions for a more typical year.

From the date of peak SWE accumulation until the snowpack isgone, we estimated the transient snowmelt rates by calculatingthe change in SWE using a daily time step. This approach does notaccount for snowpack sublimation, ablation, or the influence of addi-tional snowfall, which are processes that could be quantified using aphysically-based water and energy balance model. However, ourcalculation provides a reasonable first approximation of snowmeltrates. The calculated rates for the 2007 and 2011 water years areshown in Fig. 4b and c, respectively. The estimated rates of snowmelt

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range from less than 0.01 m/d to 0.056 m/d. During the period ofsnowpack depletion, there are some days with melt rates of zero.This indicates no net snowmelt due to additional snow accumula-tion on that day or the occurrence of air temperatures below freez-ing. Except for two low Ks values (the minimums reported for theMDI and GP in Table 1), the estimated snowmelt rates are signifi-cantly lower than our measured hydraulic conductivities. Thus, wewould generally not expect the snowmelt to produce Hortonianoverland flow.

To evaluate the potential for saturation-excess overland flow,we calculate the rate of groundwater discharge using the measuredhydraulic conductivities. We consider the geometric mean Ks val-ues from the MDI and DRI experiments (Ks(G),MDI and Ks(G),DRI). Asan approximation of the water table gradient, we use the slopeof the land surface along East Glacier Lake outlet stream betweenthe lake and the Parshall flume (Dh/Dl � 0.06; Fig. 1). Based onthe till thickness ranging from 2 to 4 m in this area (Page, 2011),as well as the presence of surface water bodies, we assume a satu-rated thickness of 2 m within the unconfined glacial till sediment.The groundwater discharge rates are estimated using Darcy’s law:

Q GW1 ¼ KsðGÞ;MDIADhDl¼ 0:024m3=d ð4Þ

Q GW2 ¼ KsðGÞ;DRIADhDl¼ 0:20m3=d ð5Þ

where QGW1 is the groundwater discharge calculated with the geo-metric mean Ks value from the MDI, and QGW2 is the groundwaterdischarge calculated with the larger-scale DRI mean value. Thesecalculations assume a unit aquifer width (width normal to thegroundwater flow direction) and therefore A = 2 m2. This assump-tion is only needed so that we can make a direct comparison tothe snowmelt-derived recharge rates; we consider snowmelt overa unit land-surface area.

As shown in Fig. 4b and c, QGW1 is within the range of theestimated snowmelt rates. Thus, a model parameterized with theMDI hydraulic conductivity value would be much more likely to re-sult in the calculation of saturation-excess overland flow that maynot be realistic for some areas. This analysis considers only onearea of the site where the till thickness and water table gradientare reasonably well constrained. Overland flow has been observedduring snowmelt in more steeply sloping areas. However, theabsence of runoff-induced erosion indicates that this effect is local-ized and highly ephemeral.

The above analysis shows the importance of hydraulic conduc-tivity for characterizing hydrologic fluxes in a mountain watershedwith glacial till sediment. Prediction of overland flow means lessgroundwater recharge, which is necessary to sustain stream base-flow in the alpine and subalpine environments.

5. Summary

In this study, we investigated the physical and hydraulic proper-ties of an alpine glacial till in the Wyoming Rocky Mountains. Par-ticle-size analyses confirm that the unconsolidated till sediment isvery poorly sorted with no representative grain size. To character-ize the field-saturated hydraulic conductivity, we conductedin situ measurements using three different experimental devices:(i) a mini-disk infiltrometer; (ii) a double-ring infiltrometer; and(iii) a Guelph permeameter. The measured hydraulic conductivitiesspan more than two orders of magnitude, with most values in therange between 0.05 and 5 m/d. The mini-disk infiltrometer gener-ally gave the lowest estimated hydraulic conductivities. Higherconductivities are obtained using the double-ring infiltrometerand Guelph permeameter. These latter two measurements providemore representative hydraulic conductivities for groundwater

calculations and the estimation of runoff. The geometric mean Ks

values obtained with the GP and DRI, respectively, are 1.35 m/dand 1.70 m/d. These values are several orders of magnitude higherthan the hydraulic conductivities typically reported for continentalglacial tills.

To explore the implications of the variable hydraulic conductiv-ities obtained with different measurements, we considered near-surface hydrologic response behavior during the peak snowmeltperiod. When groundwater discharge is calculated using a low aver-age hydraulic conductivity, the rate of shallow groundwater out-flow is not significantly higher than the snowmelt-derivedrecharge, indicating greater potential for saturation-excess over-land flow. Thus, a low Ks value obtained with the mini-disk infil-trometer or a similar measurement technique might lead to anunrealistic prediction of surface runoff. More broadly, these calcu-lations demonstrate the importance of accurately characterizingthe sediment hydraulic conductivity, particularly when there is ashallow water table. Future numerical modeling efforts will con-sider the multiple processes that control snowpack depletion,which will allow for better estimates of transient snowmelt andgroundwater recharge. The groundwater recharge that suppliesshallow saturated zones is essential for sustaining late-seasonstream baseflow. More accurate quantification of this recharge willfacilitate improved hydrologic understanding and better predictivecapability for mountain watersheds.

Acknowledgments

This work was partially funded by a research and engagementgrant from the Warner College of Natural Resources at ColoradoState University. We thank Joe Orlando for assistance with fieldmeasurements. Logistical support during field work was providedby Bob Musselman of the USDA Forest Service Rocky MountainResearch Station, Fort Collins, CO. This paper benefitted from con-structive suggestions made by the anonymous reviewers.

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