alluvial characteristics, groundwater–surface water exchange and hydrological retention in...

ALLUVIAL CHARACTERISTICS, GROUNDWATER±SURFACEWATER EXCHANGE AND HYDROLOGICAL RETENTION

IN HEADWATER STREAMS

JOHN A. MORRICE,1 H. MAURICE VALETT,1 CLIFFORD N. DAHM1

AND MICHAEL E. CAMPANA2

1Department of Biology, University of New Mexico, Albuquerque, NM 87131, USA2Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA

ABSTRACT

Conservative solute injections were conducted in three ®rst-order montane streams of di�erent geological compositionto assess the in¯uence of parent lithology and alluvial characteristics on the hydrological retention of nutrients. Threestudy sites were established: (1) Aspen Creek, in a sandstone±siltstone catchment with a ®ne-grained alluvium of lowhydraulic conductivity (1�3� 10ÿ4 cm=s), (2) Rio Calaveras, which ¯ows through volcanic tu� with alluvium of inter-mediate grain size and hydraulic conductivity (1�2� 10ÿ3 cm=s), and (3) Gallina Creek, located in a granite/gneisscatchment of coarse, poorly sorted alluvium with high hydraulic conductivity (4�1� 10ÿ3 cm=s). All sites wereinstrumented with networks of shallow groundwater wells to monitor interstitial solute transport. The rate and extent ofgroundwater±surface water exchange, determined by the solute response in wells, increased with increasing hydraulicconductivity. The direction of surface water±groundwater interaction within a stream was related to local variation invertical and horizontal hydraulic gradients. Experimental tracer responses in the surface stream were simulated with aone-dimensional solute transport model with in¯ow and storage components (OTIS). Model-derived measures ofhydrological retention showed a corresponding increase with increasing hydraulic conductivity.To assess the temporal variability of hydrological retention, solute injection experiments were conducted in Gallina

Creek under four seasonal ¯ow regimes during which surface discharge ranged from base¯ow (0.75 l/s in October) tohigh (75 l/s during spring snowmelt). Model-derived hydrological retention decreased with increasing discharge.The results of our intersite comparison suggest that hydrological retention is strongly in¯uenced by the geologic

setting and alluvial characteristics of the stream catchment. Temporal variation in hydrological retention at GallinaCreek is related to seasonal changes in discharge, highlighting the need for temporal resolution in studies of the dynamicsof surface water±groundwater interactions in stream ecosystems. # 1997 by John Wiley & Sons, Ltd.

Hydrological Processes, vol. 11, 253±267 (1997)

(No. of Figures: 7 No. of Tables: 4 No. of Refs: 38)

KEY WORDS transient storage zone; hyporheic zone; hydraulic conductivity; nutrient retention; OTIS;stream ecosystem

INTRODUCTION

Nutrient retention is a fundamental descriptor of ecosystem functioning. In streams, the ¯ux of nutrientsthrough the ecosystem is dominated by advective ¯ow, but a suite of biological, hydrological and chemicalprocesses delay the downstream transport of nutrients. The interplay of these factors determines nutrientretention and cycling. Quantitative measures of retention are useful points of comparison between streamecosystems (Newbold et al., 1981; Elwood et al., 1982; Munn and Meyer, 1990).

In this paper, we focus on hydrological retention. Stream±watershed interactions and stream surfacecomplexities entrain nutrients in ¯owpaths moving at slower velocities than those predicted by advectivetransport in the thalweg of streams. The resulting increase in hydraulic residence time is referred to ashydrological retention. We propose a conceptual model of nutrient retention where, in addition to rates ofbiological and chemical nutrient uptake, nutrient retention within a stream reach is a function of hydraulicresidence time.

CCC 0885±6087/97/030253±15 Received 13 March 1995# 1997 by John Wiley & Sons, Ltd. Accepted 25 July 1995

HYDROLOGICAL PROCESSES, VOL. 11, 253±267 (1997)

Speaker et al. (1984), Trotter (1990), and Ehrman and Lamberti (1992) found that debris dams increasedhydrological residence time and resulted in enhanced retention of particulate organic material. Triska et al.(1989) and Castro and Hornberger (1991) identi®ed interstitial zones as a signi®cant source of hydrologicalretention. The region of mixing between groundwater and surface water has been termed the hyporheiczone (sensu Orghidan, 1959) and has been identi®ed as a region of intensi®ed biogeochemical activity(Grimm and Fisher, 1984; Du� and Triska, 1990; Triska et al., 1993). Flowpaths through the hyporheic zonehave been delineated by measuring temperature gradients (White et al., 1987) and by detailed measurementsof pressure head (Harvey and Bencala, 1993). Biogeochemical conditions in the hyporheic zone may di�ergreatly from the surface waters of a stream (Triska et al., 1989, Valett et al., 1996) with importantimplications for the transformation and retention of biologically active solutes.

Solute transport models with transient storage terms have been applied to data from conservative soluteinjection experiments to provide a quantitative measure of hydrological retention (Bencala and Walters,1983). This quantitative approach may be applied to intersite comparisons of hydrological retention.Jackman et al. (1984) and Bencala et al. (1993) contend that hydraulic storage in the stream bed, orhyporheic zone, is the primary source of model-derived transient storage. Legrand-Marcq and Laudelout(1985) and D'Angelo et al. (1993) observed decreases in transient storage with increasing discharge andD'Angelo et al. (1993) found that storage was lower in stream reaches that were geomorphically constrainedthan in those that were unconstrained. While interstitial ¯owpaths are recognized as a source of hydrologicalretention in stream ecosystems, the e�ects of variation in catchment lithology and sediment characteristicson retention are not well known.

In this study, we investigate the spatial and temporal patterns of hydrological retention in three head-water streams with di�ering parent lithologies and alluvial types. We describe the rate and extent ofsurface water±groundwater interaction in the streams and alluvial aquifers and provide model parametersto quantify transient hydrological storage. We use this information to address the following questions.(1) How does hydrological retention vary between streams with di�ering alluvial hydraulic conductivities?(2) What hydrological features create heterogeneity in surface water±groundwater interactions within astream reach? And, (3) how does hydrological retention respond to variation in the annual hydrograph ofa stream?

STUDY SITES

Experiments were conducted on three headwater streams in western and north-central New Mexico, USA.The streams are in catchments with di�erent parent lithologies in which the grain size and hydraulicconductivity of alluvial sediments vary greatly.

Aspen Creek is located in the Zuni Mountains of western New Mexico. The catchment lies in Permiansandstone and siltstone of the Yeso Formation (Dane and Bachman, 1965; Stanesco, 1992). The meanparticle size distribution of alluvium at Aspen Creek is: 2% gravel, 46% sand, 42.5% silt, 9.5% clay (J. Klug,University of Wisconsin, unpublished data). The mean hydraulic conductivity is the lowest of the three sites.The catchment size is 322 ha, the overall stream gradient is 2.0% and the elevation at the lower end of theexperimental reach is 2377 m.Rio Calaveras is in the Jemez Mountains of north-central New Mexico; it lies on the western ¯ank of the

Valles Caldera at 2475 metres elevation. The catchment size is 3760 ha and the average stream gradient is1.3%. Alluvial sediments are weathering products of the Bandelier tu� produced in the eruption of theValles Caldera 1.2 million years ago (Dane and Bachman, 1965; Go� et al., 1988). The mean particle sizedistribution of alluvium at Rio Calaveras is: 36% gravel, 53% sand, 9% silt, 2% clay (J. Klug, University ofWisconsin, unpublished data). The mean hydraulic conductivity of the alluvium at Rio Calaveras isintermediate amongst the three sites.

Gallina Creek, with a catchment size of 618 ha and an average stream gradient of 11.1%, is located at2524 m elevation in the Sangre de Cristo mountains of north central New Mexico. The parent lithology isgranite/gneiss (Dane and Bachman, 1965). The alluvium at Gallina Creek is poorly sorted boulders, cobble,gravel and sands with the highest hydraulic conductivity of the three sites.

254 J. A. MORRICE ET AL.

METHODS

Field Methods

A network of sampling wells and piezometers was installed at each site. At Aspen Creek and RioCalaveras, wells were placed along ®ve transects perpendicular to the stream. The experimental reachlengths of Aspen Creek and Rio Calaveras are 175 m and 110 m, respectively. At each transect, onesampling well was placed in the stream and wells were placed on both sides of the stream at distances of 1and 3 m from the active channel. Piezometers were placed in the same con®guration 5m upstream from thesampling wells, except that piezometers within the stream and 1 m from the active channel were verticallynested. In October and November 1992, additional piezometer nests were installed in the ¯oodplains ofAspen Creek (14 pairs) and Rio Calaveras (16 pairs) to increase the resolution of potentiometric maps. AtGallina Creek, 13 wells were installed along the 190 m experimental reach. Constrained geomorphology andthe large size of alluvial material at this site made it impractical to establish a well network con®gurationsimilar to Aspen Creek and Rio Calaveras. At this site, wells were installed in the alluvial ¯oodplain atdistances ranging from 0.3 to 4m from the active channel. At each site, wells, stream channel and valleyslope breaks were surveyed and mapped.

Wells and piezometers were constructed from 5 cm (internal diameter) PVC pipe and slotted PVC wellscreen (254 mm or 0.01 inches). The screen length for piezometers and sampling wells was 15 and 50 cm,respectively. Boreholes for sampling wells were dug to ca. 60 cm beneath the water table at base¯ow toensure that the entire well screen was in the saturated zone. Piezometers were placed ca. 50 cm beneath thewater table. For the piezometer nests, a second piezometer was installed 50 cm beneath the ®rst in order todetermine vertical hydraulic gradient. Discharge was measured using cutthroat ¯umes (Baski Inc.) duringlow ¯ow conditions and by tracer dilution techniques (Stream Solute Workshop, 1990) during highdischarge.

Hydraulic conductivity was determined by slug or bail tests (Hvorslev, 1951). Slug tests were used in wellswhere the entire screen was submerged. Conversely, in wells where the water table was beneath the top of thescreen, bail tests were used. Comparative studies on speci®c wells indicated that the two proceduresproduced similar results (J. N. Morrice, University of Nevada, unpublished data). Tests were conducted on15 wells at Aspen Creek, 36 wells at Rio Calaveras and 7 wells at Gallina Creek. Geometric means ofhydraulic conductivity were calculated for each site and a one-way ANOVA on the log-transformed datacombined with a Bonferroni multiple comparison test (Sokal and Rohlf, 1981) was used to test fordi�erences between sites.

Well hydraulic heads were measured manually with a portable water level sensor (Solonist Inc.).Potentiometric maps were produced from well-head data by kriging using geostatistical software (SURFER,Golden Software 1989). Installation of additional wells in October and November 1992 greatly enhanced theresolution of potentiometric maps. Maps presented in this paper are generated from well-head data fromlong-term solute injections combined with data from the additional wells obtained on dates in 1993 whenhydraulic heads were similar to levels during the injection experiments (G. J. Wroblicky, unpublished data).

Vertical hydraulic gradients were determined from the di�erence in hydraulic head between pairs ofvertically nested piezometers. A positive vertical hydraulic gradient indicates a region of groundwaterdischarge to the stream. A negative gradient indicates a region of groundwater recharge. Vertical hydraulicgradients were determined from piezometer nests beneath the active channel and 1m perpendicular to thewetted channel.

Solute Injections

Solute injection experiments were used to characterize the surface hydrology and groundwater±surfacewater interactions at each site. A concentrated solution of sodium bromide (NaBr) was added continuouslyto the stream at a station above the experimental reach. Bromide, a biologically and chemically conservativeion (Bowman, 1984) occurring at low background concentrations (550 mg=l), was continuously injectedinto the surface waters of the three study streams following procedures outlined previously (Triska et al.,1989).

255STREAM HYDROLOGICAL RETENTION

Sampling intervals for surface and well waters during the injection were designed to characterize thebreakthrough curve of bromide-labelled water. Samples were collected in acid-washed polyethylene bottles.Well samples were obtained with a bailer and the volume of water withdrawn from wells was kept to aminimum (5500 ml) to avoid disruption of groundwater dynamics. Surface samples were collected directlyfrom the stream. Samples were immediately ®ltered in the ®eld (Whatman GF/F ®lters) and stored on iceuntil returned to the laboratory.

Table I. Dates and duration of solute additions for tracer injection experiments. Samplingperiods include background sampling prior to the start of tracer addition and the sampling

period after solute addition was stopped

Site Sampling period Duration of tracer injection

Aspen Creek 6/9/92 to 6/12/92 48 hours9/12/92 to 10/1/92 10 days

Rio Calaveras 7/14/92 to 7/17/92 48 hours8/10/92 to 8/20/92 7 days

Gallina Creek 10/3/92 to 10/17/92 7 days2/3/93 to 2/4/93 8 hours6/9/93 8 hours8/10/93 to 8/18/93 6 days

Short-term injections (i.e. 48 h) were employed for intensive sampling of tracer in surface water underbase¯ow conditions at Aspen Creek and Rio Calaveras (Table I). Long-term (i.e. 7±10 days) injections wererun at Aspen Creek and Rio Calaveras for analysis of subsurface tracer distribution and surface water±groundwater interaction. Four solute injection experiments were conducted at Gallina Creek under varyingdischarge conditions. The duration of solute injections at Gallina Creek was determined on the basis ofstream discharge. Dates for the Gallina Creek injection experiments were selected to represent seasonalvariation in the stream's hydrograph. We considered base¯ow to be the lowest point on the annualhydrograph. Unlike Aspen Creek and Rio Calaveras, where base¯ow is reached in early and mid-summerrespectively, snowmelt in the Gallina Creek basin occurs in early summer and discharge does not reachbase¯ow until late summer or early autumn.

Bromide concentration was determined by ion chromatography on a Dionex DX-100 (detection limit10 ppb). Standards were rerun periodically to ensure analytical precision and accuracy.

Data Analysis

Well analysis. Analysis of bromide breakthrough curves in sampling wells provides a measure of both theextent and rate of surface water±groundwater exchange (Triska et al., 1989). Once corrected for back-ground, steady-state concentration of conservative tracer in a well can be compared with steady-stateconcentration in surface water to determine hydrological sources of subsurface water. The wells were placedin three categories based on their degree of connection with surface water as determined by tracer experi-ments. Phreatic, or groundwater, wells had less than 10% surface water at steady state. Moderatelyconnected wells had between 10 and 50% surface water and wells with greater than 50% surface water weredesignated as highly connected.

Spatially explicit subsurface ¯owpaths are not delineated by this method, therefore it is not possible todetermine interstitial velocities. The rate of exchange between surface water and groundwater is expressed asthe nominal travel time (i.e. the time required for the bromide concentration to reach 50% of the eventualsteady-state concentration). Short travel times indicate high interstitial velocities and/or short ¯ow pathsbetween stream surface and sampling well locations.

Nominal travel time and proportion of surface water cannot be determined for wells where tracerconcentrations have not reached steady state. For example, while bromide appeared in all wells during the


injection experiment in June 1993 in Gallina Creek, the duration of tracer addition (8 hours) was insu�cientfor bromide concentrations to reach equilibrium in many wells. In these circumstances, we used the slope ofthe tracer response (proportion of surface tracer in well/time post-injection) as an alternative measure of therate of exchange between surface and interstitial water. Di�erences in slopes between high ¯ow and base¯owdischarges were compared using a Wilcoxon signed rank test (Sokal and Rohlf, 1981).Solute transport modelling. A one-dimensional transport model with groundwater in¯ow and solute storagewas used to simulate tracer transport in stream surface waters. We used the numerical code OTIS (Runkeland Broshears, 1991), which is based on the transient storage model presented by Bencala and Walters(1983). The model treats the stream as a two-compartment system, a main stream channel and a storagezone. Advection and dispersion govern solute transport in the main stream channel. Solute is transferredbetween the main stream channel and storage zone by exchange that is proportional to a concentrationgradient. There is no net movement of solute in the storage zone, which functions as a homogeneously mixed`dead zone' (Bencala and Walters, 1983). OTIS solves the following coupled di�erential equations:

@C

@t� ÿ Q

A

@C

@x� 1

A

@

@xAD

@C

@x

� �� qLIN

A�CL ÿ C � � a�CS ÿ C �

dCS

dt� a

A

AS�C ÿ CS�

where C is the solute concentration in the stream (M/L3), A the cross-sectional area of the stream channel(L2), Q the stream discharge (L3=T), D is the stream dispersion coe�cient (L2=T), qLIN is lateral in¯ow rate(L3=�T�L��, CL is the solute conc. in lateral in¯ow (M/L3), a is the storage zone exchange coe�cient (Tÿ1),CS is the storage zone solute conc. (M/L3), and AS is the storage zone cross-sectional area (L2). Given adischarge value at the head of the reach and distances to sampling sites, the parameters are iterativelyadjusted to produce a visual `best ®t' (Figure 1) of the model simulation to the bromide response from theinjection experiments (Bencala and Walters, 1983; Stream Solute Workshop, 1990; D'Angelo et al., 1993).The ®t of the model simulation to the rising and falling limbs of the tracer response were of primary concern,with particular attention paid to the regions indicated by points labelled à' and `b' on Figure 1. Point à'shows the location on the rising or falling limb where the tracer response deviates from the advection±dispersion curve, which is simulated by manipulating a (Stream Solute Workshop, 1990). Point `b' shows theregion of tailing of the tracer response curve as concentrations approach steady state on the rising or falling


Figure 1. OTIS model simulation of bromide response at a sampling station 40m downstream of the injection site during autumnbase¯ow injection at Gallina Creek. The solid line is the simulated response, crosses are observed values. Point à' and region `b' of the

tracer breakthrough curve are simulated by manipulating a and As , respectively (Stream Solute Workshop, 1990)

limb. In model simulations the extent of tailing is controlled by manipulating As (Stream Solute Workshop,1990). Hydraulic residence times in the stream (T str � 1=a) and in the storage zone (T sto � As=Aa) arederived from model parameters (Mulholland et al., 1994). Standardized storage zone area, the ratio ofstorage zone cross-sectional area to stream cross-sectional area (As/A), is mathematically equivalent to theratio of storage zone residence time to stream residence time. As/A is often used as a comparative measure ofstorage zone size (Stream Solute Workshop, 1990; Broshears et al., 1993, D'Angelo et al., 1993). Hydraulicuptake length in the stream surface (Shyd � Q=Aa) is the average distance a water molecule travels beforeentering the storage zone (Mulholland et al., 1994).

The hydrological retention of solutes is determined by the distance of downstream transport before beingentrained in a storage zone (Shyd) and the duration of storage (Tsto). We introduce the hydrological retentionfactor (Rh � T sto=Shyd), which quanti®es storage zone residence time of water per unit of stream reachtravelled. Rh is an index of retention that considers the relationship between instream transport and thehydrodynamics of the storage zone and facilitates comparisons of hydrological retention between streams.Three solute injections were performed to assess the hydrological retention under base¯ow conditions.

Model simulations were ®tted to tracer data from the June 1992 injection experiment at Aspen Creek for thethree upstream transects. Tracer responses at the three downstream transects were simulated for the July1992 injection experiment at Rio Calaveras. Model simulations of tracer response during the October 1992injection at Gallina Creek were made at ®ve locations. Comparisons between streams under base¯owconditions were based upon mean values for the model parameters.

For seasonal comparisons at Gallina Creek, model simulations were ®tted to tracer responses at a singlelocation downstream of the injection site. The distance between the injection and sampling site was 40 mduring the spring, summer and autumn injections. Deep snow cover made it necessary to relocate theinjection site 17 m upstream during the winter injection, resulting in a 57 m reach length. The relationshipbetween As/A and discharge (Q) was quanti®ed by calculating the Pearson correlation coe�cient (Sokal andRolf, 1981).

RESULTS

Aspen Creek

Discharge and average surface velocity during the June 1991 injection experiment at Aspen Creek arereported in Table II. The mean hydraulic conductivity of the alluvial sediments at Aspen Creek (Table II)was lowest of the three sites (Bonferroni multiple comparison, p < 0�05).During the twenty-day September 1992 experiment, 7 of 25 wells at Aspen Creek contained greater than

10% surface water as determined by tracer concentration (Figure 2). All but one of the connected wells wasin a location where negative vertical hydraulic gradients and/or horizontal ¯ow paths oriented away fromthe stream resulted in in®ltration of tracer-labelled surface water into subsurface environments (Figure 2).Conversely, positive vertical hydraulic gradients and/or horizontal ¯ow paths oriented towards the streamrestricted labelled surface water to the main channel near the upper two transects (Figure 2). Two wells with50% surface water were beneath the wetted perimeter of the stream. Average surface water in connectedwells was 47% (SE � 13�4%, Figure 3b). Nominal travel times to connected wells ranged from less than oneday (23 hours) to more than nine days (217 hours) and averaged 122.4 hours (SE � 31�2 hour, Figure 3a).


Table II. Hydrological characteristics of the study sites under base¯ow conditions

Aspen Creek Rio Calaveras Gallina Creek

Parent lithology Sandstone/siltstone Pumiceous tu� Granite/gneissAverage stream gradient (%) 2.0 1.3 11.1Hydraulic conductivity (cm/s) 1�3� 10ÿ4 1�2� 10ÿ3 4�0� 10ÿ3Discharge (l/s) 1.5 2.0 0.75Average surface velocity (m/s) 0.15 0.067 0.015

OTIS model simulations were ®tted to tracer data from the June 1992 injection experiment (Table III,Figure 4) indicated that model parameters were the same for all transects analysed. The dispersioncoe�cient (D) was 0.05m2/s and the storage zone exchange coe�cient (a) was 4�0� 10ÿ5 sÿ1. Storage zonecross-sectional area (As) was 8�0� 10ÿ4 m2 and stream cross-sectional area (A) was 0.01 m2.

Rio Calaveras

The mean hydraulic conductivity of the alluvial sediments at Rio Calaveras (Table II) was lower thanGallina Creek and higher than Aspen Creek (Bonferroni multiple comparison, p < 0�05). Average streamdischarge and average surface water velocity during the July injection experiment are listed in Table II.Discharge averaged 1.7 l/s during the August 1992 injection.

During the ten-day August 1992 experiment, bromide levels in 8 of 26 wells indicated greater than 10%surface water content (Figure 5). All of the connected wells, except one, were in the lower half of the


Figure 2. Potentiometric map of Aspen Creek with near-stream vertical hydraulic gradients. Equipotentials are measured in metresabove an arbitrary datum. Distribution of conservative tracer in the hyporheic zone of Aspen Creek at ®ve sampling transects (a±e)

during base¯ow as measured in 25 sampling wells


Figure 3. (a) Mean percent surface water and (b) nominal travel time in hyporheic wells (wells with >10% surface water) duringbase¯ow long-term conservative solute injection experiments. Error bars are standard errors of the mean

Table III. Mean parameters from OTIS model simulations of conservative tracer response at base¯ow

Aspen Creek Rio Calaveras Gallina Creek

D (m2/s) 0.05 0.05 0.05a (sÿ1) 4�0� 10ÿ5 5�8� 10ÿ5 6�1� 10ÿ5As (m

2) 8�0� 10ÿ4 3�0� 10ÿ3 2�3� 10ÿ1A (m2) 0.01 0.03 0.05As/A 0.08 0.10 4.6Storage zone residence time (min) 33.3 28.7 1257Stream residence time (min) 417 287 273Hydraulic uptake length (m) 3753 1154 246Retention factor (s/m) 0.53 1.50 306

Figure 4. Comparison of OTIS model simulations of conservative tracer responses 40m downstream of injection sites at Aspen Creek,Rio Calaveras and Gallina Creek during base¯ow. Model parameters used in the simulations are means from ®ve transects modelled

for Rio Calaveras and Gallina Creek and three transects modelled for Aspen Creek

experimental reach. Four wells were highly connected (450% surface water). Similarly to Aspen Creek,hydrologically connected wells were located in regions where hydraulic gradients were oriented away fromthe stream (Figure 5). Average surface water content in connected wells was 59.6% (SE � 12�9%,Figure 3b). Nominal travel times to connected wells ranged from 1.9 hours to more than ®ve days(> 176 hours) and averaged nearly four days (88.8 hours, SE � 21�6 hour, Figure 3a)

OTIS model simulations were ®tted to tracer data from the July 1992 injection experiment (Table III,Figure 4) when the dispersion coe�cient was 0.05m2/s at all transects and the storage zone exchangecoe�cient (a) ranged from 5�5� 10ÿ5 to 6�5� 10ÿ5 sÿ1. The storage zone cross-sectional area (As) was3�0� 10ÿ3 m2 and the stream cross-sectional area (A) was 0.03m2 at all three transects.

Gallina Creek

The hydraulic conductivity of the alluvium at Gallina Creek (Table II) was the highest of the three sites(Bonferroni multiple comparison, p < 0�05). Average discharge and surface water velocity during theOctober 1992 injection experiment are reported in Table II.

All 13 wells contained greater than 10% percent surface water (Figure 6). Eight wells had 450% surfacewater. Nominal travel times to connected wells ranged from 16 to 74 hours and averaged less than two days(38.4 hours, SE � 4�8 hour, Figure 3a). Average surface water in connected wells was 65.1% (SE � 7�3%,Figure 3b).


Figure 5. Potentiometric map of Rio Calaveras with near-stream vertical hydraulic gradients. Equipotentials are measured in metresabove an arbitrary datum. Distribution of conservative tracer in the hyporheic zone of Rio Calaveras at ®ve sampling transects (a±e)

during base¯ow as measured in 26 sampling wells

OTIS model simulations were ®tted to bromide data from the October 1992 injection experiment(Table III, Figure 4). The dispersion (D) was 0.05m2/s at all transects. The storage zone exchange coe�cient(a) ranged from 5�0� 10ÿ5 to 8�0� 10ÿ5 sÿ1: The storage zone cross-sectional area (As) ranged from1�8� 10ÿ1 m2 to 3�0� 10ÿ1 m2 and the stream cross-sectional area (A) ranged from 0.05 to 0.06 m2.

The storage zone residence time was longest at Gallina Creek (1257 minutes), shortest at Rio Calaveras(28.7 minutes) and of comparable duration at Aspen Creek (33.3 minutes) (Table III). The stream residencetime decreased with increasing hydraulic conductivity and alluvial grain size. The ratio of storage zoneresidence time to stream residence time (As/A) ranged from 4.6 at Gallina Creek to 0.08 at Aspen Creek(Table III). Hydraulic uptake length was longest at Aspen Creek (3753 m), of intermediate magnitude at RioCalaveras (1154 m) and shortest at Gallina Creek (246 m). The hydrological retention factor (Rh) increasedwith increasing hydraulic conductivity (Table III).

Comparisons of conservative tracer injections at Gallina Creek reveal seasonal patterns in groundwater±surface water exchange, stream hydrology and transient storage. Slopes of the leading edge of the tracerresponse curves (i.e. change in [Brÿ] vs. time) in eight wells were compared at high ¯ow and low ¯ow. Slopesduring the June 1993 injection (high ¯ow, 0�051+ 0�015; x+SE) were signi®cantly greater ( p � 0�05,


Figure 6. Potentiometric map of Gallina Creek. Equipotentials are measured in metres above an arbitrary datum. Distributionof conservative tracer in the hyporheic zone of Gallina creek at ®ve sampling transects (a±e) during base¯ow as measured in

13 sampling wells

Wilcoxon signed rank test) than during the October 1992 base¯ow injection (0�019+0�007; x+SE)indicating more rapid exchange between surface water and groundwater at high discharge.

Discharge ranged over two orders of magnitude and velocity varied 14-fold during four injections atGallina Creek (Table IV). OTIS model simulations for these injections are plotted in Figure 7. Thedispersion (D) was maximal during spring runo� when discharge was 75 l/s (Table IV), but varied littleduring other injections. The storage exchange coe�cient (a) varied from a low of 6�1� 10ÿ5 sÿ1 in autumnto a high of 5�0� 10ÿ4 sÿ1 during the winter experiment. The stream cross-sectional area (A) increased withincreasing stream discharge. Conversely, the storage zone area (As) decreased with increasing discharge. Thestorage zone residence time decreased as discharge increased and stream residence time was lowest in winterand greatest during base¯ow. The standardized storage zone area (As/A) decreased with increasingdischarge. Hydraulic uptake lengths were similar during summer, autumn and winter but increased duringspring runo�. The hydrological retention factor (Rh) decreased with increasing discharge. Following inversetransformation of discharge, the standardized storage zone area was signi®cantly correlated with discharge(As/A: r � 0�99; n � 4; P � 0�006).

DISCUSSION

Two approaches to quantifying hydrological retention were employed in this study. Analysis of tracerresponse in wells provides point-speci®c measurements of hydrological retention at the groundwater±surface water ecotone. Individual wells reveal the rate and extent of penetration of surface water into thealluvial aquifer at speci®c locations. The distribution of conservative tracer responses in our well net-works provides a qualitative measure of hydraulic retention along a stream reach speci®c to surface


Figure 7. OTIS model simulations of conservative tracer responses in Gallina Creek. Experiments were conducted under varyingdischarge regimes. Simulations of tracer responses 40m downstream of the injection site are compared

Table IV. Mean parameters from OTIS model simulations of conservative tracer responses in Gallina Creek undervarying discharge regimes

Autumn Summer Winter Spring

Q (l/s) 0.75 2.0 15.0 75v (m/s) 0.015 0.025 0.11 0.21D (m2/s) 0.05 0.08 0.05 0.40a (sÿ1) 6�1� 10ÿ5 1�2� 10ÿ4 5�0� 10ÿ4 2�0� 10ÿ4As (m

2) 2�3� 10ÿ1 1�3� 10ÿ1 1�0� 10ÿ1 4�0� 10ÿ2A (m2) 0.05 0.08 0.14 0.36As/A 4.6 1.63 0.71 0.11Storage zone residence time (min) 1257 227 23.7 9.2Stream residence time (min) 273 139 33.3 83Hydraulic uptake length (m) 246 209 220 1050Retention factor (s/m) 306 64.8 6.4 0.53

water±groundwater exchange. In contrast, the OTIS model provides an integrated, reach-scale measure ofhydrological retention. Together these methods are a strong basis for comparing hydrological retention.Hydrological retention attenuates the advective transport of water and dissolved nutrients. Results from thisresearch suggest that hydraulic transport and the associated retention of nutrients are a�ected by thehydrogeological characteristics of the stream and alluvial aquifer. The hydraulic conductivity of alluvialsediments, the direction and magnitude of horizontal and vertical hydraulic gradients, the stream gradientand the magnitude of discharge all contribute to organizing both surface and interstitial ¯ow andhydrological retention in lotic ecosystems.

While the need for a catchment-scale perspective of streams and their linkage to groundwater has beenrecognized (Hynes, 1983; Bencala, 1993), few studies have addressed di�erences in groundwater±surfacewater exchange between catchments. Kelson and Wells (1989) found that, in catchments with di�erentparent lithologies, the amount of surface runo� varied with the storage properties of the associated alluvialaquifers. Hydraulic storage in alluvium derived from crystalline parent material was higher than in alluviumderived from sedimentary parent material (Kelson and Wells, 1989). Alluvium in a stream valley resultsfrom weathering of parent geological material throughout the catchment and the transport and depositionof ¯uvial materials within the ¯oodplain. Di�erences in the hydrogeological characteristics of alluvium willdirectly in¯uence groundwater±surface water exchange.

Hydraulic conductivity exhibits great spatial heterogeneity within many aquifers (e.g. Sudicky, 1986) andcaution should be used in assuming an average hydraulic conductivity. While hydraulic conductivity isspatially variable within our sites, mean hydraulic conductivities di�ered signi®cantly between study sitesand provided a reasonable basis for establishing catchment-level di�erences between the streams. Our resultssupport the contention that average hydraulic conductivity of the alluvium is a catchment-scale determinantof the rate and extent of groundwater±surface water exchange.

It is logistically intractable to replicate these tracer experiments at the catchment scale, therefore, astatistically rigorous test of the hypothesis that, across basins, hydraulic conductivity controls the nature ofsurface water±groundwater exchange cannot be made (see Hurlbert, 1984). However, the observed trend ofincreased rate and extent of groundwater±surface water exchange with increased hydraulic conductivityfollows logically from the predictions of Kelson and Wells (1989) who showed that variation in runo� andstorage between catchments corresponds to di�erences in parent lithology and alluvial characteristics.

The patchiness of groundwater±surface water exchange within catchments is related to local variations invertical and horizontal hydraulic gradients. Others have found that surface water entered the subsurfacewhere the stream gradient increased, at the head of ri�es, and re-entered the stream where the streamgradient decreased, at the transition from ri�e to pool (Vaux, 1968; White et al., 1987; Harvey and Bencala,1993). The stream beds of Aspen Creek and Rio Calaveras do not have pronounced stair step geomorph-ology. In both streams, however, there is a shift from predominantly positive or neutral vertical hydraulicgradients in the upper reach to more negative vertical gradients in the lower reach (Figures 2 and 5; Henryet al., 1994). In addition, our water table maps of Aspen Creek and Rio Calaveras suggest lateral ¯owtowards the active channel in the upper reaches and ¯ow away from the channel in the lower reaches atbase¯ow. At both sites, hydrologically connected wells are concentrated in reaches where vertical hydraulicgradients are negative and/or horizontal gradients are directed away from the stream. In contrast, all wells inGallina Creek were hydrologically connected with the stream.

Solute transport models with dead zone storage provide a reach-averaged, quantitative measure ofhydrological retention in streams (Bencala et al., 1993). The ratio As=A standardizes the storage zone size tostream cross-section, allowing interstream comparison of hydrological retention. Broshears et al. (1993)summarized modelling results from several mid-order mountain streams and reported a 100-fold variationin standardized storage zone area between streams. Recent studies have sought to identify the features ofstream ecosystems that in¯uence and organize transient storage. D'Angelo et al. (1993) examined therelationship between stream order, reach geomorphology and dead zone storage. They reported an overalldecrease in storage zone area with increasing stream order and found that storage zone area was greater inunconstrained stream reaches than in constrained reaches. The authors hypothesized that storage zone areaincreases with increasing instream channel complexity. Mulholland et al. (1994) observed a positive


relationship between transient storage and algal biomass in laboratory streams. These studies focus on thein¯uence of surface features of streams on transient storage. In addition, Bencala andWalters (1983) suggestthat interstitial ¯ow paths are a source of dead zone storage. Hydrological retention may result from any ofthese mechanisms, the relative contribution of the di�erent sources of retention to dead zone storageprobably varies between streams.

Our research focuses on features of stream ecosystems that are likely to in¯uence subsurface dead zonestorage, in particular hydrogeologic characteristics of the near-stream alluvium. We hypothesize that theaverage grain size and hydraulic conductivity of the near-stream alluvium strongly in¯uence dead zonestorage. In our study sites, hydrological retention, as measured by ®tting the OTIS model to conservativetracer injection data, varied predictably between geological types.

At Gallina Creek, the high hydraulic conductivity site, the storage zone exchange coe�cient (a) andstandardized storage zone area (As/A) were the highest of the three streams. Storage zone residence timeswere longest and stream residence times were shortest. The average stream velocity was low, resulting in theshortest hydraulic uptake length. These results indicate extensive interaction between the advective streamand storage zone and long residence times within the storage zone.Aspen Creek, with the lowest mean hydraulic conductivity, had the lowest storage zone exchange

coe�cient (a) and standardized storage zone area (As/A) of the three sites. Stream residence time andhydraulic uptake length were longest at this site. At Aspen Creek the frequency of the stream±storage zoneinteraction, as measured by hydraulic uptake length, is lower but the duration of storage is longer than atRio Calaveras.

On the reach scale, hydrological retention is a function of the frequency of interaction between stream andstorage zone, de®ned by hydraulic uptake length, and the residence time in the storage zone. Thehydrological retention factor (Rh) integrates these elements by expressing hydrological retention as storagezone residence time per metre of stream reach travelled by stream water in the surface before entering thestorage zone. The factor Rh for Gallina Creek at base¯ow was 577 times greater than for Aspen Creek underbase¯ow conditions. The retention factor at Rio Calaveras was three times as high as that at Aspen Creek.While the di�erences between Aspen Creek and Rio Calaveras are smaller, results follow a trend of increasedhydrological retention with increasing hydraulic conductivity of the alluvium. Seasonal changes in the levelof the water table and stream discharge modify groundwater±surface water interactions and hydrologicalretention in streams (Hinton et al., 1993). D'Angelo et al. (1993) observed a signi®cant negative relationshipbetween discharge or velocity and standardized transient storage in several streams with di�erent discharges.Many of the uncertainties inherent in a cross-system approach to assessing the in¯uence of discharge ontransient storage are controlled by studying a single stream across its annual hydrological regime. Legrand-Marcq and Laudelout (1985) measured transient storage in a forested stream over a two order-of-magnituderange of discharge. They found that as discharge increased, transient storage decreased rapidly towards anasymptotic value. We conducted experiments on Gallina Creek under a similar range of discharge. Thesigni®cant relationship between As/A and the inverse of discharge is consistent with the ®ndings of Legrand-Marcq and Laudelout (1985).

The frequency of stream±storage zone exchange, as measured by hydraulic uptake length, did not varygreatly between autumn, summer and winter. Stream residence time decreased and average stream velocityincreased with increasing discharge resulting in a relatively constant hydraulic uptake length. However, thehydraulic uptake length did increase markedly during spring runo�, suggesting that a discharge thresholdexists above which the in¯uence of surface±aquifer exchange on solute dynamics becomes less signi®cant.The storage zone residence time decreased with increasing discharge and the hydrological retention factordecreased 577-fold with increasing discharge from base¯ow to snowmelt. These results suggest thatdischarge-related variation in hydrological retention at Gallina Creek is related to both changes in storagezone residence time and hydraulic uptake length.We hypothesize that saturation of the hillslope alluvium and a rising water table create strong hydraulic

gradients toward the stream during high discharge periods in Gallina Creek. The storage zone cross-sectional area (As) decreased with increasing discharge suggesting a constricted zone of groundwater±surface water mixing. However, an increase in a, the storage zone exchange coe�cient, and an increased rate


of surface water±groundwater mixing observed in wells during high discharge, suggest that while the extentof the groundwater±surface water ecotone is reduced at high discharge, the rate of exchange is enhancedresulting in lower hydraulic residence times.Bencala (1993) presented two contrasting perspectives of stream±catchment interaction. In the ®rst, the

stream is portrayed as a conduit receiving water from the catchment but not functioning as an interactivecomponent of a watershed ecosystem. With this perspective, stream channel processes account for allnutrient retention occurring in the ¯uvial system. In the second perspective, the stream is described as aninteractive component of a stream±catchment continuum. Nutrient retention results from a complex suite ofprocesses occurring in both the stream channel and groundwater environments. While most streams are bestdescribed by the second perspective, this research suggests that the degree of stream±catchment interactionand the associated retention of nutrients vary widely and, perhaps, predictably between and within streams.Local climate determines the timing of hydrological inputs and together with the geological features of acatchment, particularly alluvial hydraulic conductivity, provides a template that strongly in¯uences theinteraction between surface water and groundwater and hydrological retention in stream ecosystems.

ACKNOWLEDGEMENTS

We thank Kevin Henry, Francelia Lieurance, Michelle Baker, Joseph Morrice, Jen Klug and Phoebe Suinafor their tireless assistance in the ®eld and laboratory. Greg Wroblicky provided pieziometric maps andhydrological data. Rob Runkel and Ken Bencala generously lent their expertise in the solute transportmodelling. We also thank Pat Mulholland and Jud Harvey for comments that improved the manuscript.This research was supported by NSF grant BSR 90-20561 from the Ecosystem Studies Program in theDivision of Environmental Biology awarded to Cli� Dahm and Michael Campana of the University of NewMexico.

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