Hydrologic Influences on Groundwater-Surface Water ...

Hydrologic Influences on Groundwater-Surface Water Ecotones: Heterogeneity in Nutrient Composition and Retention Author(s): H. M. Valett, C. N. Dahm, M. E. Campana, J. A. Morrice, M. A. Baker and C. S. Fellows Source: Journal of the North American Benthological Society, Vol. 16, No. 1 (Mar., 1997), pp. 239-247 Published by: The University of Chicago Press on behalf of the Society for Freshwater Science Stable URL: http://www.jstor.org/stable/1468254 . Accessed: 02/02/2015 11:00 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp

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J. N. Am. Benthol.Soc., 1997,16(1):239-247 ? 1997by The NorthAmericanBenthologicalSociety

Hydrologic influences on groundwater-surface water ecotones: heterogeneity in nutrient composition and retention H. M. VALETT1,C. N. DAHM1, M. E. CAMPANA2,J. A. MORRICE1,M. A. BAKER1, and C. S. FELLOWS1 'Departmentof Biology and 2Departmentof Earth and Planetary Sciences, University of New Mexico, Albuquerque,New Mexico 87131 USA Abstract: The groundwater-surfacewater (GW-SW)ecotone,or hyporheiczone, is an activecomponent of stream ecosystems that influenceswhole-system metabolismand nutrientretention.Because hydrologicfluxes affect the supply of carbon,nutrients,and oxygen to the GW-SW ecotone, the biogeochemicalstructureof the ecotone (i.e., nutrientcontent) and the role of the ecotone in nutrientretentionare expected to vary under differinghydrologicconditions.In this paper,we employ an inter-basincomparisonof headwaterstreamsto assess the influenceof ecosystemhydrology on the structureand functioningof GW-SW ecotones. Specifically,we address how differingrate and extent of GW-SW interactioninfluencesheterogeneityin interstitialnutrientcontent and how variationin GW-SW interactionalters the role of the ecotone in whole-systemnutrientretention.A multiple regressionmodel derived from 6 solute-injectionexperimentsidentifiedthe extent and rate of hydrologic exchangebetween the stream and its aquiferas criticalvariablesthat determinethe retention of biologicallyimportantsolutes. This approachemphasizes that the nature of GW-SW interactionis establishedby catchmentgeology (i.e., alluvial hydrogeologicproperties),is modified by changing dischargewithin a catchment,and is a strong determinantof nutrientretention.At the landscapescale,identifyingcatchmentgeologic compositionmay be a startingpoint for comparative studies of GW-SW ecotones that could contributeto a more robust model of lotic ecosystem functioning. Keywords: groundwater,ecotone,transientstorage,nutrients,retention. In the past 30 y, spatial boundaries of stream ecosystems have been expanded to include the hyporheic zone (sensu Orghidan 1959) or groundwater-surface water (GW-SW) ecotone (sensu Vervier et al. 1992), the interstitial regions of alluvial aquifers where surface and ground waters mix (Schwoerbel 1961, Triska et al. 1989a). Current perspectives of lotic systems recognize the importance of GW-SW exchange and stress that the hyporheic zone is a dynamic region that varies among and within fluvial systems (Bencala 1993, Triska et al. 1993, DoleOlivier et al. 1994). Stream-aquiferexchange Hydrologic exchange between interstitial and surface subsystems strongly influences metabolic and biogeochemical processes that occur at the GW-SW ecotone. Accordingly, understanding the GW-SW ecotone requires appropriate characterization of the movement of water between surface and hyporheic subsystems. Although GW-SW interaction can be viewed at multiple scales, most studies of the GW-SW

ecotone have been conducted at the reach scale (sensu Gregory et al. 1991). Within a stream reach, upwelling and downwelling zones that alter the hydrologic supply of water and solutes to surface and groundwater environments are generated by changes in slope (White et al. 1987, Harvey and Bencala 1993), depth to bedrock (Henry et al. 1994), and depth of alluvium (Vaux 1968). At the landscape scale, headwater catchments of a given geologic composition will weather to produce alluvium with characteristic hydrogeologic properties (e.g., average grain size, porosity, hydraulic conductivity). Variation in catchment geology, therefore, may represent a largescale organizer of system hydrology influencing the rate and extent of GW-SW interactions (Kelson and Wells 1989, Morrice et al. 1997). As a consequence, the nature of hydrologic exchange within the GW-SW ecotone may differ less within basins of the same parent lithology than among differing geologic settings. Relatively little work, however, has employed a landscape perspective to investigate variation in ecotone structure and functioning among catchments of differing geologic composition.

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Ecotoneinfluenceson nutrient retention Material retention by streams is an appropriate measure of lotic ecosystem functioning given the tendency for export by downstream flow. The competing processes of downstream transport and in situ use are elegantly addressed by the "nutrient spiraling" concepts of Elwood et al. (1983), Newbold et al. (1983), and Mulholland et al. (1985) who described the downstream displacement of nutrient cycles that transformed them into spirals characteristic of lotic ecosystems. A useful measure of nutrient retention that has emerged from the spiraling concept is the "uptake length" (Stream Solute Workshop 1990), defined as the average distance traveled by a nutrient in dissolved form before it is sequestered by biota. Although the spiraling concept did not initially include a groundwater component, it is now recognized that the uptake length integrates retentive processes that occur along the entire flow path traversed by a nutrient in dissolved form (i.e., represents whole-system retention). The GW-SW ecotone is an active component of stream ecosystems that influences whole-system metabolism and nutrient retention (Grimm and Fisher 1984, Triska et al. 1989b, Findlay et al. 1993, Valett et al. 1994, 1996, Jones et al. 1995). Flow variations dictate the residence time for water and solutes and the duration of contact with surface and interstitial substrates (Findlay 1995, Valett et al. 1996). Increased residence time promotes interaction between the attached microflora of benthic and hyporheic sediments and enhances biological influence on solute composition and content. Over a designated period of time, nutrient retention (mass) by a lotic ecosystem can be described as the product of hydrologic residence time (t) and rates of associated autochthonous processes (mass/t). By providing for hydrologic exchange, this model of lotic biogeochemistry includes processes occurring in the GW-SW ecotone (Valett et al. 1996). In this paper, we address how variation in the hydrologic interaction between streams and aquifers influences the structure (nutrient composition) and function (nutrient retention) of the GW-SW ecotone. An inter-basin comparison of ecotone structure and functioning is provided by reviewing nearly 10 years of research on GW-SW interaction in 5 headwater streams. We hypothesize that variation in the hydrogeologic

properties of alluvial aquifers results in large scale (i.e., among-catchment) differences in GW-SW interaction. Given similar metabolic activity within the hyporheic zone, spatial variation in interstitial nutrient content should reflect water residence times (Findlay 1995, Valett et al. 1996). We predict, therefore, that hyporheic nutrient content will be less variable among streams within a single geologic setting (reflecting the similar nature of GW-SW interaction) than among streams draining catchments of contrasting alluvial composition, among which GW-SW interaction is expected to vary. Because interaction with the hyporheic zone enhances water and solute residence time, we also predict that nutrient retention will be directly related to the extent and rate of GW-SW exchange. We test these predictions by comparing interstitial nutrient content among the ecotones of 5 headwater streams distributed among 3 contrasting geologic settings and by developing a multiple regression model relating GW-SW exchange to nutrient uptake lengths. Study Sites Between 1987 and 1994, 5 headwater (orders 1-2) streams in New Mexico were studied for hydrologic and biogeochemical characterization of the GW-SW ecotone. Aspen Creek, Sawmill Creek, and Castor Creek are adjacent (35?13'00"N, 108?15'00'W), perennial, lst-order streams in sandstone catchments of the Zuni Mountains of western New Mexico. Drainage areas for the streams are similar (230-550 ha) and baseflow ranges from 0.3 to 1.7 L/s. Alluvial material is fine-grained and mean hydraulic conductivity (K) is correspondingly low (1.3 x 10-4 cm/s, Wroblicky 1995, Morrice et al. 1997). These 3 streams all drain portions of the Cibola National Forest and have experienced similar land-use practices (logging at the turn of the century, livestock grazing). The Rio Calaveras (35?56'00"N, 106?42'00"W) drains 3760 ha of Bandelier Tuff in the Jemez Mountains of the Santa Fe National Forest, New Mexico. Baseflow discharge is ca. 1.5 L/s and alluvial material is larger than in the sedimentary streams (Wroblicky 1995) with correspondingly higher mean K (1.2 x 10-3 cm/s, Morrice et al. 1997). Gallina Creek (36?35'00"N,105?,35',00"W)drains 618 ha of granitic parent material in the Sangre de Cristo Mountains of the Carson National Forest,


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GROUNDWATERECOTONEHETEROGENEITY

New Mexico. Alluvial material at Gallina Creek is poorly sorted cobbles, boulders, and interstitial gravels and baseflow is ca. 0.75 L/s. Alluvial hydraulic conductivity in Gallina Creek (4 x 10-3 cm/s, Morrice et al. 1997) is highest among the 5 study sites. Methods Data presented here were generated by hydrologic and biogeochemical surveys of 5 GWSW ecotones associated with studies of stream ecosystem functioning over the past 10 y. Detailed descriptions of instrumentation, experimental design, and laboratory methods are published elsewhere (Dahm et al. 1987, Carr 1989, Coleman and Dahm 1990, Dahm et al. 1991, Baker et al. 1994, Wroblicky et al. 1994, Valett et al. 1996, Morrice et al. 1997). Instrumentationand sampling Biogeochemistry of interstitial and surface waters of the Zuni Mountain systems (Aspen, Sawmill, Castor creeks) was analyzed monthly for a 2-y period (1987-1989). Surface water was sampled at 4 transects and interstitial samples were obtained from 15-25 lysimeters constructed of ultra-high-molecular-weight polyethylene tubing with an average pore size of 20 ,im (Carr 1989). Porous lysimeters were established in the stream bed and in the near-stream (0.5-1 m lateral) floodplain sediment. Detailed descriptions of the sites and methods are given in Carr (1989) and Coleman and Dahm (1990). In 1991 and 1992, study reaches at Aspen Creek and Rio Calaveras were equipped with 5 transects of 5-cm-diameter PVC wells (5 wells/ transect). Wells were screened (0.025 cm screen) from the water table to 50 cm depth during baseflow conditions. Transects were oriented perpendicular to surface flow and a total of 25 and 26 wells were placed in the wetted channel, 1 m and 3 m lateral to the stream along reaches of 170 m at Aspen Creek and 110 m at Rio Calaveras. During the same time period, a 125-m reach of Gallina Creek was outfitted with 4 sampling transects and a total of 13 wells established in banks and small gravel bars. For details on instrumentation, see Baker et al. (1994), Wroblicky et al. (1994), Morrice et al. (1997) and Valett et al. (1996).

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Nutrient analyses Biogeochemical characterization included quantification of nutrients and redox-sensitive solutes (e.g., nitrate-nitrogen (NO3-N), ammonium-nitrogen (NH4-N), soluble reactive phosphorus (SRP), dissolved oxygen (DO), dissolved organic carbon (DOC), and methane (CH4) in surface and groundwater samples. For earlier studies (1987-1990) of Aspen, Castor, and Sawmill creeks, qualitative assessment of anoxia in groundwater was based on a presence/absence analysis of reduced iron (an indicator of anoxic conditions, Dahm et al. 1991). Solute injections Between July 1992 and August 1995, 6 soluteinjection experiments were conducted among 3 of the 5 study systems (Aspen Creek, Rio Calaveras, and Gallina Creek). Injections introduced a biologically active solute (NO3-N) and a conservative hydrologic tracer (Br) to stream flow and were used to characterize solute transport and retention in surface and groundwater environments (Stream Solute Workshop 1990). Details on individual injection experiments are published elsewhere (Valett et al. 1996, Morrice et al. 1997). Recently, solute response curves have been mathematically represented by transport equations that contain a term for transient hydraulic storage (Bencala et al. 1984, Stream Solute Workshop 1990, Runkel and Broshears 1991, D'Angelo et al. 1993, Morrice et al. 1997). Transient storage results from the temporary routing of water and solutes to flow paths moving more slowly than the average linear velocity of the stream. As a model parameter, the storage zone is represented mathematically as a cross-sectional area (As) and has been attributed to interaction between surface and interstitial water (Bencala et al. 1993). When normalized to the cross-sectional area (A) of the stream (As/A, relative transient storage), model-derived transient storage is a good measure of ecotone extent or hyporheic size (Bencala et al. 1993, Morrice et al. 1997). The rate of GW-SW interaction is represented by the exchange coefficient (ox, s-1) which describes the rate at which solute is routed from the stream to the storage zone. The average amount of time that a particle of water remains in the surface stream (stream residence


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time, Tstr)is calculated as the inverse of the exchange coefficient (Ts, = 1/ca), and storage zone residence time (Tsto)is derived from a and the normalized storage zone size (T.to= A/AS x ox) (sensu Mulholland et al. 1994). The hydrologic retention factor (Rh, Rh = As/Q, where Q = stream discharge, sensu Morrice et al. 1997) describes the amount of time solutes and water spend in the storage zone per m of stream length traveled. Lateral inflow of water lacking tracer (QL) can be determined by longitudinal changes in steady-state plateau concentrations of the conservative tracer and is an additional measure of GW-SW interaction that emphasizes the influence of groundwater inputs. Co-injections of a conservative tracer and a dynamic (biologically active) solute (e.g., NO3-N) have been used to distinguish hydrologic and biological mechanisms of nutrient retention (Triska et al. 1989a, 1989b, 1990, Munn and Meyer 1990, Valett et al. 1996). Ratios of conservative and dynamic tracers can be compared with increasing distance downstream from an injection to calculate the nutrient uptake length (Sw). Co-injections of Br and NO3-N were conducted at Aspen Creek, Rio Calaveras, and Gallina Creek during baseflow in the fall of 1992 and during winter, spring, and summer flow conditions at Gallina Creek. Results were used to calculate NO3-N uptake lengths (Valett et al. 1996) and modeled to determine storage parameters (Morrice et al. 1997). Data analysis Variation in hydrologic and biogeochemical parameters among catchments of differing geologic composition was assessed by comparing results from Aspen Creek (sandstone), Rio Calaveras (volcanic tuff), and Gallina Creek (granite). Within a catchment, groundwater environments were designated part of the interactive hyporheic zone (GW-SW ecotone) if tracer concentrations indicated 10-99% tracer-labeled surface water (sensu Triska et al. 1989a). Biogeochemical data from ecotone wells were collected during summer and early fall preceding solute injection experiments (see Valett et al. 1996). Mean values within a geologic type were calculated from survey data (n = 20-30), resulting in a single value for each stream and each constituent (n = 1 per solute). To quantify variation

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in ecotone nutrient content among contrasting geologic types, the mean value for each constituent from each of the 3 catchments was used to generate inter-basin means and standard deviations (n = 3). To compare variation among differing constituents, the coefficient of variation (CV = [SD/i] x 100%) was calculated for each biogeochemical and hydrologic parameter. This measure represents variance (standard deviation) relative to the mean and provides a standardized measure of variability. Variation in ecotone biogeochemistry among streams within a single catchment type (within an identical geologic setting) was assessed by using the same approach described above except that CVs were calculated from mean values derived from 3 adjacent systems within the same geologic formation (Yeso sandstone, Zuni Mountains: Aspen, Castor, and Sawmill creeks). Mean monthly nutrient concentration was determined from wells (n = 15-25) during baseflow months (August, September, and October) of 1987. Mean baseflow values for each stream were then calculated as the average of the 3 monthly means for each constituent. During 1987, methane samples were not collected in September and mean concentration was the average of August and October values. Number of anoxic wells at each of the 3 streams was determined during baseflow of 1988 (Carr 1989). To test the prediction that ecotone biogeochemical heterogeneity is higher among catchments in differing parent lithologies, a dependent t-test (SAS Inst. Cary, North Carolina) was used to compare CVs for 5 constituents (NO3-N, NH4-N, SRP, DOC, CH4) common to each data set. During 1987 biogeochemical surveys of Aspen, Sawmill, and Castor creeks, interstitial oxygen status was assessed by determining the number of anoxic wells. Mean interstitial dissolved oxygen concentrations were determined during more recent studies of Aspen Creek, Rio Calaveras, and Gallina Creek. These measures cannot be directly compared and were omitted from statistical analysis of solute heterogeneity. CVs used in the statistical analysis were transformed (arcsine square-root, Sokal and Rohlf 1981) and values were paired by nutrient. Reported average CVs were back-transformed following statistical analysis. A stepwise regression model (SAS) was used to predict nitrate uptake length (Sw, m) from hydrologic variables and modeling results from


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zones, TABLE 1. Alluvial hydraulicconductivity(K),waterresidencetimes for surface(Ts,)and storage(Tsto) and the hydrologicretentionfactor(time in storageper m of streamlength traveled,Rh)forbaseflowconditions in 3 catchmentsof contrastinggeologic composition.

Site

Geologic type

Hydraulic conductivity (K, cm/s)

Aspen Creek Rio Calaveras GallinaCreek

Siltstone/sandstone Volcanictuff Granite/gneiss

1.3 x 10-4 1.2 x 10-3 4.0 x 10-3

solute injection experiments. Independent variables included stream discharge (Q, L/s), lateral inflow (QL, L/s), velocity (u, cm/s), dispersion (D, m2/s), stream cross-sectional area (A, m2), storage zone cross-sectional area (As, m2), normalized storage zone size (As/A) and the solute exchange coefficient (x, s-l). The inclusion criterion was set at 0.10. Results Water residence times varied with increasing alluvial hydraulic conductivity among the 3 geologically distinct catchments (Table 1). For * differinggeology E same geology

120 10080-

o C0

60 40- 20-

N03-N

NH4-N

SRP

DO

DOC

CH4

Solute type FIG.1. Comparisonof the Coefficientof Variation (CV = [SD/x] x 100%)calculatedfor 6 biogeochemically active solutes derived from averageconcentrations among 3 catchmentsof differinggeologic composition (solid bars, n = 3) and from averagevalues among 3 catchments of identical parent lithology (hatchedbars, n = 3). *All CVs were calculatedfrom solute concentrationsexcept for earlier studies of interstitialdissolved oxygen (DO) among 3 catchments of identical parent lithology where DO was represented by % of anoxic wells (see Methodssection for furtherexplanation).

Surface water residence Storagezone Hydrologic time residencetime retentionfactor (Rh, s/m) (TsT,min) (T,t, min) 417 287 283

33 28 1257

0.53 1.50 306

the surface stream, water residence time was longest in the sandstone system and shortest in the granitic basin. By contrast, water spent only a short time (33 min, Table 1) in the storage zone at Aspen Creek (sandstone catchment) whereas storage zone residence time in Gallina Creek (granitic catchment) was nearly 21 h. The hydrologic retention factor (Rh), which compensates for differing surface water velocity, increased 577-fold from the sandstone to granitic catchment (Table 1). Similarly, the normalized storage zone size (As/A) increased 57-fold, from 0.08 in the sandstone catchment to 4.60 in the granitic system (Table 2). At the landscape scale, the hydrologic structure of the GW-SW ecotone varied substantially among catchments with differing geologic composition. Large CVs for As/A (163%) and Rh (172%) show that under similar baseflow conditions the relative size of the transient storage zone and the amount of retention per unit stream length varied greatly among ecotones situated in sandstone, tuff, and granite catchments. As was the case for features of GW-SW interaction, there was great variation in ecotone nutrient content among catchments in differing geologic settings (Fig. 1). Average CV for 6 biogeochemical constituents was 82% and CVs ranged from 44% to 109%. Highest CVs were associated with the highly labile solutes DO (109%), N03-N (86%), and CH4 (80%). Lowest CVs among the contrasting geologic settings were calculated for SRP (53%) and DOC (44%). Concentrations of SRP averaged 19 ? 10 pbb among the 3 sites (CV = 53%, Fig. 1). Mean concentrations of N03-N (25 ppb) and NH4-N (65 ppb) were low, and calculated atomic N:P ratio was ca.10, reflecting nitrogen-limiting conditions for biotic process (Coleman and Dahm 1990, C. N. Dahm, unpublished data).


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TABLE 2. Hydrologic data and modeling results from 6 solute injection experiments carried out at Aspen Creek (sandstone), Rio Calaveras (volcanic tuff), and Gallina Creek (granite). Data are single measures used to develop a multiple regression model for nitrate uptake length (Sw) from discharge (Q), water velocity (u), dispersion (D), lateral inflow (QL),stream area (A), storage zone area (As), normalized storage zone area (As/ A), and the solute exchange coefficient (a). Details on model use are available in Morrice et al. (1997) and Valett et al. (1996).

Site

Season

Aspen Creek Rio Calaveras Gallina Creek

Summer Summer Fall Winter Spring Summer

SW (m)

Q (L/s)

u (cm/s)

D (m2/s)

(%)

A (m2)

As (m2)

As/A

ao (s-1)

1178 782 133 738 3183 205

1.1 2.0 0.8 15.0 75.0 2.0

15.0 6.7 1.3 10.7 20.8 2.5

0.05 0.05 0.05 0.05 0.40 0.08

36 13 57 3 1 38

0.01 0.03 0.05 0.14 0.36 0.08

0.0008 0.0030 0.2300 0.1000 0.0040 0.1300

0.08 0.10 4.60 0.71 0.01 1.63

0.000040 0.000058 0.000061 0.000500 0.000200 0.000120

Ecotone biogeochemistry was less variable among streams situated in the same geologic setting (i.e., same parent material) than among catchments of different lithologic type (Fig. 1). Average CV for 5 biogeochemically labile solutes from 3 ecotones of the same geologic composition was 32% and was significantly lower (p = 0.022) than average CV (76%) analyzed from 3 geologically distinct catchments. Lowest CVs for the 3 streams of similar geologic composition were associated with SRP (5%) and percent anoxic wells (12%, Fig. 1). Among the 3 sandstone streams, mean concentration of SRP varied by only 3 lig/L (25 p.g/L to 28 pg/L). On average, 18% of all tested wells were anoxic (indicated by positive reduced iron tests) with little variation among the GW-SW ecotones within this single geologic formation. CVs for CH4 (35%) and NH4-N (36%) were similar (Fig. 1) and lower than those for DOC (42%) and N03-N (51%). Hydrologic variables derived from the 6 solute injection experiments are presented in Table 2. Baseflow discharge was similar (0.8-2.0 L/s) among the 3 differing catchments. In contrast, average water velocity ranged more than

QL

10-fold, from 1.3 cm/s in the granitic catchment (Gallina Creek) to 15.0 cm/s in Aspen Creek (sandstone catchment). Lateral inflow represented between 13 and 57% of surface discharge, and stream cross-sectional area varied 5-fold among the catchments at baseflow (Table 2). Over the course of a year, discharge within the granitic catchment ranged over nearly 2 orders of magnitude, reflecting the influence of snowmelt on the hydrology of montane catchments (Morrice et al. 1997). In association with spring snowmelt, average water velocity increased 20 fold and stream cross-sectional area increased by a factor of approximately 7 (Table 2). Despite the great range in discharge, velocity, and stream cross-sectional area, results from the multiple regression analysis indicated that of the 8 independent variables loaded into the model, only the normalized storage zone area (As/A) and the exchange coefficient (oa)met the inclusion criterion. Together, these variables accounted for 98.7% of the variation in NO3-N uptake length (Table 3). While other variables (discharge and water velocity) were closely correlated to uptake length (Valett et al. 1996), including these variables in the multiple

TABLE3. Multiple regression model (n = 6) relating nitrate uptake length (Sw, m) to the normalized storage zone area (As/A) and the solute exchange coefficient (or,s-l). Nitrate uptake length and normalized storage area were In-transformed for analysis.

Step 1 2

Variable entered As/A ca

Model In Sw = -0.50(ln As/A) + 5.79 In Sw = -0.50(ln As/A) + 1750 (oa)+ 5.50

Partial R2

Model R2

0.918 0.069

0.918 0.987


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regression did not significantly improve the model fit. Discussion Parent lithology is one of the major determinants of the hydrologic character of a catchment. Features such as discharge per unit area, stream power, and sediment transport capacity vary predictably among basins composed of crystalline rock in contrast to catchments in which alluvium is derived from sedimentary materials (Kelson and Wells 1989). These controls occur at the scale of the geologic formation, suggesting that the hydrologic nature of the GW-SW ecotone should differ among catchments of contrasting geologic composition. Our results illustrate that the rate (o()and extent (As/A) of GWSW interaction increased with alluvial hydraulic conductivity among catchments with differing geologic composition where climate, gradient, and vegetation types were generally similar. Solute injections also showed that there is great variation in GW-SW interaction at the landscape scale (i.e., among contrasting geologic settings). Others have shown that the chemical composition of surface water is linked to basin geology (e.g., Garrels and Christ 1965, Dillion and Kirchner 1975, Stumm and Morgan 1981, Kruger and Waters 1983, Munn and Meyer 1990). Pringle and Triska (1991) illustrated the influence of regional groundwater geochemistry on surface water chemical composition in a lowland Costa Rican stream. Because the catchments have experienced very similar land use, we suggest that variation in interstitial nutrient concentrations over a large scale (i.e., including numerous catchments and parent lithologies) is related to differing geologic conditions that influence hydrologic and mineral properties of alluvial materials (Triska et al. 1993, Valett et al. 1996). Among streams within the same geologic setting, hyporheic nutrient content was significantly less variable than among contrasting geologic settings. While no direct measures of hydrologic storage are available for all 3 streams within the sandstone formation, we hypothesize that lower variability in ecotone nutrient content is related to similar features of GW-SW exchange. Because alluvial properties (e.g., grain size, hydraulic conductivity, sediment mineral charac-

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teristics) are similar among the 3 sandstone streams (C. N. Dahm, unpublished data), these systems probably have similar residence times for water and solutes. Similar residence times and extent of GW-SW interaction may contribute to the lower variability in interstitial nutrient concentrations. Despite the strong spatial and temporal variation in stream velocity, discharge, and crosssectional area observed among the 3 contrasting geologic settings, only the extent and rate of GW-SW interaction were statistically valid predictors of nutrient retention. Others have illustrated that surface discharge and water velocity influence nutrient uptake (DAngelo et al. 1991) and that discharge alters the extent of transient storage (Triska et al. 1990, DAngelo et al. 1993, Morrice et al. 1997). We contend that variation in alluvial hydrogeologic properties among catchments and changing discharge within a catchment, alter the nature of GW-SW interaction and lead to differences in nutrient retention. Clearly nutrient retention is related to other factors including autotrophic biomass (Fisher et al. 1982), leaf litter content (Mulholland et al. 1985), catchment vegetation (DAngelo and Webster 1991), and time since disturbance (Grimm 1987). Our model suggests that the nature of stream-aquifer interactions sets broad constraints on hydrologic retention within which biological variation may operate to determine the rate and extent of nutrient retention.

Acknowledgements The authors acknowledge the support of past researchers (Deborah Carr, Ross Coleman, Kevin Henry, Brian Horton, Teri McDonald, Joe Morrice, Phoebe Suina, Greg Wroblicky), current project members (Francelia Lieurance, Mike Marshall, Doug Moyer, Chris Peterson, Miguel Santistevan, Padinare Unnikrishna), and present collaborators at the Department of Biology, University of New Mexico (John Craig, Cliff Crawford, Steve Hofstadt, Manuel Molles, Carl White). In addition, we thank Mr. Al Bearce of the D.H. Lawrence Ranch for logistic support during studies at Gallina Creek. Research was supported by grants BSR 8616438, BSR 9020561, and DEB 9420510 from the National Science Foundation awarded to C.N. Dahm, M.E. Campana, and H.M Valett.


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