Nitrate concentration changes at the groundwater-surface water interface of a small Cumbrian river

HYDROLOGICAL PROCESSESHydrol. Process. 23, 2195–2211 (2009)Published online 28 January 2009 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.7213

Nitrate concentration changes at the groundwater-surfacewater interface of a small Cumbrian river

Stefan Krause,1* Louise Heathwaite,2 Andrew Binley2 and Patrick Keenan2

1 Department of Earth Science and Geography, Keele University, Keele, UK2 Lancaster Environment Centre, Lancaster University, Lancaster, UK

Abstract:

For an experimental field site at the River Leith, United Kingdom, the spatial and temporal distribution of nitrate was observedalong the upwelling flow path from groundwater to surface water. The study was carried out during baseflow conditions fortwo successive years. For two contrasting stream reaches, the physical and chemical characteristics of streambed sedimentcores were analysed together with observations of hydraulic head, dissolved oxygen, redox and nitrogen speciation using anarray of nested streambed piezometers.

Pressure head gradients in the streambed piezometers showed that upwelling flows dominated the exchange betweengroundwater and surface water throughout the observation period. Infiltration of surface water into the streambed was notevident at depths below 10 cm. Pore water collected from sediment cores and streambed piezometers showed spatially variableredox conditions and nitrogen speciation within up to 100 cm depth in the streambed. In particular, nitrate concentrations alongupwelling flow paths appeared to follow two opposite trends, with both decreasing and increasing nitrate concentrations beingobserved at different points in the experimental reach. The observed changes of nitrate concentrations in the upwellinggroundwater are restricted to the loose superficial sediments that overlay the sandstone bedrock and do not appear to coincidewith surface water–groundwater mixing in the streambed. The magnitude of variation in nitrate concentration along theupwelling flow path to the streambed appears to be governed by the sediment structure and characteristics in the two contrastingfield sites.

The results suggest that changes in redox status and pore water nitrate concentrations in the hyporheic may occur at depthsgreater than surface water infiltration into the streambed and may call for new conceptual understanding of hyporheic nutrienttransformations. Copyright 2009 John Wiley & Sons, Ltd.

KEY WORDS hyporheic zone; nitrate; groundwater–surface water interface; riparian

Received 16 August 2008; Accepted 23 October 2008

INTRODUCTION

Hyporheic transport and transformation of nitrate

Eutrophication of freshwater environments representsa major threat to the ecological health of surface watersin agricultural catchments. High nutrient concentrations,caused by direct inputs into the surface water or indi-rect contributions via groundwater pathways, can result inexcess algae growth and a subsequent over-consumptionof oxygen leading to a deterioration of in-stream habi-tats (McMahon and Boehlke, 1996; Brunke and Gonser,1997; Sophocleous, 2002). There is an increasing aware-ness of the ecological significance of processes at thegroundwater–surface water interface for controlling theriparian runoff generation as well as the transport andtransformation in particular, of nitrogen (Vought et al.,1995; Hill, 1996; Brunke and Gonser, 1997; Fisher et al.,1998; Duff and Triska, 2000; Pinay et al., 2002; Vidonand Hill, 2004 a,b).

Experimental studies and model-based investigationshave identified that both, physical and biogeochemical

* Correspondence to: Stefan Krause, Department of Earth Science andGeography, Keele University, Keele, UK.E-mail: [email protected]

processes at the surface water–sediment interface, con-trol the flux and transformation of nitrate. The transportand transformation of nitrate between groundwater andsurface water may influence the success of restorationmeasures to improve stream and groundwater ecologi-cal conditions (Triska et al., 1993; Findlay, 1995; Wardet al., 2001; Storey et al., 2004; Krause et al., 2007).Recently, the direct interface between the groundwaterand the surface water, the so-called hyporheic zone (HZ),has seen an increasing interest and a number of studieshave aimed to investigate the hyporheic process dynam-ics from hydrological, hydro-chemical, micro-biologicaland biogeochemical perspectives (Boulton et al., 1998;Stanford and Ward, 1988; Triska et al., 1989; Dole-Olivier et al., 1997; Soulsby et al., 2001; Storey et al.,2004).

Within the different scientific disciplines, definitionsof the HZ and its spatial extent vary. In a biogeochem-ical context, the HZ is usually described as the redoxreactive zone of the streambed were downwelling sur-face water supplies sufficient dissolved oxygen (DO),nutrients and dissolved organic carbon into the streambedto enable an intensive biogeochemical activity and hightransformation rates (Grimm and Fisher, 1984; Boulton

Copyright 2009 John Wiley & Sons, Ltd.

2196 S. KRAUSE ET AL.

et al., 1998; Mulholland et al., 2000). From a biolog-ical perspective, the HZ and its delineation are desig-nated by the abundance of groundwater organisms andkey hyporheic invertebrate communities (Stanford andWard, 1988; Williams, 1989; Hancock, 2006). Hydrol-ogists, however, usually define the HZ using the actualmixing ratio of surface water and groundwater (Harveyand Bencala, 1993; Harvey and Wagner, 2000; Storeyet al., 2003). For example, it has been hypothesizedthat the interactive HZ, characterized by sharp physical-chemical gradients, correlates with the streambed areawhich contains between 10 and 98% of surface water(Triska et al., 1989; Smith, 2005; Kasahara and Hill,2006).

Most recent hyporheic studies, investigating bio-geochemical processes and groundwater–surface waterexchange, focus on a small field scale of stream reachesor sections (Harvey and Bencala, 1993; Wroblicky et al.,1998; Butturini et al., 2002; Kasahara and Wondzell,2003; Storey et al., 2003). It has been shown that geomor-phic structures, such as riffle-pool or step-pool sequences,have a strong impact on the longitudinal pressure headgradients and therefore control the pathways and intensi-ties of small-scale exchange fluxes through the streambed(Harvey and Bencala, 1993; Kasahara and Wondzell,2003).

In-stream tracer applications, often in combinationwith transient storage models, are widely used to deter-mine the average spatial extent of the hyporheic mixingzone (Bencala and Walters, 1983; Harvey and Bencala,1993; Harvey et al., 1996; Wagner and Harvey, 1997;Worman et al., 2002; Gooseff et al., 2005). In manystudies, surface water mixing with groundwater in thestreambed has been identified to be limited to a shal-low area in the uppermost 10–20 cm of the streambedsediments. Other studies, however, found surface wateralso infiltrating into deeper parts of the streambed sedi-ments mainly as a result of topography driven advectivepumping (Hill et al., 1998; Baker and Vervier, 2004;Puckett et al., 2008). Whatever the penetration depth,a majority of investigations of HZ process studies witha surface water perspective couple their expectations ofhigher biogeochemical activity on the occurrence of sur-face water infiltration into the streambed. Previous studieshave aimed to investigate the impact of the hyporheicflow path on the streambed redox conditions and thetransport and transformations of nitrogen mainly from asurface water perspective (Duff and Triska, 1990; Holmeset al., 1996; Soulsby et al., 2001; Storey et al., 2004,1999). These studies mainly focus on the change of thesurface water chemical signature during the infiltrationinto the streambed, the concentration changes due tohyporheic processes and the exfiltration into the streamrespectively.

The efficiency of the redox-controlled nitrogen trans-formation processes along the groundwater–surface waterinterface has been found to be controlled by the abun-dance of steep redox gradients in the HZ (including the

typically complex patterns of aerobic/anaerobic condi-tions) as well as by the existence of organic matter andmicrobial activity in the hyporheic sediments (Findlay,1995; Jones et al., 1995; Fisher et al., 1998; Duff andTriska, 1990, 2000; Chafiq et al., 1999; Hinkle et al.,2001; Storey et al., 2004). The denitrification capacity,for instance, is controlled by the nitrate concentration(first-order reaction kinetics), the existence of anaerobicconditions and low concentrations of DO and the pres-ence of reductive agents acting as electron donors suchas organic carbon (heterotrophic denitrification) or pyrite(autotrophic denitrification) (Hill et al., 2000; Baker andVervier, 2004; Hill and Cardaci, 2004; Storey et al., 2004;Vidon and Hill, 2004c). The reaction efficiency is further-more a function of the hyporheic residence time, which isdependent on the length of the flow path and the conduc-tivity of the streambed material and controls the reactiontime in the highly reactive hyporheic environment (Ben-cala et al., 1993; Jones et al., 1995; Fisher et al., 1998;Duff and Triska, 2000).

The HZ has been found to function as either a sourceor a sink of nitrate. A number of case studies identifiedthe HZ to potentially attenuate fluxes of nitrate due todenitrification along the hyporheic passage (Pinay et al.,1994; Hill et al., 1998). However, other case studiesalso found processes as ammonification and nitrificationto alter the composition of dissolved nitrogen in HZ,including increases of hyporheic nitrate concentrations(Jones et al., 1995; Duff and Triska, 2000; Storey et al.,2004). The HZ has been recognized by regulatory bodiesand management agencies for its potential to attenuatestream nitrate (Smith, 2005). With nitrate concentrationsincreasing in many groundwater aquifers, mainly result-ing from the long travel and residence times, there is anincreasing interest in concepts quantifying the potentialattenuation capacity the streambed can provide along thehyporheic flow path. As many studies of HZ exchangefluxes and nitrogen attenuation are motivated by a sur-face water perspective, they mainly help to understandthe hyporheic impact on surface water infiltrating andexfiltrating the shallow streambed (Figure 1a). The fateof nitrogen in upwelling groundwater along the pathwayacross the direct groundwater–surface water interface isrelatively poorly understood. Therefore, it is still a chal-lenge to assess the implications of hyporheic processdynamics on redox conditions and nitrogen cycling inupwelling groundwaters passing the streambed.

Aims and objectives

This study investigates the impact of streambed con-ditions on the spatial distribution of nitrate at thegroundwater–surface water interface. In particular, thisstudy aims to establish if significant variations inhyporheic nitrate concentrations are occurring at depthswithin the streambed that are not necessarily penetratedby surface water (Figure 1b). Therefore, we investigatethe spatial extent of highly redox reactive areas withsteep redox gradients, high contents of organic matter and

Copyright 2009 John Wiley & Sons, Ltd. Hydrol. Process. 23, 2195–2211 (2009)DOI: 10.1002/hyp

NITRATE CONCENTRATION CHANGES IN GROUNDWATER 2197

Figure 1. Conceptual understanding of nutrient cycling along the groundwater–surface water interface: (a) Model of groundwater–surface watermixing in the HZ with a major focus on surface water infiltration and exfiltration into/from the streambed, the perspective of many HZ process

studies (b) Spatial focus of this study on the fate of nitrate in upwelling groundwater

subsequently intensive changes in nitrate concentrations,which are characteristic for the hyporheic environment.

The variation in nitrate concentration was analysedfor depths up to 1 m for the loose streambed sedimentsoverlaying the sandstone bedrock at an experimentalfield site in Cumbria, United Kingdom. Unlike in manyprevious investigations, the spatial focus of this studyis not delineated by the existence of constant mixing ofgroundwater and surface water (Figure 1a).

Within two contrasting stream reaches, the spatial andtemporal variability of hyporheic groundwater–surfacewater exchange and nitrate concentration changes areinvestigated. The impact of the HZ flow path ongroundwater–surface water exchange, hyporheic nitrateconcentrations and delivery of nitrate to the river isassessed during baseflow conditions from spring toautumn. Generally, groundwater contributions to the sur-face water have the greatest impact during low flow,when the proportion of groundwater contribution is thehighest. Such periods are furthermore characterized by aparticularly high ecological sensitivity, as even moder-ate contributions of pollutants to the river can cause highconcentrations in the surface water.

MATERIALS AND METHODS

Research area

The experimental investigation focuses on two reachesof the River Leith, a tributary of the River Eden, Cumbria,United Kingdom. The two reaches are 100 m apart andare both approximately 25 m long and 5–8 m wide(Figure 2). In this area, the Leith meanders in a narrowfloodplain of approximately 100 m width, delineatedat its sides by steep slopes with occasional outcropsof the sandstone bedrock. The channel unit scale ischaracterized by sequences of riffles and pools. Theregional groundwater aquifer consists of Permo-Triassic

Sandstone (PTS). In the area of interest, the PTS isunderlying glaciofluvial sediments of approximately 2 mdepth.

The floodplain soils origin from the glaciofluvialdeposits overlaying the PTS and are of hydromorphiccharacter with grain sizes ranging from fine silts to coarsesands. The floodplain is covered by rich grassland, whichis used for agricultural purposes as a rather extensivegrazing ground for mainly sheep and cattle. At both sidesof the river, the banks are fenced off to prevent livestockentering the water body, resulting in approximately 5-m wide buffer strips alongside the river. The relativelysteep banks are mainly covered by riparian reed andshrub vegetation. The average rainfall in the area isapproximately 900 mm per year with relatively dry,baseflow dominated periods from spring to autumn.

Measurements of river discharge and local hydraulicheads during baseflow periods, carried out by the Envi-ronment Agency (EA), revealed that over an approxi-mately 5 km river section (starting 3500 m upstream ofthe field site) of the River Leith is net-gaining groundwa-ter. This information has been used to locate the field sitethat should be well connected to the aquifer and locatedin an overall gaining reach (Figure 3).

Experimental setup

The investigations of hydrological and hydrochemicalconditions in the surface water and groundwater atthe Leith field site cover baseflow periods in 2006(August to October) and 2007 (May to October). Riverdischarge was recorded by the EA at the weir in Cliburn,approximately 50 m downstream of the research area (EAstation: 760280; 54 : 37:03N/2 : 38:23W). In this study,additionally consecutive discharge measurements havebeen carried out with high spatial resolution duringbaseflow conditions in April and May 2006, with noprecipitation occurring for a period of several days.



Figure 2. Field site location in the United Kingdom and in relation to the distribution of Permo-Triassic Sandstone aquifers (left) and the twoinvestigated stream reaches with shallow floodplain groundwater boreholes at the River Leith (right) and the location of EA groundwater (GW)

boreholes adjacent to the gauging station

Figure 3. Consecutive discharge measurements at the River Leith in May 2006 (top) and approximated rates of groundwater accretion along the riversections (bottom). The dashed line indicates the area of high groundwater accretion

These measurements showed a discharge increase ofapproximately 20% in the research area and identifiedthe approximately 200-m long selected field site as anarea of strong groundwater accretion. Figure 3 showsthe results of the consecutive discharge measurementsalong the River Leith, as well as the discharge increase,which has been interpreted as the result of groundwateraccretion in the specific river sections. It furthermoreshows that the identified field site is representative forthe gaining river stretches at the River Leith.

Within the research area, two contrasting, approxi-mately 25-m long stream reaches have been identifiedfor a detailed investigation of groundwater–surface waterexchange fluxes, streambed pore water concentrationsand redox conditions (Figure 2). The downstream site(Leith A) furthermore contains a vegetated mid-channelisland, the upstream site (Leith B) with two vegetatedsidebars.

Adjacent to the Cliburn weir, groundwater levels andchemistry have been observed in two EA groundwaterboreholes of 2 and 28 m depth. The borehole logswere used to derive information of the regional shallowgeology and bedrock surface. A topographical survey

of the riverbed was carried out using differential GPSprior to the field season in both observation years,deriving detailed information of the riverbed topography.Differential GPS was also used to produce a refineddigital elevation model of the adjacent floodplain and,together with a total station, to measure the correctheights of installed boreholes and piezometers (Figure 4).

A total of 30 cores were taken from the riverbed in thetwo stream reaches (Figure 4) using a percussion drill.The cores were taken from arrays arranged to cover avariety of different geomorphic settings of the two focusareas with riffle-step sequences in Leith A and a centralshallow riffle section (mean water level (MWL) <20 cm)delimited by two deep pools (MWL > 120 cm) in LeithB. The arrays consist of cross sectional profiles thatare connected by longitudinal transects (Figure 4). Thestreambed sediment cores ranging from 75 to 100 cm inlength were analysed in 5 cm steps for chemical parame-ters as total nitrogen and nitrate-nitrogen and ammonium-nitrogen and at usually four intervals for their grain sizedistribution. The organic carbon content was analysedin 10 cm vertical intervals. The core length was cho-sen to cover the full range of loose superficial sediments



Figure 4. Streambed topography, piezometer locations and ID’s at the sites Leith A (left) and Leith B (right) as well as the setup and design of thenested streambed piezometers. Piezometer locations correspond with the locations of the cores as the piezometers have been installed in the core

directly after drilling. Small lines perpendicular to contour lines indicate sinks in the streambed topography

overlaying the PTS. After the retrieval of the sedimentcores, in both stream reaches, furthermore a total of 84nested piezometers were installed in arrays adjacent tothe core positions. Each piezometer nest usually con-sists of three piezometers at different depths (15–30,40–60, 80–100 cm) (Figure 4) with the exact piezometerdepths varying slightly, subject to sediment heterogene-ity. This allowed the observation of vertical distribu-tions of pressure heads and nitrogen concentrations inthe streambed. Additionally, ten groundwater observationboreholes were installed (five in 2006 and five in 2007) inthe riparian floodplain at depths between 1Ð5 and 2Ð5 m toenable sampling from the shallow riparian groundwater aswell as the observation of the riparian groundwater flowfield, including possible infiltration/exfiltration into/fromthe river (Figure 2).

At the streambed piezometers and riparian groundwa-ter boreholes, hydraulic head observations by dip metremeasurements were carried out on an approximatelyfortnight basis. Vertical hydraulic head gradients werecalculated by h/l with h for the elevation dif-ference between inner and outer observed heads, andl for the distance between the mid-screen depth andthe surface water–sediment interface and indicate thedirection and the intensity of groundwater–surface waterexchange fluxes. Because sometimes turbulent flow con-ditions around the piezometer outsides may have had animpact on the uncertainty of the measurement, the accu-racy of this observation method was approximated to beC/� 5 mm head.

Laboratory analyses

The grain size distribution was analysed for sub-samples of the sediment cores using a laser diffractometer

and wet sieving techniques, considering particle sizesranging from 0Ð003 to 40 mm.

Nitrate-nitrogen and ammonium-nitrogen concentra-tions were analysed on a Dionex ICS2500 ion chro-matograph (Dionex UK Ltd., Camberley, Surrey) and aSeal AQ2C discrete analyser (SEAL Analytical, BurgessHill, West Sussex), respectively. An internal validationof the nitrate-nitrogen method gave a limit of detection(LOD) of 0Ð03 mg/l of N and a precision of 1% (at 10%of the full concentration range), whereas the publishedLOD and precision values for the ammonium-nitrogenmethod are 0Ð02 mg/l of N and between 8Ð2% (at 10%of the full concentration range) and 1Ð9% (at 90% ofthe full concentration range). For the pore water analy-sis, an extraction from the sediment samples was carriedout with 2 M potassium chloride solution under constantshaking of the samples for 1 h to ensure optimal recov-ery (Keeney and Nelson, 1987). Redox conditions as theoxidation-reduction potential (ORP), DO concentrations,pH and the electrical conductivity were measured in situwith a Hannah Instruments HI-9828/20 Multi ParameterWater Quality Meter on the freshly collected sample.

For taking pore water samples, the piezometers andshallow groundwater boreholes were purged and allowedto refill before sampling. Samples of the fresh porewater infiltrating into the piezometers were taken usinga hand-held vacuum pump and stored in 80-ml PTFEsample bottles. This procedure has been applied inorder to ensure that fresh pore water is analysed andits composition has not changed while standing in thepiezometers. In some cases, the piezometers respondedslowly and the refilling lasted 1–2 h. During this time,the sampled pore water was exposed to the atmosphere.However, as disturbances within the piezometers are



very limited, a contamination of the sample is, for suchcases, expected to be minimal (Triska et al., 1989).To avoid contamination during the sampling, the acid-washed sample bottles were rinsed with a small volumeof sample (approximately 20 ml) before the actual samplewas taken. The samples were cooled for storage andusually analysed within 48 h in the laboratory.

RESULTS AND DISCUSSION

Streambed sediment cores

The 30 cores of streambed sediments, taken in August2006 were analysed in 5 cm vertical sequences for theirnitrate, ammonium and organic carbon contents. Theresulting profiles of the 30 1-m long cores show avery diverse vertical stratification of streambed sedimentmaterial and chemical conditions.

Geomorphology. The core analysis revealed that thestreambed of the research area consists of loose sedimentscovering the in situ sandstone bedrock. The thicknessof the loose superficial sediments is spatially very vari-able. Sandstone bedrock was found in situ at depths of20–100 cm underneath the streambed surface. Figure 5shows an interpolation (interpolation method: inverse dis-tance weighting, IDW) of the depth of the loose superfi-cial streambed material overlaying the sandstone bedrockfor sites Leith A and Leith B, as obtained from thesediment cores. Both stream reaches show substantialdifferences in their sediment structures. In the upstreampart of site Leith B, the loose sediments reach depths of

up to 80 cm, whereas in the downstream part the sand-stone bedrock was found at depths of only 20 cm. Incontrast, at site Leith A, the sandstone bedrock was foundat much smaller depths of 25 cm on average (Figure 5).The only exception is represented by the mid-channelisland, consisting of up to 60-cm deep loose gravel andsand sediments. The loose superficial sediments coveringthe sandstone bedrock vary not only in their thickness butalso in material characteristics. Sediments rich in organicmaterial were found in association with fine silty mate-rial within pool sections, and coarse grit and gravel layerswere found mainly along riffles and channel bars.

Core chemistry. The analyses of the core profilesindicate that throughout the investigated locations anddifferent sample depths, nitrate and ammonium con-centrations within the sandstone bedrock were gener-ally low (Figure 6) with averages ranging well below5 mgl�1 NO3-N and 0Ð5–1 mgl�1 NH4-N.

In contrast to the spatially homogeneous, low concen-trations in the sandstone bedrock, spatially very hetero-geneous concentrations of nitrate and ammonium havebeen observed within the loose superficial sediments. AsFigure 6 shows for selected streambed sediment cores,a high horizontal variability of concentrations varyingbetween different locations as well as a vertical variabil-ity of concentrations within the same core were identified.Nitrate concentrations in the loose superficial sedimentsare found to be generally higher than in the PTS. Con-centrations in several cases change by up to 300% fromthe bottom to the top of a core.

Peaks of nitrate or ammonium concentrations werefound to correlate with higher amounts of organic matter

Figure 5. Contour map (IDW) of the thickness of loose superficial streambed sediments overlaying the sandstone bedrock for River Leith Site A(a) and site Leith B (b). Interpolation based on inverse distance weighting (IDW) of 12 points for site Leith B and 18 points for site Leith A



Figure 6a. Vertical profiles of NO3-N, NH4-N concentrations and the fraction of organic carbon (foc) for four representative boreholes from siteLeith A (a) and from site Leith B (b), taken in August 2006. The bars left of the diagrams depict the core stratigraphy and hydraulic conductivities

of the core materials

in the sediment (Figure 6). In several cases mirroredconcentration profiles for ammonium and nitrate couldbe found, with nitrate concentrations increasing whileammonium concentrations decreased or vice versa. Suchmirrored concentrations as the observed, usually indicatechanging redox conditions, e.g. from oxic to reductiveenvironments (resulting in an increase in nitrate due tonitrification) or vice versa (e.g. core 22 in Figure 6b).The generally higher organic carbon content and broaderranges of nitrate and ammonium are limited to the loosesuperficial sediments overlaying the sandstone bedrock.

Water fluxes—groundwater discharge

Hydraulic head measurements were carried out fort-nightly in the streambed piezometers in the summer of2006 and 2007 during the observed baseflow periods.The general pattern was one of upwelling flow fromgroundwater to river. However, the measured hydraulicgradients varied significantly (0–0Ð22) in space and timethroughout the observation period. Figure 7a (left-handside) shows the spatial pattern of observed hydraulic headgradients in sites Leith A and Leith B for an exemplarydate in August 2006 as interpolated contour map (IDW).The interpolation is based on the piezometer locationsshown in Figure 4 with 18 (Leith A) and 12 (Leith B)

piezometers taken into account. To show the upwellingof groundwater into the superficial streambed sediments,the hydraulic heads observed at the deep piezometers(80–100 cm) have been taken into account. Piezome-ters at these depths are located in the sandstone bedrock,which is characterized by spatially very homogeneoushydraulic conductivities.

At Leith B (Figure 7b), the hydraulic gradients at thewestern part of the stream reach are up to three timeshigher than in the eastern part, indicating higher ratesof upwelling groundwater in these sediments, as thehydraulic conductivity of the sediments 80–100 cm deeppiezometers are located in is very homogeneous and doesnot vary in the horizontal to a degree that would explainthese differences (4Ð36 10�6 ms�1 –4Ð63 10�5 ms�1).Figure 7b (right-hand side) shows the distribution ofthe hydraulic gradient at two cross-sectional transects.The box plots comprise the complete data set of the80–100 cm deep piezometers for the entire observationperiod, indicating that the spatial patterns of upwellinggroundwater are not just episodic exceptions.

The observed pattern of higher upwelling (impliedfrom the hydraulic gradients) in the western part of thestreambed can be observed throughout the monitoringperiod. Observations of the hydraulic heads at site



Figure 6b. (Continued )

Leith A (Figure 7a) show, with the exception of onepiezometer, similar positive head gradients, indicatinggroundwater upwelling. According to the contour map(Figure 7a), at site Leith A, the hydraulic gradientsin the NW downstream part are significantly higher(average 10) than in the SW upstream part (average2). However, as two cross-sectional transects for thetemporally averaged head gradients at Leith A show(Figure 7a, right-hand side), these spatial patterns seemto not persist over the entire observation period, whichis in contrast to the observations been made at Leith B(Figure 7b, right).

Groundwater/surface water chemistry

The analysis results of samples taken from the flood-plain and streambed piezometers as well as river watersamples indicate that during the research period thechemical signature of groundwater, surface water andthe streambed pore water in the research area weresignificantly distinctive. Figure 8 shows the distributionstatistics (average, range, quartiles) for nitrate-nitrogen,ammonium-nitrogen and DO concentrations observed inthe period August 2006–October 2007 within the ground-water, streambed pore water and surface water of theriver Leith. The riparian groundwater is low in DO (aver-age 2 mgl�1). Groundwater nitrate concentrations are

low (average ca 1 mgl�1 NO3-N), whereas ammoniumconcentrations in the Leith groundwater can reach rel-atively high values of up to 2 mgl�1 NH4-N (average0Ð5 mgl�1).

The Leith surface water shows high concentrations ofDO (average 11Ð5 mgl�1). Ammonium concentrations aregenerally moderate, however, maximum concentrationsjust exceed 0Ð5 mgl�1 NH4-N that can be toxic for somefish species. The generally moderate nitrate concentra-tions (average 4 mgl�1 NO3-N) are varying in time, rang-ing from 0Ð5 to 7Ð5 mgl�1 NO3-N. The streambed porewater nitrate concentrations vary even slightly more thanthe concentrations observed in the surface water (rangingfrom 0Ð2 to >9 mgl�1 NO3-N). Ammonium concentra-tions are low (max 0Ð7 mgl�1 NH4-N) and DO concentra-tions are moderate (5 mgl�1), but can vary significantlyin space and time.

A one-way analysis of variance (ANOVA) has beencarried out for the nitrate, ammonium and DO concen-trations, observed within the groundwater, surface waterand within the streambed pore water, in order to deter-mine if the chemical signature of different water sourceswere statistically different. The variance of residuals wasconstant and the ANOVA proved the means and varianceof chemical data to be significantly different at a 0Ð05significance level.



Figure 7a. Contour map (IDW) of the hydraulic head gradients observed at the 18 August 2006 in 80–100 cm depths at 18 piezometers of site A(a) and 12 piezometers at site Leith B (b) (interpolation method: inverse distance weighting) and statistics of the hydraulic head gradients for twocross-sectional transects during the entire observation period (August 2006–October 2007). Box plots are based on data from 18 sample dates per

piezometer location and show min, 5, 25, 75, 95 percentiles, mean and max values

Piezometer analysis—spatial variability of streambednitrate concentrations

Pore water nitrate concentrations in the streambedwere monitored in nested streambed piezometers at15–30, 40–60 and 80–100 cm depths (Figure 4). Acomparison of the pore water nitrate concentrations,measured throughout the observation period 2006 and2007, verified the vertical nitrate concentration patternfound in the streambed sediment cores from August 2006.

Figure 9 shows the average concentrations, 25 and75% quartiles and ranges of nitrate concentrations, mea-sured at six observation dates during the 2006 base-flow period from 12 piezometer nests for site B and18 piezometer nests at site A, respectively. At siteLeith B, nitrate concentrations in the deeper piezome-ters, most of them located in the sandstone bedrock,are generally low (average 4Ð6 mgl�1NO3-N) and spa-tially less variable (range of 3Ð5 mgl�1 NO3-N withmin 3Ð1 and max 6Ð6 mgl�1 NO3-N) than in shallowerpiezometers at 40–60 cm depth, which show a rangeof 7Ð8 mgl�1 NO3-N with concentrations extending frommin 0Ð5 mgl�1 NO3-N to max 8Ð3 mgl�1 NO3-N (aver-age 4Ð4 mgl�1 NO3-N). Within the shallowest set ofpiezometers (15–30 cm), nitrate concentration ranges of

7Ð7 mgl�1 NO3-N have been observed, with min valuesof 0Ð1 mgl�1 NO3-N and max values of 7Ð8 mgl�1

NO3-N (average 4Ð0 mgl�1 NO3-N).The range of observed nitrate concentrations in the

pore water of superficial bed sediments is more than twicethat measured within the sandstone bedrock. This showsthat at site Leith B, groundwater nitrate concentrations getsignificantly altered during the upwelling flow throughthe loose superficial streambed sediments (Figure 9b). Asthe changing ranges of observed nitrate concentrationsindicate, nitrate concentrations are increasing or decreas-ing on the flow from the deep to the shallow streambedsediments. The distribution of nitrate concentrations atdifferent depths does not show any significant skewnessthat would indicate a domination of upwards decreasingor upwards increasing concentrations.

In contrast, at site Leith A, were the vertical extentof loose superficial streambed sediments is smaller,only small changes of nitrate concentration rangesalong the upwelling flow through the streambed canbe observed (Figure 9a). In the deep piezometers, alllocated in the sandstone bedrock, the nitrate con-centrations range is 4Ð2 mgl�1 NO3-N with concen-trations extending from min 0 mgl�1 NO3-N to max



Figure 7b. (Continued )

Figure 8. Chemical signature of surface water (SW), groundwater (GW) and pore water at a depth of 20–100 cm in the streambed (HZ), based onobserved nitrate, ammonium and dissolved oxygen concentrations from six sample dates between August 2006 and October 2007 with SW sampledat 1 location, GW sampled at 10 locations and HZ sampled at 36 locations. Box plots indicate min, 5, 25, 75, 95 percentiles, mean and max values

4Ð2 mgl�1 NO3-N (average 1Ð4 mgl�1 NO3-N). In theshallower piezometers, only marginally larger nitrateconcentrations ranges were observed with a rangeof 4Ð7 mgl�1 NO3-N (min D 0 mgl�1 NO3-N, max D4Ð7 mgl�1 NO3-N) for the 40–60 cm deep piezometersand 5Ð6 mgl�1 NO3-N (min D 0 mgl�1 NO3-N, max D5Ð6 mgl�1 NO3-N) for the 15–30 cm deep piezome-ters. Average nitrate concentrations are slightly increas-ing with 2Ð0 mgl�1 NO3-N at 40–60 cm depth and2Ð4 mgl�1 NO3-N at 15–30 cm depth. The skewness ofthe distribution of the nitrate concentration data indicatesthat the maximum values presented for site Leith A rep-resent extreme values compared to the majority of data

shown by the 25 and 75% quartiles. Furthermore, themeans and quartiles indicate a small tendency for increas-ing concentrations from deep to shallow piezometers.

The range of groundwater nitrate concentrations at siteLeith B changes by >100% along the upwelling flowbetween the deep and the shallow streambed piezometers,whereas the concentration ranges change only by 25% atsite Leith A. The observed changes in pore water nitrateconcentrations from the deep to the shallow piezometers(Figure 9) correlate with the length of the flow paththrough loose superficial sediments (Figure 5). At siteLeith B, where superficial streambed sediments are withapproximately 80 cm, four times as deep as on site Leith



Figure 9. Range of nitrate concentrations in streambed pore water observed at different depths of the research area during the observation period13 August 2006–21 October 2006. Samples from piezometers at approximately 100 cm depth origin mainly from the sandstone bedrock, samplesfrom the intermediate (approximately 50 cm) and shallow (approximately 20 cm) piezometers origin from loose superficial streambed sediments

overlaying the PTS. Box plots are based on 176 samples for site A and 158 samples for site B

Table I. Ranges of sandstone bedrock (PTS) and streambed redox conditions by averaged concentrations of nitrate (NO3-N),ammonium (NH4-N), organic carbon content (Corg), dissolved oxygen concentrations (DO) and the oxidation-reduction potential

(ORP) observed in piezometers located in the loose superficial sediments and in the PTS

Loose superficial sediments (0–1 m) Sandstone bedrock (PTS)

Min Max Range Min Max Range

NO3-N (mgl�1) 0Ð02 9Ð17 9Ð15 1Ð29 6Ð88 5Ð59NH4-N (mgl�1) <LOD 0Ð84 0Ð84 <LOD 0Ð30 0Ð30Corg (mgl�1) <LOD 2Ð9 2Ð9 <LOD 0Ð37 0Ð37DO (mgl�1) 0Ð89 9Ð25 8Ð34 1Ð93 8Ð40 6Ð47ORP (mV) �107Ð2 114Ð4 221Ð6 �55Ð5 112Ð6 168Ð1

A with approximately 20 cm, also the average nitrateconcentration changes from deep to shallow sedimentswhich are significantly higher than at site Leith A. Theshorter flow path across the less deep loose superficialsediments on site Leith A is causing less alteration ofnitrate pore water concentrations than the longer flowpath observed for Leith B.

The intensity of nitrate concentration changes is corre-lated to the specific sediment characteristics, which differfor the loose superficial bed sediments and the sandstonebedrock. Hereby, type and efficiency of the hyporheicnitrate transformation processes (that are responsiblefor the observed concentration changes) are widelydetermined by the pore water redox conditions of thestreambed. As Table I shows, redox-relevant parameters,such as the organic carbon content (Corg), DO, ORP,are more divers and show a significantly larger rangewithin the loose superficial streambed sediments than inthe sandstone bedrock.

The complex changes in pore water nitrate concentra-tions along the upwelling flow path lead to an increase ofvariability in the observed horizontal spatial patterns ofnitrate concentration from the deep to the shallow sed-iments. Figure 10a,b shows contour maps [IDW of 12(site Leith B) and 18 (site Leith A) observation points] ofnitrate concentrations in the uppermost and in the deepeststreambed piezometers (located in the sandstone bedrock)

of both sites. At site Leith B, nitrate concentration pat-tern at the sandstone bedrock (Figure 10b) correlate withthe observed pattern of hydraulic gradients (Figure 7b).In the western part of the stream reach, higher rates ofupwelling nitrate-low groundwater (Table I) lead to lowerconcentrations compared to those in the south, where alsoless nitrate-low groundwater is upwelling (Figure 7b).For the loose superficial sediments at 15–30 cm depth,however, a significant increase in the variability of spa-tial patterns in nitrate concentrations can be observed(Figure 10b). This indicates that the flow through the spa-tially heterogeneous layer of superficial bed sediments issubstantially changing the spatial pattern of nitrate con-centrations in the upwelling groundwater. Similarly tosite Leith B, also at site Leith A, a clear spatial pat-tern of nitrate concentrations in the deeper piezometerswas observed, with higher concentrations in the NW andlower concentrations in the SE (Figure 10a). The concen-tration range in the observed pattern is slightly smallerthan the one observed for site Leith B. In contrast tosite Leith B, the observed concentration pattern in theloose superficial bed sediments of 15–30 cm do not dif-fer substantially from the pattern observed in the deepersediments of 80–100 cm (Figure 10a). Apparently, thesmaller depth of loose superficial bed sediments is result-ing in less alteration of the spatial pattern of nitrateconcentrations in the upwelling groundwater than it hasbeen observed for site Leith B.



Figure 10. Contour map (IDW) of nitrate concentrations in deeper streambed sediments of 0Ð5–1Ð0 m and in shallow streambed sediments at0Ð15–0Ð3 cm depth (interpolation method: inverse distance weighting) for 18 piezometer locations at site Leith A (a) and 12 piezometer locations at

site Leith B (b)

Substantial changes of nitrate concentrations in the

upwelling flow path of site Leith B are furthermore

shown in Figure 11, which presents the fate of pore water

nitrate concentrations for a longitudinal streambed tran-

sect. The interpolation of the nitrate concentrations is

based on observations at 18 piezometer locations cov-

ering three depths. Flow direction and intensities are

indicated by the direction and strength of the flow vectors

given in Figure 11. The most intensive upwards concen-

tration changes can be observed at riffle crests and in



Figure 11. Contour map (IDW) of nitrate concentrations along a longitudinal cross-section of site Leith B including the location of piezometers usedfor the interpolation (interpolation method: inverse distance weighting), arrows indicating upwelling flow intensities of the groundwater (based on

the observed pressure head gradients)

pool sections, whereas within the central riffle sectionsthe concentrations in the upwelling groundwater stay rel-atively constant. The pattern of vertical concentrationchanges shown for this exemplary transect, with decreas-ing nitrate concentrations at riffle crests and increasingnitrate concentrations within pool sections, seem to becharacteristic for the investigated stream section and havebeen found in other locations at the field site as well.

The data for both sites, Leith A and Leith B, showthat changes in nitrate concentrations in the upwellinggroundwater can be manifold with decreasing concen-trations in some areas and increasing concentrations inothers. Figure 12 shows the change of nitrate concen-trations from deep to shallow piezometers for differentlocations at site Leith B. In 2006, for some locations(e.g. piezometer locations 27 and 28 in Figure 12) nitrateconcentrations strongly increase along the upwelling flowpath, whereas for other locations decreasing concentra-tions from the deep to the shallow piezometers have beenobserved (e.g. location 21, 31 in Figure 12).

Piezometer analysis—temporal variability of streambednitrate concentrations

The nitrate concentrations along the upwelling flow-path do not just vary in space, but the found spatial pat-tern itself can be of temporal variability. Locations witha broad range of observed nitrate concentration changeswithin the same year (e.g. piezometer 29 and 30 in 2006and piezometer 29 and 20 in 2007) indicate a strongtemporal variability of nitrate concentration changes dur-ing the same observation period. Furthermore, compar-ing the upwelling nitrate concentration changes for dif-ferent locations during the baseflow periods in 2006(Figure 12a) with conditions in 2007 (Figure 12b), it canbe seen that, although for some locations the concentra-tion changes remain widely constant during both seasons

(e.g. piezometer locations 27 and 28), at other locationsthat showed decreasing concentrations in 2006, concen-trations are increasing in 2007 (piezometer 24, 26, 31)and vice versa (piezometer 20 and 25). These results indi-cate that nitrate concentrations and their change along theupwelling flowpath do vary in space and time.

A comparison of the streambed nitrate concentra-tions with the in-stream concentrations in the studyarea reveals that the overall impact of nitrate contri-butions from the streambed to the river varies signif-icantly in time. As Figure 13 shows, at site Leith B,during summer baseflow conditions, pore water nitrateconcentrations in the streambed are usually higher (aver-age 4–5 mgl�1 NO3-N) than the concentrations observedwithin the river (average 1Ð5–3Ð5 mgl�1 NO3-N). Duringthis period, the streambed pore water discharging intothe river is increasing the in-stream concentrations andthus represents a net contribution to the surface waterwithin this stream reach. However, during high flowevents at the end of the baseflow period, the propor-tions of the pore water and the surface water nitrateconcentrations appear to change. While nitrate concen-trations in the streambed pore water remain relativelyconstant (5 mgl�1 NO3-N), concentrations in the riverare rising above pore water levels (7Ð8 mgl�1 NO3-N).As a result, during high flow events, streambed porewater contributions to the river rather dilute the in-stream nitrate concentrations. During baseflow conditionsin 2007, streambed pore water concentrations, similarlyto the observations of 2006, are higher than surfacewater concentrations again. In contrast, at site LeithA, nitrate concentrations in the streambed pore waterare usually smaller (1Ð5–2Ð2 mgl�1 NO3-N) than sur-face water concentrations (1Ð5–3Ð5 mgl�1 NO3-N). Thiscondition remains stable also during high flow eventswhen streambed pore water concentrations increase to



Figure 12. Range of nitrate concentration changes in the upwelling groundwater from deep to shallow piezometers of site Leith B for the observedbaseflow periods in 2006 (a) and 2007 (b). Positive values indicating increases and negative values indicating decreases of nitrate concentrationsalong the upwelling flow. Box plots are based on 60 (2006) and 67 (2007) calculated differences of nitrate concentration between bottom and top

piezometers and indicate min; 5, 25, 75, 95 percentiles; mean and max values

2Ð5 mgl�1 NO3-N, but concentrations in the river rise upto 7Ð9 mgl�1 NO3-N. Throughout the observation period,at site Leith A, streambed pore water contributions to theriver dilute the in-stream nitrate concentrations.

CONCLUSIONS

Significant spatial variation in nitrate concentrationsin upwelling groundwater was observed across thegroundwater–surface water interface in the streambedof the study area. The observed changes in nitrate con-centrations were not limited to an area of permanentgroundwater–surface water mixing in the streambed,classically understood as the HZ, but were recordedup to 1 m deep in the superficial sediments that over-lay the in situ sandstone bedrock. Nitrate concentra-tions have been observed to change significantly at thegroundwater–surface water interface with concentrationsaltering by up to 300%, depending on the depth ofthe loose superficial streambed material. Thereby notonly a reduction of pore water nitrate concentrationsin the streambed has been observed, but also increasesof nitrate concentrations along the upwelling pathwayhave been found frequently at both sites in the researcharea. The magnitude and direction of the spatially veryvariable concentration changes along the hyporheic flow

path appear to mainly depend on the spatial pattern ofupwelling groundwater, flow rates and superficial sedi-ment depth overlaying the sandstone bedrock, and hasbeen shown to be highest in areas with intensive redoxreactivity, due to high contents of organic matter and DO.

The streambed impact onto nitrate concentrationchanges within the upwelling groundwater differsbetween the two experimental reaches. The compositionand depth of the loose superficial streambed sedimentsappear to be an important control on the variation innitrate concentration at both sites. While the ranges ofnitrate concentrations changed by more than 100% inLeith B with superficial sediments overlaying the sand-stone bedrock with up to 80 cm, at Leith A, with a shal-lower superficial sediment layer (average 20 cm), nitrateconcentrations did not change similarly intensive alongthe upwelling groundwater flow through the streambed(less than 25%).

As the study reaches, the pattern of variation in nitrateconcentrations in the streambed appears to be relatedto the variation in organic carbon and DO in the loosesuperficial sediments, and not to the actual infiltrationof surface water into the streambed. Large nitrate con-centration changes indicating high transformation rateshave been found to extend to the full depth of loose sed-iments overlaying the sandstone bedrock with evidently



Figure 13. The range of nitrate concentrations observed within the streambed piezometers compared to nitrate concentrations in the surface waterand river discharge for the observed baseflow periods in 2006–2007 at site Leith B (a) and the baseflow period in 2006 for site Leith A (b)

no surface water–groundwater mixing occurring at thesedepths. Although the experimental setup of this study didnot cover the top 5–10 cm of the streambed sedimentsand thus, local surface water infiltration at this depth can-not be ruled out, during the observed baseflow periods,no groundwater–surface water mixing in the streambedcould be observed at any locations below 10 cm depthbut still, intensive alteration of nitrate concentrations wasfound in these deeper areas of the streambed. This appar-ent contradiction highlights some inconsistency in alter-native HZ concepts of different scientific communities.For the presented case study, the typical hydrologicaldefinition of the HZ by the actual mixing of surfacewater and groundwater in the streambed fails to predictthe spatial extent of the HZ as a redox-reactive transfor-mation zone in the streambed, which coincides with thebiogeochemical definition of HZ activity. Hence, investi-gations of the hyporheic biogeochemical process dynam-ics, which spatially delineate the HZ by the evidence ofsurface water mixing into the streambed, may tend tounderestimate its redox activity and turnover capacity. Ithas furthermore been shown that the hyporheic alterationof nitrate concentrations in the streambed as well as thedelivery of groundwater nitrate to the surface water at thefield site is temporally variable (a) during different flowconditions and (b) within the same baseflow period.

Further work is needed to establish whether episodicmixing of surface water and groundwater in the streambed

could be sufficient to supply the organic matter and DOneeded for denitrification and which role denitrificationplays in explaining the spatial pattern of nitrate concentra-tions in the streambed. Future studies may aim to quantifyto what degree high nitrate concentrations in some partsof the streambed result from effective nitrification alongthe upwelling flowpath, nitrate being transported fromthe surface water into the streambed during high flowsand flow inversion or the breakdown of organic nitrogencompounds in the organic rich streambed sediments. Interms of the capacity of the HZ to ameliorate ground-water nitrate fluxes to the river, our results suggest ahigh complexity of spatial N variation to depths evenon the small scale of this investigation. Further researchis needed to upscale from this small scale experimentallygained knowledge to make assumptions on the HZ impacton a larger sub-catchment scale.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the German ResearchFoundation (DFG) for supporting SK on this project (KR3305/2-1). We furthermore would like to thank D. Kaeser(Lancaster University) for providing data on hydraulicconductivities, grain size distributions and streambedtopography and J. W. N. Smith (formerly of the ScienceGroup of the Environment Agency of England and Wales)for providing the data of organic carbon contents in the



sediment cores and for his field support. Many thanks alsoto S. Skradde (formerly University of Potsdam, Germany)and S. Nutter (Glasgow University) for their support inthe field and in the lab. The authors would like to expresstheir gratitude to the anonymous reviewers for their veryhelpful and constructive comments that greatly helped toimprove this paper.

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Nitrate concentration changes at the groundwater-surface water interface of a small Cumbrian river