Discrete wetland groundwater discharges revealed with a three-dimensional temperature model and...

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Hydrogeology JournalOfficial Journal of the InternationalAssociation of Hydrogeologists ISSN 1431-2174 Hydrogeol JDOI 10.1007/s10040-015-1242-5

Discrete wetland groundwater dischargesrevealed with a three-dimensionaltemperature model and botanicalindicators (Boxford, UK)

Andrew R. House, James P. R. Sorensen,Daren C. Gooddy, Andrew J. Newell, BenMarchant, J. Owen Mountford, PeterScarlett, et al.

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Discrete wetland groundwater discharges revealedwith a three-dimensional temperature modeland botanical indicators (Boxford, UK)

Andrew R. House & James P. R. Sorensen &

Daren C. Gooddy & Andrew J. Newell & Ben Marchant &J. Owen Mountford & Peter Scarlett &Peter J. Williams & Gareth H. Old

Abstract Wetlands provide unique goods and services, ashabitats of high biodiversity. Hydrology is the principalcontrol on wetland functioning; hence, understanding thewater source is fundamental. However, groundwaterinflows may be discrete and easily missed. Researchtechniques are required with low cost and minimal impactin sensitive settings. In this study, the effectiveness ofusing a three-dimensional (3D) temperature model andbotanical indicators to characterise groundwater dischargeis explored at the CEH (Centre for Ecology andHydrology) River Lambourn Observatory, Boxford, UK.This comprises a 10 ha lowland riparian wetland,designated for its scientific interest and conservationvalue. Temperature data were collected in winter atmultiple depths down to 0.9 m over approximately3.6 ha and transformed into a 3D model via ordinarykriging. Anomalous warm zones indicated distinct areasof groundwater upwelling which were concurrent withrelic channel structures. Lateral heat propagation from thechannels was minimal and restricted to within 5–10 m.

Vertical temperature sections within the channels suggestvarying degrees of groundwater discharge along theirlength. Hydrochemical analysis showed that warmer peatwaters were akin to deeper aquifer waters, confirming thetemperature anomalies as areas of groundwater discharge.Subsequently, a targeted vegetation survey identifiedCarex paniculata as an indicator of groundwater dis-charge. The upwelling groundwater contains high concen-trations of nitrate which is considered to support thespatially restricted growth of Carex paniculata against abackground of poor fen communities located in reducinghigher-phosphate waters.

Keywords Wetlands . Groundwater/surface-waterrelations . Temperature . Biological indicator . UK

Introduction

Wetlands serve important environmental, cultural andeconomic functions. Particularly sensitive to environmen-tal change, groundwater-dependent wetland ecosystemsare identified as priority habitats under the EuropeanHabitats Directive (EEC 1992). Consequently, there is agrowing realisation of the need for sustainable wetlandmanagement (Maltby and Acreman 2011). The establish-ment and maintenance of wetlands depends primarily onhydrology (Mitsch and Gosselink 2007), which is a keycontrol on vegetation (Baldwin et al. 2001; Wheeler et al.2009), fauna (Ausden et al. 2001; McMenamin et al.2008) and biogeochemical cycling (Lischeid et al. 2007;McClain et al. 2003).

Groundwater discharge to riparian wetlands can be asignificant component of the water balance (Krause andBronstert 2005). However, groundwater influxes aretypically complex and spatially heterogeneous (Huntet al. 1996; Lowry et al. 2007); being dependent upontopographical, geological and climatic factors(Sophocleous 2002; Winter 1999). Investigative tech-niques are required that can capture processes at a highspatial resolution. Such techniques should be non-

Received: 17 October 2014 /Accepted: 14 February 2015

* Springer-Verlag Berlin Heidelberg 2015

Electronic supplementary material The online version of this article(doi:10.1007/s10040-015-1242-5) contains supplementary material,which is available to authorized users.

A. R. House ()) : J. O. Mountford : P. Scarlett :G. H. OldCentre for Ecology and Hydrology,Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB, UKe-mail: [email protected]

A. R. HouseDepartment of Geography, University College London, 26 BedfordWay, London, WC1H OAP, UK

J. P. R. Sorensen · D. C. Gooddy · A. J. Newell · B. Marchant ·P. J. WilliamsBritish Geological Survey, Crowmarsh Gifford, Wallingford,Oxfordshire, OX10 8BB, UK

Hydrogeology JournalDOI 10.1007/s10040-015-1242-5

Author's personal copy

http://dx.doi.org/10.1007/s10040-015-1242-5

destructive and minimally invasive due to the sensitivityof many wetland environments; furthermore, resourcerestrictions often compel economical approaches.

Temperature is a useful natural groundwater tracerwhich can identify small-scale zones of groundwaterdischarge (Anderson 2005; Conant 2004). A significantbody of research is centred on identifying and quantifyinglocalised groundwater flux in the hyporheic zone (Briggset al. 2012; Constantz et al. 1994; Hannah et al. 2009;Keery et al. 2007; Krause et al. 2012; Schmidt et al.2007). Small-scale variability in temperature has beenconsidered along the horizontal plane of a cross-fault ditch(Bense and Kooi 2004); however, applications to wetlandsare fewer, with temperature observations in such studiesalso restricted to a limited number of point measurements(Bravo et al. 2002; Hunt et al. 1996).

Botanical indicators of groundwater also have thepotential to provide a cost-effective means of sitecharacterisation (Lewis 2012). Nevertheless, links be-tween phreatophytes (plants which draw water from thewater table) and groundwater discharge are moreestablished in arid and semi-arid regions than more humidregions (Batelaan et al. 2003). Plants have been regardedas indicators of groundwater discharge in the Netherlands(Grootjans et al. 1988; Klijn and Witte 1999; Lucassenet al. 2006; Schot et al. 1988; van Diggelen et al. 1988;Wassen et al. 1988; Wierda et al. 1997) and Minnesota,USA (Almendinger and Leete 1998; Batelaan et al. 2003;Glaser et al. 1990; Goslee et al. 1997; Rosenberry et al.2000). Research in the UK has focussed on general waterrequirements of wetland plant communities and species(Gowing et al. 2002; Newbold and Mountford 1997;Wheeler et al. 2009). Specifically, botanical indicators forgroundwater are not defined in the UK.

This report aims to demonstrate the potential of 3Dtemperature models in effective and economical concep-tualisation of groundwater/surface-water interaction, witha focus on lowland riparian wetlands. The potential ofbotanical indicators of groundwater discharge is alsofurther explored.

Study area

Site descriptionThe Centre for Ecology and Hydrology (CEH) RiverLambourn Observatory (51.445° N 1.384° W) encom-passes ca.10 ha of riparian wetland bordering a 600 mreach of the River Lambourn (Fig. 1). The wetland andRiver Lambourn are designated as a Site of SpecialScientific Interest (SSSI) and Special Area of Conserva-tion (SAC), due to their importance for Vertigomoulinsiana (Desmoulin’s whorl snail), Lampetra planeri(brook lamprey) and Cottus gobio (bullhead). This is inaddition to the river habitat (Annex 1 habitat from EUHabitat Directive: Watercourses of plain to montane levelswith Ranunculion fluitantis and Callitricho-Batrachionvegetation) and terrestrial plant communities—MG8

vegetation community of the UK National VegetationClassification (Rodwell 1991).

The site is underlain by the Seaford Chalk Formation, auniform soft to medium-hard chalk with frequent flintnodules, which dips at 1–2° to the southeast (Allen et al.2010). Up to 9.1 m of river terrace deposits and alluviumoverlie the Chalk, and consist primarily of coarse gravels,which are on average 4–5 m thick with local thickeningand thinning. In their lower layers there is often a highproportion of reworked chalk material. Additionally, adiscontinuous layer of highly weathered and low perme-ability ‘putty chalk’ (Younger 1989) approximately 5 mthick exists between the gravel-chalk interface andcoherent chalk bedrock. The alluvial cover overlying thegravels ranges up to 2.7 m thick, mostly consisting ofpeat, with intermixed occurrences of silt, sand and gravel(Allen et al. 2010). Within the wetland, these deposits aretypically around a metre thick (Chambers et al. 2014).

The site is located 13 km downstream from theephemeral source of the River Lambourn at Lynch Wood,Lambourn (51.512° N, 1.529° W). The river drains the

Fig. 1 Map of the UK, showing the location of CEH RiverLambourn Observatory and the instrumentation network

Hydrogeology Journal DOI 10.1007/s10040-015-1242-5


Chalk of the Berkshire Downs and is characterised by alarge baseflow component. At Shaw, 5 km downstream ofthe observatory, the river has a base flow index of 0.96and mean discharge of 1.73 m3/s (Marsh and Hannaford2008). The wetlands to the west of the river are dissectedby the Westbrook Channel, which separates the site into anorthern and southern meadow.

Historic maps dating back to the 1880s show a networkof predominantly linear conduits, sluices and aqueductscharacteristic of a managed water meadow system(Everard 2005). In particular, a channel through the northmeadow displays a sinuosity and irregularity indicative ofnatural origin. Most of these channels have naturallyinfilled and are absent from current maps.

Initial conceptual modelWetland water levels have been inferred to be principallycontrolled by the River Lambourn and Westbrook stage(Atkins 2005; Musgrave 2006; Old et al. 2014). Ground-water contributions are regarded as more significant in thesouth meadow, though the possibility of groundwaterupwellings at the northernmost extent of the site has beensuggested from hydrochemical analysis (Musgrave 2006).Such deductions were, however, based on an arbitrarilypositioned transect and cluster of randomly locatedpiezometers in each meadow.

The low permeability ‘putty’ chalk is considered to actas a confining layer to the Chalk aquifer, and its unevendistribution has implications for exchange betweengroundwater and surface water (Chambers et al. 2014).Studies at a nearby site 100 m upstream of theObservatory, part of the Lowland Catchment Research(LOCAR) programme (Wheater et al. 2007), haveindicated variable hydraulic connection between theChalk, gravels and surface water (Abesser et al. 2008;Allen et al. 2010; Lapworth et al. 2009). The extent of theinteraction remains unclear.

Methods

3D peat temperature modelTemperature of the peat was characterised in daylighthours over 5 days in February 2013 when the difference insurface temperature and groundwater temperature wasexpected to be pronounced. The north meadow wassurveyed between 11–13th February and the southmeadow between 18–19th February. An Oakton™ TypeT thermocouple probe connected to a thermocouplethermometer was inserted into the peat at 1,056 locationson an approximate 5 m × 5 m grid (Fig. 2a). These weregeoreferenced with differential GPS (dGPS). Measure-ments were taken at depths of 0.15, 0.30, 0.45, 0.60, 0.75,and 0.90 m below ground level (bgl) resulting in a total of5,109 temperature measurements, with the peat less than0.90 m thick in places. Dense scrub and watercourseboundaries confined the survey extent.

Fig. 2 a Locations of 1056 peat temperature profiles b Variogramsin the horizontal and vertical directions



Temperature data were imported into the 3D visualisa-tion and analysis application Paradigm SKUA 2011.3™.This is a powerful modelling package widely used in theearth sciences, allowing the integration and analysis ofborehole, geophysical and other subsurface data. A 3Dgrid was constructed which encapsulated the temperaturemeasurement points and had cell dimensions of 1 m × 1 min the horizontal plane and 0.05 m in the vertical direction.The north and south meadows were kept separate. Thegrid was positioned so that the temperature measurementpoints coincided with the centre of grid cells. It wasdeformed to align each layer of cells parallel to theundulating surface topography. Measured temperaturevalues were assigned to the grid cell in which theyoccurred and, following variogram modelling (Fig. 2b),were interpolated throughout the gridded volume usingordinary kriging (Webster and Oliver 2007).

The spherical 3D variogram model was estimatedwithin Paradigm SKUA 2011.3™ by the method ofmoments. The software includes the capability to includeanisotropy in the variogram model. This is where theexpected squared difference between a pair of observa-tions is a function of both the length and the direction ofthe vector which separates the measurement locations. Incontrast, isotropic models assume that the variogram ispurely a function of the length of the vector.

In the horizontal plane, any directional-dependenceappeared to be caused by a single feature in the northernmeadow. The temperature was less variable parallel to thisthan perpendicular to it. Since the feature dissected themeadow, it was not possible to represent this anisotropyby a geometric distortion (stretching or contraction) of thevariogram range in a single direction. Any attempt to doso would have led to artefacts in the predicted temperaturesurface. There was a clear difference between thevariograms in the horizontal and vertical planes; thus,the estimated model was isotropic in the horizontal planewith geometric anisotropy in the vertical plane (Websterand Oliver 2007). The estimated spherical variograms areshown in Fig. 2b. The estimated nugget and sill variancesare 0.01 (°C)2 and 0.52 (°C)2, respectively. The range ofthe model is 35.5 m in the horizontal direction and 0.3 min the vertical direction.

Site instrumentationAn existing gridded piezometer array was numbered 1–13(Fig. 1). Supplemental piezometers were added in May2013 to target observed temperature anomalies (locations14–19; Fig. 1). All locations comprise separate peat andgravel piezometers, with the exception of location 8 wherethe peat was too thin to complete an installation. Gravelpiezometers are screened approximately 2.5–3.5 m bgl,whilst peat piezometers are screened across the entire peatthickness. Chalk boreholes are also located at sites 3, 20and 21; these are screened at 9.5–10.0, 8.0–9.0 and 5.0–6.0 m bgl, respectively.

Groundwater heads are routinely measured manuallyby dipping observed water levels. Moreover, gravel

groundwater temperatures are monitored every 5 minusing either In-Situ Level Troll® 500s or SWS Divers®installed to a consistent depth of 3 m bgl in piezometers1–13. The River Lambourn temperature is measured every15 min at monitoring site SW1, with a Druck® PDCR1830 (Fig. 1). Channel stage is regularly observed at fourstage boards along the River Lambourn (a–d) and two inthe Westbrook (e–f). Continuous 15 minute averaged airtemperature data and subsurface temperature at 0.1, 0.3and 0.5 m depths are recorded at an automatic weatherstation (AWS) using CS215™ and 107-LC™ temperaturesensors, respectively.

Water sampling and analysisGroundwater samples were collected in May 2013 fromall gravel and chalk piezometers once stable fieldmeasurements were obtained for dissolved oxygen (DO),pH, specific electrical conductance (SEC), redox potential(Eh) and temperature. These measurements were collectedusing Mettler-Toledo™ probes contained within a flow-through cell to inhibit any contact with the atmosphere.Samples from all peat piezometers were obtained follow-ing a single purge, as the transmissivity of the peat wastoo low to sustain continuous abstraction with a peristalticpump (0.1–0.5 l/min) in piezometers 1–13. Hence, onlyfield measurements of pH and SEC were collected in thepeat. Surface water samples were also obtained from theRiver Lambourn at SW1 and Westbrook at SW2 (Fig. 1).

Laboratory sample preparation and analysis wereperformed following the procedures outlined by Nealet al. (2011). Major anions and dissolved metals weredetermined via Dionex™ liquid chromatography and ICP-OES, respectively. Alkalinity was determined by titrationwith hydrochloric acid. Dissolved organic carbon (DOC)analysis was undertaken with a Thermalox™ C analyserfollowing acidification and sparging. Total phosphorus(TP) was determined by the method of Eisenreich et al.(1975). Soluble reactive phosphate (SRP) and ammonium-nitrogen were determined colorimetrically.

Linking vegetation coverage to hydrologyVegetation species were identified at a subset of thetemperature survey positions to identify potential botani-cal indicators of groundwater upwelling. The temperaturesurvey positions were selected by stratified randomsampling (De Gruijter et al. 2006) where the stratacomprised deciles of the observed temperature at 0.15 mdepth (Table 1). Twelve of the positions within eachstratum were selected at random. Vegetation species wereidentified at these 120 locations in addition to thelocations of the paired piezometers.

The survey was conducted with a 2 m2 quadrat in July2013 when stands were mature enough to allow easieridentification. Each quadrat was aligned north–south andlocated at its southwest corner using dGPS. For eachquadrat, individual species and their percentage coverwere indentified. These results were then used to allocate



the location to a particular community of the NationalVegetation Classification (NVC; Rodwell 1991) using theTABLEFIT procedure (Hill 1996). This establishes thedegree of agreement between species coverage in eachquadrat and the association tables in British plantcommunities. Only where goodness of fit values were atleast 50 %, were samples allocated. Further, the positionsof all Carex paniculata, easily recognisable in densetussocks up to 1.5 m tall and 1 m in diameter, wererecorded and matched to the local temperature decile.

Results

Temperature

3D peat temperature modelOverall temperature ranges between 2.1 and 10.3 °C in thepeat, with a mean temperature of 6.4 °C (σ=1.7 °C). In thenorth meadow, the mean temperature was 6.5 °C(σ=1.76 °C), whilst in the south, mean temperature waslower at 6.0 °C (σ=1.32 °C). The model shows discretewarm temperature anomalies (>9 °C) correlating withhistorical channels (Fig. 3), which have infilled and are nolonger present (Fig. 1). In the north meadow the anomalieslargely correspond to the path of the sinuous relic channel,whilst the main anomaly in the south meadow lies withina relic channel draining to the Westbrook. The anomaliesare tightly constrained with heat only propagating 5–10 mlaterally from the channel centre.

Temperature gradients vary widely across the site, andcross-sections through the model show they are relativelyconstant with depth at some of the strongest temperatureanomalies (Fig. 3c). Overall the mean temperatureincreases from 4.7 °C at 0.15 m to 7.9 °C at 0.9 m, whileσ drops from 1.3 to 0.9 °C. The decreasing σ with depthreflects a decreasing range from between 2.1 °C and10.0 at 0.15 m, down to between 5.7 and 10.3 °C at 0.9 m.

Awarm temperature anomaly is located at the source ofthe main relic channel in the north meadow, with furtheranomalies spread along its course (Fig. 3d). Anomalieswithin the peat frequently extend toward the surface, butthen gradually dissipate with distance. Greatest changes intemperature occur at shallow depths. Below 0.7 m warmanomalies are persistent along the entire channel lengthbefore dispersing at around 370 m from the source.

Logged air, river, peat and gravel temperatureAir temperature ranges during the peat temperaturesurveys were −0.3 to 2.6 °C and −5.0 to 10.6 °C for thenorth and south meadows respectively (Fig. 4). Thesefluctuations were significantly dampened through the peat

depth, with total variations of only 0.8, 0.3, and 0.1 °C at0.1, 0.3 and 0.5 m bgl, respectively.

Groundwater temperature within the gravels rangedfrom 9.4 to 10.2 °C between piezometers in the northmeadow, with the exception of piezometer 6, which wasaround 7.3 °C. There appears to be a temperature gradientacross the meadow with the warmest water located furthernorth. Within the south meadow, the gravel groundwaterwas cooler at 8.3–9.0 °C, while the River Lambourntemperature varied between 7.7 and 8.2 °C, with a meanof 7.9 °C, during the north meadow survey. During thesouth meadow survey the variation was 7.3 to 9.4 °C, witha mean of 8.4 °C.

Piezometric surface elevations (heads)Vertical upward head gradients from gravel to peat persistat sites 14–19 (Fig. 5), which are drilled into the relicchannels containing the warm temperature anomalies.Upward gradients also exist at sites 5 and 12, which arein close proximity to these channels, whilst elsewhere,predominantly downward gradients are present. Withinthe north meadow, there also appear to be highergroundwater heads in the peat and gravels in the vicinityof the relic channel. Chalk heads are higher than adjacentgravel and peat heads in the north meadow, but lower inthe south meadow.

Peat and gravel heads are significantly higher than theadjacent River Lambourn and Westbrook levels in thenorth meadow. In the south meadow, there is nosignificant gradient towards Westbrook. Generally,groundwater head contours follow a south-westerlydirection at an overall gradient of 0.003 in peat, gravel,and chalk (Fig. 5).

HydrochemistryWater chemistry from the peat piezometers falls into threedistinct groups, initially distinguished on the basis of theirSEC (Fig. 6). Group 1 comprises piezometers 14–19targeting the temperature anomalies, where peat waterchemistry is akin to the chalk waters. Group 3 encom-passes piezometers 1 and 4–13 where peat waters arecharacterised by elevated alkalinity, Ca, DOC, Si, NH4,TP, SRP, Fe and Mn, whilst they are depleted with respectto SO4 and NO3. Group 2 includes piezometers 2 and 3,which are considered intermediate waters between groups1 and 2. pH does not vary between groups with a mean of7.1, 7.0 and 7.1 for groups 1, 2 and 3, respectively.

There are strong positive correlations between alkalin-ity, DOC and Ca in group 3 (Fig. 7). Group 1 and chalkwaters, in contrast, are clustered, with group 2 watersfalling between. Gravel groundwater is reasonably well

Table 1 Boundaries of temperature deciles

Decile 1 2 3 4 5 6 7 8 9 10

Min temperature (°C) 0 3.4 3.7 4.0 4.2 4.4 4.6 4.9 5.5 6.8Max temperature (°C) 3.39 3.69 3.99 4.19 4.39 4.59 4.89 5.49 6.79 10.0



Fig. 3 Subsurface temperature across wetland: a 3D temperature model with ×20 vertical exaggeration, b plan view of temperature at0.7 m bgl with historical channel layout from 1882 displayed, c temperature cross-sections across wetland, and d cross-section alonginfilled relic channel in north meadow



mixed across the site, although alkalinity, TP and NO3

show some variation between groups. The gravel watersshow a similarity to chalk waters. Nevertheless, in places,the gravel waters display occasional characteristics ofgroup 3 peat waters. This is most notable at piezometer 6,which has comparably high SEC (0.7 mS/cm) andalkalinity (6.3 mEq/l), high concentrations of Ca(132.8 mg/l), DOC (5.7 mg/l), Si (8.9 mg/l), Mn (0.1 mg/l),and is depleted in SO4 (4.3 mg/l) and NO3 (2.6 mg/l).

Vegetation coverageIn terms of NVC plant communities the S28 Phalarisarundinacea tall-herb fen community dominates the site—Table S1 of the electronic supplementary material (ESM).Other prevalent communities include the OV24 Urticadioica–Galium aparine community, OV26 Epilobiumhirsutum community, S7 Carex acutiformis swamp, S6Carex riparia swamp, and S5 Glyceria maxima swamp.These communities are all distributed evenly across thetemperature deciles. Plant communities unclassified by theTABLEFIT procedure are also distributed evenly. Smallsamples of the communities S23 Other water-marginvegetation, and W6 Alnus glutinosa–Urtica dioica wood-land are observed in the warmest decile.

Amongst individual species, Glyceria maxima, Carexacutiformis, U. dioica, Phalaris arundinacea, Irispseudacorus and Galium aparine prevail (Table S2 ofthe ESM). These show no preference for areas withtemperature anomalies. Where species appear to show anaffinity to areas with warmer temperatures, such as Salixtriandra, S. fragilis, Carex paniculata, Caltha palustris,Typha latifolia and Lamium album, sample numbers arelimited (1–3 locations). Poa trivialis appears to show theconverse preference towards cooler areas, but based ononly four samples. The exhaustive Carex paniculatasurvey identified 55 of 59 individuals located in temper-ature deciles 9 and 10 (Fig. 8). Furthermore, Carexpaniculata were abundant in areas of the north meadow,but restricted to only a single individual in the southmeadow.

Discussion

Groundwater dischargeWarm temperature anomalies in this winter survey areconsidered to relate to areas of groundwater discharge.This is supported by groundwater head and hydrochemicaldata which indicate vertical upward head gradientstowards the peat, and peat water chemistry representativeof chalk waters in such areas. Discrete areas of ground-water discharge into infilled relic channels occur withinboth the north and south meadows.

In the north meadow, peat and gravel groundwaterheads are consistently higher than the river and Westbrookstages. These heads are likely to be supported byupwelling groundwater from the Chalk. This is borne outby warmer temperatures in both the peat and gravels. The

Fig. 4 River Lambourn temperature at SW1, with air and soiltemperature at AWS. A grey background indicates records notrelating to the period of the temperature spatial survey

Fig. 5 Groundwater head map for peat and gravels with verticalgradients (red upward, blue downward), chalk heads, and channelstages. Elevations in m above ordnance datum (mAOD)



heads around the main relic channel indicate this is likelyto be the focus of upwelling. Such small-scale variationsin gravel head have also been noted 100 m upstream ofthe wetland where they were attributed to probableupwelling of chalk waters associated with geologicalheterogeneities (Allen et al. 2010). An absence of either

relatively impermeable putty chalk at the chalk surface, orreworked chalk towards the base of the gravel matrix,would account for these observations (Fig. 9).

The warm temperature anomaly within the sinuousrelic channel of the north meadow is maintained at depth,yet loses constancy towards the surface. This is likely to

Fig. 6 Boxplots showing selected hydrochemistry of peat, gravels, and chalk. Peat and gravels are split into three groups to reflectvariations in chemistry within the peat. Chalk waters encompass both groundwater and surface waters. SEC specific electrical conductance,SRP soluble reactive phosphate, TP total phosphorus

Fig. 7 Grouped peat and chalk water concentrations of Ca against Alkalinity and DOC



be a result of groundwater discharge of varying magni-tudes. However, during hydrochemical sampling when thepeat was saturated across the site, piezometers away fromthe channel pumped with intermittent flow. Peat piezom-eters within the channel, however, sustained continuousabstraction indicating a higher permeability. Localisedupwellings and resultant increases in hydraulic headwould promote preferential lateral flow through morepermeable sediments of the relic channel. The observedheat transport could then encompass an element of lateraladvection induced by relic channel flow.

Hydrochemical evidence suggests some lateral ground-water movement away from the vicinity of relic channels,which is supported by the intermediate group 2 waterscontained within peat piezometers 2 and 3. Peat piezom-eter 3 is located down gradient of the northern relicchannel, from which water is likely to have at leastpartially originated given the lack of upward hydraulicgradient at the site itself. A relic channel exists 5–8 msouthwest of piezometer 2, yet beyond the survey extent.

At a distance of approximately 370 m along the relicchannel temperature anomalies cease. Further south thereis a large inversion in the vertical head gradient indicatingthe potential for downward movement of peat waters intothe gravels. Downwelling of peat waters within thisvicinity is further supported by hydrochemical andtemperature data at gravel piezometer 6.

In the south meadow, the main warm anomaly iscontained within a minor relic channel within 30 m of theWestbrook. This anomaly is relatively cooler compared tothose in the north meadow indicating less significantgroundwater discharge. Moreover, there is no significantgradient between the groundwater and surface waters.Therefore, groundwater is not considered to be a majorcontrol on peat heads in the south meadow. Heads arelikely to be supported by river stage and/or rainfall.

Biogeochemical wetland processesThe wetland appears to be acting as a highly dynamicbiogeochemical reactor (Prior and Johnes 2002). Upwellingareas are delivering waters rich in NO3 and SO4, which areremoved away from the temperature anomalies throughreductive bacterial processes (Fig. 9). On the other hand, theupwelling waters are depleted with respective to SRP andTP. Nevertheless, SRP concentrations (x = 53.1 μg/l,

Fig. 8 Distribution of Carex paniculata occurrence across tem-perature deciles

Fig. 9 Conceptual vertical section through the River Lambourn Observatory



σ=76.4 μg/l) suggest that P is not a limiting nutrient for thewetland ecosystem. These biogeochemical processes andnutrient fluxes create distinct chemical environments, whichhave implications for ecological response.

The reductive processes and chemical observations canbe explained through Eqs. (1)–(6). High alkalinity andreducing conditions away from the upwelling waters may beexplained in part through the oxidation of organic matter:

CH2Oþ O2→CO2 þ H2O ð1Þ

This dissociates to:

CO2 þ H2O→HCO3 þ Hþ ð2Þ

The free proton is then able to liberate calcium fromthe calcite in the peaty matrix:

CaCO3 þ Hþ→HCO−3 þ Ca2þ ð3Þ

The much lower NO3 concentrations away from theupwelling waters provide some evidence for denitrification:

5CH2Oþ 4NO−3→2N2 þ 4HCO−

3 þ CO2 þ H2O ð4Þ

and nitrate reduction to ammonia (DNRA) with higherNH4

+ and DOC concentrations:

4NO−3 þ 4H2 þ 2Hþ→3H2Oþ NHþ

4 ð5Þ

which both further contribute to the alkalinity. Although anumber of studies have supported the premise that highorganic carbon availability favours DNRA over denitrifica-tion (Megonigal et al. 2003), the higher proportion of NH4

+

is likely due to mineralisation of organic N in the peat.Denitrification could also produce N2O as the final product.

Similarly, lower SO4 concentrations could be frombacterial sulphate reduction:

2CH2Oþ SO−4→H2Sþ 2HCO−

3 ð6Þ

which, again, adds to an increase in the alkalinity. Similarsequences of redox chemistry have been observed in othercalcium carbonate environments when stimulated by highloadings of organic carbon (Gooddy et al. 2002).

Botanical indicators of groundwater dischargeThe NVC plant communities identified for the site arespecies poor and collectively typical of mesotrophic toeutrophic soils and waters. There is little discerniblepreference to water source as indicated by temperature.

Some individual species seem to correspond to areas ofgroundwater discharge in the main vegetation survey, butsample numbers are low. These include Caltha palustriswhich has been recognised elsewhere as a possibleindicator of groundwater discharge (Klijn and Witte1999; Rosenberry et al. 2000; Wierda et al. 1997).

The affiliation of Carex paniculata to warm tempera-ture anomalies is marked. An explicit association betweenthe presence of the species and temperature is, however,doubtful. Carex paniculata is part of a widespreadEuropean temperate element in the UK flora, as definedby Preston and Hill (1997). There is little suggestion thatthe species has a tight temperature requirement, thoughEllenberg (1988) has it as an Bindicator of fairly warmconditions from lowland to high mountain sites, butespecially in submontane to temperate regions^. Theoccurrence of this sedge is more likely to be determinedby other factors linked to the temperature pattern.

The upwelling areas deliver waters relatively low inminerals Ca, Si, Fe and Mn, and nutrients TP and SRP, yetrich in NO3 and SO4 (Fig. 9). A preference for Carexpaniculata to Ca poor groundwater with higher NO3 andSO4 concentrations has been shown on the Pleistocenesands of central and northeast Netherlands (Grootjanset al. 1988; Wassen et al. 1988). Furthermore, Carexpaniculata has been shown to prefer waters with low Caand relatively high NH4 concentrations on the graveldeposits of the River Meuse in southeast Netherlands(Lucassen et al. 2006). However, groundwater Ca con-centrations are higher across the study site, given itssetting within a chalk valley, than those found in theNetherlands, by around 30–70 mg/l, as are Si concentra-tions by 3 mg/l. Fe and Mn concentrations are compar-atively lower, by around 1–12 and 1.7 mg/l, respectively.

At this site, it is considered that nutrient rather thanmineral supply is likely to be the limiting factor for Carexpaniculata, particularly given it is a sizeable species withhigh nutrient demands. Furthermore, it is plausible that NO3,rather than P, is the limiting nutrient for Carex paniculatadistribution. Addition of N to calcareous fens has beenshown to increase the biomass of some Carex species,without changing the species composition (Pauli et al. 2002).Increased P has also been found to promote the growth ofpoor fen species, especially more competitive grasses, to theexclusion of other species (Hoek et al. 2004). These findingsare consistent with evidence ofCarex paniculata surroundedby a variety of poor fen species to situations with the influxof base-rich groundwater and eutrophication (Rodwell 1991;Sinker 1962). This suggests the species may have potentialas an indicator of groundwater discharge across lowlandchalk wetlands, as chalk groundwater is generally high inNO3 throughout much of Western Europe due to historicalagricultural loadings (Aguilar et al. 2007; Wang et al. 2012).

The value of Carex paniculata as an indicator specieslies in its use for initial hydrological site appraisals anddirecting further study. The presence of the species or ofgroundwater does not guarantee the other, despite themajority of stands found in the groundwater dependentnorth meadow. Rather, tussocks of Carex paniculata



indicate adjacent conditions arising from a distinctchemical environment. These may result from groundwa-ter discharge in a particular geological context or fromanother supply mechanism (Wheeler 1999). Furthermore,other factors than water source are influential, principallylight availability (Goslee et al. 1997), along with soilconditions, air quality and seed availability. Supportinginformation is invariably required. Wider areas may becharacterised as potential areas of groundwater discharge,although it is not possible to delineate discrete areas byvegetation alone.

Value and limitations of a high-resolution 3Dtemperature modelA detailed 3D temperature model could be a useful precursorto the targeted deployment of sensor arrays. One-dimensional vertical temperature arrays are becomingincreasingly commonplace as a means of estimating ground-water fluxes (Anibas et al. 2009; Voytek et al. 2013). Suchestimates require profile time series to solve analytical fluxrepresentations (Briggs et al. 2012; Hatch et al. 2006).Sensors could be positioned to sample temperature gradientsrepresentatively across an entire site to estimate totalgroundwater influx. Furthermore, their deployment couldalso be based upon an understanding of the flow field, whichis important to avoid misinterpretation of temperature timeseries (Cuthbert and Mackay 2013). For example, at this sitethere is evidence for non-vertical flows which have beenconsidered the greatest source for error when implementing1D solutions (Lautz 2010).

Temperature data may be gathered simply and eco-nomically over large areas. In this study, the totalequipment cost was GBP 250 and a team of two covered0.1 ha/h. Limitations of the technique include the need fora high water-table elevation and penetrable soils, althoughthese are often distinguishing features of wetlands.Furthermore, it is possible that groundwater dischargemay exhibit seasonality or dynamically respond to intenseprecipitation events, which would not be captured in asingle temperature survey.

Other approaches to detect groundwater upwellinginclude water budgeting (Acreman and Miller 2006),intrusive investigation, and remote sensing by thermal ormultispectral imaging (Becker 2006). Water budgetinglacks spatial definition and requires quantifiable boundaryflows which are often not applicable to wetlands. Standardintrusive exploratory techniques include drilling and trialpitting which are disruptive to sensitive ecosystems andare spatially restricted (Baines et al. 2002). The influenceof air temperature at the ground surface renders remotesensing uncertain, as groundwater temperature signals areseen to fade noticeably in the upper 0.5 m. Moreover,commonly used satellite techniques do not possess therequisite spatial resolution and have limited penetration ofthe subsurface (Becker 2006). The method presented isable to encapsulate the necessary scale, depth andresolution with minimal intrusive impact.

Conclusions

A high spatial resolution 3D temperature model provides arapid, economical tool for detailed investigation withminimal intrusive impact. It is applicable worldwideprovided there is a sufficient contrast between air andgroundwater temperatures at the time of the study. Thetemperature model, here, delineates discrete areas ofgroundwater discharge in areas which had not previouslybeen considered to be groundwater dependent, withimplications for future sustainable management of thesite. The lateral continuity of the temperature anomaliesrevealed upwelling was generally associated with infilledrelic channel structures. Such features are likely to bewidespread within riparian lowland wetlands given thedynamic nature of river channels and could be pivotal inthe hydrological conceptualisation of many sites. Further-more, vertical temperature sections within the relicchannels indicate the magnitude of groundwater dischargevaries along their length.

A temperature model is an effective precursor to thetargeted installation of more costly monitoring, e.g. anarray of piezometers in this instance. Such targetedpiezometer arrays should be favoured in these heteroge-neous riparian settings where data from dense randomclusters or gridded arrays have been demonstrated to beinsufficient for successful site characterisation.

The new site conceptualisation also revealed the preva-lence of Carex paniculata in areas of deeper groundwaterupwelling suggesting that it could be a useful botanicalindicator of groundwater dependence. This correlation isconsidered to be a result of the relatively NO3 and SO4 rich,Ca and P poor, waters associated with the deeper ground-water, as opposed to a causal link to temperature itself.Carexpaniculata is likely limited by N rather than P. Surroundingreducing waters low in NO3 and SO4 and high in P promotepoor fen communities. A need for hydrochemical orhydrogeological supporting information limits the use ofCarex paniculata as an indicator species to preliminary sitewalkovers. Nevertheless, in similar geological settings, thespecies may help determine further investigativerequirements.

Acknowledgements Thanks go to Henry Holbrook for illustrationof Fig. 9. Funding for the study was provided by the NaturalEnvironment Research Council (NERC). J. Sorensen, D. Gooddy,A. Newell, B. Marchant and P. Williams publish with thepermission of the Executive Director of the British GeologicalSurvey.

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