HYDROLOGICAL PROCESSES.VOL. 9, 99 IIO (1995)
HYDROCHEMICAL AND WATER SOURCEVARIATIONS ACROSS A FLOODPLAIN MIRE, INSH MARSHES,SCOTLAND
IAN C. GRIEVE, DAVID G. GILVEAR AND ROBERT G. BRYANI Department of Environmental Science, University of Stirling, Stirling FK9 4LA, UK
ABSTRACT
Groundwater heads and chemical composition were measuredat approximately two week intervals during the summer of 1993along a I km transect acrossthe Insh Marshes floodplain mire in Inverness-shire,Scotland. Groundwater heads were generally higher near the valley side slope, with lower pH values and greater dissolved organic carbon, A1 and Cl concentrations. In the centre ofthe transect, upward groundwater headswere identified anC pH, conductivity and con- centrations of base cations were much greater. Near the River Spey, pH and basecation concentrations decreasedand A1 and Cl concentrations increased.Deep groundwater followed a similar spatial trend but was generally more base- rich than shallow groundwater. These variations reflect the influence of three major water sources with different chemical signatures.Runofffrom the valley side slope increaseddissolved organic carbon and Al in the shallow groundwater, the upward flow of groundwater increasedthe pH and Ca concentration and inundation near the river decreasedthe base status and increasedCl and A1.
KEy woRDS Floodplain mires Groundwater Hydrochemistry Wetlands
INTRODUCTION
Wetland hydrology and hydrochemistry are controlled by inputs and outputs from precipitation, evapo- transpiration, groundwater and surface water flow (Orme, 1990). Relatively small changesin the balance of water volumes and chemical composition between these inputs and outputs may induce shifts or even give rise to a loss of plant species(Van Wirdum, 1982).The hydrochemistry of a wetland is an important interface between the hydrological processesthat occur there and the types and abundances of wetland vegetationcommunities (Wassenet al., 1988).To managefreshwater wetlands successfully,an understanding of their hydrology and hydrochemistry is therefore needed. Despite the widespread realization that hydrology and hydrochemistry are important controls on wet- land vegetation, few studieshave examined the hydrology of floodplain wetlands, or investigatedassociated surface and groundwater chemistries. Notable exceptions include Giiler and Wheeler (1988), Wassen e/ al. (1990),Wassen and Barendregl (1992) and Gilvear et al. (1994).Traditionally, the hydrology and hydro- chemistry of floodplain wetlands has been viewed as solely relating to hillslope and riverine water sources. Increasingly, however, the importance of groundwater inputs to floodplain wetlands has been realized (Siegel,1988a; Siegel and Glaser, 1987).These inputs are significantnot only as a water inflow, but also in determining wetland hydrochemistry (Wassen et al., 1990).Previous work has focused largely on base- rich mires such as the fens of East Anglia (Giller and Wheeler, 1988).The purpose of the researchdescribed here was to gain an understanding of the hydrology and hydrochemistry of a base-poor system, a small floodplain mire in Inverness-shire,Scotland. This paper presentsthe initial results of hydrochemical studies and relates the findines to water sources. ccc 0885-6087i9s I 010099 -12 Received4 January 1994 O 1995by JohnWiley & Sons,Ltd. Accepted 12 April 1994 100 I. C, GRIEVE EZ II,.
STUDYAREA Location The River Speyrises about 30km westof Newtonmorein the MonadhliathMountain rangeand flowsin a north-easterlydirection towards the Moray Firth. It is joined by severalmajor tributaries,notably on the left bank by the Calderand Dunlin, and on the right Uan[ Uythe iruim, Tromie,Feshie. Druie and Nethie. In the studyarea, between the town of Kingussieand Loch Insh,the Speyflows through a largearea of flat, poorly drainedland calledthe Insh Marshes(Figure l). The Insh Marshesare widely recognizedas a wet- land siteof internationalimportance, providing habitats for a wide varietyof flora and fauna.In 1963this sitewas designated a Siteof SpecialScientific Interest (SSSD and is currentlymanaged by the Royal Society for the Protectionof Birds. Along the southernbank of the Spey,the Insh Marshesare approximately 7.5 km in length,although the sinuosityof the river hereresults in a channellength of l0 km. Threemajor tributariesflow over the marsh into the Spey,the Gynack,Tromie and Raitts,and severalsmall streams drain from the surroundinghills into the marsharea (Figure 1).within the studyarea, all of the major river channelshave man-made banks on either side.These were constructed in the late 18th and 19thcenturies as flood preventionstructures (Gordon, 1993).Several lengths of bank were also constructedacross the marsh. On the south side of the river a large lateraldrainage ditch and a seriesof internalinterconnected drainage ditches were con- structedto removeexcess water from the marshlandsinto Loch Insh and also to collectwater draining off the higher ground to the south (Gordon, 1993).The lateral drainageditch originally had u on"-*uy flap valve installedat the Coull Culvert preventingriver flood watersflowing into the marshes(Figurl 1). This valveis now in disrepair(Johnson et al., l99l).
Figurel. Map showingthe locationof the Insh Marshes VARIATIONS ACROSS A FLOODPLAIN MIRE 101
rP5 "9 .JU rP4 .P3 \.1 I
StratigraphicSurvey :::::::::::;:::::::::Embankment Pl Piezometer Cl Compartment DrainageDitch t:ffi openwater
Figure2. Map showingthe locationof the piezometertransect and the Instituteof Hydrologyshallow stratigraphic survey. See Figure I for locationof map
REGIONAL AND LOCAL GEOLOGY The SpeyValley is boundedto the north by the Monadhliath Mountains and to the south by the Cairngorm Mountains.The geologyof theseupland areas,which locally riseup to 1200mOD, is characterizedby a rangeof schistsand gneissesof Precambrianage (Moinian Series)and their associatedigneous intrusions (Young, 1978).Between Laggan and Aviemore,the floor of the SpeyValley slopesgently from 250 to 200m OD and is commonlyat least500m wide,broadening to 1500mbetween Kingussie and Loch Insh. The valleyfloor is infilled with Pleistoceneto Recentfluvioglacial deposits of interbeddedsands and gravels, overlainby alluvialdeposits (Grant and Birse,1953). A shallowstratigraphic survey of the Insh Marshes south-eastwardsfrom NH 80450280to NH 81200210(Figure 2) was carriedout in l99l (Johnsonel a/., 1991).Near to the river (Stationsl-3) shallowsediments were largelymade up of interbeddedorganic silts.At Station4, the siltsgraded into alternatinglayers of dark brown silty peatand brownishgrey silty 102 1. C. GRTEVEET AL mud. It was suggestedthat this station representeda section through former river channel and open water oxbow lake deposits. Further from the river (Stations 5-7) large proportions of peat and mud were found. Mud at Stations 5-7 was dark brown and predominantly organic, containing wood fragments. The peat at Station 6 was found to contain Phragmites remains. These sedimentswere thought to representdeposition within an open water environment with fringing or floating reedswamp.Towards Station 8 the muddy sedi- ments were found to change abruptly to peat. Between Station 8 and the Insh drainage ditch the peat was found to be dark and fibrous with little or no mineral content, containing remains of Phragmites and wood fragments.
FIELD AND LABORATORY METHODS
During the summer of 1993piezometers were installed in a transect acrossthe Insh Marshes perpendicular to the flow of the Spey. The transect ran south-east to north-west with the first piezometer (Pl) at NH 81300215and the final piezometer(Pl7) at NH 80550285(Figure 2). The surfacetopography and accurate location of all piezometerswas surveyedusing a WILD EDM (Figure 2). The transect crossedthree marsh compartments (Cl, C2 and C3) which were separatedby two drainage ditches (Dl and D2) and the main Insh lateral drainage ditch (D3). Initially (May 1993) the transect consisted of 17 PVC piezometer tubes with a diameter of 0'07m measuring groundwater heads and sampling at 0'50m depth below the ground surface. These were installed at approximately 60m intervals along the transect. In August 1993, nests of piezometerswere installedat Pl, P5, P8, Pl1 and P17. In eachcase two PVC tubes of 0'05m diameter were insertedto depths of l'5 and 2'5m. BetweenJune and October 1993,water levelsin the piezometers were recorded every 14 days. Samples were collected from each piezometer in a 1000-ml glass flask using a suction pump and trans- ferred to 250-ml polythene bottles for transport. On return to the laboratory, pH and conductivity were measured on aliquots of the sample using laboratory pH and conductivity meters. Samples were then filtered through glass fibre filters (Whatman GFiC) to remove suspendedmaterial and analysed for major cations and anions. To examine possible lag effectsassociated with exchangebetween the piezometersand local groundwater, sampling was repeatedone day after the full transect sampling. No significant difference in water chemistry between the two setsof sampleswas found. It can thus be assumedthat the piezometer samples replicate the groundwater chemistry at the time of sampling. Ca and Mg were determined using flame atomic absorption spectrometry (AAS). Samples were dosed before analysis with Sr(NO3)2 solution (final concentration 0'4o/o)to suppressinterferences. K and Na were determined using flame pho'ometry. Fe and Al were determined by graphite furnace AAS. Samples were diluted where necessaryand acidified before analysis with nitric acid to a final concentration of r, l%. Approximate detectionlimits for Ca and Mg were 0'02mg I for Na and K 0'l mg l-l and for Fe '. and Al 10pg I Anions were measured by ion chromatography. Samples were eluted with Na2CO3/NaHCO3 solution and the conductivity suppressedwith dilute H2SO4. Cl, NO3 and SOa concentrations were determined l. from conductivity peak heights from the chromatogram. Detection limits were approximately 0'02mg I Dissolved organic carbon (DOC) concentrationswere measuredusing a TOCSIN aqueouscarbon analyser. DOC is oxidized at high temperaturesto CO2, reduced to CH4 and carbon measuredby a flame ionization detector.The detectionlimit was approximately0'1mg l-'.
RESULTS
H y elrologic al backgr ound The Insh Marshes are seasonallyinundated due to flooding from the River Spey, flood dischargesfrom streams draining adjacent hillslopes and high water levels within Loch Insh downstream. During times of low flow in the River Spey and low water levels in Loch Insh, there is still direct hydraulic continuity betweenthe marshesand thesewater bodies via a networth of surfacedrainage ditches.Hydraulic gradients VARIATIONS ACROSS A FLOODPLAIN MIRE 103
-1.5'
-2.0
E E-z.z o 2 -z.t () -2.6 A(6 6 f -z.a .9 f -s.o -3.2 1100 8oo 7oo 600 500 400 300 DistanceAlong Transect (m)
the gradient from valley side to the River Spey Figure 3. Shallow groundwater head gradients- for June and september 1993 showing and local groundwater mounds and drawdowns
Groundwater Head Relative to Ground Surface (m) -0.8 -0.6 -o.4 -o.2 0 0.2 0.4 0 6 BelowGround Above Ground
Sept€mber1993
in vertical groundwater head gradients Figure 4. Groundwater heads in three piezometer nests l, 1l, 17 showing differences levelswithin the and water-tableson the marshare thereforeultimately controlled by the respectivewater precipita- River Speyand Loch Insh and waterinputs to the marshfrom other sourceswhich could include tion, groundwaterand surfacerunoff from adjacenthillslopes' (Figure revealed Examinationof shallowgroundwater head data collected during the summerof 1993 3) groundwater that the piezometricsurfacJ dipped towards the River Speybut wascomplicated by localized weremain- moundsand drawdownadjacent to drainageditches (Figure 3). Generally,groundwater heads in -tainedclose to the wetlandsurface adjacent to the valleysides, within the centreof compartmentI and werealways at the areaof compartment2 closestto D3. Adjacentto the River Speygroundwater heads ground- greaterdepths und lo*.. absoluteelevation. For example,in compartment3 and at Pl2 and Pl3 dry periods, water headsfell to a depth of greaterthan 0'70m (>2'75m below arbitrary datum) during 0'35m whereasshallow groundwater heads close to the valley sides(P1 and P2) weremaintained within 104 I. C. GRIEVE . T IZ of the ground surface (<2'5m below arbitrary datum). Groundwater heads in P8, P9 and Pl0 were also always within 0'45m of the local ground level. The variation in the piezometric surface suggestedthat water from the adjacent hillslopes was an important source of water and that the River Spey was a dischargezone. Upward head gradients adjacent to the valley side and in compartments I and 2 (Figure 4) also demonstrated the potential for groundwater contributions to the wetland water balance. In compartment 3 downward head gradients and lower groundwater heads suggestedlateral leakage to the River Spey (Figure 4). Leakage towards the river may also account for the low groundwater heads in the area of compartment 2 closest to the river. However, this may also be the result of high evapotranspiration due to the overlying willow and alder carr. The maintenance of a high groundwater head adjacent to D3 most probably related to lateral seepage from the drainage ditch; the water level in the drainage ditch was maintained at a higher level than the surrounding groundwaters by surface water from elsewherein the marsh discharging towards the River Spey via this drainage ditch. Indeed, the groundwater hydrology adjacent to all of the drainage ditches was complex in that seepageof water from the drainage ditches through surface layers caused the near-surface groundwater head to be higher than the surrounding areas, indicating local recharge, but the groundwater heads at depth were unaffected. Such localized recharge has been observed at P8.
Spatial variations in shallow groundwater chemistry Initial examination of the data set indicated that spatial variations among the different sampling points and variations with depth were large, whereas temporal variations were much smaller. Table I shows the matrix of correlation coefficients for the entire data set. Significant correlations were noted for several groups of variables.pH, conductivity, Ca and, to a lesserextent, Mg concentrationswere positively correlated with one another. DOC, Cl, Na and Al concentrations were also positively correlated with one another and these were each negatively correlated with pH, most strongly in the caseof DOC (r : -0.704). From this analysis two initial groups of interrelated variables were identified for examination of the variations across the marsh transect. One-way analysis of variance showed that variations in the chemical composition of the water were significantly different among the surface samplers at the p < 0.001 significancelevel for all determinands except Na and NO: (p < 0'05) and K, SOa and Fe (not significant at p - 0.05). Table II shows the means and standard deviations for all determinands at the 27 piezometers,the drainage ditches and the River Spey. Figure 5 shows graphically the variation in pH, Ca, Cl and DOC across the marsh for the surface samplers,ditches and river. The initial subdivision of the variablesinto two groups basedon the correlation analysis was also evident in thesedata. Variations in pH, conductivity and Ca concentrations followed a similar trend, with small values at each end of the transect and much larger values near the centre. The mean pH was smallestat Pl (4.6), increased across compartment I to the maximum of 6'5 between samplers P7 and Pll, and then decreasedacross compartment 2 to around 5'2 at Pl5-17 in compartment 3. The only departure from this trend was at
Table I. Correlationmatrix of maior anionsand cations
pH Conductivity Ca Mg Na K CI NO: So+ AI
Conductivity 0.562 Ca 0.554 0.956 Mg 0'271 0'537 0.381 Na -0'376 -0.058 -0.256 0.228 K 0.169 0.348 0.238 0.003 *0.032 cl -0.480 -0.248 -0.460 -0.t64 0.535 -0.038 NOr 0.103 0.341 0.103 *0.124 0.062 0.182 0.213 so+ 0.2t1 0.088 0.014 -0.226 -0.213 0.281 0.064 0.229 Al -0.427 -0.342 -0.308 -0. I l5 0.429 -0.064 0'475 0'243 -0.r42 DOC -0.704 -0.296 -0.287 -0'009 0.451 -0.076 0.531 -0.039 -0.1830.392 VARIATIONS ACROSS A FLOODPLAIN MIRE t0s
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\o F- € o \o F +'aF;r b n':-'.o !a'+'in'?'' i-{ ?-l h -,9 l: a r o'5 3 $ + Ar 6 + N 6 N 6 m v L; + ; 6 i\i ; J -; s .{ o c'.1O,a S C'l 9 T. T, T. ll't ^ - FF I : ,l ^ ;. :A ---Ja999evJ9VAA A ^,^ ^ A ^ A - i, e vl9 -;- -'+ ro'l - - oo n - \o O. m oo C\| 09 al O co O\ N Q I I q .. :- -' * r c r- !.) 6 ts o a - N r a v -: ;i & 5 N 6 $.t + r- oo ni o s ct -' + 9 ? 9 n'o h'o'b'o'o U I E ; ;; ; ;; 6 6 ; o o o 6 o o'b'o'o n n 6 h n Fi 0) 0) € ! .eE c'9 ^-pF b E > 'o > c 6 fo- o*J-'l"rdh\q\FF*ol .? F U) ++rgrsrirsrJnnnEFrrrrs;;;EEEEEEEEEoot 106 I. C. CRIEVE Er IZ.
UgAIU UlIAIU E Ltd Ltd g 9td |.# 9ld g std g 9td d g fld g ,td l'.+ €ld fld |+ ztd o B ztd |+l IId N |+ 0td 0td ,+l 6d 6d # v8d v8d |+ 8d 8d d |'.'ff tvuq NIVUq I |"-.ff u Ld, @ B 9d |'-ff 9d E B sd |'..€l 9d U l-g vd ffi ,d g €d }E--r €d g zd P+---<- zd l...H NIVU(I NIVUq E Id |...... ,.....€- Id I TR ti* ! Eg Ro bo tltueJ v?u coc (J
I
UlIAIU +- U!IAIU bo t+{ LTd r-----'---.E-l Ltd + 9td F<--t 9td Q |+ 9td |'..ff std |+ fld HI ttd F & €td l+i gtd + ztd l..".....H cld
IId |"-.# Itd ci |+ 0rd |....ff 0rd l..q 6d |+ 6d |.''."....H v8d |.'.ff v8d + 8d +.€- 8d |.+ NIVUC |....'.'# NIVdO +l u l+ Ld @l 9d |.+ 9d |+t sd +l sd + td ?d, +l €d €d 'I |+ zd ed |....# NIVUC NIVd(I ri |.....ff Id td R9=:I= Hd s t/8u tc VARIATIONS ACROSS A FLOODPLAIN MIRE 107 p8, besidethe centralditch, wherethe pH was0'5 units smallerthan at the neighbouringsamplers, Trends in conductivityand Ca weresimilar, with minimum valuesat samplersnear the hillslopein comPartment,l and near the river in compartment3. Mean Ca concentrationsfor theseareas ranged from 3 to 6mgl-" with conductivityrangingfrom 5 to 8mSm-1. Both theseranges were similar to the meanvalues found for the River Spey and the ditches.Maximum Ca and conductivity occurredin the area of compartment 2 nearestthe centralditch, with the largestvalues at P9 (Ca 34mgi-I, conductivity16mS m-l), but again smallermeans at P8 than at neighbouringsites. Variationsin Na, Cl and Al concentrationsshowed a consistentpattern, with generallygreater values at the endsof the transectand smallervalues in the centre.The patternwas clearest for Cl, concentrationsof which rangedfrom l4mgl-l at samplerpl through 5-7mgl-r at the samplersnearthe centralditch to 14mgl-r i't pfO. Mean Cl concentrationsin the ditchesand river were9 and llmBl-', resPectivelY.Al concentrationsranged from 500p,gl-r at eachend of the transectto a minimum of 140pgl-' at site P9. Al concentrations However,the largestmean Al concentrationof 960pgl-t .*ut found at Pl6. The mean in the ditchesand river rangedbetween 160 and 210p'gl-'. Variations in DOC acrossthe transectwere slightly different from theselatter three determinandsand displayeda simplerpattern of variation.DOC concentrationswere very large(l00mg1-') at the hillslope end of the transectand decreasedrapidly acrosscompartment 1. Mean concentrationsin compartments2 and 3 rangedtiom 5 t" f Z-ti:t. ttre taieratditch and River Speyhad meanDOC of around6mgl-r, but the centralditch had a much smallermean DOC. Mean concentrationsof K, SOaand NO3 weregenerally small in compartmentI and largerin compart- ments2 and 3. Mean concentrationsin the River Speyand the ditches(1'0mgl-' for both K and NO3 and 2.8-3.7 mgl-l for SOa)were similar to the meansfor sitesin compartments2 and 3.
Variationsin groundwaterchemistry with depth Figure 6 showsthe variationsin selectedconstituents of groundwaterchemistry at the three different depthsfor the siteswhere a nest of piezometerswas installed.The depth profileschanged with distance u"io5 the marsh,but the 5,u-. gro.tpingof variableswas evidentin thesedata. pH increasedmarkedly with depth at sitespl and Pl'l at the edgesof the transect,but was essentiallyconstant at the other greater sites.Ca concentrationsand conductivityincreased with depth at all sites,but meanvalues were in the middle of the transect.Cl and Al concentrationsdisplayed a differentpattern, with concentrations greatestat the surfacefor thosesites with largesurface Cl and Al concentrations.Values did not change tuch with depth at P8 and Pl l in compartment2. DOC concentrationsalso decreasedwith depth, particularlyat P1. Other determinandsdisplayed no regularpattern (K and NO3) or remainedconstant (So+). The spatialvariation of the chemicalcomposition of the deepsamplers across the transectwas very similar to that for the surfacesamplers, but the variation was much more heavilydamped. Thus deep groundwaterpH increasedfrom around 6 at the outer edgesof the transectto almost 7 in the centre' whereasCl decreasedfrom l0mgl-l at site Pl to just over 5mgl-l at Pll. Deep groundwaterDOC and Al wereboth greatestat Pl and Pl7.
DISCUSSION
Variationsin the groundwaterchemical composition across the marshand with depthcan be explainedin relationto four sourcesof waterinputs. Drainage of surfacewaters from the valleyside slopes is likely to be acid and base-poor,but enrichedin dissolvedorganic carbon, complexed Al and ions from rainfall suchas Na and Cl. Upwelling of groundwaterwould supplywater enrichedin basesfrom rock weatheringand thereforeof greaterpH, Ca and Mg concentrations.The chemicalcomposition of inflows "onductivity, from the -ajo. ,iuei, the Spey,would be similar to hillslopeinputs, but with higher pH and smaller DOC given the sizeof this rivir. Finally, rainwaterinputs havevariable pH and high concentrationsof sea-saltions suchas Na and C|. Both the spatial,across marsh, pattern and the variationwith depth are consistentwith inputs from thesesources. 108 I. C. GR.IEVE ET AL.
PH Cl mg/l DOC mg/t
E 3
Pl _a
a o
U
P53 5 6
;
P8a i a o
U
Pll
6
Pt7 B
6
Figure 6. Chemistry of groundwater samples (Ca, pH, Cl, DOC) taken from piezometer nests along the transect
Near the hillslope(samplers Pl-3) the shallowgroundwater was acid and enriched in DOC andAl. Acidity and DOC decreasedmarkedly with depth.These trends are consistent with the shallowgroundwater being dominatedby the hillslopeinputs. The influenceof hillslopewater inputs decreased across the transectfrom Pl to P6,away from the valleyside slope. It is probablysignificant that the substratein this areaof the marsh is dominatedby deeppeat deposits, although the markeddecrease in DOC and Al acrossthe zonemakes it unlikelythat the marshpeat is the sourceof the acidityand organiccontent of the groundwater.Similar pH, Ca, DOC and Al concentrationsto thoseat the edgesof Insh Marsheswere reported for peaty wetlands underlainby glacialsediments in Alaska (Siegel,1988b). Giller and Wheeler(1988) also reportedlow pH and Ca concentrationsin shallowgroundwater and similar increaseswith depth in acid Sphagnumdominated areasof fen in Norfolk. Betweensamplers P7 and Pl I therewas a clearzone of base-richshallow groundwater. Surface concen- trationswere similar to thoseat depthand pH, conductivityand Ca wereat their maxima,whereas Cl was at its minimum. Examinationof the groundwaterhead within this zoneof the transectindicated that there wascommonly an upwardhead, and thereforea significantinflow of deepergroundwater to thesesites. The base-richchemistry of this groundwaterprobably originatedfrom weatheringof more base-richrocks, although thesehave not been located.Silts, sandy silts and silty clays were more common substrates than peat in this centralsection of the transect.The basecation concentrationsand pH of the shallow VARIATIONS ACROSS A FLOODPLAIN MIRE 109 groundwatersin the centreof the Insh Marsheswere, however, much lessthan thosereported for most other studiesof mire groundwater,which havelargely focused on calcareousfen systems(Boyer and Wheeler, 1989). From this centralzone towards the Spey,the chemicalcomposition, particularly of the shallowground- water,changed again towards base-poor, Cl-rich water. This changewas not accompaniedby an increasein DOC, and the overallchemical composition of the shallowgroundwater became more similarto that of the River Spey.The mostlikely sourceof shallowgroundwater near the river is the Speyitself. Shallow ground- water rechargeprobably takes place during periodsof inundation,followed by drainageback to the river. Deepergroundwater was still base-richhere, but upwardsgroundwater heads were less common near the river. Similarinundation from and subsequentdrainage to the centralditch may alsoexplain the lowerpH, Ca and conductivityat site 8, locatedvery near an old channeladjacent to this ditch. Similar gradientsin groundwaterchemistry have been identified by other workers from mire systemswith very differentchemical inputs. Wassen et al. (1990)reported a doublingof Ca and an increasein pH by 0'5 units betweenthe valley margin and the river leveein a mire systemin the Biebrza valley Poland, due principallyto inputs of groundwaterrich in Ca. Gradientsin nutrient statuswere related to the influence tf floodwaterrich in K and to low P concentrationsin groundwaterdischarge. In the Catfieldand Irstead Fensin Norfolk, Giller and Wheeler(1988) identified clear concentration gradients in dissolvedcations, althoughinputs to the systemwere much more base-richthan in the Insh Marshes.Movements of water into areasof fen near surfacedrainage ditches were responsible for strongdepletion gradients in cations away from the drainageditches. Progressive base depletion of the upper peat in areasaway from base- rich water sourceswas also evident.Also on CatfieldFen, Gilvear et al. (1994)found variationsin Ca and pH due to localizedupwelling of groundwater. The overallcontrol of shallowgroundwater chemistry which emergesfrom this analysisis thus a balance amongthe threemajor sourcesshown schematically in Figure7. Groundwaterinputs (G) supplybase-rich waterin the centreof the marsh,supplemented by surfacewater inputsrich in DOC and Cl from the slope at one end (Sh) and by inundationand drainageof Cl-richwater from the river at the other (Sr, Gr). Such an analysisis alsoconsistent with the generallydomed nature of the piezometricsurface across the marsh, with maximum elevationin compartment2. The ditch pattern within the marshpartially explainsthe spatial
tdal surfarc flow dd @hsgc frcm thc G crcundwarerMhdge ild discharge St
G' Croundwabr flow to dd from River Spcy P Pmipihtion Ga Ground*atq flow to and from drainage clitchcs Et Evapomnspimtion w.n3!p7 t&al ccology Sn Surface watcr flow frcm upled ed
Figure 7. Schematicdiagram of water inflows and outflows within the Insh Marshes ll0 I. C. GRIEVE , T IZ.
pattern of groundwaterchemistry piezometric and surface,as compartment I is crossedby a densenetwork of ditches,whereas compartment 2 in particularhas few ditchesand ihereforeis largelygroundwater-dominated. Locally aroundthe ditches groundwatermovements (Gd) may alsoaffect watei asat samplerpg. Direct rainwater inputs (P) "i"-irtry to and evapotranspirationlosses irom (Et) the marsh systemmay also influence the shallow groundwaterchemistry, particularly in areasaway from groundwaterrecharge (Figure 7), but thesecannot be positivelyidentified on the basisof the existinedata.
CONCLUSIONS Latge variationsin groundwater chemistryoccur within the base-poormire systemin the Insh Marshes, both spatiallyacross the marsh and with depth.These are primarily relatedto the varyinginfluence of hill- slopedrainage, river inundation and groundwaterdischarges, the balanceamong which affectsthe concen- trationsof bases'Na and Cl and DoC and associatedmetals. Differences in shallowgroundwater chemistry are sufficientlylarge to have a potentialeffect on vegetationcommunities and thus on the conservation statusof the system.Changes in the hydrologyof the systemmay thushave implications beyond the simple effecton wetness,but to predict theserequires more detailedeiamination of ihe relationshipsamong hy- drology,hydrochemistry and vegetation.
ACKNOWLEDGEMENTS We aregrateful to the Royal Societyfor the protectionof Birdsfor accessto the siteand to Zul Bhatia,site warden,for assistance. Financialsupport from the CarnegieTrust for the Universitiesof Scotlandis grate- fully acknowledged. We thank Chris Andersonand Dave Harrisonfor technicalsupport in the laboratory and in the field.
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