Hydrochemical and Water Source Variations Across A

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 concentrations 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 wetland 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 hydrochemistry 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 wetland 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

Load more