archers (Teveldal et al.

LElF BASBERG, ATLEDAGESTAD&PETERENGESGAARD NGU-BULL 434,1 9 9 8 - PAGE35

Geochemica l modelling of natural geo-/hydrochemical stratification dom inated by pyrite oxidation and calcite dissolution in a glaciofluvial Quaternary deposit,

Garde rmo en, Norway

LEIFBASBERG,ATLEDAGESTAD&PETERENGESGAARD

Basberg,L.,Dagestad,A.&Engesgaard,P.1998:Geochemical modellingof naturalgeo-/hydroc hemicalstrat ificat ion dominatedbypyriteoxidation and calcitedissolut ionina glaciof luvialQuate rnary deposit,Gardermoe n, Norway.

NorgesgeologiskeundersekelseBulletin434,35-44.

ThePHREEQC geochemicalmodelhasbeenused to reprod uceon a geological time scaleanabruptshift between anoxicand ananoxiczone,in the Gardermoen glacioflu vialdelt adeposit. Groundwa ter in the aquiferhasamaj or shiftinchemicalsignat urethreemet res belowthe groundwa tertable indicatin gthatcalcit edissoluti on andpyrite oxidat ionarethedominatingweat heringprocesses.The resultsof themodellinghavebeen comparedwit h thoseof wate rsamplescollected frommult i-level samplerswit h microscreenslocated onemet reapartfrom the groundwa- tertableand fivemetresdown.Thesamplingscheme wasnot detailedenough toresolvethe exact locationof the weat hering horizons.The mod el suggests that the calcite dissol utionfront islocatedslightly belowthe pyriteoxida- tion front.This causes asignificant dropin predictedpH inazonewhereothe rbuffe ringmechanismsthancalcite dissolution maybecomeactive.Calciteandpyritein the sedime ntsareofsuch concentrat ionsthat theirrespecti ve dissolutionfront smoveatapproxi matelyequalratesin theaquifer.Thedepletionofthese mineralshas caused adis- tinct chemical separationin the aquifer betweenan oxic/low ionicst rengt hand anoxic/highionic st rength zone, that will determinethe fateofa contaminantenteringthesat urated zone.

LeifBasberg&AtleDagestad,Faculty of AppliedEarth Sciences,Divisionof Geologyand MineralResourcesEngineering, NorwegionUniversity ofScience ond Technology (NTNU),Trondheim,Norway.

PeterEngesgaard,Department ofHydrodynam icsandWoter Resources,Bldg.175,Technical University of Denmark,DK- 2800Lyngby,Denmark

Int rod uction

The capabilitiesof ageochem icalmodel have been explo - red with the aim to use the resultsin a coupledgeochemical flow and solute transport model for the Gardermoen aquifer in Norway. Beforefuture scenarioswith regard to the hydro- chemical development of groundwater affected by natural andanthropogenicevents can be hypothesized,it is neces- sary to understand the existing natura lgeo-/hydrochemical environment.One-dimensionaladvective flow coupledwith a selection of geochem ical react ions provides a tool in which the most

reactive

minerals in theaquifermatrixcan be identified, and theproposed simplification can be tested againstfielddata to checkthe validity of the proposedmo- del.Identificationof natural weat hering processes is the first step towardsmodellingthe movement of a leachateplume at the Gardermo enaquifer,since themost activemineralsin the weat herin g processes and their weat heri ng products arelikelytorespond asleachateent ers the aquifer,and be- cause the natura lweat hering processes mayhaveresulte d in a horizontal stratificati on wit hin the aquifer, establ ishing units wit h differentgeochemi calpropert ies.

Field studies of mineral weat hering processes in the Gardermoen aquifer havebeencarriedout byseveralrese-

archers (Teveldal et al.

1990,Jorgensen et al. 1991,Dagestad in prep.).WhileTeveldal et al.(1990)focusedon weathering

processes in the soil zone at Nordmoen(Fig. 1),J0rgens enet al.(1991) perform eda mass balance study for thewholewa- tershed.Adetailedfieldstudy was performed by Dagestad (in prep.)at Moreppen II(Fig.1)where water and sediment samples were collected at one metre intervals.Based on ob-

servations

made by Dagestad (in prep.),a model was con- structed to try to reproduce the observed weathering hori- zons, and the results of this st udyare reportedin this paper.

Postm a et al. (1991)modelledthe reductionof nitrate by pyrite oxidation in aDanish aquifer.Asimila rapproach has been adoptedinthis study to modelthe weat hering proces- ses in a sandy sediment unaffectedby anthropogenic activi- ties. One of theaims of the study has been to determine if the concentrationof pyrite in the aquifer sediments andit s oxidationdue to infiltrating oxygen controlthe propagation of the calcite dissolution front. Although the mineralogyof the aquifer sediments is expected to be similar over the whole aquifer,the individual mineralconcentrations might showsome variat ion,but no data to quantify thisspatialva- riat ion are available.

Geology

The study site is located on the Gardermo en glaciofluvial delta, near thenew Oslo airport60 km northeast of Oslo(Fig.

NGU- BU L L434,1998 - PAGE36 LEIF BASBERG,ATLEDAGESTAD

s

PETER ENGESGAARD

terp retat ion in this report.If the current groundwaterlevel is atan alltime low,then the predicted weathering rate sare slig ht ly underestimated;if not, the rates are overestimated (i.e., if the saturated zone has been free to atmospheric oxy- gen for a considerable time period and over a great depth).

Hydrogeology and geo- /hyd rochemistry

TheGardermoen aquifer is a phreatic aquifer that is rechar- ged by infiltrating rain.The initial model setup uses values for meanyearly amou nts of infiltra tio nthat are distributed evenly throu gh ou t the yearfor the entire simulation period.

Henceforth, fluctuations in yearly precipitation, seasonal precip itat ion and the effects of snow cover,and the subse- quent high spring infiltration,have been neglected.A yearly average evapo transpira tionof400 mm has been reported byJorgensen&0stmo(1990)calculated from pan evapora- tionmeasurementsduring a summer with a yearly precipita- tion of800 mm.The same evaporation was also calculated using chloride as a conservative tracer,as suggested by Appelo& Postma(1993),with a net vertical transport of0.9 m/yr.

Monitor ing of groundwa ter levels has been carriedout at two locati onsat theGardermoen aquifer over the last thir- ty years (Kirkhusmo & Sonsterud 1988).These data show that the present location of the groundwater table islow.

The depth of the groundwatertable varies significantlyin thest udyarea,from a fewmet res at the groundwa ter divid e and up to 30 m in distal areas.Seasonal fluctuations are more significant alongthe fringes of the groundwater aqui- fer.

Thedissolved concentrationsof oxygenand carbon dio- xidewillbedet erm ining factorsfor the modelled reactions.

Recharg e water that crosses the water table is saturated with respect to oxygen and the dissolved carbon dioxide pressure has been measured in the field to be approximate- ly ten times the atmospheric pressure,but wit h seasonal fluct uat ion s (Sw endesen et al. 1997, Dagestad in prep.), Unfortunately,the carbondioxid e pressurein theunsat u ra- ted zonehas not been measured deeper than 2 mbelow the surface.Nevertheless,measurements madein the lower part ofthe 2 m-deep unsaturated profile agreewellwit h measu- rement sinthe upperpartofthe saturated zone. As expec- ted, the carbon dioxide pressure stabilizes with depth (Reardo netal.,1979).

Teveldal et al.(1990)performed a detailed study of the silicateweathering rates in a podzol profile with no signsof cult ivat ion atNordmoen. The highest weathering rates were found nearthesurface,decreasing to0.7m below the surfa- ce (rn.b.s.),wherethe silicate com po sit ion of the sediments wassta bilizedand remained constant down to two rn.b.s..

The mostimpor tantweathering processes in the upper soil horizon are the complete breakdown of chlor iteand biotite and the transformation of muscovite to vermicu lite and smect it e.

LEGE:-lD

I

o .HAUERSETER

~

-.>

/

~Rock-cover

- -Groundwater -di..-ide -.. -_. Topographicboundary ...Hydrogeol ogicalboundary

Railroad Rood

- - River

o

:-;ORD~fOE"

lJ·\···

^<.

_~C?

N '. 0 -.

2000m

- ---

6 . ..••

,'- !\~ \J "

Fig .1. Study area, Gardermoen.

1).Thesediments in the Gardermoen deltawere depositedc.

9500years ago,and geolog icalhistoryof thedeltahas been describ ed byseveralauthors (e.g. Longva 1987).The hydro- str atigraphyofthe delta aquiferconsistsof three main units (Tut t le&Aagaard1996).The middlehydrostratigraphicunit, corresponding to thedeltaforeset beds,is the mostimpor- tant unit in terms of groundwaterflowsince the highl y per- meableupper(topset)unit is unsaturated.The groundwater drainage pattern isradial (0stm o1976),wit h groundwat er flow ing towards a number of spring ravi nes discharging at the edge of the deltadeposit. The sediments in the area are fairlyuniform, and the underlying bedrock consitsmainlyof gneissesand some granitepegmat ites (Longva198 7).

Aft erthe finaldegla ciati on, theground wate r tablewas probablylocatedclosetothe surface,but due to the subse- quentisostatic uplift thegroundwatertablehas been lowe- red.The shorelinedisplacement has been estimatedto0.17 m/yr just after the deg laciat ion,decreasing to 0.12 m/yr 9000 years before the present (Longva 1987).Thecurre nt rate of isostati c uplift in the area is0.003to 0.004 m/ year (Andersen& Borns 1994).Astheisostaticuplift decreased, the groundwater table became more dependent upon landslides occurring along the edges of the deltaand also on recharge.Theshiftin thecontrolling mechanism forthe location of the ground w ate rtable must have occurredafter the mostacti veisostat icupli ft ended (Jorgensenet al.1991). The location ofthe ini ti alwate r tableisim por t ant for thein-

JESSHEI!l.1

LEIF BASBERG,ATLE DAGESTAD&PETER ENGESGAARD ^{NGU-BULL434,1 99 8 -PAGE 37}

pH

Fig.2.Modelset up and expectedhorizontalstratification.

A geoch emica lanalysis of theupper 8 mofthe soil profi- leatMoreppenI(Fig. 1)revealedasignifi cantcontentofiron oxyhydroxides (Skaarsta d 1996). This isin accor dancewith other field st udies (Reardon et al. 1979, Postma &

Brockenhuss-Schack 1987).Gustafson et al.(1995) also re- ported on thelikely format ionofimogolit einsediment sin northern Scandinavia.Theinclusion of imog oliteisnecessa- ry in ord er to perform massbalancesofweat hering in the upperzone or massbalancest udiesfor the wholewate rs- hed.

As the water percolates further down through the unsa- turatedzone and intothe upper saturated zone, only small changesin the waterchemistry are noted. When groundwa- ter reachesthepyrite/calcite horizon,it is expectedthat pyri- teoxidat ion willconsume all the available dissolved oxygen and releaseproto ns.The lowering of thepH will potentially giverise to silicate weathering,but thisis probably confined to a smallarea justbelow the pyrite oxidationzone due to thebuffering of theacidification by calcite dissolution.The calcite weatheringhorizonis believed to be located just be- low the pyriteoxidati onzone as showninFig. 2.

Ground w ate rcollect edfrom wells located in areas wit ha differen t thicknessofthe unsaturatedzonerevealsa similar chemic al signat ure;an upper oxic/low ioni c st reng t hzone anda lower anoxic/highionicstreng t hzone.Thelow er zone has a chemi cal signat ure with high calcium and sulphate concent rationsthatstronglysuggests that pyrite oxidation and calcit e dissoluti on are the most importantweathering processes(Tab le 1).The strongest indication of thisis found at Morep pen 11 (Dagest ad in prep.) where both sediment samples (Fig. 3),and water samples (Fig.7)from the same profile have beencollected at regularintervals.The data do not resolvethe locationof the pyrite and calcite weathering horiz ons relative to each ot her,only that they are located between 7.5and8.5 m.b.s.(- 3 m below the water table).A st udy byRudo lph-Lund(1997)at MoreppenIII(Fig. 1)using multi-levelsamplers locatedfrom 4-7m.b.s.showsno signs of pyrit e or calcit e weat hering . Except for studies at Morepp enIIandIIIsam plesarecollecte d from wellswith fil- terscreen svarying in lengthbetw een one and six met res.

Theinterp retat ion with respect to horizontalst rati ficati on maytherefo rebeseriously distorted dueto sam p leavera- gingover thelengthof thefilter(Martin-Hayde n&Robbins 1997). and the interpretation of the hydroc hemistry must consider possib lemixingof gro undw aterfro m diffe rentho- rizons.At Morepp enI(Sw endesenetal. 1997).water samples

tt,"

~ \--pj,

~~

_Initialcond itions Pyrite Calcite Goethite Anoxicdeepgrw

+ +

+ •

+ •

4 6 10

Oxygen (mgll) + pH

• DO

+

Silicate~eathering&ionexchange

•

P .te oxidation +

aCle wea enng Silicateweathering

• +

- ""' - - -- - - ; - - - , , - /

11 10

Parameter/ Precip itati on Percolation Unsaturated Satu rated zone Satu rated zone

unit s in mmol/I water zone /upper /low er

pH 4.38 3.76 5.88 6.55 7.85

Chloride 0.014 0.028 0.044 0.039 0.079

Nit rate 0.032 0.111 0.0 11as N

Sulfate 0.022 0.044 0.043 0.057 0.149

Sodiu m 0.012 0.024 0.062 0.079 0.125

Potassium 0.002 0.004 0.027 0.014 0.027

Calcium 0.003 0.006 0.052 0.069 0.662

Magnesium 0.002 0.004 0.019 0.030 0.105

Amm onium 0.024 0.000 0.00 14 asN

Alkalinit y 0.06 0.131 1.336

Silicon 0.19 0.1 31 0.158

P02 10-0.7 10-0.7 10-0.7 10-0.7

PC02 10-3.5 10-3.5/-1.5 10-2.0/ -2.8 10-2.4

Table1.Chem ical developmen tofwateratGardermoen.Average values from:Je rqensen et al.(1991), Skaarstad(1996).Basberget al.(1998) and Dagestad (in prep.).

NGU-BULL 434,1998 - PAGE38 LEIF BASBERG,ATLE DAGESTAD&PETER ENGESGMRD

0.15 Nordmoen

0.60 0.80

.., 1

I

cBJ

0.05 0.10

o

Pyrite mol/kg

o

C!!lc ite mol/k g

0.20 0.40

0.00

0.0

83 '---- - - -- - _ --...!-

-5.00 ~ []

-35.00

-40.00

-45.00

f

-50.00 -

c r=

-55.00! -

-60.00

~'---'--- !

-'---'----'---'_L...-."---'--'---'----'---'---'---'---'

0.00 -10.00

=--

.; _{f~ [J~~}

[I]

-25.00 \

I

-30.00

I

s:

'E.

..

c

0.15 Moreppen11

Q05 Q10

o

^Pyritemol/kg

o

Calcite mol/kg

0.20 0.40 0.60 0.80

~

I

IT]

[5J

o

\

o ,

/

[Z]

G_____________

/

8~ 5J

\

" l

0.00

v'

., ,1

;----:~---++---~-

~

^.

-10.00 s:

'E.

..

c

-15.00 L...-."---'--'---'----'---'---'---'---'----'---'_L...-..L--"---'

0.00

Fig. 3. Observed sediment concentrations,Datafor Moreppen 11aft er Dage sstad(inprep.)and data from Nordmoen after Jorgensenet al.1991.

collectedfrom filterslocated2-7and 6-9m.b.s.showno tra- ces of pyriteor calciteweathering.Ground water from awell located at 34-36 m.b.s. hasachemical signat urethat sug- geststhat these weathering processes have occurred,but unfortunately no samples have been collected between 9 and 34 m.b.s.lnconnectionwith the descriptionof alandfill leachate plumeatTrandum(Seet her et al.1992,Basbergetal.

1998),watersamples havebeencollectedfrom wellsunaf- fected by ant roph ogenicactivit ies.Thedepths to the pyrite oxidation zone and the calcite dissolutio n front in these wellsare also suggestedto be approximately three metres belowthegroundwatertable.

The sedimentsampling intervalatNord moenistoo lar- ge to confirm if thesehorizons are located at the same depthatthis locality.However,collected data suggestthat the calciteweathering horizonislocat edat approximate ly7 m below the wate r table and thatthepyrit e horizonis less than16m belowthe watertable(Jerqensen etal.1991).

In general,the vertical resolution is not fine enough to determine,with confidence,the depth to the weath erin g horizon except in the profile measuredby Dagestad(1998) at Moreppen 11.However,allsites indicate a horizonta lstrati- fication of the aquifer with respect to pyrite and calcite. In the remainder ofthis report theciteddissolved concentrati- ons from the upperoxic/low ionicstrengthandlower ano- xic/hig hioni cstre ngth zon es are average concent rat ions ta-

ken from available sources using selectedwells(Table 1).As for the sedimentconcentrations,a selectionof minera lsand valuesfrom Jerq ensen etal.(199 1) and Dagestad(in prep.) has been used.

Model set-up and processes

The sharp boundaries observed between the calcite/decal- cified zoneandthe reduced/o xid izedzone suggest that the dissolut ion ofcalciteand oxidation of pyrite occurat a rate thatismuch higher than that of the downward transport of water. Conseq uent ly,the assumption of equilibr ium seems justi fied(Rubin 1983).Anequilibr ium code such as PHREEQC (Parkhurst 1995)can be used to assist in the interpretati on ofthe weath eringprocesses.The programincludes asuite of aq ueous geochemical reaction models that can be used for theanalysisofa wide varietyof geochemicalproblems.Only afew of theoptions availableare usedhere.These options include;speciation and saturation index, mineral and gas equilibria,and advective-transportmodelling.Evenif equili- brium not can be assumed,the use of such codes can provi- de valuable insig ht into the reaction mechanisms,and can often be a valuable tool for assessing possible weathering scenarios.

A modelaquiferhas been constructed based on infor- mation on the groundw ater chemistry and mineralogy of

LEIFBASBERG,ATLE DAGESTAD&PETER ENGESGAARD

theGarde rmoenaquife r.Fort y mixing cells were usedrepr e- senting a6m-d eep profile.Aselection of minerals has been chosen from the unweat her edsedim entminer alogy,as gi- ven in Table 2,and hor izons observedat Moreppen 11will be used to check the simulation results of the proposedmodel.

The unsaturated zone has been assumed to be deplete d of calcite and pyrite,and this is also indicated by the collec- tedsedimentsamples.Modelledrecharge waterintroduced int o the'fi rst unweatheredcell' is ineq uili briumwit h expec- ted oxygenand carbondio xid epressures in thelower part of the unsaturated zone(Table 3). The composit ion ofinfi l- trat ingwater isconstant throughoutthe sim ulati o n, and the compositionis the same as thatfoundin the decalcified/py- rite-freezone in the uppersaturated zone(Tab le 1).Theinfil- trating water is transported int o the first cell, then water fromthefirst cellintothe second,and so forth.The cell shift s aresim ulati ngad vect ive transpo rt.

Vert ical pore water velocities are calculatedfromobser- ved average groundwater flow.The vertical component of groundwater velocities variesgreatly across the aquifer,but values between 0.6 and 1.2 metre s/yr are represent at ive (Jo rge n sen & 0stmo1990, Basberg etaI.,1998).Thedimensi- ons of the computationa lcellsare0.15*0.15*0.15m,and the effectiveporosityis 0.3.

Sedimentanalyses of the calciteand pyrite contents in weathered and unweatheredsamples from two differentlo- cations at theGardermo en aquiferare shown in Fig3. The difference inweat hered and unweathe red calcit e conce n-

Group Mine ral Wt.%

Primary

Amphibole Horn blende 2.0

Chlo rit e 7.0

K-Mica Biotite 2.0

K-Mica Muscovite 12.5

K-Feldspar 17.5

Plagioclase 8.0

Quart z 48.0

Calcit e 2.5

Pyrite 0.5

Secondary

Vermiculite Imogolite Oxides Carbo nates Oxyhydroxides

Table 2.MineralogyatGardermoen,modified after:Teveldal et al.(1990).

Jorgensen etal.(1991) and Dagestad(in prep.)

NGU-B U L L 434,1998 -PA G E39

trat io ns is approxi m ately0.30mol/kgwith a variati on of 0.1 mol/kg. Sed im ent samples from Moreppen 11, Nor dmoen and Trand um all reveal sim ilar chang es in con cen trations, and theadopt ed initialvalueof 0.30mol/kgfor thereactive part of the calcite is therefore believed to be representative forthe Gardermoen aquifer.Mineralconcentrationsineach cell are given in moles.Henceforth,the reportedvalues must bemultiplied by the bulk density.Report eddry bulk densit i- es vary between 1400-1700 kg/m' (Jorgensen et al. 199 1, Dagest ad in prep.).The calciteconcentrat ion sper cellrange fro m 0.9to 2.2molespercell andaconcentrat ion of 1.275 moles hasbeen chosen.The reportedpyrit econcentrat ion s arenotasco nsist ent as the calcite co nce nt rat io ns;rep o r- ted valuesat Moreppen are approximatelya factor of two higher than the concentrations found at Nordmoen.

Reportedpyriteconcentration rangesfrom 0.03mol/kgto 0.1 mol/kg,giv ing a concentration per cell ranging from 0.15 to 0.62moles.In this study,a valueof 0.214moles has been chosen.

Pyrite oxidation has been studiedintens ivelyby several authors (Nicho lso n et al. 1988, Postma etal.1991,Appelo&

Postm a 1993, Stumm& Mor gan 1995,Elb erling 1996). The proce sses includedin the modelare simplifi ed by assuming that oxidation proceeds atredoxequilibrium and equilib ri- um dissolution .The follow ing reacti o ns(Appelo & Postma 1993)describe the processof pyriteoxidation andprecipita - tion of iron hydr oxides .

FeS,+7/20,+H,O

=

^{Fe" +2S0/ + 2W (i)}

FeS,+14Fel++8H,0

=

^15Fe" +2S0/

+

16W (ib) Fe" + 1/40,+W= Fe" +1/2H,0 (ii) Fe" +1/2H,o=Fe(OH),+3W (iii) Fe" +2H,0

=

^{FeOOH+3H' (iiib)}

In the model, goethite (iiib)has been chosen to precipit at e rather than ferrihydrite(iii).

The energy yield of(i)is much higher than(ii),so incom- plet e oxidatio n mayoccur and a solution rich in dissolved Fe" will resultif the pHis notincreased.Thelimiti ng st ep for pyrit e oxidati o nis believedto bethe oxidationof Fe" (ii), At a pH of around4,the kineticcontrol shifts:

1. At pH<4,the reactionis slow and independent of pH.

2. At pH>4,the reaction rate increases rapidly,but is limited by increasinginso lubil it y of iron oxyhydroxidesatincre- asing pH. A secondary mechanismfor pyrite oxidation is by Fe" (ib), but the solubilitydecreaseswit h a pHincrea- se to the power of 3, so it is only significantatlow pH.

Cell

o

1-40

Pyrite mo les

0.2142

Calcitemoles

1.275

Goeth it emoles PC02 10-2.4 0.002

P02 10-0.7

grw co m positio n upper

deep

Table3.lnitial cond it ions at6°Celsius.

NGU-BUL L434 , 1998 - PAGE 40 LEIF BASBERG, ATLEDAGESTAD&PETER ENGESGAARD

The infiltrating wateris equilibratedat a givencarbon dioxi- de pressure to givea dissolved carbon dioxide concentrat i- on.As the water enters the saturated zone,the system can be characterised as a closed system dissolution of calcite and theamount of calcite that dissolves is determ ined by the amount ofinfil t rati ng carbon dioxide.A simplified reacti- on representing calcite dissolutio n is (Ap pelo & Post ma 1993);

uncertaintiesare clearly associated wit h the simulated re- sults.

Results

The development of the weat herin g hori zon sis depicted using cell2(i.e.0.15- 0.30m belo w the water table)as the fronts develop from 752 to 833 years(Figs.4 and5).This pe- riodisselected sinceit coinc ides wit h the majorgeochem i-

2 J 4 5

Cell(1cell,0.15m)

9.0E-4-

7.0E-4- 0.20-

L

1 2 J 4 5

c.n(1ee1~0.1Sm)

2 J • 5

Cell ('ee1~O_ISm) Pyrite

I.DE - 0.25

7.DE-4- Co 0.05-

0.00- - - 1

0.05

5.0E.~'--'-'L--- 6.0E-4-

5.DE-4 6.DE-4

2.DE-4- :::I!8.0E-4- ::0.15-

~_0.10-

4.DE-4-

,.

3.DE-4-

2 J S

Ceu(1 cell,0.15m)

l' ^{J 4 5}

Cell (1 cell.0.15m) 1.00

1.40.----Calctte

1.20-

0

0.40r-

»:

5.00--"'=--

1 2 3 4

c.n(1een,^0.1Sm)

0.20-

O.OOY~~~'t----

5.0E_S·o'-'---'-"-'_ _----'-_--'--_

1

2 J 4 5

Cell (1 cell,0.15m) :0.80

~^{060l -}

lE-4- 7"'\1

lE.5...Y^e .:;..

lE-6l -

lE-7r- :i lE·e-

~ lE-9r-

0I'E-10"-

! lE-ll-

,. lE·12-

lE-1J- lE· 14-- lE·l&c- rr-

1E·I5-iL.---'--L-- - ~---'---

Dissolved cone.majorIons(timein years:752 .n3.^{806 - 833)}

2.5E-4

sO'

^{r-r- 1.1E·J -}

P~f-f!}---i®-CO C4

2.0E-4j 10E-3-

~1.5E-4l -

1.0E-4

. 0.15- .!~_0.10- 7.00-I

8.00- PH/o-1j>} @

0 ®

025, Gael " " .

m

0.20-~-

• c :=

pH and sedimentconc.(timeIn yea rs:752 •n3 -806• 833)

.

^~

6.00-

Fig_4.Sediment concentrationsand pH.

Fig.S. Dissolvedconcentrat ions.

CaCOj +CO,+H,O=Ca"+2HCOj- (iv)

If a source of carbon dioxideis present,dissolution of calcite will take place.The solubilityof calcite will not solely be con- trolledby the supp lyof dissolved carbondioxide.As pyrite is oxidized,the released protonscan associate with the carbo- nate ion and increase the solubility of calcite.An interpreta- tion of different calcite dissolution mechanisms is given by Plummeret al.(1978):

CaCOj +H·= Ca" +HCOj- at pH<3.5 (v) CaCOj +H,COj•=Ca"+2HCOj- at 3.5<pH <7.0(vi) CaCOj +H,O=Ca"+2HCOj-+OH at pH>7.0 (vii)

Calculations to determine the dominat ing calcit edissolut i- onmechan ism indi cate that calcit edissolutio ndue to buffe- ring ofexcess protons frompyrite oxidation and from infil- trat ingcarbondioxide accountsfor approximatelyone half each of the totaldissolution.

The saturation ind ex(SI)forpyrite andcalcite is setto - 0.6 and -0.5.These levelsofundersaturat ions are prima rily used to correctthe pH value.Sincethe modelonly includes a selection of minerals,the undersaturation may bejust ified as a means to estimate actual values as would be expected in a systemin which the entire mineral matrixwasincluded.

Skaarstad (1996) found iron oxyhydroxides in the upper eight metres of the soil profile,and it is postulated that these iron oxyhydroxidesare precipitated at the calcitedissolution frontafter oxidationofpyrit e since the pH lowering is buffe- red due to calcite dissolution.Postma et al.(1991) suggested

«t hat easily extractableiron oxyhydroxidies may accumulate above the redox line, but no obvious relationshipexists». In the modelapplicat ion reported by Postma et al.(1991) un- dersaturation of goethite was used to simu latedissolut ion of morestableiron oxyhydroxidesthan goethite, whichac- ted as a pH buffer.In the currentmodel, goethite isinclude d in order to precipitate iron oxy hydroxides.Fielddata fro m Moreppen didnot revealhighdissolved iron concentratio ns, suggestingthatiron quickly precipitates.

When the physical and chemical characteristics of the sediments and groundwater have been determined,theini- tial conditions and length of simulation period must be se- lected. The aim of the modelis to reproduce the depth of the weathering horizons since deglaciation,and the simu lat ion period is therefore set to 10,000years.Sincethe weathering processes have occurred over such a long time span,many

LEIFBASBERG,ATLE DAGESTAD&PETERENGESGAARD ^{NGU-BULL 434, 1998 -PA GE 41}

Weatheringratescalcite and pyrite

Fig. 6. Rat es of calciteandpyrite depl etion.

3 2

:Y=3.96*10-4*X-3.43*10-3 :Y = 3.74*10-4*X-3.57*10-3 Calcite dissoluti on

Pyrit eoxid at ion

4 4

o 1000 2000 3000 4000 5000 6000 7000 8000 9000 1000011000 Years exposedtoweathering

1year=6 cellshifts(at 0.9rnIyearverticalflowvelocity)

o 1000 2000 3000 4000 5000 6000 7000 8000 9000 1000011000

o 0

result is a complete pyrite oxidation.

Variatio n in saturation indices influe ncesthe simulated pH value in the differenthorizons.Thisinfluence is most cri- ticalforcalcit e (Table 4).For pyrit e andgoeth it e,the range of sat urat ion indices used doesnot inf luence the pH to the same exte nt, and the variat ions only result in a minor change in pH.The simulated pH valuesfor the basecase agreewell wit h the observed pHin thecompletely weathe- redand in the unweath ered zones. Unfortunately,no measu- rements havebeenmade in theintermediatezone.

The ratesof calciteand pyrite depletionare showniFig.

6.Both ratesare almostlinear,but it was expectedthat the lowering ofpH in thedecalcified zone might have increased theweat heringratesovertime.This influenceon the overall ratescan'tbe detected in these simulat ions. The regression equationsdescribing thepropagation of thepyrit eoxidat i- on and calcitedissolutionfronts aregiven by:

caldevelopm ents in the column corresponding to:1. initial condition,2.calcite deplet ion, 3.pyrit e depletio n,and 4.

completely weat heredcondition.The ot her cells aredeple- ted in thesame sequent ialfashion dow nthrough the co- lumn,andaconstantrateof increasing calcite depleted/Iow pH zone over timeis quickly approached.

The sediment concent rations and groundwate r pH are shown in Fig.4.Alowering of pH as a result ofcalcite depleti- on and ongoing pyrite oxidation isseenin cell 2 until inlet conditionsare establi shed.When the cell is depletedof calci- te,goet hitecan act to buffer the acidity, but goethite dissolu- tion isnot seentoany significant degree.In cell 2,in which calcitehas beendepleted,a slightincrease in pyriteoxid ati on due to the increaseddissolved Fe'· concentrations is seen from thecalculation s.Since calcit edissolut ion is partly con- trolledby pyriteoxidation,itwas expected thattheincreased pyrit eoxidation would affect the calcit edissoluti on in the next cell,butasigni ficant increasehasnotbeen observed.

From these simulations with a flow velocity of 0.9m/yrit would take 9000 years to weather a 3.2 m-thick profil e.

Hencefort h,the simulated results suggestthat theselected average verticalvelocity of 0.9m/yris slightly higher than the actual vert ical velocity in the area.The solubilityofdis- solved iron is strongly dependentupon pH. Dissolved Fe'·

concent rations will be stoichiome tri cally controlled by in- completepyrite oxidationat low pH.At lowpH,the energy differencesbetwe entheoxidationof ironandsulphidewill be determiningfor thedissolvedironconcent rat ion, butas pH increases,iron will precipitatedueto insolubilit y of iron oxyh ydroxides resulti ngin smalldissolvedcon centr ati ons.

Dissolved concent ration sof selected species between 752 and 833 yearsare shown in Fig. 5. The concentrati ons show an increasedamount of dissolved iron, and the shift coinc id es wit h the depl etion ofcalcit ein thecell and the as- sociated lowering of pH to approximately 5.An increased pyriteoxid ation due to lackofcalcit ebufferin g isnot ed in the calculations,anda slight increasein aqueous sulphate can be seen. The aqueou sFe'·concentrations are closerto the calculatedvaluesusingincompletepyriteoxidationdue to infiltrating dissolved oxygen (equati on:i)in theIow-pH zone.Asthesolut ion is transpor ted into the calcitedissolut i- onzone,equili brium is shiftedasthe pHincreases, and the

pyrit e&calcit e calcit e unweathered free zone freezone

Observed 6.55 n.a.- 7.85

SIPyrit e SI Calcit e SIGoethite

-0.6 -0.5 -1.5 6.55 5.14 7.85

0.0 0.0 0.0 6.55 5.82 8.40

0.0 0.0 -1.5 6.55 5.1 6 8.40

0.0 -0.5 -1.5 6.55 5.1 6 7.85

-0.6 0.0 -1.5 6.55 5.1 4 8.40

Tabl e 4.Observe d(average)andsimulated pHatdiffere ntSI.

NGU-BULL 434,1998 -PAGE 42

whereY isthethi ckn ess of weathering horizon(met res).and Xisthe time expos edto weat herin g(years).

The influence of different sediment concentrations is also depicted in Fig.6,where a doubling ofthe pyr ite con- tentlead s to a decrease in the movement of the pyrite oxi- dation front,as expected.The rate of propagation is onehalf of the original propagation rate,and generallywe can say that therate of propagation is linear.

Je rqensen et al.(1991)sugges ted that three metres of theaquifer should be free of pyrite,whilea 10 m-thickhori- zon should be decalcified based on mass balancestudies.

The PHREEQC simulationsyieldresults that arein good agre- em entwit h the depth of thepyrite -freezone.Unfortu nate ly, the good agreement is partly due to a coinc idence.

Jerqensenet al.(1991)used an oxygenconsumption of 0.25 mmol/I while the current study suggests that theoxygen consum ption sho uldbe high er,app rox ima tely0.32mmol/I;

henceforththe current study should predicta deeper pyrite- free zone,but the pyrite concentrations used in thetwostu- dies are such that the oxidationfrontis located at the same depth (sincedifferentdry bulk densit ies have been used).

Jerqensen et al.(199 1)suggestedfro m mass balance studies that approxima tely50%of the bicarbonatein the aquifer re- sults fro m calcite dissolutio nand carbon dioxi de.The same distrib ut io n was foun din this st udy, butthethickn ess of the decalcifiedzones does not agree inthe two studies.The de- viati o ns can'tbe explainedby differentdry bulkdensit ies.A

LEIFBASBERG,ATLEDAGESTAD & PETER ENGESGAARD

morelikely source of the error is that ofpro blems with an unknown source ofcalcium.Tevelda l et al.(1990)concluded that there must be anot her sour ce for calcium .The same problem may have caused an overestimation of the thick- ness ofthe weathering horizons,assugg est ed by Jerqensen et al.(1991).

The pyrite concent rat ion of 0.03 mol/kg is approximately ten tim esthe concent rat io n reportedbyPost m a et al.(199 1), and the propagationof the pyrit e-fr eehorizonisappro xima- tely ten tim esslow erat Garderm oe n,anda much shallowe r pyrit e-fre ehorizon isfound.The thinner oxic-saturated zone makes the aquifermore vulnerabletopossib le contamina ti- on.

The observed horizo ns versus predict ed horizons are shown inFig.7.The verti cal velocityhas been corrected to 0.8m/yrin order to obt ai nanalm ost exactfit.The vertical velocitiesare one of thekey parametersin themodel.With the reported veloc it iesrang ing from0.6to 1.2 m/yr for the Gardermoenaquifer,a reasonablefit can be obtained.

Discussion and conclusion

It em s thatwill beimp o rt antfor the modellingresult sare;va- riation in the mineralogy,partial gaspressureat the water table, aqueous species ininfiltrating wate r,groundwat ervel- ocit ies,flo w pattern,location of thegro und w at ertable in a geo log ica ltime perspective,and variation in precipitation,

mmol/l mmol/l mmol/l -log(H30+). mmol/l

0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0 o.s 1 1.5 5 6 7 8 0.0 0.2 0.4 0.6 0.8 1.0

I 1 1

0 21

1 1

5 0 4

1 I I I I 1 I I 1 1 1

C a

HC03- pH

.~

-5 f-- 1 - I ^{I- I ' -}

T

e- I

+

I

,

. '

^{C. I}

,

^{I I}

-6 I- -

,

f-- I -

,

, , •

g . , _{• .' 0'} ^,

CJ 1 1

, ^{- ,}

u -7 f-- ^1-

1 ^{' - -} 1

~^r..~ e rI

• -,

1 ^(e'

•

^I

~ ^:-

_- - - - - - - _{" - --} -

^J

- - - -

0 -8 ^I-

- ^- _- ^-

0:; I I

- - ^{- -} - ' .

.c

., .,

I °

1-

¹

^{p. ,0} •

~ ^{, .)}

¹

C.

I

~ ^,

CJ -9 1- - 1 -

,

I

Cl _I

~. ^• , , ^• ' . ^{' 0}

t

1

,

1 1 I

I - I-

-1O -r

I I

,

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l.

c. ,

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,

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Sampled cone.

- -

0 Simulated cone.

Fig.7.Observed vs.calculated dissolvedconcentrations.

LEIFBASBERG,ATL EDAGESTAD &PETERENGESGAARD

bothseasonal fluctuations and fluctuatio nson a geologi cal time scale.Areas inwhich therepo rt ed modelcanbe imp ro- vedbyfurtherfield work arein thedet ermination ofminera- logy and dissolv ed gas concentrations.The reportedvalues show some variation,and it would be beneficial to obtain moreconcl usivefielddatafromtheGarderm oen reservoir.

Inthe model,pyrit e andcalcit eweathe rin gin theunsa- turated zonehas beenneglected since theseprocesses have occurredso fastthat theycanbe assumed to beinstantane- ous ona geologica l tim escale.It issuggestedthatincreased oxidat ionand dissolutionoccurr edjustafterthedeglaciati- onwit ha rapidlowering ofthewate r table andexposure of the freshly grained surfacesto the atmosphere.This may haveprodu cedahydrochemicalenvi ronme nt thatis diffe- rent fromtheoneseen today,dueto more ext reme conditi- ons.Thedeterminati on of themaximumdepthto thewater tableon a geologicaltim escaleiscrucialtoobtaining reaso- nabl eresult sfromthemodelsince the pointofzero prop a- gation is assumed at the water table.With the availablein- format ion on ground waterfluctuation s and controlsonwa- tertablelocation,themeasuredwatertablefrom 1996 se- emsto be a reasonab ledividebetweenan unsatu rated and sat uratedzone.

Sediment concentration s are anot her factor that will co nt rol the simulated result s(Post maet al.1991),asdepic- tedin Fig. 6,whereadoubling ofthe pyrit econcent ration is show n togethe r wit hselected concent rat ions.Theincreased pyrite concentration did not affect the down ward move- ment of thecalcitedissolutionfront.If the relative concen- trations of calcite and pyrit e inthe sedime nt saresuch that thetwo fronts are not located at approximately the same depth,itwould be expected thatsomeotherbuffe ring me- chanism than calcit e would beactivein thedecalcifi ed zone, or that aIow-pH zoneshouldbefoundin the aquifer.From samples collected across the aquifer no Iow-pH zone is found, and the hydrochemical signat ure is fairly unifor m wit h eithercalcite and pyrit e weat hering produ ct sor lowio- nicst rengt h water, suggest ing thatnosignifi cant pyrit eoxi- dationorcalcite dissolutionhave occurred.The fieldobser- vationssupportthesimulatedresult s that the pyrit e and cal- citetransition zonesare locatedat approximatelythesame depth .Theresult s alsosuggested that the calcit efront is lo- catedslig ht ly below the pyrit efront. If the calcite front was located aboveorat the samedept h asthepyrit efront,the alkaline environment would negate pyrit e oxidation (Nicholson et al.1988)and the pyrit e-de pletedzonecould not have been reprodu ced using the reported approa ch.

Thereare some uncertainti es associated wit h the selected pyrite concentration due to an inco nsistency in rep orted concentration s.However, whenevaluated togeth er wit hthe calcite content , forwhichthe analytic alresult s aremorecon- sisten t, theselecte d concentr at io n for pyriteseemsreasona- ble.Thisdoesnotexclude thepossibilitythat theremaybe areal variations in sediment concent rati on s and that the depthtothedepl eted zonesmayvary across theaquifer, but

NGU-BULL 434,1998- PAGE43

itissuggested herethattherelativ e concentratio nof pyrite to calcit e willbe fairlyunifor macross theaquifer.This might be explained by the mineralogical analysesthat suggest that both mineralsresidein the shalefract ion of the sedi- ment s.ltwouldbe instructi ve toobtainmorefielddat a in or- der todiscloseanyareal variati on, determ inetheconcentra- tionwit hahigh er levelof accuracy,and disclose possibleva- riat ionsbetwee n the differentdepositionalstr uct ures.

Silicate weat hering isnot accounted for in the model, and henceforth its contribution to theaqueo uschemistryis notevalu atedquantitatively.The exclusion of these minerals may also leadto an overest imation of calcite weat hering, since possiblebuffering of pHby silicatesisneglect ed. The hydrochemi stry indicates that the contr ibutio n to wate r chemistr y acrossthe weat heringhorizon isprim arily derived frompyriteoxid ation or calcite dissolution.Increase inspeci- esindicating silicate weat hering are order sof magn it ude smaller. Inaddit io n to silicateweathering,anobservedincre- asein manganese across theweat heringhorizon maybe ex- plained byadditional buffering by manganeseoxides.It is expect edthat the addi tionalbufferin g will bemostacti vein thedecalcifi edzonejust belowthepyriteoxid ation zone. In the reduc ed leachateplume originati ngfrom theTrandum landfillareductionofsedime nt-boundironand mang anese has been observed, suggesting that there are considerable ironand manganese oxidesand oxyhydroxid es available in theaquifersedim ents (BasbergetaI.,1998). Formine ralsfor which eq uilibri um can't be assumed a kinetic approach must beemployed.Nyst rern et al.(1995) have reported a modelinwhichchemica lkinetics of aluminumsilicatemine- rals areincluded.Their modelhas been construct ed based on thePHREEQE mod el,anda similarapproach could be em- ployed inafuturestudy, wherea selectionof themostactive minerals could be tested against observed weathering in thefield.

Whenthefocusis shifted to themoveme ntofant hro po- geniccontamina nts, natu ral weatheringprocesses will con- tribute significant ly less than anthropo gen ically induced changes.However,an understanding ofthe natural geoche- mical enviro nmentin theaquifer formsthe basis foraneva- luation of the movement ofant hropoge nic cont aminants.

Thedistinct separation andthicknessobservedoftheoxic andanoxic zone willinf luence leachate migration,depen- dinguponwhic h zonetheleachate residesin.The velociti es and flow patternin the area,especially inthevicinit yofthe Trandum landfill,arecomplexdue to highlyhete roge neous natu reof the sediment s in this area(Basberget al. 1998).To assess the importanceof theflow patt ernit isnecessa ry to coupleflow andsolutetranspor t with thegeoch emi cal reac- tion s.These result swill provid ea betterunderstand ing of howthedepth of thepyrite and calcite horizons varieswit h the hydrogeolog ical flowpattern.

Interp retation ofthe current simulation sreq uiresthat theuser isaware of thecon straint sthat are imposed on the model. Nevertheless,the simulatio nsdomimic thefieldob-

NGU-BULL 434,1998 - PAGE44

servations, and themainfeaturesof propagation of the cal- cite-and pyrite -depleted horizon s seem tobe capturedby themodel. Observ ed weat hering horizon s at3mbelow the wat er table of pyrite and calcite have been reproduced using the PHREEQC geochemical model on a geolog ical time scale.The model alsopredicted a low pH anddecalcifi- ed zone located between the completely weat hered and unweath ered zone of the aquifer.Thiszone wasexpected, but no evidencehasbeen collected sincethe microscreens wereplacedvert ically too far apart.The determ ination of the dominating nat ural geo-/hydrochemical environment will be the startin g pointfor future investi gati ons oflandfilll ea- chate migration in the aquifer attheTrandum landfill,inclu- ding simulati onsof coupled flow, solutetranspo rt andgeo- chemistry.

Acknowledgments

We thank the Research Councilof Norway for funding.Crit icalreading of the manuscriptby ala MagneSzetherand Steven A.Banwarth are greatlyappreciated,as their suggestionshave greatlyimproved this pa- per.Special thanks are extended to Kirsten Djorupand DavidRoberts for their linguistic comments on the manuscript.

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Manuscriptreceived January 7998:revisedmanuscriptacceptedJune7998.