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Prepare�a��Publi�he��i�h�heSupp�r��

THESCOTTCOUNTYBOARDOFCO��SS�ONERS

SUBSURFACERECHARGEANDSURFACE�NF�LTRAT�ON

By

RobertG.Tipping

2006

COUNTYATLASSER�ES

ATLASC-17

Pla�e 6—Sub�ur�aceRechargea��

Sur�ace ��il�ra�i��

GEOLOGICATLASOFSCOTTCOUNTY,MINNESOTA©��

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�NTRODUCT�ON

Thesemapsdescribeland-surfacewaterinfiltrationandpotentialrateofrechargetoburiedglacial

andbedrockaquifers. Infiltration, recharge,and runoffarecomplexphenomena involvingdepth to

water,permeabilityof soilandsubsurfacematerial,degreeof saturation, topography, landuse,and

otherfactors(Allerandothers,1987).Thisplateprovidesaqualitativeanalysisofverticalinfiltration

andrechargethatislargelymaterialsbased. Soiltypes,surficialgeologicmaterials,andthemapped

sequenceofburiedglacialdeposits areused toestimate relative ratesof infiltrationand recharge.

The term"infiltration" isused todescribe thedownwardmovementofwater throughtheunsaturated

(vadose)zone. The term"recharge" isused todescribedownwardmovementofwater through the

saturated(phreatic)zone.Themethodsusedherearebasedonverticalmovementofwateronly,and

doesnotconsiderlateral(horizontal)flow,whichmaydominateinmanysettings(GeologicSensitivity

Workgroup,1991). Therechargemapsdealprimarilywith the larger,mappableunitsofburiedsand

andgravel.Therearemanyothersmallerunitsthatmayimpactrechargerateslocally.

SUBSURFACERECHARGE

�ETHODS

MapsshowninFigures1through4arebasedonmethodsdevelopedbyBerg(2006). Avertical

successionofoverlapping, sometimes intersectingburiedaquifersare thought toprovideacomplex

pathwayforwater tomovedownward throughunconsolidateddeposits. UsingBerg'sapproachofa

"rechargesurface,"eachburiedaquiferreceivesfocusedverticalrechargefromthebaseoftheoverlying

aquifer ifan interveningconfining layer is thinorabsent. Thisconceptualapproach is illustrated in

Figure5.

Calculationsusedtoconstructthesemapsweredoneusingthedigitalsurfacesconstructedaspart

ofthethree-dimensionalgeologicframeworkmodel(Plate4).Recentinfiltrationfromprecipitationthat

reachesthewatertablemovesdownwardthroughsandandgraveldepositsuntilitintersectsaconfining

unit(buriedtill)orreachesthebottomofasurficialsanddeposit(rechargesurface1).Inplaceswhere

theintersectedtilllayerislessthan10feet(3meters)thickorabsent,watermovesdownwardintothe

underlyingsandbodyandbecomesrechargesurface2.Theprocessisrepeateduntileitherthebedrock

surfaceisreachedortherearenolongeranyless-than-10-foottilllayers.Itshouldbenotedthattheunit

"undefined"belowtill3andsand4isarbitrarilytreatedasaconfininglayer,althoughthecomposition

ofthisunitisunknown(Plate4).

Areasofburiedaquiferswithfasterrechargeratesareshowninredororange.Theseareportions

oftheburiedaquiferthatareeitheroverlainbyacombinationofsandandgraveldepositsand/orthin

tillunitsthatextendbacktothelandsurface.Theratesofrechargeareadaptedfrompreviousmapping

wherequalitativeestimatesareinverselyproportionaltothethicknessoftheoverlyingconfininglayer

(Fig.6;forexampleGeologicSensitivityWorkgroup,1991).Usingthisapproach,thoseareaswhere

thethicknessoftheconfininglayerbetweenthetopofaburiedaquiferandthenextoverlyingrecharge

surfaceislessthan10feet(3meters)receivea"veryfast"rating;wherethethicknessisgreaterthan40

feet(12meters),theareareceivesa"veryslow"rating.Intermediateratingsandassociatedthicknesses

areshowninFigure6.

RESULTSANDD�SCUSS�ON

InnortheasternScottCounty,a seriesofoverlappingand intersectingsandandgraveldeposits

fromrepeatedglacialadvancesextendsdown to thebedrocksurface (seePlate4). Thesedeposits

provideapathwayformorerapidrechargetomoveverticallytodeeperburiedsandandgravelandto

thebedrocksurface,asillustratedinFigures1through4.Inseries,themapsalsoshowadecreasein

rechargepotentialwithdepth.

Existingwaterchemistryandisotopicdatasupporttheconceptualmodelofenhancedverticalrecharge

inthenortheasternpartofthecounty.Burman(1995)foundevidenceofrecentrecharge(wateryounger

than50years) inbothbedrock(thePrairieduChienGroup)wellsandwellscompletedintheburied

bedrockvalleyinthevicinityofPriorLake.ChloridelevelsfromBurman(1995),commonlyusedas

indicatorsofrecentrecharge,werealsoelevatedinthispartofthecounty.MinnesotaDepartmentof

Healthisotopicdataalsoindicaterecentrechargeinthenortheasternpartofthecounty(S.Robertson,

unpub.data).AdditionalrecentrechargewasdetectedinwellsalongtheMinnesotaRiver.Similarpatterns

ofenhancedverticalrechargetothenortheastandalongtheMinnesotaRiverwerefoundinnitratedata

providedbytheScottCountyEnvironmentalHealthDepartment(P.Schmitt,unpub.data).

SURFACE�NF�LTRAT�ON

�ETHODS

ThemapshowninFigure7combinesNaturalResourcesConservationServicehydrologicgroup

ratings(NaturalResourcesConservationService,2006a)fromadigitizedversionoftheScottCounty

soilsatlas (MetropolitanCouncil,1998),with texturesderivedfromthesurficialgeologymap(Plate

3). Thisapproachhasbeensuccessfullyapplied toanalyzesurfacewater/groundwater interaction

elsewhere in themetroarea,andprovidesan importantcomponent toground-water rechargemodels

(ZhuandMohanty,2002;Barr,2005).Themapunitsareusedtorepresentvadose(abovewatertable)

conditions. MuchofScottCounty iscoveredwithclay-richglacial till that restricts thedownward

movementofwater.Asaresult,perchedwatertableconditionsarepervasiveandanintegratedwater

tableaquiferdoesnotexistcountywide.

Becauseofthelackofanintegratedcountywidewatertable,infiltrationrateswereestimatedtoa

hypotheticalwaterdepth(vadosezonethickness)of10feet(3meters).InfiltrationratesfromNatural

ResourcesConservationServicepublications (NaturalResourcesConservationService,2006b)were

appliedtotexturesfromthesoilsmapandthesurficialgeologymap(Table2)toconstructtherecharge

ratingsshown inFigure8. Ifnohydrologicgroupratingwasavailable foragivenareaon thesoils

map,thenthesoilzoneestimatewasmadeusingtheinfiltrationrateassignedtothesurficialgeology

maptexture.Peatonthesurficialgeologymapwasassignedanintermediateinfiltrationrateof0.15

inch(3.8millimeters)perhour.Theseratingsprovideanestimatedrateofinfiltrationthrough3feet(1

meter)ofsoilzoneand7feet(2meters)ofunderlyingparentmaterial,assumingthatthetotal10foot

thicknessisunsaturated.ThismethodfollowstheapproachappliedtoNaturalResourcesConservation

Servicedualhydrologicgroupratings,whereratingssuchasA/D,B/D,andC/D,showninTable2,are

givenforcertainwetsoilsthatcanbeadequatelydrained;thefirstletterappliestothedrainedcondition,

and thesecond to theundrainedcondition(NaturalResourcesConservationService,2006a). In this

way,theinfiltrationmap,liketherechargemap,isbasedprimarilyongeologicmaterialsonlyanddoes

notaccountfortemporalhydrologicconditions.

RESULTSANDD�SCUSS�ON

TexturesofQuaternarydeposits in theeasternandnortheasternpartsof thecountyarecoarser-

grained than the restof thecounty. Superiorprovenance till andDesMoines lobe tillmixedwith

Superiorprovenancetilltotheeasthavegreatersandcontentthanothersurficialtills(seePlate3).As

aresult,infiltrationratingsarehigherintheeasternandnortheasternpartsofthecountythaninplaces

wheretillispresentatthelandsurface.

Dualhydrologicgroupratingsforhydricsoilsresultinfastinfiltrationratingsinareasthatoverlie

unitsmappedaspeatonthesurficialmap(Plate3).Asdiscussedabove,dualhydrologicratingsresult

inhighinfiltrationratings,eveninareaswherewaterisnearoratthelandsurfaceasindicatedbyhydric

soilsandthepresenceofpeat.Thisistoaccountforareaswherewatertableconditionscouldchange

duetodrainagemodifications,pumping,ornaturalchangesinthehydraulicgradient.

Ratingsdonottakeintoaccountsecondaryconditions,suchasoxidationandfracturing,thatcan

affecttillpermeability.Oxidationintillscommonlyextendsdownwardsasmuchas49feet(15meters),

increasingpermeabilityofthetillbyasmuchasthreeordersofmagnitude(SchillingandTassier-Surine,

2006).InScottCounty,areasratedslowmayexperienceinfiltrationatgreaterratesthanmappeddue

tosecondaryporosityandpermeabilityinsurficialtills.Othersecondaryconditionsnotincludedinthe

ratingmethodarethepresenceofdraintiles,whichalsowouldresultingreaterinfiltrationratesthan

areshownhere.InScottCounty,tilingoccursprimarilywestofMinnesotaHighway21(P.Beckius,

ScottCountySoilandWaterConservationDistrict,unpub.data).

SU��ARYANDCONCLUS�ONS

Therechargetoburiedaquifersandinfiltrationmapsonthisplateprovideaqualitativeassessment

ofhowquicklywatermovesverticallydownward into thesubsurface. Theestimatesareconsidered

qualitativebecause theyhavenotbeen fieldverified,donotconsiderheterogeneities in thesoilsor

subsurface,anddonotaccountforactualwatertableconditionsorlateralwatermovement.Thegeologic

modelofunconsolidateddeposits,basedonourcurrentunderstandingofglacialstratigraphywithinthe

county,agreesfavorablywithexistingwaterchemistryandisotopedata.Specifically,enhancedvertical

rechargeoccursinthenortheasternpartofthecountyandalongtheMinnesotaRiver,wheresequences

ofburiedsandbodiesprovidepathwaysforwatertomovemorequicklytobedrockaquifers.Thereis

lessverticalrechargeinthecentralandwesternpartsofthecounty,wherethicktillsequencesandlack

ofwell-connectedburiedsandbodiesrestrictthedownwardmovementofwater.

Themapsdonotaccountforallwaterwellsinunconsolidateddeposits;manywellswerecompleted

insandsthatarenotshownasburiedsands1,2,3,or4(seediscussionofbuildingthethree-dimensional

model—Plate4).Instead,themapsprovideguidanceforwheremorerapiddownwardverticalmovement

ofwaterisexpectedtooccur,andwhererechargetoburiedaquifersisrestrictedduetothepresenceof

thickoverlyingglacialtills.

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��NNESOTAGEOLOG�CALSUR�EY

Harve�Th�rlei��,Direc��r

LO�A�IO�DIAG�A�

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Sl��—Severalmonths

�er��l��—Monthstoayear

��era�e—Amonth

Fa��—Weekstoamonth

��il�ra�i��ra�i�g�—Estimatedvertical

traveltimeforwatertomovefromtheland

surfacetoadepthof10feet(3meters).

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REFERENCES

Aller,L.,Bennett,T.,andLee,J.H.,1987,DRASTIC:Astandardizedsystemforevaluatinggroundwaterpollutionpotential

usinghydrogeologicsettings:EnvironmentalProtectionAgencyReportEPA/600/2–87/035.

BarrEngineeringCompany,2005,Intercommunitygroundwaterprotection:'SustaininggrowthandNaturalResources,inthe

Woodbury/AftonArea':ReportondevelopmentofagroundwaterflowmodelofsouthernWashingtonCounty,Minnesota:

Minneapolis,Minn.,<http://www.co.washington.mn.us/client_files/documents//ENV-LCMRModel.pdf>.

Berg,J.,2006,Sensitivitytopollutionoftheburiedaquifers,pl.9ofGeologicatlasofPopeCounty:MinnesotaDepartment

ofNaturalResourcesCountyAtlasC-15,pt.B,9pls.

Burman,S.R.,1995,Pilotstudyfor testingandrefininganempiricalgroundwatersensitivityassessmentmethodology:

Minneapolis,UniversityofMinnesota,M.S.thesis,256p.

GeologicSensitivityWorkgroup,1991,Criteriaandguidelinesforassessinggeologicsensitivityofground-waterresources

inMinnesota:MinnesotaDepartmentofNaturalResources,DivisionofWaters,122p.

MetropolitanCouncil,1998,Digitalsoilsurvey—ScottCounty[representingfieldconditionswhenthesurveywascompleted

in1955]:St.Paul,Minn.,<http://geogateway.state.mn.us/df/documents/show.php?nd=0&n=5>.

NaturalResourcesConservationService,2006a,Soilpropertiesandqualities:Washington,D.C.,<http://soils.usda.gov/

technical/handbook/contents/part618p2.html>.

———2006b,Hydrologic soilgroups: [NationalEngineeringHandbook (NEH-4), filedas210-630 (NEHPart630),

<ftp://ftp.wcc.nrcs.usda.gov/downloads/hydrology_hydraulics/neh630/hydro_soil_groups.pdf>.

Schilling,K.,andTassier-Surine,S.,2006,HydrogeologyofPre-IllinoiantillattheI-380reststopsite,LinnCounty,Iowa:

IowaDepartmentofNaturalResources,IowaGeologicalSurveyTechnicalInformationSeries51,53p.

Zhu, J., andMohanty,B.P.,2002,Spatial averagingofvanGenucthenhydraulicparameters for steady-state flow in

heterogeneoussoils:Anumericalstudy:VadoseZoneJournal,v.1,p.261-272.

Figure1. Estimatedrateofrechargeforburied

sand2.Morerapidrechargeisestimatedforthe

easternandnortheasternpartsofthecounty,where

sand2isclosertoordirectlyconnectedtosurficial

sandandgraveldeposits.Sandunitshigherinthe

stratigraphicsequenceareshownbyapattern.


sand3.Rapidrechargetosand3islimitedtothe

northeasternpartofthecounty,whereitisclose

toor indirectcontactwithoverlyingsand2or

surficial sandandgraveldeposits. Sandunits

higher in the stratigraphic sequenceare shown

byapattern.


sand4. Rapid recharge to sand4 is limited to

areaswhereitisclosetoorindirectcontactwith

portionsof sand3 receiving rapid recharge,or

surficial sandandgraveldeposits. Sandunits

higher in the stratigraphic sequenceare shown

byapattern.

Figure 4. Estimated rate of recharge for the

bedrocksurface. Rapidrechargetothebedrock

surfaceislimitedtothoseareaswhereitisatthe

landsurface,orisclosetoorindirectcontactwith

surficialsandandgraveldeposits.

Figure5.GeneralizedcrosssectionofnorthwesternScottCountyshowingconceptualrechargeforburiedsandandgravelandbedrock

aquifers. Recent infiltrationfromprecipitation thatreaches thewater tablemovesdownwardthroughsandandgraveldepositsuntil it

intersectsaconfiningunit(buriedtill)orreachesthebottomofasurficialsanddeposit(rechargesurface1).Inplaceswheretheintersected

till layer is less than10feet (3meters) thickorabsent,watermovesdownward into theunderlyingsandbodyandbecomesrecharge

surface2.Theprocessisrepeateduntileitherthebedrocksurfaceisreachedortherearenolongeranyless-than-10-foottilllayers.It

shouldbenotedthattheunit"undefined"belowtill3andsand4isarbitrarilytreatedasaconfininglayer,althoughthecompositionof

thisunitisunknown(Plate4).

Table1.Rechargerateconceptualmodelmatrix.Oneofthevariables

thatdetermines the rateof recharge is the thicknessof theconfining

layerbetweenthetopoftheaquiferandtheoverlyingrechargesurface

asdefined inFigure5. Theseestimatesarequalitativeandbasedon

comparable travel time estimates from previous studies for similar

geologicmaterials(Berg,2006).Figure6. Recharge ratings forFigures1

through4asdefinedbyverticaltraveltime.

This rating comes from earlier geologic

sensitivity ratings (Geologic Sensitivity

Workgroup,1991). Ratingsarebasedon

thetimerangerequiredforwateratornear

thesurfacetotravelverticallydownwardto

anaquifer. Tritiumandcarbon-14studies

indicate the relativeagesofgroundwater.

Ground-wateragedatawerenotcollected

as part of this study; these estimates are

qualitativeandbasedoncomparable travel

time estimates from previous studies for

similargeologicmaterials(Berg,2006).

Figure7. Surface infiltrationmap. Thismapshows the

estimatedrelativerateofinfiltrationthroughthevadose,or

unsaturatedzone.Unitsonthismapweredefinedbasedon

NaturalResourcesConservationServicehydrologicgroup

ratingsfromtheScottCountysoilsatlas(NaturalResources

ConservationService,2006a), andunit textures from the

surficialgeologymap(Plate3).

Table2.InfiltrationratesforFigure7forsoilhydrologicgroupsandsurficialgeologymaptextures.The

rateofinfiltrationthroughthevadosezoneisestimatedusingNaturalResourcesConservationService

soilsatlashydrologicgroupratings(NaturalResourcesConservationService,2006a)andtexturesfrom

thesurficialgeologymap(Plate3)asamodelformaterialsbelowthesoillayertothewatertable.

Figure8. Infiltration ratingsasdefinedbyvertical

travel time. Ratingsarecalculated fromminimum

estimatedtransmissionratesforsoilhydrologicgroups

(Natural Resources Conservation Service, 2006b)

appliedoveradistanceof10feet(3meters).

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