subsurfacerechargeandsurface nf ltrat onbloximages.newyork1.vip.townnews.com › swnewsmedia.com ›...
Post on 27-Jun-2020
3 Views
Preview:
TRANSCRIPT
������
�������
�������
�������
�������
�������
�������
������ ������ ������
������
������
��
��
¤���
���
¦��
������
��������
���������
�����������
��������������
������
�������
�������
�������
�������
�������
�������
������ ������ ������
������
������
��
��
¤���
���
¦��
������
��������
���������
�����������
��������������
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©������������������������������������������
����������������������������������������������������
�����������
�������������������������������������������������������������������
�����������������������������������;������������������������������
������������������������������������������������������������
�������������������������������;�������������������������������������
��������������������fi���������fi���������������������������������
�������������������������������������������������������������������
������������������������������������������������������������������
������������������������������������������������������������������
������������������������������������������������������fi�����fi�������
��������������������
�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.
������
�������
�������
�������
�������
�������
�������
������ ������ ������
������
������
��
��
¤���
���
¦��
������
��������
���������
�����������
��������������
������
�������
�������
�������
�������
�������
�������
������ ������ ������
������
������
��
��
¤���
���
¦��
������
��������
���������
�����������
��������������
������
�������
�������
�������
�������
�������
�������
������ ������ ������
������
������
��
��
¤���
���
¦��
������
��������
���������
�����������
��������������
��NNESOTAGEOLOG�CALSUR�EY
Harve�Th�rlei����,Direc��r
LO�A�IO�DIAG�A�
��������������������������
����������������������������������
��������������������������������������������
�������������������������������
���������������������������������
�������
�������������
���������
���������
�����
�����
�����
�����������
�����
�����
�����
�����
�er��a��—Hourstoweeks
Sl��—Severalmonths
�er��l��—Monthstoayear
���era�e—Amonth
Fa��—Weekstoamonth
���il�ra�i��ra�i�g�—Estimatedvertical
traveltimeforwatertomovefromtheland
surfacetoadepthof10feet(3meters).
���� ��� ����
��������������������������
������������
���������
�������
����� ���� ������ �������
��
��
��
��
���
���
������������������������������
�����
� � � � � ���
����
��������
��������
����
��������
�������
����������������
������������� �������
�����������
���������������
������
���������� ���������������
������������
���
���
���������� �������������� ����
�������������� ������������� ����
����
������
��������������
��������������
�����������������������
����
������������
������������
��������������
���������������
�����������
��������������
�������������
���
��
�������������������
�������
���������������������
�����������
�������������������
��������������
��������
�������
�������
����������������
�������������
���� ��� ���� ����� ���� ������
����
���
���
���
���
���
����������������������������
�����
� � ���
����
��������
��������
����
��������
� � � � � � ������
�����������
�������������
� � � � � � � � �
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.
Figure2. Estimatedrateofrechargeforburied
sand3.Rapidrechargetosand3islimitedtothe
northeasternpartofthecounty,whereitisclose
toor indirectcontactwithoverlyingsand2or
surficial sandandgraveldeposits. Sandunits
higher in the stratigraphic sequenceare shown
byapattern.
Figure3. Estimatedrateofrechargeforburied
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).
����
�������
���������
������������
������������������������������������������
���������������������������������������������
����� ������������
������
����������������������
����������������������������
�����������������������
�����������������������
�����������������
�������� ��� �������������� ��� � ����
������ ���� �������� ������ ���� ��� ����
������������������������������������������
� � � � � � � � � � �� � � � � � � � � � � � ��� �
�������������������
������������������������������������
�����
������������������������������������
�����
������������������������������������
���������������������������������
�����������
top related