ML21119A066

Uranium Travel Time and Estimated Maximum Concentration in the Cimarron River

ML21119A066
Person / Time
Site:	07000925
Issue date:	03/31/2004
From:	- No Known Affiliation
To:	Office of Nuclear Material Safety and Safeguards
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Download: ML21119A066 (8)

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Uranium Travel TimeandEstimated MaximumConcentration in theCimarron River Burial Area#1Groundwater Analytical Modeling ForCimarron Corporation's Former Nuclear FuelFabrication Facility

Crescent, Oldahoma Introduction Burial Area#1 trenches received uranium-impacted wastefrom1966until thetrenches wereclosed with asoil cover in1970.

Soilsettlementinthetrench areaultimately ledto thedecision toexcavate the buriedwaste material

- aprocess which occurred during the time period 1986through 1988.

These trenches stayed openfrom1986(when excavation began) towhenNRCconfirmatory sampling in1992showed theareawas decommissioned inaccordance with releasecriteria.

Subsequent groundwater sampling north oftheburial areaconfirmed the presenceof contaminated groundwater.

Kerr-McGee hasattempted toreconstract the releasehistory considering thefact that the furthest extent oftheuranium plume hastraveled some500 feet tothenorth ofthetrench area(Cimarron Corporation, 2003).

Thepresent-day centroid of theplumeexists inthe areaofwell TMW-09(with aconcentration of5,039 pCi/1 total uranium inAugust 2002).

Onepossible explanation, whichisconservative, isthat the release history consisted of three stages:

1)movementofimpacted groundwater fromthe source(trench) areadueto recharge during thetime thetrenches wereopen(1986-1992) northward 100 feettothe areaclose tomonitoring well TMW09;2-)

movementfromTMW09 northward an additional 200feet under amuchless steep gradient tothearea close tomonitoring well 02W05;then3)movementanadditional 250feet duenorth under only aslight groundwater gradient (and sometimes reverse groundwater gradient) withonly rninimal dispersion butagreater interaction between theCimarron River andthealluvium aquifer.

A simplified schematic isattached toillustrate STAGE1,STAGE2,andSTAGE3 migration.

STAGE1:Early onintheplume's migration, theheadcreated bywaterinthetrenches andasteep groundwater gradient asthegroundwater discharged fromthebedrock into thealluvial material wassufficient tobethedriving force forinitial plume movement.

STAGE2:Groundwater entered thealluvial material with aheadbehind itandinto material having amuchhigher hydraulic conductivity (0.001 to0.01cm/sec) than where theplumeoriginated.

Alluvial materials aremostly clay butwith thin sandy stringers.

A groundwater gradient ontheorder of0.01 isestimated fromthepresent northwesterly extent oftheplume.

Advection, with longitudinal dispersion, becomes animportant transport mechanism.

STAGE 3:Thefurther awayfromtheoriginal source (trench) areatheplume moves,theslower the groundwater velocity becomes (several orders ofmagnitude slower) asthegroundwater gradient flattens.

A gradient ontheorder of0.001 isestimated fromthepresent northern extent ofthe plume tothe CimarronRiver.

Diffusion nowbecomes animportant transport mechanism-and advection and dispersion become muchless important duetoslower groundwater movement.

Analytical Modeling A one-dimensional advective-dispersive analytical model wasused to"affirm" therelease and leaching scenario.

Themodel used (Ogata 1970) ispresented inEquation 9.5inFreeze and Cherry (1979).

Analytical modeling isused to"validate" the hydraulic conductivity value usedintheStage 2

transport mechanism.

Themodelisthenused tosimulate thetransport mechanism under the Stage 3scenario.

STAGE2Modeling "validation":

A dispersivityof20 feetwasused inthemodel bytaking 10%oftheplume length frommonitoring well TMW-09 totheareabetween monitoring wells 02W04and02W05where the2000-3000 pCi/1 contourlines arelocated. Thelongitudinal dispersion coefficient, Dx,isestimated tobe1ft2/day andiscalculated bymultiplyingthe dispersivity bytheretarded average linear velocity

(=0.05 ft/day*20 ft).

Thefollowing isalist ofinput parameters usedinaffirming the measured hydraulic conductivity.

Plumelength L = 200ft(from TMW09totheareabetween wells 02W04and 02W05)

Dispersion coefficient Dx= 1ft2/day Distribution coefficient Kd= 3ml/g Effective porosity n = 0.25 Groundwater gradient i= 0.0375 Retardation factor R= 1+Kd*p/n

= 21A(p

= 1.7g/cm3)

Hydraulic conductivity used

= 7.5ft/day Average retarded contaminant velocity V = K*i/n*R

= 0.05ft/day Withthese assumptions

andinput, itcanbecalculated that itwould take 10years (1992 to2002) fordissolved uranium totravel northward 200feet fromtheTMW09location towhere auranium concentration ofabout 2,750pCi/1 would bepredicted.

Thisconcentration anddistance are consistent with theplume mapdrawn onthealpha specanalytical data(Cimarron Corporation, 2003)

- andthus affirms areasonable plume migration scenario foruranium transport under the Stage 2transport scenario.

Thevalidated hydraulic conductivity isreasonable notonly toplume maps,butalso todatagenerated fromaslug test conducted onTMW09.

STAGE3Modeling scenario:

Under theStage 3transport scenario (where potentiometric maps showanextremely flat groundwater gradient of0.001 orless),

itispredicted that itwould take approximately 1900years forthetipofthedissolved uranium groundwater plume(here defined

as 1% ofthesource atthearea between 02W04and02W05orequal 30pCi/1 ofdissolved uranium)toreach totheCimarron River.

(Please seetheattached Mathcad worksheet forthe detailed calculation).

This scenario assumes that thesource iscontinuous andnoremediation ofthegroundwater is undertaken byCimarron (note:

itisbelieved that thesource wasapoint source leaching asthe source wasremoved). Thistime estimate shows theslowmigration ofthedissolved uranium plume inthe aquiferunder flat groundwater gradient conditions.

Calculations forSTAGE3scenario follow.Thegroundwater potentiometric gradient is estimated tobenotmorethan 0.001 (August 2002data).

Thedistance fromtheareaofthemid-level concentration (area between wells 02W04 and02W05) totheedge oftheCimarron River is approximately 1,400 feet.

Theestimated retarded contaminantvelocity is0.0014 ft/day (all input parameters remain thesameexcept the groundwater potentiometric gradient flattens to0.001).

Thedispersion coefficient used Dxis 0.024 ft2/day (=17.3ft*0.0014 ft/day)

- twoorders of magnitude less than thedispersion coefficient used undertheStage 2migration.

Adispersivity value of17.3 ftwasestimated using Xuand Eckstein (1995) equation asdocumented intheUS EPABioscreen Natural Attenuation Decision Support System User's Version 1.3(;Environmental Protection

Agency, 1995).

Finally, inorder tocalculate themaximum concentration to reach the CimarronRiver during the Stage 3transport, Equation 9.7inFreeze andCherry (1979)isused assuming Dxis0.024 ft/day, Dxis0.024ft2/day, Dyis0.024ft2/day Using atotal uranium mass oflessthan20pounds within theplumefootprint, theestimated maximum concentrationto reach theriver isless than 2

pCi/1 after 2,740years ofmigration (1400 ft/0.0014 ft/day),

assumingthat themaximum concentration islocated atthecenter oftheuranium plume.

A Mathcad worksheet isattached fordetailed calculation.

Conclusion According totheanalytical

modeling, itisbelieved that theleading edgeoftheuranium-impacted groundwater plume isnotexpected toreach theC:imarron River forover1000years of migration, andeventhan theconcentration inthegroundwater will beless than 2pCi/1.

This scenario assumes that noremediation ofthegroundwater isundertaken bytheCimarron facility.

Thehighest concentration ofuranium inthegroundwater that

will, ifever, reach theCimarron River isless than 2pCi/1 after 2,740 years ofmigration.

Theseconcentrations donotrepresent theuranium concentration oftheriver after the groundwater hasdischarged into it, buttheconcentration ofthegroundwater.

Theconcentration increase intheriver would notbemeasurable duetodilution fromtheriver.

References Baca,E., "On theMisuse oftheSimplest Transport Model,"

Ground

Water, Vol37,No.4,July-August1999, p.483.

Cimarron Corporation, 2003,"Burial Area#1Groundwater Assessment Report".

Environmental Protection

Agency, BIOSCREENNatural Attenuation Decision Support System User's Manual Version 1.3, EPA/600/R-96/087, August

1996, p.17.

Freeze, R.A.andJ.A.

Cherry. 1979.

Groundwater.

Englewood

Cliffs, NewJersey:

Prentice-Hall.

Steep gradient V

highconcentration

~ T

~~

relatively high velocity

/ /'

.-V

/

C plume alluvial bedrock a

m 4

backfilled 0.01gradien lowpermto ighperm

w#

p-advection prdominates

- 'f;.,..;a7

'^

=

i

^

e a

cm-

/

7,-

y c

+

e c

flat gradient very lowflow

...-..-.,r..

diffusion

&dispersion dominate

4 1

Burial Area

1. 1Grounwater Analytical Modeling This model isaonedimensional advectivedispersive analytical equation used to affirmthe release andleaching scenario under Stage 2.

LUPEfEAmgters L:plume lengthin ft i:groundwater gradient K:hydraulic conductivity in ft/day Kd:distribution coefacient in mL/gm n:porosity p:density inmL/gm Dx:dispersion coef6cient v:average retarded contaminant velocity inft/day t:time oftransport inyear Lm200R Kd:=38 K:=7.51 I:=0,0375 gtn day n:=0.25 p =I.7@

Dx:=1-

'#Y' day v:=

1

= 0.053 ap n.1+-

ft day QhtigaLM9Ael Crelative

=0.51

- erf/ld

+exp@

1-erD (2.Q Dx/b )2-@

Crelative

= 0.546 Crelative5039

= 2.75=103 This istheuranium concentration (in pCi/L) expected between wells 02W04and02W05.

Burial Area#1Groundwater Analytical Modeling l

This model isaonedimensional advectivedispersive analytical equation used to estimate how longitwould take forthe1%plume tip toreach theCimarron River under Stage 3.Notethat this isacontinuous source model andisused toestimate the "ball-park" plume migrationtimeframe.

blpAttalgletelji L:plume length inft i:groundwater gradient K:hydraulic conductivity inft/day Kd:distribution coefficient inmL/gm n:porosity p:density inmL/gm Dx:dispersion coefficient v:average retarded contaminant velocity inft/day t:time oftransport inyear 1"'

l"#^

Kd:=38 K:=7.51 i:=0.001 gm day n:=0.25 p:=1,7 Dx:=0.024.

day I-

= 1.402*10-3 dp n 1+-

ft n /

gj Q8a#ll9291AoAbdicaLM9Ani Crelative

=0,5.1-er@

+exp bl-erf!3-2bt g

DxJ( (2-@x.t Cretative

= 0.01 Crelative3000

= 30.238

Burial Area#1Groundwater Analytical Modeling Thefollowing model, apoint source

model, isfrom Equation 9.7intheFreeze and Cherry Groundwater (1979).Themodel isused toestimate themaximum uranium concentration toreach theCimarron River.

if na 32 Umass=20lb Dx:=0.024-Dy:=0,024-Dz:=0.024-t:=2740yr day day day Cmax:=

U*ff Cmax 2

1.932 Theestimated maximum 8 (n t)2Q@

30.001 dissolvedoranium concentration liter isleas than 2pCi/1 after 2,740 years ofmigration.

l

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