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r 

p . 

NUREG/CR-3585 

De Minimis Waste Impacts 

Analysis Methodology 

Manuscript Completed: December 1983 

Date Published: February 1984 

Prepared by 

0. I. Oztunali, Dames & Moore 

G. W. Roles, USNRC 

Dames and Moore 

20 Haarlem Avenue 

White Plains, NY 10603 

Prepared for 

Division of Waste Management 

Office of Nuclear Material Safety and Safeguards 

U.S. Nuclear Regulatory Commission 

Washington, D.C. 20555 

NRC FIN B7318 
P.3 

6.0 POST DISPOSAL IMPACTS 

The previous chapters considered radiological impacts which would result 

from transporting de minimis waste to a treatment or disposal facility as 

well as the impacts that could result as part of operating the waste 

treatment or disposal facility. This chapter addresses the potential 

radiological impacts that could occur long after the waste has been 

disposed into the ground. In this chapter, three sets of generic scenarios for human exposure are considered, including: 

1. "Intrusion scenarios" involving human intrusion into the waste; 

2. "Groundwater scenarios" involving groundwater migration and 

leachate accumulation; and 

3. "Exposed waste scenarios" in which disposed waste is assumed 

to be brought to the surface and is dispersed by wind or water. 

Impacts calculated for the first set of scenarios (the intrusion scenarios) depend upon the concentrations of the radionuclides within the de 

minimis waste streams considered. Impacts calculated for the last two 

sets of scenarios depend upon the total radionuclide inventory and waste 

volume disposed at the disposal site. 

A brief overview of the post-disposal exposure scenarios considered in 

this report is presented in Section 6.1, followed by Sections 6.2 through 

6.4, which discuss the calculational procedures for these scenarios in 

more detail, and address intrusion scenarios, groundwater and leachate 

accunulation scenarios, and exposed waste scenarios, respectively. 

6.1 Overview 

As addressed in the following sections, the calculational procedures and 

assumed parameter values vary for the different exposure scenarios 

depending upon the following considerations: 

o the design and operation of the particular disposal facility; 

o the environment in which the disposal facility is located; and 

o the waste form and packaging characteristics of the 

de minimis waste stream. 

Five different disposal facilities are considered in this report, and any 

one may be considered by the user of the computer codes. These include an 

on-site disposal facility, two disposal facilities for municipal waste, 

and two disposal facilities for hazardous waste. Parameter values for the 

on-site disposal facility must be supplied by the user of the code, since 

the on-site facility may be of any size or configuration. For the other 

four facilities reference site parameter values are assumed in the 

computer codes as discussed in the appropriate sections of the report. In 

addition, three reference disposal site environments are considered in 

this report, and the user of the code may specify the particular environment to be considered. The scenarios and calculational procedures are 

discussed in terms of the above considerations. 

6-1 
P.4 

Afte r calculatin q the average radon qas concentratio n (C) withi n the 

buildino, annual imoacts are determined as: 

H = £ C EDF PDCF-3 (6-18) 

n 

where H i s in mrem/.yr and C is as discussed above. The exposure duration 

factor, EDF, i s obtained from the assumed activit y distributio n presented 

at the beginning of this section for the individual intruder (i.e. , EDF = 

4380/8760 = 0.5). PDCF-3 i s the appropriate pathway dose conversion 

facto r fo r radon-222. Impacts are summed over al l waste streams containinn radium-226 at the time that the intrude r scenario i s initiated , 

and added t.o the other impacts calculate d for the intruder-agriculture 

scenario. 

6.3 Groundwater Scenarios 

These scenarios calculate the potential impacts resulting from qroundwater 

migration of radionuclides from the disposed wastes to three biota access 

location s downstream in the direction of the groundwater flow: (1) a well 

located at the boundary of the disposal area, (2) a wel l locate d between 

the disposal facilit y and the surface hydrologi c boundary, and (3) a 

stream located at the surface hydrologic boundary. Different, pathway dose 

conversion factors are used depending on whether the access location i s a 

well or a stream (see Appendix D). An idealized map showing the qeometric 

relationships hetween the disposal facilit y and the biota access locations 

is shown as Figure 6.3. 

As shown i n thi s figure , the main streamline passing underneath the 

disposal facilit y has been straightened out (the longitudinal coordinates 

are measured along thi s streamline). The overall area of the disposal 

facility , which influences the migration calculations , varie s depending 

upon the particula r facilit y considered. For municipal waste disposal 

(either a sanitary landfil l or an open dump), the disposal area covers 

approximately 300 m by 340 m, while fo r hazardous waste disposal, the 

disposal area covers approximately 665 m by 910 m. For on-sit e disposal , 

the disposal area dimensions (e.g. , length and width corresponding to the 

above 300 m and 340 m) are specified by the user of the code. I t may also 

be observed that the disposal area in Figure 6.3 has been divided into 10 

sectors. In the calculation, the total radionuclide auantity disposed i n 

the disposal site i s divided by 10, and the resultant tenth i s assumed to 

he released as a point source into each sector. The tota l impacts from 

mioratio n from each poin t source are summed. The following equation i s 

used to calculate human exposures which may resul t from the wel l access 

Groundwater scenarios: 

H « I Z VdiVifs i Cin PDCF-6 (6-19) 

i n 

6-18 
Ol 

I 

10 

DTH 

•3-

• 

| — 

X 

o 

¥ 

—> 

X 

1 .ENGTH 

IIII I 

DI[SP0SAL_AREA1 

X X X X 

u\ 

II —> 

X X X 

I 

vr J-1 > 

INDIVIDUAL-WELL 

POPULATION-WELL 

MAIN STREAMLINE 

POPULATION-SURFACE 

WATER 

GROUNDWATER FLOW 

DIRECTION 

SITF BOUNDARY 

KEY: 

X SOURCES 

0 ACCESS LOCATIONS 

GEOMETRIC RELATIONSHIPS 

OF DISPOSAL AREA AND DISCHARGE LOCATIONS 

FIGURE 6. 3 
P.6 

where H i s the annual dose rate in mrem per year during the 50tHn ^peair of 

exposure, PDCF-6 is the radionuc l ide-specifi c pathway dose co Rimers lion 

facto r discussed and presented in Appendix D, Ci n is the concentration!, of 

the radionuclide in the de minimis waste stream considered , 1 <dlem.ot.es 

summation over al l the de minimis waste streams, and n denotes stumnratnon 

over al l the radionuclides. (Note the difference between the caimceimtiratio n C-jn and the concentration Cw considered in the intrusion sceui.ain.os.) 

For a surface water access location the dose conversion facto r PBECF-7 i s 

substitute d instead of PDCF-6. The values of the transfer factors are 

presented below. 

The time delay factor f 0 and the sit e design and operation facto r ff<gf are 

assumed to be unity . This merely means tha t the groundwater nmiigratiori 

scenario i s assumed to be initated at the close of the operational! [period 

and that the influence on the calculations resultin g from design* o f th e 

disposal facilit y i s considered as part of the waste form airo<8 pe.cfec.ge 

factor and the sit e selection factor. 

I t i s also important to recognize tha t due to certai n aspects off the 

disposal facilitie s and sit e environments considered in thi s report,, some 

judgement i s required by the code user in interpreting result s off ground 

water migration calculations. The ground water migratio n calcuuTattftens 

depend upon an assumption that the rate of water percolating tUnrouiigKn the 

disposal cel l covers is less than the rate of water infiltratin g ouut th e 

bottom of the disposal cells . I f not , then the percolating waiter can 

accumulate within the disposal cell s as leachate. This can hapipem, fo r 

example, at a disposal facilit y located at a humid sit e hawiiraig wery 

impermeable soil s (or an impermeable liner) , and the permeabilit y off th e 

disposal cel l covers exceeds that of the soi l (or liner) . It. this case 

impacts associated with thi s leachate accumulation would be more ffi|ppn!D[!i.riate to consider than those associated with ground water migratiiomi- Tffeese 

leachate accumulation impacts are addressed in Section 6.3.3. 

6.3.1 Source Term 

The source term i s represented by the waste form and package facto r ff ., 

which has unit s of m^/year, and which denotes the annual woilutnrae' of 

contaminated liqui d tha t passes int o the aquife r beneath th e «ii'ris|pxosal 

facility . This factor is given by the formula: 

fwi = U Vw f c , (((&-20) 

where f-j i s the fractio n of the total disposed waste volume (<dte rmiinnii,mis 

plus non-radioactiv e waste) tha t comprises the (i) t n de miraiirmiis waste 

stream; Vw i s the annual volume of water tha t percolate s tHi.(rau#i the 

disposal facilit y cover and contacts the disposed waste/soil mixtiumae^ -and 

f c i s the fraction of the waste radionuclide concentration traiusffenrad to 

the leachate. 

6-20 
P.7 

The firs t factor f-j is self-evident; i t is the rati o of the volume of the 

de minimis waste stream being considered to the entir e volume of waste 

disposed. Clearly , the variabl e (Vw) i s simply the percolating infiTitratio n (p) multiplie d by the effective disposal sit e surface area (Sf}, 

which is the surface area of the disposal cells . (The space between th e 

disposal cells , i f any, i s not included). The effective disposal sit e 

surface area is given as follows: 

Sf = Vt o t / (EMP x EFF) (6-21) 

where 

^tot

 = total volume of waste (de minimis and other waste) disposed at 

the site , in m^; 

EMP = waste emplacement efficiency (dimensionless) which i s defined 

to be volume of waste emplaced in the disposal cell s divide d 

by the total volume of available disposal space; and 

EFF = volumetri c disposal efficienc y (i n m) which i s defined t o be 

the volume of availabl e disposal space i n a disposal cel l 

divided by the surface area of the disposal cell . 

In the above equation, al l three parameters are supplied by the user fo r 

on-sit e disposal . For municipal (sanitar y landfil l and open dump) and 

hazardous waste disposal V^0-t is the average annual waste volume times the 

operating life , which is assumed to be 20 years. Reference parameters fo r 

municipal and hazardous waste disposal facilitie s are shown in Table 6-4. 

TABLE 6-4 . Reference Disposal Parameters 

Municipal Hazardous 

Parameter Waste Facilit y Maste Facilit y 

Vt o t (m3) 592,000 1,827,000 

EMP 0.8 0.75 

EFF (m) 7.31 4.37 

There may be- several differen t techniques for calculating the parameter 

(p) (also called PERC i n several references). One of these methods i s 

usuall y calle d the water-balance technique (Refs. 8 and 9 - also see 

Appendix C). The water-balance technique yields percolation components o f 

about 74 mm of water per year for the northeast site , 180 mm of water per 

year for the southeast site , and 1 mm of water per year for the southwest 

site . These values are applicable to surface conditions at the sit e prio r 

to construction and operation of a disposal facility . The volume of water 

percolating in thi s case wil l be denoted by VI . 

6-21 
P.8 

For cases where there exists special disposal facility covers containing 

low permeability layers, and where the integrity of the covers can be 

assumed, the percolation component may be determined by the Darcy velocity 

of the least permeable stratum between the waste and the atmosphere (Ref. 

10). The Darcy velocity of a material, with hydraulic conductivity (K) in 

units of m/yr and unit hydraulic gradient (the most conservative assumption), is equal to K m3/m2-yr. This number, however, should be modified by 

the fraction of each year durina which there is at least 0.01 inch of 

precipitation. Therefore, in this latter case, (p) will be calculated from 

the followino eouation: 

p = K (w/365) (6-22) 

where (K) i s the hydrauli c conductivit y of the leas t permeable laye r 

covering the waste, and (w) i s the mean annual number of days with 0.01 

inch or more of rainfal l (see Reference 10). Assuming that a permeability 

of 3xl0" 7 cm/sec (about 0.3 ft/yr ) is applicable for the least permeable 

stratum of the disposal facilit y cover, and assuming appropriat e values 

f o r w fo r each of the reference sites , thi s yields estimated percolation 

components of 36 mm, 30 mm and 1 mm, respectively , fo r the northeast , 

southeast and western sites . The volume of water percolating in thi s case 

wil l be denoted by V2. 

The specifi c percolation value used in the calculations depends upon the 

particular disposal facilitie s considered. For on-sit e disposal , p i s 

supplied by the user of the code. For municipal waste disposal, p varies 

depending upon whether one is considering a sanitary landfil l or an ope,n 

dump. For a sanitar y landfill , p i s assumed to be equal to VI while fo r 

an open dump, D i s assumed to be equal to 2V1. This basicall y assumes 

tha t in a well-operated sanitary landfill , percolation into the sit e wil l 

be no larger than that which existed prior to construction of the disposal 

facility . An open dump i s assumed to be operated in a much less satis -

factory manner than a sanitary landfill , including poor compaction of th e 

waste, an inadequate, poorly compacted final cover, and failur e to perform 

maintenance activitie s such as repair of subsidence depressions . Thi s 

result s i n increased percolatio n int o the waste, (the open dump i s 

assumed to be characterized by potholes and subsidence depressions , 

resulting in concentrated sources of rain water infiltration) . 

Two optional values fo r the percolatio n are also availabl e fo r th e 

hazardous waste facility--!'.e. , the user of the code may select a percolatio n value eaual to eithe r V2 or VI . The former could represent a 

possible case in which the synthetic membrane in the disposal cel l covers 

fai l to functio n as expected (perhaps due to damage), and rainwate r 

percolates through the compacted clay layer beneath the synthetic membrane. (See Sections 5.5.1 and C.4.2 for a description of the hazardous 

waste facilit y design). Thelatte r could possibl y represent a sit * 

operating under interim status standards (See Section C.4.2). Unlike ti e 

open dump, however, al l hazardous waste facilitie s are assumed t o be 

operated in a reasonable manner. 

6-22 
P.9 

The factor fc represents the fraction of the activity that is transferred 

from the waste to the leachate. It may be calculated usino the following 

formula: 

fc = M0 tc 10d-IA) (6-23) 

where M0 i s the fractio n of a specifi c radionuclid e transferre d from 

disposed waste to trench leachate due to contact of water at continuous 

ful l saturation; t c is the fraction of a year that the infiltratin a volume 

of water i s in contac t wit h the waste; and the facto r 10^ 1-IA> *j s the 

accessibilit y multiplie r (see Section 2.2.4). These are discussed below. 

The facto r M0 can be estimated by many methods. In thi s report , the 

average upper bounds of the leach fraction are estimated assuming that the 

leachate/waste condition s at the Maxey Flats disposal facilit y may be 

used to approximate thi s bounding fraction . As discussed extensivel y i n 

Reference 1 , radionuclid e concentrations in leachate were obtained from 

leachate samples obtained from several disposal trenches . These radio -

nuclide concentrations were then ratioed to the radionuclide concentrations in the waste disposed in the disposal trenches. The ratios obtained 

from each trench were then averaged over al l trenches sampled. The ratios 

thus obtained are liste d in Table 6-5. 

The primary rational e for thi s approach i s that under specified chemical 

conditions there i s an upper limi t to the solubilit y of al l elements. The 

above disposal site , because of the presence of organic chemicals and 

chelatin g agents and because waste trenches can be assumed to be a t 

continuous ful l saturation , may be assumed to represent extreme leachabilit y conditions . This estimate also takes int o consideratio n the 

fractio n of the leached radioactivit y that may be reversibly adsorbed by 

the interstitia l trench soils . Some researchers believe that, use of Maxey 

Flats estimates represent the best that can be achieved with the available 

experimental data (Ref. 4). 

The use of the facto r M0 , however, necessitates a correction factor to 

take into account the expected transient and partiall y saturated conditions . This correction factor is approximated through t c . This fraction 

depends on the contact time between the waste and infiltratin g water. 

Assuming tha t leaching at partia l saturatio n i s proportiona l to the 

moisture content, the fraction (t c ) may be expressed as the fraction of a 

year that the percolation component calculated above takes to pass through 

a aiven horizontal plane, i.e. , 

t c = p/(nv) (6-24) 

where p is the precipitation (i n m/yr) tha t infiltrate s and comes int o 

contac t wit h the waste, n is the waste cel l effective porosity, and v i s 

the speed of the percolating water (i n m/yr). The waste cel l effectiv e 

6-23 
P.10 

TABLE 6-5 . Radionuclide Partition Ratios 

Between Waste and Leachate 

Element 

H* 

C* 

Na 

P 

S 

Cl 

Ca 

Sc 

Cr 

Mn 

Fe 

Co* 

Ni 

Zn 

Se 

Rb 

Sr* 

Zr 

Nb 

Mo 

Tc 

Ru 

Ratio 

1.15E+0** 

5.76E-3 

1.62E-4 

1.26E-3 

1.20E-2 

1.15E-1 

9.86E-3 

1.11E-4 

1.26E-3 

1.26E-3 

1.48E-2 

1.48E-2 

1.48E-2 

1.48E-2 

1.20E-2 

1.62E-4 

9.86E-3 

9.86E-3 

1.11E-2 

1.15E-1 

1.15E-1 

1.15E-1 

Element 

Aa 

Cd 

Sn 

Sb 

I 

Cs* 

Ba 

La 

Ce 

Eu 

Yb 

Pb 

Po 

Ra 

Ac 

Th 

Pa 

U* 

Np 

Pu* 

Am* 

Cm 

Cf 

Ratio 

1.62E-4 

1.48E-2 

1.62E-4 

1.62E-4 

1.15E-1 

1.62E-4 

1.11E-4 

1.11E-4 

4.67E-4 

1.11E-4 

1.11E-4 

4.11E-3 

4.11E-3 

4.11E-3 

4.11E-3 

4.11E-3 

4.11E-3 

1.25E-4 

4.67E-4 

4.67E-4 

4.11E-3 

4.67E-4 

4.11E-3 

* These ratios are obtained from Reference 3, and 

were calculated based on experimental data. The 

remaining ratios are estimated based on chemical 

similarities between nuclides. 

** Exponential notation, i.e., 1.15E+0=1.15xlO+0. 

6-24 
P.11 

porosity can conservatively be assumed to be about 25% (partially compacted soils are likely to have higher porosities resulting in lower 

contact times). The value of v depends on the interstitial soils; a ^ery 

conservatively low value of 1 ft/day (corresponding to a permeability of 

about lxlO-^cm/sec), an effective porosity of 0.25, and a hydraulic 

gradient of unity will be assumed in this report. The values for tc for 

each of the reference sites are presented in Table 6-6. (The contact time 

fraction for the on-site facility is specified by the user of the codes.) 

TABLE 6-6 . Values of Contact Time Fraction 

Northeast 

Southeast 

Southwest 

Volume of Water 

VI 2 VI 

2.66xl0~3 5.39xl0-3 

6.47xl0-3 1.29x10-3 

3.60xl0-5 7.19x10-3 

V2 

1.29x10-3 

1.08x10-3 

3.60xl0-5 

It should be noted that an increase or decrease in the volume of percolating water affects the contact time linearly, which is further incorporated 

into the formulation of the source term fw-j. Therefore, the source term is 

a quadratic function of percolation. It should also be noted that the 

contact time fraction approximation should only be considered reasonable 

over a relatively small range of percolating water speeds. Use of the 

contact time fraction also implies that the disposed waste is not in 

continuous full saturation. 

The last factor in equation (6-23), lo( 1-IA), is the multiplier due to 

the relative inaccessibility of radioactivity in activated metals waste 

streams. The multiplier for activated metal waste forms has been discussed 

in Section 2.2.4. Use of the accessibility factor in the estimates of the 

groundwater migration source term is discussed in Reference 1. 

6.3.2 Migration Reduction Factor 

The waste form and package factor, as expressed above, yields the total 

source term (in nr/yr) that can be expected from a given de minimis waste 

stream, and the product of the radioactive concentration with the source 

term gives the annual release (in Ci/yr). This source term must be 

related to the radionuclide concentrations at the groundwater biota access 

locations. This relation is expressed through the site selection factor 

(fs) in units of yr/m3. This factor, which has also been referenced as the 

"confinement factor" or "reduction factor" (Ref. 1), is the ground water 

migration analog of the (X/Q) dispersion factor in meteorological diffusion calculations. 

6-25 
P.12 

In thi s report , a variation of the migration model utilize d in the EIS's 

in support of the Part 61 rule is used (Refs. 11, 12). This model assumes 

tha t the porous medium consist s of an unsaturated and a saturated zone, 

each of which is stationary , homogeneous and isotropic , and the flui d 

moving through these zones is incompressible and of constant viscosity . 

Also very importantly, use of thi s model assumes tha t any liner s placed 

beneath the disposal facilitie s are non-functional , or at least only 

partl y functional . Hazardous waste facilitie s are required by EPA 

regulatio n to instal l liners , but to date there is littl e data regarding 

their overall effectiveness. Similarly , sanitary landfill s may or may not 

incorporate liners , and lik e liner s for hazardous waste facilities , only 

time and accumulated experience wil l demonstrate their effectiveness. The 

on-sit e disposal facilit y and open dump wil l probably not have liner s 

installed in any case. 

The source term i s assumed to be given by J 0 (which i s equal to fwlmultiplied by the waste radionuclide concentrations), whose unit s are i n 

Ci/yr . The source term i s assumed to exis t during the source duration 

time (T). A geometry of the migration problem is shown in Figure 6.4. 

The measurable hydrogeological parameters tha t must be included in a 

simulation of mass transport are: the geometry of the problem (e.g. , the 

trave l distance, x, to a biota access location), the hydraulic velocities 

of the flui d (e.g. , v), the dispersion characteristics of the medium, and 

the retardatio n coefficients of the soil s through which radionuclides are 

migrating. The space- and time-averaging of the above parameters, i f 

necessary, may be accomplished in a straightforward manner (see Ref. 1). 

As discussed in Section 6.1 , i t can be shown that the time dependent sit e 

selection factor is given by (Ref. 1): 

U, = f>g /Q] £ r t i j (6-25) 

J 

where (Q) is the dilutio n factor in units of volume/time; the factor rg is 

the time-independent reduction factor due to the geometry of the problem 

(i.e. , the spatia l relationshi p of the disposal facilit y and the biota 

access location) ; j denotes the longitudina l sectors of the disposal 

facilit y shown in Figure 6.3 ; and r-tj j i s the reduction factor due to 

migration and radioactive decay which depends on both space and time , 

includin g the sectors of the disposal facilit y and the source duration 

time (T). 

Geometric Reduction Factor - rg 

This reduction factor is assumed to be independent of the characteristics 

of the waste streams. I t is also independent of the longitudinal relation -

ship of the disposal facilit y wit h the biot a access location. In thi s 

report, i t is conservatively assumed that the geometric reduction facto r 

rg is it s maximum value, unity . 

6-26 
ro 

UNSATURATED ZONE 

PARAMETERS: 

w, z0, Ru 

D

u Du Du 

J0(x",y",t") 

BOTTOM OF THE 

DISPOSAL CELL 

HYDRAULIC VELOCITY 

M 

STREAMLINE 

HYDRAULIC VELOCITY (v) 

SATURATED ZONE PARAMETERS: 

V, X, Rs, 

D

s ns ns 

INTERFACE OF THE 

SATURATED AND 

UNSATURATED ZONES 

J(x,t) 

DISCHARGE SURFACE 

GEOMETRY OF GROUNDWATER MIGRATION 

FIGURE 6, 4 

T_ 
P.14 

Dilution Factor - 0 

The dilution factor is independent of the characteristics of the waste 

stream and the geometrical relationship of the disposal facility with 

respect to access location. The factor Q may be the pumping rate of a well 

or the flow rate of a stream. 

In this work, the dilution rates assumed are 200,000 nvVyear (about 100 

gom - gallons per minute) for the population well scenario and 4.5 x 10° 

n.3/year (about 5 cfs - cubic feet per second) for the surface stream 

scenario. Small farming communities that utilize groundwater for their 

needs usually have wells that range from 100 gpm to 1000 gpm depending on 

the population. A stream flow rate of about 5 cfs is selected since a 

stream with flow rate below this value is very unlikely to be used for 

human consumption. For example, Rock Lick Creek nearby the Maxey Flats 

disposal facility has an annual average flow rate of about 7 cfs, but it 

is not used for human consumption; it is used only for livestock. 

For' the intruder well scenario, Q is given by the assumed total volume of 

precipitation infjltrating through the disposal facility area under 

natural site conditions. In other words, the source term, J0, calculated 

in the previous section is diluted by a minimum volume of water (infiltrating through the disoosal area and rechargino the groundwater) equal to 

the product, of the site percolation (VI) with the total surface area of 

the disposal facility. The total surface area consists of the surface 

area of the disposal cells plus any space between the disposal cells. It 

is calculated by dividing the effective surface area (defined in Section 

6.3.1) by the surface utilization efficiency (SEFF), where SEFF is defined 

as the ratio of the surface area of the disposal cells to the surface area 

of the disposal cells plus any space between the disposal cells. SEFF is 

provided by the code user for on-site disposal, and is assumed to be unity 

for municipal waste disposal and 0.9 for hazardous waste disposal. 

There is a lower bound, however, for the value of the dilution volume for 

the intruder well. Otherwise the above technique would give invalid 

results for disposal facilities located in regional environments in which 

the natural percolation is low, e.g., a semi-arid environment. The lower 

bound dilution rate in this report is taken to be 7700 m3/year (3.84 qpm), 

which represents the ,minimum, practical needs of a single person living in 

a rural area (Ref. 3). <•/*. 

< — N: I !)•£ y,,,. .,•>•.:,'>! 

Migration Reduction Factor - rt-j-j 

This factor depends on the time, after initiation of the scenario, that 

the exposure is assumed to occur, the duration of groundwater travel 

between the .itn longitudinal sector making up the disposal facility and 

the biota access location, the retardation capability of the soils 

(radionuclide dependent), the duration of the assumed source term, and the 

waste stream characteristics. The longitudinal extent of the disposal 

facility is considered by dividing the facility into 10 sectors and 

summing the contributions from each sector (assumed to be equal in size) 

6-28 
P.15 

to obtain the concentrations at the biota access location. In thi s work, 

the following formula is used for the migration reduction factor r-fjj : 

r t 1 j = [exp(-Xt)/(JxT*j)] x [ Fj(t) - Fj(t-Ti) ] (6-26) 

where X is the decay constant of the radionuclide; t is the time at which 

the migration reduction factor is applicable; J is the total number of 

longitudinal sectors the disposal site has been divided into, which is 10 

in this work (see Figure 6.3); Tj is the source duration time for the it n 

de minimis waste stream; and j denotes the sector of the disposal site. 

The function Fj(t) is given by the following formula (see Refs. 1,2): 

Fj(t) = 0.5 x U(t) x [erfc(X_) + exp(Pj) erfc(X+)] (6-27) 

x+= ^_J_±t/lRt^l _ (6.28) 

2 Vt/(Rtwj) 

where U(t) i s the uni t impulse functio n tha t i s zero fo r a negative 

argument and i s equal to unit y otherwise ; t W j is the water travel time 

between the disposal sector being considered and the biot a access loca -

tion ; Pj i s the Peclet number for the distance between the disposal sector 

and the biota access location; R i s the retardatio n coefficien t of the 

radionuclide ; and erfc(x ) i s the complement of the error function and is 

given by the formula (Ref. 12): 

x 

erfc(x) = 1 - / (2/ir) exp(-t 2) dt (6-29) 

/ 

The retardation coefficients (R) that are utilized in the above equations 

depend on the radionuclide as well as the geochemistry of the soils and 

the transporting groundwater. They are indicative of the reversible ion 

exchange capability of the soils and represent the ratio of the radionuclide velocities in the soil to the groundwater velocities. The cation 

exchange capacity of the soils is a parameter which can be used to 

estimate the retardation coefficients of the soils, since retardation 

coefficients are usually linearly dependent on the cation exchange 

capacity. Five optional sets of retardation coefficients, presented in 

Table 6-7, may be selected by the user of the code. 

The clay and mineral content of the soils, in addition to the groundwater 

chemistry, significantly affects the retardation capability of the soils. 

The retardation coefficients given in Table 6-7 span the general range of 

values that are encountered in groundwater migration calculations. The 

first set is representative of coefficients for sandy soils with low to 

6-29 
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moderate cation exchange capacities, and is assumed to represent the lower 

bound of retardation coefficients used in this analysis. The fourth set is 

representative of coefficients for clayey soils with moderate to high 

cation exchange capacities, and is assumed to represent the best conditions that can be routinely achieved. In between these two sets, two 

other sets have been postulated and have been calculated utilizing the 

geometric mid-points of sets 1 and 4. A fifth set of coefficients has 

been also calculated for use in special cases. 

Any one of the five sets of retardation coefficients may be selected by 

the user of the code for a given site. As a reference, however, the third 

set is assumed in this report for the northeast site, the second set for 

the southeast site, and the first set for the southwest site. The choice 

of these three sets reflects consideration of the probable presence of 

chelating agents or organic chemicals in the disposed waste. 

The source duration time Tj for the it n de minimis waste stream is 

determined by dividing the total activity in the stream with the annual 

release fraction which is given by the factor fw*{ multiplied by the radionuclide concentration. This calculation conservatively neglects the 

depletion of the radionuclide inventory at the disposal facility by 

previous releases, and results from the manner in which the radionuclide 

source term is modeled. That is, each year a fixed "slug" of activity is 

released until the radionuclide inventory is depleted after T-j years. No 

"exponential" source term, in which the fraction of the initial site 

inventory released each year is exponentially reduced, is considered. 

The groundwater travel times twj depend on the distance between the 

disposal facility sector being considered and the biota access location. 

The travel time between waste disposed in the first sector and a biota 

access location is denoted by tw i, and is given as the sum of the time 

reguired to traverse the unsaturated zone with the time required to move 

horizontally through the groundwater to the biota access location. It is 

assumed for both the northeast and the southeast sites, for example, that 

groundwater takes 10 years vertically to traverse the unsaturated zone. 

the assumed values of t ^ for the hazardous and municipal waste disposal 

facilities are presented in Table 6-8 for each reference site. The 

qroundwater travel time between the center lines of two adjacent sectors 

is also shown in Table 6-8. To determine the groundwater travel times to 

biota access locations from the other sectors, an appropriate multiple of 

the travel time between sectors is added to t^. 

The groundwater times for the sanitary landfill and hazardous waste 

facility are believed to be reflective of good site selection procedures, 

and correspond to groundwater speeds of 0.2 m/yr for the northeast site, 

1.25 m/yr for the southeast site, and 10 m/yr for the southwest site. The 

open dump, however, is assumed to be sited in a less careful manner. The 

vertical travel time between the waste and the groundwater table is 

assumed to be zero for both the northeast and the southeast sites, and the 

horizontal groundwater speeds for these two sites are assumed to be 1 m/yr 

and 5 m/yr, respectively. For the southwest site open dump, however, the 

6-31 
P.18 

TABLE 6-8 . Reference Travel Times and Peclet Numbers 

Biota Access Location Northeast 

Travel Time (years) 

Between Sectors - SLF* 

- OD 

- HWF 

to Intruder Well - SLF 

- on 

- HWF 

to Population Well - SLFftHWF 

- OD 

to Surface Stream - SLF&HWF 

- OD 

170 

34 

455 

95 

17 

237.5 

2500 

500 

5000 

1000 

Southeast 

27.2 

6.8 

72.8 

23.6 

3.4 

46.4 

400 

100 

800 

200 

Southwest 

3.4 

3.4 

9.1 

280 

280 

280 

580 

580 

NA** 

NA 

Peclet Number 

Between Sectors - SLF^OD 

- HWF 

to Intruder Well - SLF&OD 

- HWF 

to Population Well - all 

to Surface Stream - all 

340 

910 

170 

455 

10000 

20000 

680 

1820 

340 

910 

10000 

20000 

340 

910 

170 

455 

30000 

NA 

* SLF 

OD' 

HWF 

** NA 

Sanitary landfill 

Open dump 

Hazardous Waste Facility 

Not Applicable 

6-32 
P.19 

aroundwater speeds and travel times are assumed to he the same as those 

for the sanitary landfill. This is due to the thickness of the unsaturated zone, uniformity of topography and aroundwater regimes, and the 

semi-arid nature of the southwest site with the ensuing low stream 

densities and high evaporation durina periods of hiah precipitation. 

For the on-site disposal facility, the ground water travel times, twi, 

from the first sector to the intruder well, population well, and surface 

stream, and the travel times between two adjacent disposal site sectors 

must be supplied by the user of the code. 

The Peclet number, P-j, is the distance from the disposal sector to the 

biota access location 'divided by the longitudinal dispersivity of the 

medium. Longitudinal dispersivities for the northeast, southeast, and 

southwest sites are assumed to be 10 cm, 5 cm, and 10 cm, respectively. 

Peclet numbers for the distances between the first sector and the biota 

access locations, and between two adjacent sectors, are determined in a 

manner similar to the travel times. The Peclet numbers for each reference 

site and disposal facility are given in Table 6-8, along with the Peclet 

numbers corresponding to the distances separating the centerline of each 

sector. The user of the code must supply the four appropriate Peclet 

numbers for the on-site disposal facility. 

The discussion presented above for the variation of travel times is 

applicable to the selected Peclet numbers as well. In this manner, the 

unsaturated and saturated zones are considered as a single unit. The 

primary justification for this approach is the generic nature of the 

analysis. Moreover, as long as the groundwater travel time in the unsaturated zone is added to the saturated zone travel time, and the Peclet 

numbers for the two zones are added, the above is a reasonable approximation to the alternative of considering saturated and unsaturated zones as 

two units with the ensuing complications. Such.a treatment can be found in 

a previous work by the authors (Ref. 1). 

It may also be pointed out that using aroundwater travel times (and the 

Peclet numbers discussed above) as the primary variables on which the 

migration analysis is based implicitly allows for a sensitivity analysis. 

Sites with differing environmental parameters may lead to similar radionuclide concentrations at the access locations. For example, similar 

results would be obtained if the groundwater velocity is twice as high and 

the distance to the access location is twice as large. Similarly, a larger 

unsaturated zone travel time (water speeds of the order of 10"^ ft/year 

are common) would compensate for a shorter saturated zone travel time. 

6.3.3 Disposal Cell Leachate Treatment and Overflow 

One of the difficulties faced when modeling groundwater impacts associated 

with de minimis waste disposa.1 into municipal landfills or hazardous t 

waste facilities is that the desions of these disposal facilities frequently include installation of disposal cell liners. These liners are 

frequently described as being "impermeable" and are intended to prevent 

6-33 
P.20 

migration of toxic materials out of the waste and into the aquifer beneath 

the disposal facility. The existance of the liners, or alternatively the 

possibility of disposal of waste into highly impermeable soils, complicate 

groundwater migration calculations, since percolating water can potentially accumulate in rather than flow through the disposal cells. 

This "bathtub" effect has been observed at low-level radioactive waste 

disposal facilities in the past, and if the water is allowed to continue 

to accumulate so that the disposal cell fills up with leachate, the 

disposal cell could overflow, in which case potential public impacts could 

occur. Alternatively, the disposal site operators could pump the accumulating leachate from the disposal cells, send the leachate through a 

treatment process, and then release the treated leachate to the environment. The leachate treatment process would also involve potential public 

impacts, since no treatment process will be fully effective in removing 

the toxic constituents from the leachate. 

An order of magnitude estimate of potential impacts from leachate treatment and overflow is thus provided. This estimate is adopted from a 

calculational procedure outlined in Reference 12. Briefly, three 

scenarios are considered: 

1. During operation of the disposal facility, accumulating leachate is 

continuously removed by site operators, processed in some manner, and 

released into a nearby stream. The contaminated stream water is 

consumed and used by an individual, and impacts to the individual are 

calculated in units of mrem/yr. 

2. Accumulating leachate is allowed to completely fill up the disposal 

cells to the point that the leachate overflows the disposal cells. 

The overflowing leachate is carried to a nearby stream, and impacts 

from consumption and use of the contaminated stream water are again 

calculated in units of mrem/yr. 

3. Rather than overflow, the accumulated leachate in the second scenario 

above is assumed to be processed through an evaporator. Radionuclides 

released through the evaporator stack impact the surrounding population (in person-mrem/yr). 

The first scenario would correspond to a situation in which a disposal 

facility is designed for leachate collection and removal. This would be 

the case at a hazardous waste disposal facility (see Section C.4.2 of 

Appendix C), where in accordance with EPA regulations liners are required, 

and no more than 1 ft head of leachate is allowed to buildup on the liner. 

Example treatment processes could include filtration and ion exchange 

systems. The second scenario corresponds to a situation in which insufficient attention is given to accumulating leachate, which could 

potentially occur at some time after the disposal facility is closed. The 

third scenario corresponds to a situation in which remedial actions are 

taken at a closed disposal facility. 

Impacts from the first two scenarios {H\ and H2) and the third scenario 

(P) are thus given as: 

6-34 
P.21 

Hi = I I ^o (Ain/Q) fin

 pDCF-7 (6-30a)" 

i n 

H 2 = I I f0'(Ain/Q) ^2n PDCF-7 , (6-30b) 

i n 

p = Z I fo'Ain frn P0P PDCF "3 (6-30c) 

i n 

with: 

Ai n = Ci n fi VL M0 tc IO*1 " TM) (6-31) 

where: 

f0 = decay correction factor for the first scenario (see below); 

f0' = decay correction factor for the other scenarios (see below); 

A-jn = activity (Ci) contained within leachate volume 

(VL) annually generated or processed; 

Q = flow rate of surface stream (assumed to be 

4.5x10° m3/year - see Section 6.3.2); 

fin = radionuclide release fraction from operational 

leachate treatment process; 

f£n = radionuclide retention in soil during overland flow; 

frn = radionuclide fraction released into the air 

. due to evaporator operation; 

POP = population-weighted atmospheric dispersion factor 

(See Section 4.1.5): 

C-jn = concentration (Ci/m3) of nt n radionuclide 

in i*h de minimis waste stream; 

f-j = fraction of the total site waste volume that is 

composed of the it n de minimis waste stream; 

VL = volume (m3) of leachate annually released or processed; 

M0 = radionuclide-specific leach fraction (see Table 6-4); 

tc = fraction of a year that the leachate is in contact 

with the waste; and 

IA = accessibility index (see Section 2.2.2). 

Parameters requiring additional explanation include f0, fin, f2n» frn» VL, 

and tc. The decay correction or the .time delay factors, f0 and f0', 

account for radionuclide decay. The first scenario is conservatively 

assumed to occur during the last year of waste disposal. Consequently, 

this factor is given by the following: 

f0 = [l+exp(-xn)+exp(-2xn)+---+exp(-(ILFE-l)xn)]/ILFE (6-32a) 

where xn is the radionuclide decay constant (yr-*) and ILFE is the 

facility life in years. Each term above accounts for the decay of the de 

minimis waste disposed in a specific year, e.g., exp(-xn) is the decay 

factor for the de minimis waste disposed during the penultimate year of 

6-35 
P.22 

VL i s difficul t to estimate , but fo r thi s report wil l be assumed to &e 

equal to the tota l volume of water percolating into the disposal cell s per 

year. This volume i s given as the percolation (p) times the effectiv e 

disposal sit e surface area (Sf), where Sf is given as equation 6-21. This 

also gives conservative results , since i t corresponds to the las t year o f 

facilit y operatio n when the disposal facilit y wil l be fille d to capacity 

with waste. The percolation (p) is a function of the design and operation! 

of the disposal facility . Default values for p for municipal and hazardous waste disposal are discussed in Section 6.3.1 . For on-site disposal,, 

p i s supplied by the code user. 

The contac t time fraction , t c , also differ s depending upon the scenarioFor the firs t scenario, leachate i s assumed to be removed continuousl y 

during operation, and so percolating water is assumed to flow through the 

waste rather than saturate the waste continuously. Thus t c for the firs t 

scenario is given as a function of percolatio n as discussed i n Section. 

6.3. 1 and liste d i n Table 6-6. For the second and thir d scenarios, the 

waste is assumed to be in continuous saturated contact with the leachate „ 

and so t c for these las t two scenarios is assumed to be unity . 

6.4 Exposed Waste Scenarios 

In these scenarios , the earthen cover over the disposed waste is assumed 

to be totall y or partiall y removed, resulting in exposure of the waste a rudi 

dispersal by wind or water. Two possible mechanisms are considered fo r 

removal of the cover: (1) the actions of a potential inadvertant intrude r 

(see Section 6.2), and (2) erosion of the cover by wind or surface waterFor each removal mechanism, two radiologica l exposure scenarios ar e 

considered. One involves suspension and airborne dispersal of contaminated particulates to the area surrounding the disposal site , leading t® 

exposures to the surrounding populatio n resultin g from i nhal atioin.,, 

immersion and other pathways. The second scenario involve s transport of 

radionuclides via surface water runoff to an off-sit e surface water body* 

leading to exposures to individual s using the contaminated water bodj -

This results in four exposed waste scenarios depending upon the particula r 

cover removal mechanism and radionuclid e transport agent considered : 

intruder-air, intruder-water, erosion-air, and erosion-water. 

The following equations are utilize d to calculate human exposures result -

ing from these scenarios. For the water transport and access scenarios: 

H • I I (Vdfwifs)wa t Ci n PDCF-7 (6-33)) 

i n 

and, for the air transport and access scenarios: 

P= [ [ (Vdfwifs)ai r Ci n

 pDCF-8 (6-34)) 

i n 

6-37