Some solute transport attenuation properties of the Bunter sandstone, England

The Science of the Total Environment, 66 (1987) 245-261 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

245

SOME SOLUTE TRANSPORT ATTENUATION PROPERTIES OF THE BUNTER SANDSTONE, ENGLAND

T.M. MIMIDES* Department of Geological Sciences, University of Birmingham, Birmingham (United Kingdom) (Received November 24 th, 1986; accepted March 2 nd, 1987)

ABSTRACT

Laboratory parameters of secondary mechanisms of pollution such as porosity, density, primary permeability, formation factor, and diffusion exhibited good correlating straight line relationships for triassic Sherwood sandstone. These relationships supported the hydrogeopollution studies of the porous medium and revealed its good uniformity and homogeneity suitable for mathematical formulation. For the study of the mechanisms of attenuation, such as rates and heats of adsorption, static equilibrium techniques were employed with satisfactory results. Concerning toxic metal pollutants, exothermic reactions were involved with Freundlich-type isotherms. With most heats of adsorption found to be < 8 kcal mol-' (physical adsorption), and due to the high values of distribution coefficients, the formation proved to be an excellent retardation unit for the disposal of toxic metal pollutants. INTRODUCTION P o l l u t a n t s move t h r o u g h a s u b s u r f a c e unfissured p o r o u s flow system by a complex i n t e r a c t i o n of four processes: c o n v e c t i o n , dispersion, diffusion and h y d r o g e o c h e m i c a l a t t e n u a t i o n . F o r p r i m a r y m e c h a n i s m s , s u c h as c o n v e c t i o n and dispersion, the l a r g e pores are m a i n l y i n v o l v e d and c o n t a m i n a n t effluents t r a v e l with the same r a t e of flow and in the same d i r e c t i o n as the c a r r i e r fluid. Dispersion t a k e s place u n d e r the influence of the r a n d o m i n t e r a c t i o n of fluid flow p a t h s (tortuosity) and m e d i u m grains; it exerts an i m p o r t a n t diluting influence in t r a n s p o r t p h e n o m e n a of liquid toxic wastes t h r o u g h p o r o u s media, with some p o l l u t a n t s a r r i v i n g a h e a d of the c o n v e c t i v e f r o n t and some l a g g i n g behind. M a n y a u t h o r s h a v e i n c l u d e d m o l e c u l a r diffusion in t h e i r dispersion expressions, but all c o n c l u d e t h a t its d i l u t i n g influences are u n i m p o r t a n t in c o m p a r i s o n with dispersion. Diffusion, therefore, comprises a s e c o n d a r y mecha n i s m b u t in c o m b i n a t i o n with a t t e n u a t i o n m e c h a n i s m s becomes extremely i m p o r t a n t in t h a t p o r t i o n of the m e d i u m w h e r e small d i a m e t e r pores predomin a t e t o g e t h e r with the clay portion, l e a d i n g to some d i v e r g e n c e from the idealised t r a n s p o r t process. * Present address: Department of Ore Deposits and Applied Geology, National Technical University of Athens, 42 Patision Str., GR.10682, Greece.

0048-9697/87/$03.50

© 1987 Elsevier Science Publishers B.V.

246

Lower~L. ias ,'.

"'

0

./

ono Coat •

----=

M ea,.s.u r e s Boreholes

. . . .

River

0 I

~ Scale

5kin I

Fig. 1. The geological map around Nottingham with locations of the sampling boreholes.

This paper attempts to formulate some inter-relationships between physical properties of a porous medium and pollution processes. The intention was to throw some light onto the assimilation capacities and magnitudes of triassic Sherwood sandstone as an acceptor of liquid metal wastes under laboratory conditions. LABORATORY TECHNIQUES

Techniques employed in measuring parameters such as porosity, density, primary permeability, resistivity and diffusion, making use of sandstone plugs of certain diameter and length, are fully described and analysed by Barker and Worthington (1973), Worthington and Barker (1972) and Mimides (1981). The plugs were cored from sandstone collected from six boreholes around Nottingham (Fig. 1) and from eight boreholes of the triassic basin of north Shropshire (Fig. 2). For the geology, hydrogeology, mineralogy and chemical characteristics of the two areas and samples refer to Mimides (1981). A simple four-electrode cell (Rust, 1952) saturated with 5500 and 26500 ppm NaC1 was used for the measurement of the sandstone plugs' resistivity, Q~. The procedure is similar to that adopted by Worthington and Barker (1973). Effec-

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ii:

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ppm

15ppm

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TIME (hOL~tSj

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25ppm

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15ppm 5ppm

o~ _ . . . .

~-;,,;~, o~,

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Sppm 19 - -

8

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218

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TIMEthours]

Fig. 4. Rates of adsorption of iron and copper at two temperatures.

Equilibrium techniques Five toxic metals (copper, nickel, cadmium, cobalt and iron) were selected in order to develop adsorption isotherms. Different concentrations of solutions of the above metals were brought into contact with different amounts of crushed samples at two different bath temperatures, 20 and 40°C. In order to ensure complete mixing the flasks were shaken periodically over 2 days. At various time intervals an aliquot of supernatant solution was withdrawn and analysed using atomic absorption analytical equipment. Equilibrium was reached between 2 and 50 h for all toxic metals used. Figures 3 and 4 present the equilibrium adsorption curves obtained by plotting the amount adsorbed as a function of time for the two temperatures. All the curves are similar in shape, exhibiting a rapid initial rate of adsorption which declines to zero as equilibrium is approached. By assuming that the rate of adsorption is proportional to the proximity of the system to equilibrium, a rate constant, i, for the adsorption may be calculated from the expression: dX d--}- --- ~ ( C -

X)

O)

where C is the concentration of the solution in equilibrium with the rock at

249 TABLE 1 Rate constants for each toxic metal tested at specific initial concentration and temperature Metal

Initial conc. (ppm)

~l (1 h 1)

T (K)

£ (1 h -1)

T (K)

Cobalt

5.00 15.00 25.00

0.117 0.132 0.156

293 293 293

0.094 0.106 0.126

313 313 313

Nickel

5.00 15.00 25.00

0.150 0.154 0.166

293 293 293

0.086 0.145 0.140

313 313 313

Iron

5.00 15.00 25.00

0.071 0.063 0.047

293 293 293

0.064 0.051 0.038

313 313 313

Copper

5.00 15.00 25.00

0.089 0.044 0.036

293 293 293

0.040 0.024 0.026

313 313 313

different time i n t e r v a l s t (ppm), and X is the a d s o r p t i o n (ppm). T h e s o l u t i o n of the above differential e q u a t i o n is X = C (1 - e~t), w h i c h r e d u c e s to: = ~ In - - C t C-X

(2)

The c a l c u l a t e d r a t e c o n s t a n t s for e a c h sample were found to be valid for

B1S 8 1 . 5 5 - 81.70 •

-~-=0.45

C1 ° °

r,098

B/,S A s t l e y D.C 19.50-19.70 * - -x = 0 . 6 3 C T M m r • 097

B35 2 5 . 1 0 - 2 5 . 3 0 o -ZL- = 0 . 2 8 C°~2 m

r.0g6

Wo|fordS

82.10-82.60

O - ~ - = 0.13 C1°3 r = 0 93

lOO

01

EOUSLIBRIUM

LIII CONCENTRATION.

Fig. 5. Freundlich isotherms for cobalt solution (20°C).

C (ppm)

250

AION •

40.10-4020

__x = 0 . 9 4 C °g7 m r= 1 O0

51.10-51.25 x 0.13 C ° 7 8 -~-=

/f

SIS

r=0g9

A11N O

m

72.50-72.65

= 0.06

C~7

r=099 w(

i.

A18N 27.65-27.90 x =0.06 0%--

.=

C1s°

,"=0 9g

i p I"

.J .J

100 0.1

EQUILI

BR#UM

I I I I I Illl, CONCENTRATIO N. C (ppm)

Fig. 6. Freundlich isotherms for cadmium solution (20°C). 75-80% of the total adsorption rate. At this point, the rate constant decreases, indicating that the relationship can no longer be expressed by a single mechanism. Desorption or interference from adjacent adsorbed molecules causes a decrease in the rate as the system approaches equilibrium. The rate constants for the sample tested are listed in Table 1. Freundlich-type isotherms (Figs 5--7) expressed by the equation X / M = Kc" (k and n constants) fit the experimental data. MATHEMATICAL INTER-RELATIONSHIPS

Density-porosity relationships For an homogeneous consolidated porous medium, density and porosity are related according to the following equation: dw = de + dm ( 1 - ¢)

(3)

where dw is the wet density of the medium, d the density of the saturating fluid (1.0 g cm-3), d~ the matrix density of the medium, and ¢ the porosity. When the matrix material is calcite, d~ = 2.17 g cm 3, while for the Bunter sandstone the presence of quartz (Mimides, 1981) as the predominant mineral modified dm to 2.65 g cm -3. Nafe and Drake (1975) have shown that for soils, shale, limestone and sandstone Eqn (3) takes the form: dw = 2.68 - 1.68 ¢

(4)

Barker and Worthington (1973) working on sandstone samples from northwest England obtained dw = 2.60 - 1.60 ¢.

251

Copper s o l u t i o n B1S 5 1 . 1 0 - 5 1 . 2 5 • ~ = 1.99 C°'ss m r=o.gm

,'

u

o

/

o

.I0

z

Iron solution WQifordS 82.10-B2.60

/"

1.&l

X

E

o - - = 0.08 C m r= 0.g3

o w

Nickel s o l u t i o n

o

B1S 51.10- 51.25 . x_K. = O. 31 C°'e~ m

o

r=Og?

i

I

I I i I

0.1

F i g . 7. F r e u n d l i c h

EOUILIBRIUM

I I I I CONCENTRATION

l I I

, C

(ppm}

i s o t h e r m s f o r t h r e e m e t a l s (20°@).

Figure 8 shows the variation of porosity with wet density for Bunter sandstone, S symbolizing samples from the Shropshire area and N those from the Nottinghamshire area. A least square fit to the 49 points yields the expression: d , = 2.60 - 1.70 q~

(5)

A mean value of 2.57g cm 3 was found for dm. Equation (5) satisfies the boundary conditions at ~b = 0 (actually for porosities between 0.05 and 0.29). The other boundary condition at ~b = I is nearly satisfied and therefore the two lines are parallel. This small discrepancy is most probably due to unconnected porosity.

Porosity-true formation factor Archie (1942) and Wyllie and Gregory (1953) studied and carried out experiments on clean sandstones and aggregates, obtaining the relationship: A F = ~b--~

(6)

where F is the true formation factor and A and h are constants (A = 1 for Wyllie and Gregory). Values of A and h determined for various sandstones generally vary from 0.6 to 2.5 and from 1.3 to 3.0 respectively (Winsauer et al., 1952; Keller, 1966; Carothers, 1968). Equation (6) assumes t h a t the rock contains non-conducting minerals and electric current is transmitted only by the interstitial electrolyte of uniform concentration. For rocks with a conducting matrix and resistivity Qs, an apparent formation factor, F,, is measured at any given electrolyte resistivity, Q,. Patnode and Wyllie (1950) proved that:

252 Bes~ f i t : d = 2 5 0 - 1 . 7 0 ~ Points plotted: 49

2.7

.S *N

"',

L2.6

÷

"" / 2.5

~

- 2.4

~

'~°"'°'°"' "'.

•

°

""~.

*

*

"

~ .2.2

-2.1

•

..

-

:

"

"

*

*

-2.0

-1.9

0.1

1.8

0,2 I

J

0i.3

J

POROSITY

0.0

Fig. 8. Variation of porosity, ¢, with wet density, dw.

F = lim Fa -- lim e, ewe0

Qw~0

(7)

~w

Barker and W o r t h i n g t o n (1973) s h o w e d that o n l y the plotting of the true formation factor and porosity on bi-logarithm paper gives a straight line. Figures 9 and 10 present the variation of ¢ and a formation factor with electrolyte c o n c e n t r a t i o n s 26500ppm (Qw = 0.220ohm-m) and 5500ppm Plotted -30

c e

9 8

°

•

S

•

N

points

z, 2

÷ "~

v 6 E ~5

-3

Z_ 2 e2

1 I, 1 01

I

I

I

~ i i L J IJ I l l l l P ~ Porosity

j03

1

Fig. 9. Variation of porosity, ~b, with true formation factor, F.

253 Ptotted • S • N

30

point

42

-20

~9°

• • .. ,'~

•

~.

5

v

E

°

S

3 2 a

02 I

1

I

f

I

I

]

~ I L I~J

0.3

i=lLIztf

Porosity

0.1

Fig. 10. Variation of porosity, ~b, with apparent formation factor, F~.

(~w = 0.962 ohm-m) NaC1 respectively• Because the value of 0.220 is very close to Patnode and Wyllie's value of 0.1 ohm-m, with a very small error we can assume that the straight line of Fig. 8 is the plotting of porosity and true formation factor with a correlation coefficient 0.85, and Eqn (6) can take the form F = 2•3/(b11•

Porosity, permeability-effective diffusion coefficient Figures 11 and 12 show the correlation between the effective diffusion coefficient and porosity. The semilogarithm plots give straight lines with cor-03

Diffusing Plotted

system:

Air-H

2

points :l.0

.'." ~ - > ~

Best" " fit : ~, =o.as.o.~alog, p.

•

.* *

•

-0.2

o a_

~0.1

0]02

0.03 EFFE[CTIVE

.

L

0.05 I

71

[

0.08 I

I

0.1 (

DIFFUSION(cmlSJe¢)

0.01

Fig. 11. Variation of porosity, ~b, with effective diffusion coefficient, D~ (air H2)

254 Diffusing

0.3

Plotted • S *

system: Air-CO 2

points:

40

N

Best fit:+

=0 1.1*0.1310gDe ,, •

°

~

, •

(,÷

÷

/

/

÷° •

~-0.2

°

o

0-

0.1

o.o2 I

o.

(~o3 I

0.01

I

EFFECTIVE

DI

o,os I FFUS

I0

]

noa (11 1 I )

I,

NlcmZ/$e¢)

Fig. 12. V a r i a t i o n o f p o r o s i t y , ¢ , w i t h e f f e c t i v e d i f f u s i o n c o e f f i c i e n t , D e ( a i r - C O 2 )

relation coefficients of 0.74 and 0.71 respectively. The o n l y difference between the two graphs is that in the first the effective diffusion was measured using a i r - H 2 (D O = 0.691cm 2 s - l ) , while in the second air-CO2 (D O = 0.159cm 2 s -1) was used. The mathematical e q u a t i o n s are of the form:

% u

z

o"

o

" •

,

.• °

o w

>

Di ffusing system:Air-H z Plotted points: 38

~,

us u.

-0.01

Best fit: D,=o.OglK~

uJ

0.01 l 0'00(~.00~

~0A

r=0.65 • S ,N

I

I

I I I I tl

0.1 I l I I I I I I] PE~MEABILITY-POROSI

1 I I I l J ii ] T Y PRODUCT. K,~

1

i

i

i

10 i i i ll]

(mdorciesi

Fig. 13. R e l a t i o n s h i p b e t w e e n t h e e f f e c t i v e d i f f u s i o n c o e f f i c i e n t a n d t h e w e t p e r m e a b i l i t y - p o r o s i t y product.

255

4 4 i

0.1 I •

÷

=,

֥

÷

w > , ,..,

Diffusing

-0.01

system

Air-CO

w

Plotted

2 points:36

Best

fit:

D~=0.O6IK~,]

°'is

r=0.60

0.001

I

L

I

I il

1001 ~1

=

J

[

I 1111(

PE R HE A Sl

OO01

0.1

L

I

L I T Y-POROSI

1 J I i I I []

TY

PRODU

I

•

S

*

N

J

/, l

C T . Kx~ (mdorcil~l

Fig, 14. Relationship between the effective diffusion coefficient and the wet permeability-porosity product.

~b = H + G l o g D e

(8)

The slope G = 0.13 is the same for both lines (parallel) and the intercepts H exhibit very similar values (D e is independent of the gas system used). The scattering of the data from the correlation curves is due to the incompleteness of the parameter ~b as a full descriptor of the interstices with regard to diffusion. Figures 13 and 14 present a plot of the effective diffusion coefficient for dry plugs versus the (wet) primary permeability-porosity product (K¢). The best fitted lines are again parallel with very similar intercept values. The general equation is of the form: Diffusing system Air- CO~

L02

Plotted • *

poin|s 36

S N

d oo5

-001 0007

I 01

J

I

i

075

I

I

0 2 I I POROSITY

03

~ ~ I I ~ I I I ]

Fig. 15. Diffusion factor versus fractional porosity (air CO2).

256 0iffuslng

system:Air-N

Plotled ~5

points

z

38

• S + N

•

02 J

001

l

°

f

/

/

/

.

I

I

I

03

I PO,~O$1TY

01

Fig. 16. Diffusion factor versus fractional porosity (air-H2). De = d (K¢) ~

(9)

where J and fl are constants. The broad scattering of the data is believed to be due, in part, to the uneven changes in pore structure from the dry state for diffusion measurements to the wet state for permeability measurements. Klinkenberg (1951) and Wyllie and co-workers (1952, 1955) discuss a method for obtaining a tortuosity factor (Mimides, 1981) from electrical conductivity measurements by assuming that the electrical conductivity and molecular diffusion of a porous medium can be equated: Ue U0

De Do

1 F

~¢b

(10)

where U0 = 1/Q~ is the electrolyte conductivity (mho-m-*), U~ is the core plug conductivity, 7 a constant varying from 0.14 to 1.0, and b a constant varying from 1.0 to 2.0. According to Figs 15 and 16 we obtain respectively:

De Do

-

0.24 ¢,.~0

De

D--~ = 0.60

(~1.20

(11) (12)

Most frequently 7 equals 1/q (Van Brakel and Heertjes, 1974); q is called the matrix factor (T1/2). A general form of Eqns (11) and (12) is: De Do

_

(~1.20 T1/~

(13)

257 where ~ is the tortuosity factor (). However, Eqn (6) in this case can take its final form:

Go u0

1

F

- 0.43 ~b11° = ~

~b11°

(14)

Equations (13) and (14) prove the validity of Eqn (10). The value of r has been found to be 5.67 for conductivity measurements and 5.41 for resistivity measurements. ADSORPTION Charles (1977) gives a full interpretation of the phenomenon of adsorption breaking it down into several physical processes. The most important criterion for differentiating between physical and chemical adsorption is enthalpy. The energy evolved when physical adsorption occurs is ~ 2-6 kcal mol 1 and the process is exothermic. The enthalpy change accompanying chemisorption is significantly greater than t h a t of physical adsorption and often lies between 10 and 50kcal mol 1.

Heat of adsorption Consideration of the intercept k and slope n of the Freundlich isotherms (Figs 5-7) provides some quantitative information about the nature of adsorption. Haque and Coshow (1971) and Crisp (1953) discuss the relationship of k to the relative free energy changes t h a t occur during the adsorption process. The surface site equilibrium is expressed by the equation: C + S m Cs

(15)

If k~ is the equilibrium of reaction (15), then:

(c~)

(S) = ke (C)

(16)

where S is the surface of the catalyst, (C) and (Ca) the activity of a component in solution and of the same component adsorbed on a surface respectively (for very dilute solutions activities approach concentrations). Haque and Coshow point out t h a t Eqn (16) is a special case of the Freundlich isotherm where n is equal to unity; in such cases ke is equal to k. Values Of n greater than unity are indicative of sandstones t h a t are rich in clays indicating concave (S-type) isotherms. Such isotherms arise from strong adsorption of solvent, strong 'intermolecular attraction within the adsorbed layers, penetration of the Solute in the adsorbent and the monofactional nature of the adsorbate. Values of n less t h a n one probably represent convex (L-type) isotherms. Values of n ranging between 0.8 and 1.0 indicate intermediate behaviour. Hence for n = 1, the intercept logk (Figs 5-7) provides an indirect measure of the relative surface free energy changes according to AG ( = - R T l n ke). In all cases, as the

258

TABLE 2 k and n Freundlich isotherm values for Bunter sandstone Metal

Temperature

Freundlich constants

(°C) k

n

Cadmium

20 20

0.13 0.94

0.78 0.87

Cobalt

2O 20 20 40 20 20

O.3O 0.28 0.20 0.15 0.13 0.45

0.91 0.92 1.10 1.03 1.03 1.00

Copper

20 20 40

1.99 1.57 1.03

0.55 0.57 0.68

Iron

20 40

0.19 0.12

0.95 1.05

Nickel

20 20 40

0.13 0.50 0.20

0.83 0.57 0.75

t e m p e r a t u r e i n c r e a s e s k d e c r e a s e s ( T a b l e 2), c h a r a c t e r i s m g t h e e x o t h e r m i c n a t u r e of adsorption. T h e e x p e r i m e n t a l w o r k w a s c o n d u c t e d a t f o u r t e m p e r a t u r e s i n o r d e r to c a l c u l a t e isosteric h e a t s of a d s o r p t i o n w h i c h are i n d i c a t i v e of the n a t u r e of the s u r f a c e b o n d i n g . T h e y a r e d e f i n e d as t h e d i f f e r e n c e s b e t w e e n o n e m o l e of + BIS

/ k

- A H so: 7 5 E oWatfoed

~21~'- 625C

Ccppe, -Z~H,ss~B£O

o

l

5110-5125

Iron

-1

/ i

0.1 0.001

i~

1 ¢ p t[llll

0.01

oI f

I

1 ] Itlll

1 /T

Fig. 17. Graph of Freundlich adsorption constant, k, versus reciprocal of absolute temperature (iron, copper).

259 ~O ,Waifor~ 8ZJ0 - 82.60 Nickel -/~H = 1 5 1 0 /

/ O

l

A 2 N 13L 4 0 - 1 3 & . ? 0 Ccbatt

/

/

.? t 01

I

1001 t

I IIIJ

01301

0.1 I 1 I T

I

t

I ~rilf

Fig. 18. Graph of Freundlich adsorption constant, k, versus reciprocal of absolute temperature (nickel, cobalt).

adsorbent at any activity and one mole of adsorbate in equilibrium with the adsorbant. Thus: AHi~o = ~

- / ~ or A/-r~° = R

~lnk ~(1/T)

(17)

where ~ and H ° are the partial molal enthalpies of adsorbate on the surface and in the liquid states, R the gas law constant (1.987 cal mol 1 ), and T the temperature (K). Figures 17 and 18 show the log plots of logk versus 1/T. AHi~o can be calculated from the slopes of the curves. The heats of adsorption exhibit values < 8kcal mol 1, with only one having a high value of 15kcal moF1; therefore, the most probable mechanism of adsorption is Van der Waal-type bonding or physical adsorption.

Distribution coefficient Vermeulen and Hiester (1952) and Hajek (1969) proved that the retardation of the pollutant relative to the bulk mass of water is described by the relation: V dm Vii = 1 + ~ - g d

(18)

where V is the average linear velocity of the water, Vi the velocity of the C/Co = 0.5 point, and /Ca the distribution coefficient (mg g - ' ) (/Ca = k in Freundlich isotherms where n = 1). Kd is a valid representation of partitioning between liquid and solids only if the reactions t h a t cause the partitioning are fast and reversible (physical adsorption) and only if the isotherm is linear. The term 1 + (dm/¢) Kd in Eqn (18) is referred to as the attenuation factor and its reciprocal as the relative velocity. For Bunter sandstone Eqn (18) takes the form: V -

v, -

= (1 + 9 g d ) t o (1 + 53Kd)

(19)

260

for porosities ranging between 0.05 and 0.30 and matrix densities between 2.65 and 2.79. From Eqn (19) it is apparent that if K d = 1 the mid-concentration point of the solute would be retarded relative to the bulk water flow by a factor between 10 and 54. Distribution coefficients for reactive toxic materials range from values near zero to 103 or greater (Freeze and Cherry, 1979). The Bunter gave values between 600 and 20000 ml g - ' for inorganic toxic metal contamination. Most of the contaminant mass, therefore, will migrate only a very short distance from the input zone during a specified migration period. CONCLUSIONS

It is apparent that the problems of groundwater contamination by toxic metals are highly complex. All of the parameters involved in the two secondary mechanisms of diffusion and hydrogeochemical attenuation must be considered. Not only must these hydrogeopollution parameters be identified but also their inter-relationships must be assessed. An important contribution of this paper is that it tries to relate parameters such as porosity, density, primary permeability, formation factor and diffusion. The straight line relationships proved the uniformity and homogeneity of Bunter sandstone as a consolidated porous medium which can be easily described with the help of mathematical formulation. Although some lines do not seem to exhibit good correlation coefficients, this is due to heating of the core plugs and resaturation. Some plugs were damaged by heating which increased interconnected porosity (by reducing unconnected porosity) and this might have caused a detectable reduction in permeability and resistivity. Adsorption on the solid surfaces of a consolidated porous medium is a very complex process. Variations in energy, crystal structure and chemical composition will occur as a species moves about on the porous surface. In spite of this it is generally possible to divide all adsorption phenomena involving solid surfaces into two main classes: physical adsorption and chemical adsorption (chemisorption). Physical adsorption arises from intermolecular forces involving permanent dipole interactions; it involves Van der Waals or secondary valence forces. Chemisorption involves a chemical interaction with attendant transfer of electrons between the adsorbate. The static equilibrium techniques that have been employed to investigate mechanisms of adsorption proved very successful. The rate of adsorption in the Bunter is proportional to the solution concentration and inversely proportional to the temperature. Toxic metal pollutants follow Freundlich-type isotherms, with exothermic reactions taking place. With heats of adsorption < 8kcal mol-', we are dealing mostly with physical adsorption and with distribution coefficients lying between 600 and 20000 ml g-1. The formation is an excellent medium with respect to adsorbing capacities under laboratory conditions.

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