This is an updated version of Joachim Gruber, "Contaminant accumulation during transport through porous media", Water Resourc. Res., 26, 99 - 107, 1990.

Contaminant Accumulation During Transport Through Porous Media

by

Joachim Gruber

Address at the time this paper was finished:

Los Alamos National Laboratory, MS J-495, P.O. Box 1663, Los Alamos, NM 87545, USA

ABSTRACT

When a soil, rock or sediment is exposed to changes of its chemical environment, e.g. by infiltration of water, the new chemical composition propagates as a wave through the porous medium. The corresponding transport processes are described using concepts of equilibrium multicomponent ion chromatography with interaction (method of characteristics with hydrodynamic dispersion being neglected). The mathematical model is a system of n coupled quasilinear partial differential equations in one space dimension and time, n being the number of interacting chemical components. This model allows to quantify an interesting remobilization scenario for a deposit of trace contaminants adsorbed on soil particles: the contaminant inventory desorbing behind the front of a remobilizing wave will accumulate at the front if the velocity of the front is intermediate between the contaminant velocities upstream and downstream of the front. Oxide adsorbents (described by the Triple Layer Model) are used as examples in the computations.
The analysis suggests that the potential hazard associated with a contaminated soil or sediment cannot be derived from the contaminant adsorption isotherms alone. The additional data needed are the velocities of the possible waves.

INTRODUCTION

The processes operative when a fluid migrates while in contact with a solid with which it interacts, may be divided into two types - local and transport. Local processes can counteract transport processes such as diffusion, hydrodynamic dispersion or other concentration reducing transport processes, and can thus cause accumulation.

The spectrum of examples is as diverse as the nature of possible local processes:
These examples have the following in common: A local process changes the ratio CM/cM (abbreviated later as kd) of a species M across a boundary, where CM and cM are the concentrations of M associated with the immobile and mobile phases, respectively. M is transported across this boundary. Conservation of mass causes M to accumulate at the front.

This paper is concerned with adsorption as a local process and aqueous solute transport in a porous medium containing a deposit of adsorbed M. M accumulates at the fronts of waves of solution composition changes propagating through the deposit and triggering spatially correlated remobilization [Gruber, 1994].

The concentration maxima observed in soils by Killey et al. [1984], Serne, Peterson and Gee [1983], Peterson, Martin and Serne [1986], Dubrovsky et al. [1985] and White et al. [1984] can be interpreted as a result of such remobilizing waves.

The former group [Killey et al., 1984] investigated a desposit of Co-60 near a low-level waste infiltration pit at Chalk-River Nuclear Laboratories, Ontario, Canada. About two decades before the authors started their investigations of the contaminated porous medium (a sandy aquifer), nitric, tartaric, and oxalic acids had been discharged at the waste site. The then five years old Co-60 deposit had thus been remobilized by complexation. The authors report fronts of elevated concentrations of Co-60 at some depth beneath the surface.

Of the latter groups Serne, Peterson, and Gee [1983] and Peterson, Martin and Serne [1986] worked with laboratory soil columns and the other groups [Dubrovsky et al., 1985; White et al., 1984] reported on mill tailings. They observed the effect of a migrating low-pH front on the spatial element distribution in the tailing piles. The front acts as "chemical dam" [Guy, 1986], behind which elements pile up, among others cobalt, lead, cadmium, zinc, nickel and the uranyl ion. Similar accumulations arise when exhausted chromatographic columns are regenerated by infiltration of a remobilizing solute [Helfferich and Klein, 1970]. This demonstrates that precipitation as well as sorption can lead to element reconcentration.

If in addition to the physical laws of solute transport the equilibrium adsorption isotherms Ci = fi(c1, c2, ..., cn) for all n interacting components i are known ("chemical adsorption model"), the detailed evolution of the accumulation process at the front can be computed. Various models and solution techniques have been developed to represent transport under the influence of interference in multicomponent systems [Helfferich and Klein, 1970; Theis, Kirkner, and Jennings, 1982; Benhadid, 1984; Grove and Stollenwerk, 1984; Lichtner, 1985; Cederberg, Street and Leckie, 1985; Foerster, 1986; Ortoleva et al., 1986; Novak, Schechter and Lake, 1988; Hwang, Helfferich and Leu, 1988; Charbeneau, 1988; Kirkner and Reeves, 1988; Reeves and Kirkner, 1988; Amir and Kern, 2010; Prigiobbe, Hesse and Bryant, 2013].

Rather than presenting site specific transport simulations of accumulation scenarios, this paper will discuss which chemical factors lead to the development of accumulations.

The first part of this paper reviews one possible mechanism of accumulation of a trace M ("contaminant"), known in chromatography [Helfferich and Klein, 1970]. Here the terminology will be defined, and the description will use basic concepts and the language of multicomponent ion chromatography with interference [Hwang, Helfferich, and Leu, 1988]. The accumulation conditions will be stated in terms of distribution coefficients kd of M and a suitable secant of the isotherm of the remobilizing chemical f. These system properties can either be taken from empirical compilations or can be computed based on a suitable adsorption model.

In the central part of this paper ("Accumulation Potential") it is shown that once these simple system properties are known, one can decide whether or not a constant kd model (or a model assuming a stochastic kd distribution) may be used to compute contaminant transport, thus ignoring the possibility of accumulation. Contaminant kd's alone do not provide sufficient information to make that decision. Cation adsorption on oxide mineral surfaces will be used to illustrate this point.

MECHANISM OF ACCUMULATION

One-dimensional transport is determined by conservation of mass of the chemical components i. In the absence of sources or sinks the change of the total amount of i in any volume is equal to the net influx of i:

(1)

eq.1

where Cti = Ci + ci is the total concentration of i, v0 is the velocity of the liquid and D is the coefficient of hydrodynamic dispersion. Influx of non soluble phases of i is not considered in (1). In this paper it will be assumed that the dispersive influx term is small compared to the advective one represented by the first term on the right hand side of (1). Disregarding the dispersive term, (1) reduces to

(2), a hyperbolic set of conservation laws

eq.2

In the case of adsorption with an isotherm Ci = fi(c1, c2, ..., cn)

time derivative of isotherm

where the summation Σj is done over all chemical components j. This can be written in matrix form as

time derivative of isotherm, vector form

Here, C and c are vectors with components Ci and ci, respectively. Q(c) is the Jacobi matrix of the multicomponent isotherm C(c), with elements

Jacobi Matrix of Isotherm

In this notation the hyperbolic set of conservation laws (2) becomes

(3)

hyperbolic set of conservation laws

with I being the unit matrix. (In multicomponent chromatography and contaminant transport I + Q = R is called "retardation matrix".)

The set of quasilinear conservation laws (3) is hyperbolic, meaning that the inverse of the matrix (I + Q(c))/v0 has n real, distinct eigenvalues λj(c), (j = 1, n). Customarily, they are arranged in increasing order

λ1 ≤ λ2 ≤ ... ≤ λn.

A line in {x, t} space (Fig. 1) with slope λk(c) is called "k-characteristic at concentration c". It is the path in this space of a concentration vector c being a solution of (3) corresponding to the eigenvalue λk(c). This solution is called k-rarefaction wave, the name referring to the properties

  1. "wave", i.e. the concentration c varies continuously across the wave (e.g. along a path perpendicular to the characteristics) and propagates with time. Usually the variation is not periodic.
  2. "rarefaction", i.e. the fan of characteristics spreads with increasing time t.
Figure 1 shows a schematic example of a 4-component system (n = 4). The 1-, 2- and 4-rarefaction waves are plotted together with some of their characteristics (light and heavy lines). The speeds of the rarefaction waves are

Fig. 1: Schematic of Solution
FIGURE 1: Schematic of the solution of hyperbolic system (2) with 4 components (vectors ci have 4 components: ci = {ci1, ci2, ci3, ci4}), represented in {x, t} space. System solution consists of 4 constant states, c1, c2, c3, c4 = c", the characteristics of which are the heavy lines. Constant states 1 through 3 are bounded by the 1-, 2-, and 4-rarefaction waves (heavy solid lines) and the 3-shock ("line of discontinuity, L", heavy, dashed). Rarefaction waves are pictured by a fan of some arbitrarily chosen characteristics (light lines), each representing the path of a concentration c within the wave.

Insert: Characteristics 1', 2', 3', 3", 4" are defined as

There exist also piecewise smooth solutions of the system of conservation laws (2). Here the concentration changes by a finite amount rather than by differential amounts as it does across a rarefaction wave. Let P be a point on the line of discontinuity L and let c' and c" be the values of the solution on the left and right side of the discontinuity, respectively. Accordingly, the mass conservation law involves the differences

(4)

Δc = c' - c"
ΔC = C' - C"

rather than differentials ∂C and ∂c. For every component i we have a conservation law similar to (2)

(5)

discontinuity condition

After multiplying with Δx

discontinuity condition

we can solve for Δx/Δt = σk, the speed of the k-discontinuity:

(6)

In {x, t} space, σk is the slope of the line of the k-discontinuity. Equation (6), the so-called "Rankine-Hugoniot relation", has n physical (piecewise smooth) solutions. Any such solution is called a k-shock (also called "front", "self sharpening wave" or "migrating boundary") if it meets the following "entropy" conditions (from Greek εντρεπειν (entrepein) = to turn -or run- into) [Lax, 1957]:

  1. Draw, issuing from P in positive t-direction, the k-characteristics of constant states c' and c" (in the example of Fig. 1 (insert) these are the dashed thin lines 3' and 3"). The characteristics need to cross line L.

    (7)

    λk(c') > σk > λk(c"), (k = fixed, in our example: k = 3).

  2. Draw, issuing from P in positive t-direction, those k-characteristics with respect to the state c' which stay to the left of L, and those with respect to the state c" which stay to the right of L (thin lines in box in Fig. 1). The total number of characteristics drawn in this way needs to be n-1.

    (8)

    λk-1(c') ≤ σk ≤ λk+1(c"), (k = fixed, in our example: k = 3).

    Because the λs are arranged in increasing order this implies

    λ1(c') ≤ λ2(c') ≤ ... λk-1(c') ≤ σk ≤ λk+1(c") ≤ ... ≤ λn(c")

    or in our example:

    λ1(c') ≤ λ2(c') ≤ σ3 ≤ λ4(c").

Condition (7) makes sure that the k-wave "breaks", meaning that it resembles a water wave breaking on its path onto the shore, its top running faster than its bottom part. The physics of concentration waves (c ≥ 0) allows this breaking process to proceed only until all parts of the wave have arrived at the same line. At this line the concentration jumps from ck-1 to ck (in our example from c2 to c3). This concentration jump Δc = ck-1 - ck is the k-shock.

In chromatography [Helfferich and Klein, 1970, Charbeneau, 1988] it is customary to use the matrices I + Q(c) instead of the above mentioned inverse (I + Q(c))-1 referred to in gas dynamics.

The eigenvalues

Obviously

(9)

λj(c) = v0/(1 + kdj(c)) = v0/Rj(c),

thus

(10)

kdj(c) = v0j(c) - 1.

The retardation ρk of a k-shock is defined analogously to (9)

(11)

σk(c) = v0k(c).

The Rankine Hugoniot condition (6) tells us that the retardation of the k-shock is

(12')

ρk = 1 + ΔC/Δc.
where C is any component Ci of the vector C and c is any component ci of the vector c. (In a k-shock ΔC1/Δc1 = ΔC2/Δc2 = ... ΔCn/Δcn)

In the rest of this chapter we will apply the theory to show how a contaminant will accumulate during migration.

Contaminant Shock Wave

Consider a natural environment containing contaminant M as a trace. By adding a remobilizing chemical f to it we can trigger a shock wave of M. Actually, removing -or reducing the concentration of- a chemical component f can generate a shock wave of M as well (in this animated example f is sulfate. More examples are presented below).

The retardation of this shock is

(12)

ρf = 1 + ΔCf/Δcf.

Because the concentration cM of the contaminant M is small, trace M moves on the background of the natural components and f, without being able to influence the chemistry of the background. Its speed is approximated by λM given in (9) with the vector c in kd(c) not containing the concentration cM:

(9')

λM(c) = v0/(1 + kdM(c)),

with

(13)

c = {c1, ..., cM-1, cM+1, ..., cn}

Condition (7) for the contaminant is

(14)

kdM(c') ≤ ΔCf/Δcf = kdf ≤ kdM(c").

This means: An observer located at P on the remobilizing shock will see species M coming towards her both from upstream and downstream locations. The front carries the inventory of the remobilized contaminant with it. The degree of accumulation can be calculated [Gruber, 1984].

The following limiting case of this situation is interesting as well: The remobilizing front moves without retardation (σf = v0 = λM(c') or kdf(c') = ΔCf/Δcf = kdf = kdM(c') = 0):

(15)

0 = kdM(c') = kdf(c') ≤ kdM(c").

The contaminant is remobilized by the moving front but cannot travel across the front. Thus, the remobilized contaminant inventory will stay in solution.

Whenever (14) or (15) is met, the decrease of the distribution coefficient (14) must not be disregarded, and transport models based on the assumption of a constant kd or a spectrum of stochastic kd 's are inappropriate. In that case the transport model has to contain a chemical adsorption model. Any of the chemical models meeting that requirement can be chosen, if it is used within the composition space in which it has been calibrated.

ADSORPTION ON OXIDES

Often, oxides on soil or rock particles are the dominant adsorbents and determine the trace metal concentrations in solution [Jenne, 1968]. Their adsorption properties vary strongly with the composition of the surrounding solute. Remobilization and accumulation of adsorbed trace metal deposits on oxides are therefore of particular interest here. The corresponding adsorption model is the surface complexation model. Of this family of models the triple layer model [James and Parks, 1982; Kent et al., 1986] gives us an adsorption isotherm CM = f(c1, c2, ..., cn) in which the major ion concentrations c1, c2, ..., cn appear explicitly.

In surface complexation models adsorption is described as complex formation on hydrolyzed surface sites, S-OH, for example

(16)

S-OH + M++ + H2O = S-OMOH + 2H+.

Consistent with the assumption of M being a trace element, only mononuclear surface complexes of M need to be considered. The concentration of a surface complex s containing M, CM,s, is then expressed as the product of a side reaction coefficient, σs, and the free concentration of the adsorbing ion, [M++ ], (definition of σs ):

(17)

CM,s = σs[M++ ].

Similarly, the concentration of a soluble complex mononuclear in M, [Ql], is the product of a corresponding side reaction coefficient, σl, and [M++ ]. This coefficient is a function of the concentration of a background element L that forms a soluble complex with M:

(18)

[Ql] = cM,l = σl(L) [M++ ].

The major chemical components of the system are also subject to adsorption and complexation but without restriction to mononuclear reactions. The appendix gives the sources of the data considered in the following examples.

Above pH = 7 adsorption of the divalent transition elements Pb, Cd, Cu, Zn, Ni and of UO2++ on a metal oxide can be represented by one single surface complexation reaction of type (16) [Hohl and Stumm, 1976; Vuceta, 1976, James and MacNaughton, 1977; Benjamin, 1978; Davis and Leckie, 1978; Richter, 1978; Balistrieri and Murray, 1979, 1981, 1982; Altmann, 1984; Tripathi, 1984]. The corresponding adsorption side reaction coefficient is

(19)

eq. 19

where {H+} is the activity of the proton in solution. Because of its surface charge, the oxide is surrounded by an electrostatic potential field, Ψ. The exponential term in (19) takes account of the work necessary to assemble the surface complex. There are two major adsorbing locations on the surface, the 0-plane (the surface itself), which loses one proton upon adsorption of one M, and the β-plane (located at some distance from the surface), which receives the (single) positive charge of the MOH complex. e, Ψ0 and Ψβ are the charge of the electron and the values of the electrostatic potential in the 0- and β-plane, respectively. K is the equilibrium surface complex formation constant. kT is the product of the Boltzmann constant and the absolute temperature of the system. With

(20)

CM = σs [M++ ]

and Σl meaning the summation over l, the soluble complexes of M and

(21)

cM = (1 + Σl σl ) [M++ ]

the distribution coefficient of M can be rewritten as a ratio of the side reaction coefficients:

(22)

eq. 22

Combining (22) with (19) gives the final expression for kd of M:

(23, Triple Layer Model)

eq. 23

Major processes and environmental variables influencing kd are now represented by individual terms in (23):


When for example major ions in solution forming surface complexes [S-Xi] compete with M for adsorbing sites, the concentration of sites available for M decreases because the total number of adsorbing sites, Sr , is limited:

(24)

Sr = [S-OH] + Σl [S-Xl] + σs [M++].

ACCUMULATION POTENTIAL

Fig. 2 is a graphical representation which allows to evaluate (23) with respect to accumulation conditions (14). The left ordinate gives K of the adsorption reaction (16).

The right coordinate represents the factor SrK/{H+}2 for pH = 7. This factor is kd for a non-complexing solution (i.e. σl = 0) at pH = 7, when both competition and electrostatic contributions to adsorption are negligible ([S-OH] = Sr and exponential term in (23) is unity).

Sr = 0.1 moles/l has been chosen using the simple relationship

(25)

Sr = ns f v ρ (1 - Φ) = 0.1 moles/liter.

where ns is the number of sites per unit area of the oxide. To simplify the discussion ns = 10-5 moles/m2 is used for all oxides in Fig. 2, because the variation of Sr due to the variation of ns [James and Parks, 1982] is small compared to the variability of the other terms in (23). f is the fraction of the soil particle covered with the oxide (0.2 is assumed). v is the surface to weight ratio of the soil particle (30 m2/g is assumed). ρ is the density of the soil particle (2.6 g/cm3 assumed) and Φ is the average volumetric water content of the porous medium (to simplify notation, Φ = 1 was used in chapter "Mechanism of Accumulation"). Sr = 0.1 moles/liter is consistent with the estimate used by Anderson [1979].

Triple Layer Model Distribution Coefficients for Oxides

FIGURE 2. Distribution coefficients (kd, right ordinate, (-) means dimensionless) calculated from adsorption constants (K, left ordinate) for 6 divalent cations and 5 oxide adsorbents ("contaminants M"). Data and assumptions (see (23)):

The retardation of the contaminant M is (1 + kd) (see (9')). One can see that for the majority of cations M and adsorbents the retardations range between 1 and 100.

Accumulation of M occurs when a remobilizing agent shifts the corresponding symbol in Fig. 2 from its pre-remobilization position kd" = kdM(c") (e.g. the one used in the figure) down to a position kd' = kdM(c'). According to (14) the kd of the shock kdf = ΔC f/Δc f is intermediate between kd' and kd". So for the majority of cations M and adsorbents in Fig. 2 retardations of shocks range between 1 and 100:

1 ≤ ρf ≤ 100.

The remainder of this chapter will show that such shifts of the symbol locations in Fig. 2 are well possible (processes (a) - (d)). The element nickel is chosen as a representative example (because (16) describes adsorption in the pH range of interest, 4 <= pH <= 10 [Richter, 1978]). The computer code used is MINEQL-SGMA (Stanford Generalized Model of Adsorption), a computerized version of the surface complexation model [Davis, James and Leckie, 1978; Davis and Leckie, 1978].

Finally, also the model parameter K and the exponent of {H+} in (19) vary slightly with changing environmental conditions ("residual variation") [Benjamin, 1978; Honeyman, 1984; Altmann, 1984].

TABLE 1. (Negative logarithm of) Total concentrations of major ions (moles/l) used in computations of Figs. 3 - 6. Total concentration of adsorbing sites is 0.25 moles/l except for Figs. 4 and 6, where it is 0.0025 moles/l. Total concentration of Ni is 10-8 moles/l, except in Fig. 5 (intruding water), where it is 3.8 x 10-10 moles/l.


Figure Ca Na CO3 Cl SO4 pH
3 and 4 (curves 1, Remob) 2 6 6 2 6 -
3 and 4 (curves 2, IC) 6 6 6 6 6 -
3 (curve 3) 6 2 6 2 2 -
3 (curve 4) * 6 * 6 2 -
5 (intruding water) 6 6 6 6 6 4
5 (undisturbed environment) # 2 # 2 3 7
6 (intruding water) 2 6 6 2 6 8
6 (undisturbed environment) 6 6 6 6 6 8

Legend:

Based on the preceding quantitative discussion of processes (a) - (d) one can expect critical kd shifts in Fig. 2 (i.e. the ones necessary to meet accumulation conditions (14)) even in the case of a poorly retarded front (ΔC f/Δc f = kdf (appr.=) 0). The deposits on γ-Al2O3 and amorphous Fe2O3 will resist accumulation less than the ones on α-SiO2, because the former are more easily remobilized.

EVOLUTION OF REMOBILIZATION

A numerical solution of (2) or (3) with prescribed initial data has to start with establishing a difference scheme which can be used to approximate the set of partial differential equations. Lax and Wendroff [1960] have investigated a fairly large class of stable schemes with small truncation errors.

In this paper a very crude difference scheme will be shown to be able to illustrate two simple cases (a much more sophisticated model based on the philosophy presented here has been published by Amir and Kern, 2010, in cache). In both cases, remobilization of a contaminant deposit leads to a migrating contaminant wave carrying the remobilized inventory. The difference scheme meets the requirements of von Neumann stability and small truncation error [Lax and Wendroff, 1960], while using a negligible fraction of the total computing time.

The set of difference equations used in the model (MINTRANS) is

(26)

(Ctikm+1 - Ctikm) / Dt + v0 (cikm - cik-1m)/Dx = 0.

k denotes a position and m a time step:

ci(kDx, mDt) = cikm.

The set (26) expresses the mass balance for transport in a chain of cells k, each cell having a length Dx and spatially uniform equilibrium concentrations Ctikm, cikm (with Ctikm = Cikm+cikm). Dt is the time necessary to exchange the soluble phases of the cells:

(27)

Dx/Dt = v0 ≥ vj for j = 1, 2, ..., n.

vj is any possible concentration velocity in the chain. Because of (27) the scheme (26) satisfies the Courant-Friedrichs-Lewy convergence condition [Lax and Wendroff, 1960].

Equation (26) is an appropriate difference scheme for (2) in the following two situations:

  1. Vertical transport in unsaturated homogeneous field plots under periodic rain or irrigation.
    Jury and coworkers [Jury, 1982; Jury and Sposito, 1985; Butters, Jury and Ernst, 1989] have shown for their field that up to a depth of several meters the solute can be viewed as moving in a system of non-interacting stream tubes or flow paths. The lengths of the stream tubes leading to a given depth are statistically distributed. The solute moves only when water enters a flow path at the soil surface. In the time between rain events the solute is stationary. If the duration Dt of single rain events is short compared to the interval between them [Eagleson, 1978] and compared to desorption times, the spatial concentration profile (along a flow path) is better represented by a series of steps than by a continuous function. The distance between adjacent steps is Dx = ioDt/Φ, where ioDt is the height per rain storm. A stream tube can then be approximated by a chain of cells with uniform concentrations, and (26) is applicable for every flow path. Transport in the entire field plot is the linear superposition of the transport through individual stream tubes.

  2. Transport over microscopic distances.
    While the set of difference equations (26) describes a macroscopic profile in the above case of a series of discrete rain events, it illustrates the microscopic profile when water input is continuous. In this case it is the discrete spatial structure of the solid matrix of the porous medium which allows the simple set of difference equations.

    The set of hyperbolic differential equations (2) is valid for continuous space. Let d be the distance at which the solution of (2) breaks down because of the discrete structure of the pore network. In this paper it is assumed that solute transport within d is random. This includes that the starting and ending points of transport trajectories larger than d are randomly distributed within volumes Vmin = F d, where F is the cross section of a flow channel. This causes mixing of the pore water within d. The mixing process is assumed not to allow the development of any structure within Vmin. (The cells Vmin resemble "continuously stirred reactors"). The microscopic concentration profile along a flow path is therefore rather a sequence of steps than a continuous curve, and consequently the set of mass balance equations (2) can be approximated by (26) with Dx = d.

Figures 5 and 6 show simulated migration in two remobilization scenarios.

The cells contain an adsorbent (goethite) and an aqueous solution (no gas phase). The chemical components are Ca2+, Na+, CO32-, Cl-, SO42-, and Ni2+, the latter being the contaminant. The appendix gives the data used in the calculations. The chemical model is the triple layer model as implemented in MINEQL-SGMA. The initial states of the chain of cells is given in Tab. 1.


In the first scenario (Figs. 5a and b)
The dissolution of calcite in the system (closed to the atmosphere) consumes protons and therefore raises the pH. When calcite has totally disappeared in a cell, the pH drops to the value in the incoming water and the contaminant (nickel) is mobilized. Next the remobilized inventory crosses the pH step and becomes re-adsorbed immediately thereafter. Thus nickel accumulates at the downstream flank of the pH step.

At pH = 4 the kd of nickel is well below 1, thus the accumulated nickel can easily follow the relatively slowly advancing pH front (condition (14) is met). In a more realistic model, accumulation would be limited by precipitation of Ni and other processes not considered here, such as the concentration dependency of the parameters of the chemical model, dispersion or kinetic effects.

Fig. 5a: Concentrations in Column with pH Buffer at Time 0
Fig. 5b: Accumulation Profiles in Column with pH Buffer
FIGURE 5. Spatial concentration, kd and pH distribution in a nickel deposit adsorbed on goethite. Environment contains calcite as pH buffer. Upper part of figure: undisturbed environment. Lower part: environment after intrusion of remobilizing water having pH = 4 (see Tab. 1). The front of the intruding water has arrived in cell 21. Dotted curve gives CNi(c)/cNi(c). Curve representing soluble nickel coincides with abscissa. Shaded area marked (-) is remobilized Ni-inventory, shaded area marked (+) is accumulated Ni-inventory (secondary repository).

Contrary to scenario 1, in scenario 2 (Figs. 6a and b) the remobilizing front is poorly retarded. The intruding remobilizing water has the same composition as the water in the undisturbed deposit except for its higher calcium chloride concentration (10-2 M). Fig. 4 shows the variation of the kd of the contaminant in the deposit prior to and after remobilization (curves IC and Remob, respectively).

As mentioned above, calcium competes here with the contaminant for adsorption sites at high pH (process (d), above), because CtCa (appr.=) Sr. At low pH, the adsorbed calcium ions affect contaminant adsorption via the electrostatic field they build up at the surface (process (c), above).

When the calcium rich water intrudes into the initially calcium poor deposit, calcium adsorbs and increases the (positive charge repelling) surface potential. This, in turn, leads to the desorption of protons into the pore water. The result lies between two extremes, depending on the efficiency of the proton buffer in the system:

  1. The system has an effective proton buffer: Each adsorbed calcium ion displaces one proton from the surface. The desorbed protons disappear in the buffer pool. The concentration of protons that this pool will have to accommodate is approximately equal to the total concentration of soluble calcium ions in the intruding water, CtCa, because at the pH of interest (pH > 7) a large fraction of the intruding calcium ions become adsorbed.
  2. The system has no proton buffer: Because CtCa is large compared with the activity of the free proton, the pH drops rapidly. This stops desorption of protons even before the surface proton concentration has changed by more than a small fraction of CtCa.

Solute transport is considered here for case 2. In Fig. 4 one can see that kd = RNi - 1 << 1, i.e. the remobilization meets conditions (15), indicating that a soluble concentration wave of the contaminant is to be expected (in contrast to an adsorbed one as in the previous scenario). This is indeed what the simulations show (Fig. 6).

A similar scenario would be one in which the goethite surfaces release protons upon desorption of sulfate or chloride ions. This happens when the intruding (remobilizing) water has a smaller concentration of these anions than the initial pore water in the deposit.

Fig. 6a: Concentration Profiles in Column without pH Buffer at Time 0
Fig. 6b: Accumulation Profiles in Column without pH Buffer
FIGURE 6. Spatial concentration, kd and pH distribution in a nickel deposit adsorbed on goethite, no pH buffer. Upper part of figure: undisturbed environment. Lower part: deposit after intrusion of remobilizing water. Intruding water front is in cell 21. pH in intruding water is the same as in undisturbed environment. Ni inventory has been remobilized (shaded area, -) and put into solution (shaded area, +).

CONCLUSIONS

Remobilization of a deposit of an adsorbed contaminant results in a migrating wave of possibly largely elevated contaminant concentration either adsorbed or in solution, if the remobilized contaminant inventory can follow the movement of the remobilizing front on its way through the deposit.

Oxides of iron, aluminum, titanium, and silicium are of particular interest, because their adsorption properties vary strongly with solution composition. The waves of major species, such as protons, released from oxide surfaces upon disturbance by infiltration of remobilizing water can amplify contaminant remobilization if buffers are insufficient.

An experimental example of a contaminant wave has possibly been shown in silicate soil underlying contaminated soils (S. Eberle (local link), see also: Sickerwasserprognose in "Sickerwässern auf der Spur: Tagung der Gesellschaft für Hydrogeologie über Wasserwirtschaft und Umweltschutz in München", Deutschlandfunk, Forschung Aktuell, 21.4.2004)

This paper has shown that investigations of the groundwater pollution potential of contaminated adsorbents have to start by determining which remobilizing fronts can develop in the environments of concern and with what velocities vf these fronts might migrate. Subsequently, the compositions upstream and downstream of these fronts have to be used in batch experiments to determine the contaminant adsorption isotherms. This sequence of investigations will enable us to single out critical remobilization processes, i.e. those leading to contaminant build-up at migrating remobilizing fronts.

ACKNOWLEDGEMENT

Part of this research was sponsored at Los Alamos National Laboratory by the "Toxic Substances Research and Training Program" of the University of California at Riverside.

Notation

C = {C1, C2, ..., Cn} = vector of adsorbed concentrations, unit: M = moles/l, first time used

Cj = adsorbed concentration of component j, unit: M, first time used

Ctj = total concentration of component j = cj + Cj, unit: M, first time used

c = {c1, c2, ..., cn} = vector of soluble concentrations, unit: M, first time used

cj = soluble concentration of component j, unit: M, first time used

c' = soluble concentration upstream of the remobilizing front, unit: M, first time used

c" = soluble concentration downstream of the remobilizing front, unit: M, first time used

D = combined coefficient of diffusion and hydrodynamic dispersion, unit: m2/s, first time used

Δ = difference operator, Δc = c' - c", ΔC = C' - C", first time used

D = cell size operators in difference scheme (26), Dx = size of spatial cell, Dt = size of time step, first time used

∂ = partial derivative, first time used

f = remobilizing agent, first time used

fj = multicomponent adsorption isotherm of component j, unit: M, first time used

I = unit matrix, an (n x n) diagonal matrix with 1's in the diagonal, 0's off diagonal, first time used

i = index meaning chemical component, first time used

K = equilibrium surface complexation constant, unit: M, first time used

k = index for wave, k = 1, 2, ..., n, first time used

k-characteristic = line in {x, t} space on which c is constant, i.e. distance x that a fixed concentration cj travels on k-wave during time t, first time used

kdk(c) = k-eigenvalue of Q(c), generalized distribution coefficient in k-wave at concentration c defined as normalized adsorbed concentration (normalized with respect to soluble concentration), (unit: L/kg), first time used

kd = distribution coefficient of contaminant M, dimensionless, first time used

k-rarefaction wave = continuous solution of (3), first time used

k-shock = piecewise smooth solution of (3), first time used

kT = product of Boltzmann constant and absolute temperature, unit: eV, first time used

λk(c) = speed of concentration (vector) c in k-rarefaction wave, unit: m/s, first time used

l = index meaning soluble complex, first time used

M = unit mole/L, first time used

M = contaminant, present at trace concentration, first time used

[M++] = concentration of free ion M, unit: M, first time used

n = number of components in geochemical system (n = maximum of j), first time used

Q = Jacobian matrix of multicomponent isotherm, unit: dimensionless, first time used

[Ql] = concentration of soluble complex l containing contaminant M, unit: M, first time used

qfj = partial derivative of isotherm Cf with respect to concentration cj, unit: dimensionless, first time used

Rk(c) = 1 + kdk(c) = k-eigenvalue of (I + Q(c)), retardation of concentration (vector) c in k-rarefaction wave, unit: dimensionless, first time used

ρ = mass density of soil particle, unit g/cm3, first time used

ρk(c) = retardation of concentration (vector) c in k-shock wave, unit: dimensionless, first time used

s = index meaning adsorbed complex, first time used

σk(c) = speed of concentration (vector) c in k-shock wave, unit: m/s, first time used

σl(L) = side reaction coefficient for formation of a soluble complex MLx containing M and a background element L. It is the concentration of the soluble complex MLx divided by the concentration [M++] of the free (i.e. non complexed) ion M++, unit: dimensionless, first time used

σs = side reaction coefficient for adsorption of M. It is the concentration of the adsorbed complex S-OMOH divided by the concentration [M++] of the free ion M++, unit: dimensionless, first time used

S-O = adsorbing surface site, S being the oxide lattice and O the surface oxygen layer, unit: M, first time used

S-OH = hydrolyzed adsorbing surface site, unit: M, first time used

S-OMOH = adsorbing surface site the proton of which has been replaced with the MOH complex, unit: M, first time used

Sr = total number of adsorption sites, unit: M, first time used

Σj = summation over j, first time used

t = time variable, unit: s, first time used

Φ = is the average volumetric water content of the porous medium (to simplify notation, Φ = 1 was used in chapter "Mechanism of Accumulation"), first time used

Ψ0 = electrostatic potential at surface, unit: V, first time used

Ψβ = electrostatic potential in beta plane, unit: V, first time used

v0 = flux of water in direction of x, unit: m3/(m2 s) = m/s, first time used

x = spatial variable, unit: m, first time used

{x, t} space = space with abscissa x and ordinate t, first time used

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APPENDIX: Sources of Data Used in Computations

Complexation & Precipitation
Element Reference
Ca 1, 2
Cd 1, 3, 4, 5
Cu 2, 4
Mg 1, 2, 3
Ni 1, 2, 3, 6, 7, 8
Pb 1, 2, 3
Zn 1, 2, 4
Cl, CO3, PO4, SO4 4
UO2 9

Adsorption
Element Adsorbent Reference
Ca, Mg, Na G 11
Cd, Cu, Pb, Zn G 12
Cd, Cu, Zn, Pb A, F, Q 13
Cd, Mg T 13
Ni G 14, 15
Cl, SO4 G 11
UO2 G 9

Legend


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