Semianalytic solution for transport of a two-member decay chain in discrete parallel fractures

Authors

  • Scott K. Hansen

    Corresponding author
    1. Department of Environmental Science and Energy Research, Weizmann Institute of Science, Rehovot, Israel
    • Corresponding author: S. K. Hansen, Department of Environmental Science and Energy Research, Weizmann Institute of Science, Rehovot 76100, Israel. (scott.hansen@weizmann.ac.il)

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Abstract

[1] A wide variety of analytic solutions have been developed for 1-D contaminant transport, but to date the author is aware of none modeling a decay chain in parallel discrete fractures in porous media. In this note, the derivation is presented for a two-species first-order decay chain in such an environment, with an arbitrary concentration history specified upgradient, fracture advection, and diffusion into the porous matrix. The solution is presented in brief, followed by corroboration of its numerical implementation against two different existing numerical codes. An appendix contains a detailed derivation of the solution, and a Mathematica notebook that implements it and may be used by practitioners is enclosed as supporting information.

1. Introduction

[2] Previously, there has been interest in analytic solutions for transport of decaying contaminants in fractured porous media, and solutions have been derived in a number of simplified geometries. In the literature, those solutions have been widely cited by authors working in the transport of radionuclide chains in fractured rock, as well as by authors verifying various numerical codes. These solutions are also applicable to transport and first-order biological decay of organic compounds in fractured porous media. A good introduction to currently existing solutions may be found in chapter 6 of Rumynin [2011]. The decay of a single species has been modeled in a single fracture by Tang et al. [1981] and in a set of parallel fractures by Sudicky and Frind [1982]. A two-species decay chain was also later considered in a system with a single isolated fracture in Sudicky and Frind [1984], modified in a special case by Cormenzana [2000], and reconsidered for longer chains by Sun and Buscheck [2003]. However, no solution the author is aware of exists which accommodates both a decay chain and a set of parallel discrete fractures, despite zones of multiple fractures being common in reality. The work presented here fills that lacuna. (See Table 1 for a comparison of approaches.)

Table 1. Comparison of Various 1-D Transport Laplace-Domain Analytic Solutions for Two-Component Decay Chains
 Single FractureParallel Fractures
Single SpeciesTang et al. [1981]Sudicky and Frind [1982]
Decay ChainSudicky and Frind [1984] and Cormenzana [2000]This paper

[3] In this section, the exact problem formulation is provided. In section 2, the Laplace-domain solution is presented (without derivation), along with discussion of its form. In section 3, computer implementation of the given solution is discussed, and results of trials of a computer implementation of the new solution against existing codes are given. Full derivation of the new solution is presented in Appendix A, and a computer implementation is enclosed as supporting information.

1.1. Exact Problem Formulation

[4] This document presents a Laplace-domain expression for the concentration histories of two chemical species comprising a straight, mother-daughter decay chain in an infinite set of parallel fractures, under uniform groundwater flow down the fractures, given an arbitrary Type I (specified concentration) source at the origin. Assumptions made are that: (1) groundwater flow is known, constant, and is longitudinal along the fracture direction, (2) groundwater flow is sufficient for transport in the fractures to be advection dominated, (3) in the porous matrix, diffusion is the only transport process, and it occurs only orthogonal to the fracture planes, (4) geometry is uniform, with fractures of uniform width, evenly spaced, and of infinite extent, (5) all decay processes in the chain can be modeled as first order or pseudo first order, and (6) sorbed and free solute concentrations are proportional, allowing use of constant retardation factors. In Figure 1, the geometry of the fracture system is shown. Note that the origin for the x coordinate is at the edge of the fracture, rather than the midpoint. This is slightly different from the convention used by Sudicky and Frind [1982 and 1984], whose x ran from 0 to inline image, rather than 0 to L. This change was made in order to simplify some algebra when deriving the solution.

Figure 1.

Schematic diagram of fractured rock, with coordinate system overlaid. Only two fractures are shown, but the system is taken to be of infinite extent in the x direction, so that lines of symmetry exist in the middle of each porous matrix block. Note that the fractures are shown horizontal, but the solution derived here is valid regardless of the orientation of the x axis relative to gravity.

[5] In the fracture, the governing equations for the various species are the standard advection equations with first-order decay, plus a sink term corresponding to matrix diffusion in and out of the fracture walls. The governing equations for the two species are (variables are defined below):

display math(1)

and

display math(2)

where c1 is the concentration of species 1 (the mother in the decay chain) in the fracture, and c2 is the concentration of species 2 (the daughter). These are solved subject to the following initial and boundary value equations, which define fractures initially devoid of solute, with arbitrary type 1 (specified concentration history) boundary conditions specified at the upgradient boundary for both the mother and daughter.

display math(3)

[6] In the matrix, advection is not a factor, and diffusion is the dominant mechanism, so the governing equations for the two species are simpler:

display math(4)

and

display math(5)

where the prime (i.e., inline image) symbol indicates a concentration in the matrix, and the numeric subscripts 1 and 2 again identify the mother and daughter species, respectively. The matrix solutions must satisfy the following initial and boundary conditions, which enforce continuity of concentration across the fracture/matrix boundary and also no flux across symmetry boundaries:

display math(6)

[7] Finally, the sink terms in the fracture (representing flux outward to the porous matrix), as seen in (1) and (2), are given by the following expression:

display math(7)

[8] In all the above equations, the physical meanings of the variables and parameters are outlined in Table 2.

Table 2. Catalog of Symbols Representing Physical Parameters and Quantities Defining the Transport Problem
SymbolMeaning
cn and inline imageMass concentration of species n [M L−3] in, respectively, the fracture and the matrix
vGroundwater velocity in fractures [M T−1]
inline imageUpgradient concentration of species n in the fracture (at coordinate z = 0)
inline imageFirst-order decay constant for species n
DnEffective (tortuosity included) diffusion constant for the porous matrix
Rn and inline imageRetardation factor for species n in, respectively, the fracture and the matrix
θMatrix porosity
tTime since first release of solute (all concentrations are zero at and before t = 0)
xSpatial coordinate: distance into matrix from nearest fracture wall
zSpatial coordinate: distance along fracture from location where the inline image are specified
LHalf width of matrix block between two adjacent fractures
bHalf width of each fracture

2. Summary of the Solutions

[9] The solutions in the Laplace domain can be expressed in terms of the Laplace transforms of the boundary conditions, the spatial variables, and a number of simplifying auxiliary functions (which are defined below). The solutions are summarized in Table 3. As in the work of Cormenzana, two different forms are given for the concentration of species 2 in the fracture, on account of potential zero division when the auxiliary functions inline image and inline image are equal.

Table 3. Summary of Solutions in Terms of the Laplace Transform of the Concentration Function for Each Species, in Both Domains
 Species 1Species 2
Fracture inline image inline image
Matrix inline image inline image

[10] The functions employed in the solutions summarized in Table 3 are defined below. The location-independent quantities are defined:

display math(8)
display math(9)
display math(10)
display math(11)

[11] The location-dependent functions are defined:

display math(12)
display math(13)
display math(14)
display math(15)

3. Numerical Results and Solution Corroboration

[12] The analytic solution presented briefly above (and completely in the appendix) is in the Laplace domain, where time has been transformed to a nonlocal variable, p. In order to use the solutions to determine the concentration history at a given spatial location, it is necessary to invert the Laplace transform. While analytical inversions are sometimes possible, using a computer for evaluation of the solution is inevitable given the symbolic complexity here. Thus, it is simpler to numerically invert the Laplace transforms directly. A computer code (Parfrac) that does this inversion to calculate concentration profiles downgradient from the source, in the fractures, was written. The computer implementation was slightly simplified, assuming only the mother species is present at the source. This is in keeping with computer implementations of prior solutions, though there is no technical reason why the program could not cover the full range of solutions outlined above, if needed. Three trials were run to test the code implementing the new solution against two previously implemented solutions in order to compare fracture concentration profiles generated. Since the fracture solutions are derived from the matrix solution, this testing has the potential to indirectly corroborate them, also. The trials were:

[13] 1. Comparison with Chainf, an existing code written by C. Neville which implements Sudicky and Frind [1984] and Cormenzana [2000] solution for a two-species decay chain in a single fracture. This was run for against Parfrac for a very large fracture spacing (so the parallel and single fracture approaches converge) for inline image and inline image both distinct. Concentration upgradient was 1.0 for the first 100 days and 0.5 thereafter.

[14] 2. A repetition of the above with intrinsic parameters altered to make inline image and inline image identical.

[15] 3. Comparison with Craflush, coded by E. A. Sudicky, which implements the Sudicky and Frind [1982] solution for decay of a single species in parallel fractures. This was compared, for a narrow fracture spacing, with the output of Parfrac for the mother species (species 1) only. Concentration upgradient was 1.0 for all time.

[16] The hydrogeologic parameters used in the simulations are given in Table 4. Results for the three trials are shown in Figures 2 and 3, respectively. As is apparent, correspondence between output from Parfrac and the existing codes is excellent in all cases.

Table 4. Parameters for Simulation Trialsa
 Trial 1Trial 2Trial 3
  1. a

    Units are not used by any of the implementations, but for the sake of concreteness may be thought of as meters and days. Boldface parameter values are distinct from the ones employed in trial 1.

Velocity v0.10.11
Fracture Width inline image1.0e-41.0e-41.0e-4
Fracture Spacing inline image10100.1
Porosity θ0.010.010.01
Matrix Diffusion Coefficient D18.64e-68.64e-68.64e-6
Matrix Diffusion Coefficient D24.68e-68.64e-6 
Fracture Retardation Coefficient R1111
Fracture Retardation Coefficient R211 
Matrix Retardation Coefficient inline image111
Matrix Retardation Coefficient inline image11 
First-Order Decay Constant inline image3.798e-44.220e-43.798e-4
First-Order Decay Constant inline image4.220e-64.220e-4 
Figure 2.

Comparison of concentration profile predictions from Parfrac (solid lines) and Chainf (hollow markers). Square, cross, and circle markers, respectively, indicate profiles at 1000, 10,000, and 100,000 days. Black curves and markers are for species 1, gray for species 2. The top axes show output of trial 1 (distinct inline image and inline image), and the bottom axes show output of trial 2 (identical inline image and inline image).

Figure 3.

Comparison of concentration profiles at three different times, generated for the mother species (species 1) for trial 3. Lines represent output from Parfrac and markers represent output from Craflush. Square, cross, and circle markers, respectively, indicate profiles at 1000, 10,000, and 100,000 days.

4. Summary

[17] A Laplace-domain analytical solution to the problem of a straight two-species decay chain in a set of parallel fractures was derived. This solution allows for a broader range of calculations than are possible using existing analytic solutions. It may be of use for making engineering estimates for natural attenuation problems, particularly concerning the transport of radionuclides, and also for verifying numerical models. A computer implementation of the model was generated for the case of a two-species decay chain, with only the parent species present at the upgradient source. Both the analysis and its computer implementation have been corroborated by empirical comparison with existing solutions using a few different sets of fracture and solute properties. The implementation has been included as supporting information to this note.

Appendix: Derivation of the Solution in the Laplace Domain

[18] First consider the governing equation for species 1 in the matrix:

display math(A1)
display math(A2)

and define inline image. By inspection, the above has the solution

display math(A3)

Differentiating and applying the type II BC at x = L yields inline image. Substituting back into inline image, evaluating at x = 0, and applying the type I BC yields

display math(A4)

So,

display math(A5)

Substituting this all back into (A3) yields

display math(A6)

where we may define the function inline image to represent the term in square brackets. Now we have an expression for the Laplace-transformed concentration in the matrix of species 1 in terms of its concentration in the fracture. To determine the concentration in the fracture, we need to determine the flux into the matrix, to determine the sink term there, and then solve. We determine this by differentiating spatially, yielding

display math(A7)

where we may define the function inline image to represent the term in the square brackets. Then, evaluating inline image yields

display math(A8)

[19] Consider now the transform of (1) for the mother species in the fracture. Substituting in the above equation for inline image yields

display math(A9)

where we define M1 to be equal to the square-bracketed quantity. Since the above is a homogeneous first-order equation, employing the boundary condition (3) yields

display math(A10)

which is the Laplace transform of the solution for species 1 (the mother) in the fracture. To complete the analysis, this can be plugged back into the expression for species 1 in the matrix, yielding

display math(A11)

[20] Now, we can do a (more complicated) analog of the above analysis, for the daughter species. Beginning again in the matrix, Laplace transforming (5) and rearranging yields

display math(A12)

where we define the square-bracketed term to be inline image. The above is a second-order linear nonhomogeneous equation, which may be solved by an operator factoring approach. Using operator notation, with inline image (note this is not the Laplacian), we rewrite the problem:

display math(A13)

where we again make a simplifying definition, defining Q(z) to represent the square-bracketed term. Since inline image has an explicit integral form for any real value a, we may write down the following expression, using the indefinite integral form and arbitrary constants of integration b1 and b2:

display math(A14)

Rearranging yields:

display math(A15)

where the first term represents a particular solution to the ordinary differential equation (ODE), and the remaining terms representing the solution to the associated homogeneous equation. It is possible to evaluate the integral explicitly, but two separate cases must be considered, depending on the relative values of inline image and inline image. For the present, we will assume that they are distinct, and then modify this solution to account for the case when inline image.

A1. The Case of Distinct inline image and inline image

[21] This case will be most common. Since p is a variable on which each inline image depends, both inline image and inline image for inline image and inline image to be generally equal. If either equation is unsatisfied then there are distinct inline image and inline image, except possibly at isolated points in the p plane. In that case, this double integral in (A15) (which we may term inline image) may be determined explicitly to be

display math(A16)

Consequently,

display math(A17)

Applying the boundary conditions, it may be shown that

display math(A18)

As an aside, note the similarity of the first term to inline image. Essentially, the solution has been divided into two additive terms, the first representing the behavior of the daughter species on its own, and the second representing a source due to the decay of the mother species into the daughter species. Finally, we may use this expression in the matrix to determine the sink term for the daughter species in the fracture. We may write

display math(A19)

Substituting this into the Laplace-transformed governing equation for the daughter species in the fracture yields

display math(A20)

where we define M2 to represent the square-bracketed term. Then, substituting in for both inline image and Q(z), we derive

display math(A21)

where we may make a final simplification, defining W to represent the square-bracketed term. This leaves us with the following differential equation for inline image:

display math(A22)

[22] To solve the above, first-order, nonhomogeneous ODE is straightforward. In the case when two distinct values of inline image and inline image exist, one is justified in stipulating that inline image, since otherwise it would be necessary that inline image for some constant inline image, for all values of p. It can be shown this is not possible. Thus,

display math(A23)

for some γ. Applying the boundary condition when z = 0, (3), yields

display math(A24)

[23] We have thus derived Laplace-transformed solutions for both the mother and daughter species in both the fracture and the matrix.

A2. The Case of Identical inline image and inline image

[24] Because the solutions derived above contain terms with inline image in their denominator, we cannot directly apply them in the case when inline image. Fortunately, these terms are indeterminate ( inline image, and so L'Hospital's rule can be applied. Let us begin by considering the equation (A18), which was derived assuming inline image and inline image distinct. Defining inline image and inline image, then taking the limit as inline image

display math(A25)

yields the following solution in the matrix:

display math(A26)

[25] An identical procedure can be adopted in the fracture, beginning with (A21)

display math(A27)

leading to

display math(A28)

We may make a simplification, defining X to represent the square-bracketed term. This leaves us with the following differential equation for inline image:

display math(A29)

It is easy to show that when inline image for all p, inline image for all p. To solve the above first-order, nonhomogeneous ODE is then straightforward. The solution is

display math(A30)

for some γ. Applying the boundary condition when z = 0, (3) directly yields

display math(A31)

[26] Again, we have derived Laplace-transformed solutions for both the mother and daughter species in both the fracture and the matrix, this time for the special case where inline image.

Acknowledgment

[27] I am extremely grateful to Christopher Neville of S.S. Papadopulos and Associates, who suggested this work and also provided his implementation of Cormenzana's single fracture solution in source code form. I would not have undertaken this project without his impetus.

Ancillary

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