Modeling Salt Redistribution in Fractured Porous Media Caused by

1

Modeling Salt Redistribution in Fractured Porous
Media Caused by Convection Driven Evaporation
Within the Fracture Christopher Graham1 Maria
Ines Dragila1, Clay Cooper2, Noam Weisbrod3 (1)
Department of Crop Soil Sciences, Oregon State
University, USA (2) Desert Research
Institute(3) Department of Environmental
Hydrology Microbiology, Institute for Water
Sciences Technologies, BIDR, Ben-Gurion
University of the Negev, Israel
METHODS
RESULTS
FURTHER RESULTS and CONCLUSIONS
INTRODUCTION

• Effect of Permeability Modification
• Permeability reduction greatly reduces cumulative
  evaporation. 100 day cumulative evaporation
  decreases 56 with a reduction of permeability
  from 1E-13 to 1E-15 m2.
• Salt flux to the fracture is similarly reduced,
  with a 88 reduction in salt flux over the 100
  day simulation.
• Matrix permeability clearly has a large impact on
  evaporation and salt flux dynamics.

Simulation Fracture Salt Total Evaporation
(Y/N) g/L of Control
Control N 0 100.0
Fracture Y 0 120.5
Low Salt Y 15 116.6
High Salt Y 100 100.1

• Objective
• Numerical investigation of solute redistribution
  within the porous matrix caused by evaporation
  from a fracture in the matrix.

Permeability 100 Day Total Evap Average Daily Evap Solid Salt Precipitation Near Fracture
(m2) (kg) (mm/day) ( Pore Space)
1E-13 1.96 0.65 3.10
1E-14 1.13 0.38 0.81
1E-15 0.85 0.28 0.35

• Evaporation Dynamics
• Evaporation rate and total evaporation were
  enhanced by the presence of the fracture.
• Daily evaporation rate and total evaporation were
  reduced with increasing solute concentration.
• Water content decreased with increased proximity
  to fracture, with greater decrease seen with
  reduced salt concentration

• Justification
• Evaporation from fractures that are open to the
  atmosphere will lead to salt crusting along the
  surface which can then either close the fracture
  or be flushed down to the aquifer during rain
  events.

Table 1 Simulation initial conditions and
results.
Table 1 Permeability simulation results.

• Questions investigated
• How does evaporation from fractures and salt
  concentration affect total soil evaporation?
• How does evaporation from fracture affect salt
  redistribution?

Figure 3 Physical model based on Ritchie and
Adams (1972) weighing lysimeter. 5 cm slice
represents entire lysimeter due to symmetry.
Full depth is modeled.
Cumulative Evaporation (kg)
Water Content

• Boundary Conditions
• Lower Constant Head (Gravity Drainage)
• Upper Constant Head (Atmospheric Boundary)
• Right No Flow (Lysimeter edge)
• Upper Left Constant Head (Fracture Atmospheric
  Conditions)
• Lower Left No Flow (for Symmetry)

• Model Details
• Soil lysimeter is modeled using a representative
  5 cm slice, 60 wide and 120 cm deep.
• Hydraulic properties of Houston Black Clay are
  modeled using the van Genuchten curves using
  parameters for clay from Carsel and Parrish.
• Lower boundary is constant head, allowing for
  gravity drainage, right and left boundaries are
  no flow, for symmetry. Upper boundary and
  fracture are constant head, representing a well
  mixed atmosphere.

A
B
C
Time (days)
Distance from fracture (m)
Figure 8 Cumulative evaporation for 100 day
simulations, with and without fracture and with
and without salt.
Figure 7 Water content with distance from
fracture at end of 100 day simulations, with and
without fracture and salt
Figure 12 Cumulative evaporation for 100 day
simulations, with varying matrix permeability.
Figure 13 Total solid phase salt precipitation
over time with varying matrix permeability
MODEL CALIBRATION AND VALIDATION

• Salt Flux
• Salt content in the 1 cm nearest the fracture
  increased over 300 over the 100 day simulations.
• A larger increase of 350 was seen in the low
  initial salt concentration simulation (not
  shown).
• Salt flux is a combination of positive advective
  salt flux and negative diffusive flux.
  Throughout the simulation advective flux
  overwhelms diffusive flux, though net flux
  decreases throughout the simulation.
• Flux is greatest nearest the fracture at early
  time, reversing later in the simulation.

• Conclusions
• 100 day cumulative and daily evaporation
  increased with presence of fracture.
• Fracture evaporation caused increased drying at
  all depths in soil column.
• Increased pore salinity reduced daily and
  cumulative evaporation.
• Reduced soil permeability decreased evaporation
  and salt flux.
• Salt levels increased near the fracture during
  course of simulations due to transport with water
  driven by matric potential gradient caused by
  evaporation.

• Laboratory Data
• While the EWASG module of TOUGH2 has been shown
  to accurately model saturated and unsaturated
  water and salt flux, and solid phase salt
  precipitation, its capabilities regarding
  evaporation are untested.
• A small scale evaporation experiment was
  constructed and simulated using the EWASG module
  of TOUGH2.
• A hydrometer cylinder was filled with saturated
  sand and allowed to evaporate for 80 days.
• Simulated results closely matched experimental
  data with minor modifications to the standard
  modeling procedures.

Figure 2 A) Fracturing and salt precipitation
driven by evaporation in clay soil. B) Extreme
salt precipitation on sandstone surface. C)
Salt precipitation on sandstone and chalk
surfaces.

• Implications
• While the fracture increased evaporation by less
  than the amount predicted by Ritchie and Adams
  (1974), this amount can have a significant impact
  on agricultural water balances, especially in
  arid ecosystems, which account for one third of
  the Earths landmass.
• Enhanced evaporation due to soil fractures has
  implications on water balance for agricultural
  lands as well as natural arid ecosystems such as
  shallow seasonal lakes in arid environments.
• Salt on the surface of fractures lies in
  preferential flow paths to the subsurface. In
  areas such as the Negev Desert of Israel, salt
  filled rock fractures penetrate meters deep into
  the subsurface, potentially creating preferential
  flow paths to the aquifer. This situation
  creates the possibility of salt contamination of
  subsurface aquifers.

F. Botts

• Numerical Simulator TOUGH2
• TOUGH2 is a three dimensional, multiphase,
  multicomponent, finite difference porous media
  simulator capable of handling severe permeability
  differences between elements.
• The EWASG module of TOUGH2 is designed to
  simulate water, saline and air fluxes in porous
  media. EWASG models salt in both the aqueous and
  solid phases.

Distance from fracture (m)
Figure 4 Simulation and experimental results.
First stage evaporation duration and second stage
evaporation rates are accurately modeled.

• Evaporation and Salt Diffusion
• Evaporation and Salt Diffusion are modeled as
  Fickian Diffusion between elements, according to
  a soil matrix dependent version of Ficks Law

• Moisture and Advective Salt Flux
• Soil pore water flux is governed by Darcys Law,
  and advective salt flux is a function of this
  flux

Figure 5 Scale and hydrometer lab setup

• References
• Carsel, R.F., and R.S. Parrish. 1988. Developing
  joint probability of soil water retention
  characteristics. Water Resources Research
  24755-769.
• OHara, S. L. 1997 Irrigation and land
  degradation implications for agriculture in
  Turkmenistan, central Asia. Journal of Arid
  Environments 37(1)165-179
• Pruess, K., C. Oldenburg, and G. Moridis. 1999.
  TOUGH2 User's Guide, Version 2.0 Lawrence
  Berkeley National Laboratory, Berkeley.
• Ritchie, J.T., and J.E. Adams. 1974. Field
  measurement of evaporation from soil shrinkage
  fractures. Soil Science Society of America
  38131-134.
• van Genuchten, M.T. 1980. A closed form equation
  for predicting the hydraulic conductivity of
  unsaturated soils. Soil Science Society of
  America 44892-898.
• Weisbrod, N., M. Dragila, C. Graham, and J.
  Cassidy. 2005. Evaporation from fractures exposed
  at the land surface impact of gas-phase
  convection on salt accumulation Dynamics of
  Fluids and Transport in Fractured Rock. American
  Geophysical Union, submitted August 2004.

• Field Data
• Based on soil fracture and lysimeter described in
  Ritchie and Adams (1974).
• Lysimeter is filled with Houston Black Clay
• Soil lysimeter has cross sectional area of 1.83 m
  x 1.83 m and depth of 1.2 m.
• fracture is 50 cm deep, with surface width of 5
  cm, running from corner to corner of lysimeter.

f diffusive flux f porosity t0matrix
dependent tortuosity T saturation dependent
tortuosityD diffusion coefficientDX change in
mass fraction salt between elements.
F advective fluxk(q) relative permeabilityA
cross sectional areaX mass fraction salt in
elementDy change in matric potentialDx
distance between elements
Figure 6 Water content iso lines Ritchie and
Adams field data (left), and TOUGH2 numeric
simulation (right).