REGRESSION
  CELL_IDS
    1
  /
END

#=========================== numerical methods ================================
NUMERICAL_METHODS TRANSPORT

  LINEAR_SOLVER
    SOLVER DIRECT
  /

END

#=========================== discretization ===================================
GRID
  TYPE STRUCTURED
  NXYZ 1 1 1
  BOUNDS
    0.d0 0.d0 0.d0
    1.d0 1.d0 1.d0
  /
END

#=========================== fluid properties =================================
FLUID_PROPERTY
  DIFFUSION_COEFFICIENT 1.d-9
END

#=========================== material properties ==============================
MATERIAL_PROPERTY soil1
  ID 1
  POROSITY 0.25d0
  TORTUOSITY 1.d0
END

#=========================== output options ===================================
OUTPUT
  PERIODIC_OBSERVATION TIMESTEP 1
END

OBSERVATION
  REGION all
END

#=========================== times ============================================
TIME
  FINAL_TIME 25.d0 y
  INITIAL_TIMESTEP_SIZE 1.d0 s
  MAXIMUM_TIMESTEP_SIZE 2.5d-1 y
END

#=========================== regions ==========================================
REGION all
  INFINITE
END

#=========================== transport conditions =============================
TRANSPORT_CONDITION background_conc
  TYPE ZERO_GRADIENT
  CONSTRAINT_LIST
    0.d0 initial_constraint
  /
END

#=========================== condition couplers ===============================
# initial condition
INITIAL_CONDITION
  TRANSPORT_CONDITION background_conc
  REGION all
END

#=========================== stratigraphy couplers ============================
STRATA
  REGION all
  MATERIAL soil1
END
