Transcript Groundwater flow modeling of an abandoned mine lands site scheduled for reclamation Robert C.

Groundwater flow modeling of an abandoned
mine lands site scheduled for reclamation
Robert C. Waddle, CGDA and Department of Geological Sciences, Indiana University
Greg A. Olyphant, CGDA and Department of Geological Sciences, Indiana University
A tool to evaluate probable outcomes of reclamation design
Minnehaha
abandoned
mine lands
site
Potentially
unstable levee
and the
contribution
of AMD
Scheduled for
reclamation
by the IDNRDOR
seep
What should reclamation accomplish?
Current goals of IDNR-DOR
• Want to re-direct AMD away from seep for on-site
treatment
• Want to lower the water table (levee stability)
Do both while minimizing volume of sediment disturbance
Why use a groundwater flow model for AML
reclamation?
Cost effective way to preview hydrologic outcomes
under different reclamation scenarios
Modeling Approach
• Fully 3-dimensional, transient model,
incorporates effects of local topography
• Treats the groundwater system as a whole
(specifically the unsaturated-saturated zone as
a continuum)
• Any configuration of boundary conditions
Theory: Darcy’s Law and Mass Balance
Freeze (1971)
K = hydraulic conductivity
C = specific moisture capacity
θ = volumetric moisture content
ψ = pressure head, where h = ψ + z
ρ and ε are the density and compressibility of water
η and β are the porosity and compressibility of the porous media
Unsaturated Hydraulic Parameters
van Genuchten (1980)
effective saturation
van Genuchten characteristic
equation parameters:
K0  n θs θr and m=1-1/n
L assumed to be ½
hydraulic conductivity
specific moisture capacity
Solution Method
Implicit finite-difference
approximation
Block-centered nodal grids with
variable size
Surface boundary condition
driven by time-dependent
meteorological data
Freeze (1971)
upper boundary (ground surface)
142.3 m
125.6 m
129.3 m
∆x = ∆y = 5.0m
∆z = 1.0m, 0.5m
Total grid cells = 2,408,000
lower boundary (clay)
124.2 m
Monitoring wells, sediment types, and boundary conditions
Calibration Procedure
Weather driven transient simulations
• Initial K0 estimates obtained from slug and pumping test
data and van Genuchten parameters from literature
• Started with most sensitive parameter (K0) then proceed
with other parameters
• Vary hydraulic parameter values until minimum RMSE
(simulated and observed water levels) for a period of 50
days
Calibration Results
Model Parameters
K0 (cm/s)
α (cm-1)
n
θs
θr
RMSE (m)
1.0E-03
0.035
3.18
0.375
0.053
0.505
Model Parameters
K0 (cm/s)
α (cm-1)
n
θs
θr
RMSE (m)
5.0E-4
0.035
3.18
0.375
0.053
0.296
Model Parameters
K0 (cm/s)
α (cm-1)
n
θs
θr
RMSE (m)
5.0E-04
0.025
2.25
0.46
0.065
0.195
Resolved flow at water table: current
Proposed Valley Network
Main segment follows
existing ditch
Three additional
branches extending
toward levee
Graded at 1:500
down to Mud Creek
elevation (NW)
35,000 cubic meters
sediment excavation
Resolved flow at water table: valley network
Difference in pre and post reclamation water tables
Fine-grained refuse average water table
pre: 130.7 m
post: 129.7 m
Conclusion
• Study shows that an appropriate GW model can
predict the probable outcomes of reclamation
experiments
• In this case, it appears that a simple construction
implementation could accomplish the goals of
reclamation as specified by the IDNR-DOR
• Currently exploring additional applications of this
model to the hydrology of reclamation sites.
QUESTIONS?
Acknowledgements
Funding for this project was provided through a contract with the Indiana
Department of Natural Resources – Division of Reclamation. The
contributions of Dr. Sally Letsinger (GIS analysis), John T. Haddan (field
work) Center for Geospatial Data Analysis, Indiana Geological Survey are
especially appreciated.
Blackfoot
Example Models
• MODFLOW (USGS): 2D (quasi-3D), heterogeneous, saturated,
numerical
• GFLOW: 2D, homogeneous, saturated, analytical element model
• Freeze (1971): fully 3D, heterogeneous, sat/unsat, numerical
References
Freeze, R. A. 1971. Three-dimensional, transient, saturated-unsaturated flow in a
groundwater basin. Water Resources Research 7(2), 347-365.
van Genuchten, M. T. 1980. A closed-form equation for predicting the hydraulic
properties of unsaturated soils. Soil Science Society of American Journal 44, 892898.
Schaap, M.G., J.L. Feike, and M.T. van Genuchten. 2000. Estimation of the soil
hydraulic properties. In: Looney, B.B., Falta, R.W. (Eds.), Vadose Zone: Science and
Technology Solutions, vol. 1. Battelle Press, Columbus, OH, pp. 501-509.
Equations
 
   
   

 

K
(

)

K
(

)

K
(

)

1

C
(

)


x 
x  y 
y  z 
t
 z

𝜕
𝜕𝛹
𝜕
𝜕𝛹
𝜕
𝐾 𝛹
+
𝐾 𝛹
+
𝐾 𝛹
𝜕𝑥
𝜕𝑥
𝜕𝑦
𝜕𝑦
𝜕𝑧
𝜕𝜃 𝑚𝑛𝛼 𝑛 ∙ 𝜃𝑠 − 𝜃𝑟 ∙ 𝛹
𝐶 𝛹 =
=
𝜕𝛹
1 + 𝛼𝛹 𝑛 2
𝑛−1
𝜕𝛹
+1
𝜕𝑧
1
1 + 𝛼𝛹
𝜌𝜃 𝛹
=
𝜀 + 𝜂𝛽 + 𝜌𝐶 𝛹
𝜂
𝑚−1
𝑛
𝜕𝛹
𝜕𝑡
More Equations
𝜃 𝛹 − 𝜃𝑟
1
𝑆𝑒 =
=
𝜃𝑠 − 𝜃𝑟
1 + 𝑎𝛹
𝐾 𝛹 = 𝐾0 𝑆𝑒
𝐿
1 − 1 − 𝑆𝑒
𝑚
𝑛
2
1 𝑚 𝑚
𝜕𝜃 𝑚𝑛𝑎𝑛 ∙ 𝜃𝑠 − 𝜃𝑟 ∙ 𝛹
𝐶 𝛹 =
=
𝜕𝛹
1 + 𝑎𝛹 𝑛 2
𝑛−1
1
1 + 𝑎𝛹
𝑚 −1
𝑛
K = 10-3 cm/s
n = 3.18
α = 0.035 cm-1
θs = 0.375
θr = 0.053
Tailings
Gob
Soil "clayey"
Theta_s (poros)
0.46
0.42
0.48
Theta_r
0.065
0.053
0.09
alpha
0.025
0.03
0.011
n
2.25
1.9
1.53
K0
5.00E-04 7.50E-03
1.00E-05
Resolved flow at water table: valley network