PERMFILE121316

• Permit No: C1982056
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: EXHIBIT 39 MINE INFLOW ESTIMATES
• Media Type: D
• Archive: No

Getty Task 2 Page 2 March 1, 1983
• 2.0 AfODEL DESCRIPTION
The finite difference model utilized in this study is a modified version
~ of program GRWATER developed at Colorado State University. The model is
1' capable of handling confined or unconfined conditions and can also simulate
the special case when a confined agaifer becomes unconfined. Using known
initial conditions, the model solves for the distribution of heads in
horizontal (z,y) plane is response to stresses imposed oa the aquifer.
To use the model, the area to be modeled is overlain with a system of
grids. It is assumed that the physical and hydraulic properties within each
grid can be defined with single values which represent the average value for
the grid. The model uses the backward difference method to solve the finite
difference form of the groundwater flow equation. Formulation of the
coefficient matriz is based on a balance of mass, i.e, the sum of flows plus
changes in storage for all grids most be equal to zero. The total number of
grids for the system is equal to the number of grid rows (NR) multiplied by
the number of grid eolmm~s (NC). A subroutine in the program develops the
coefficient matriz, which is a square, pentadiagoaal matriz containing (NR -Z)
z (NC - 2) equations and unknowns. This system of linear equations is solved
by Gauss elimiatioa. Solutions are obtained at discrete time steps using the
known conditions at the end of the previous time step as initial co aditions
i for the current time step.
Physical and hydraulic data that describe the area to be modeled must be
supplied to the program. These data are stored in computer memory as
I numerical arrays. Aa array D% (containing NC elements) is used to store the
grid spacing along the columns of the grid system while array DY (NR elements)
is used to specify the row spacing. Ia addition to horizontal space
dimensions, the model requires the following input data:
FB - hydraulic conductivity (ft/day)
Z - elevation of the lower impermeable boundary (ft)
G - elevation of The upper impermeable boundary (ft)
~. S - confined aquifer storage coefficient
SY - unconfined aquifer apparent specific yield
OC - net recharge (+) or discharge (-) (acre-ft/day)
H - initial piezometric head elevation (ft)
• Boundary conditions must also be known and are specified by special coding of