1988-04-11_PERMIT FILE - C1981008A ‎(15)‎

• Permit No: C1981008A
• IBM Index Class Name: PERMIT FILE
• Doc Date: 4/11/1988
• Doc Name: Probable Hydrologic Consequences
• Section_Exhibit Name: Tab 17
• Media Type: D
• Archive: Yes

Model Design. Two models were developed. The model of the overburden was designed
to examine long-term, far-field effects due to mining, and does not consider the shallow
overburden materials (except as a recharge source), or Tuttle and Calamity Draws. The
model for the coal represents only the coal aquifer.
The hydrogeologic system was represented in each model as a confined, one-layer,
homogeneous unit. The generalized ground water flow direction was along the secondary
model axis from northeast to southwest. The San Miguel River was defined as a
constant-head boundary along the downgradient edge (Figure 17-1). The three other model
boundaries represented no-flow conditions. The mine pit was represented as a rectangular
set of grid cells 2400 ft x 150 ft (360,000 ft2) in the center of the model grid. These
dimensions were based on the approximate pit width, and the greatest length of pit
expected during the five-year period simulated. Figure 17-2, a model grid overlay, is
provided to orient the reader to the actual scale locations of the Nucla East mining area
and the San Miguel River.
The drain option in MODFLOW was used to simulate face seepage and dewatering of the mine
pit. The ten model cells defining the pit were designated as drain cells. The DRAIN
package was used to simulate the pit for two reasons. First, it allowed the pit to be
modeled as a constant-head boundary for discharge of water, but did not allow the pit to
become a constant-head recharge boundary. Second, it allowed for separate accounting of
the water discharged into the pit. The drains maintained predetermined water elevations
for the duration of a given stress period (length, 1 year). The elevations of the drains,
and hence the drawdown in the pit, was changed on a yearly basis, based on projections of
the depth of the pit beneath the existing water table.
Water was added to the model using two methods: the RECHARGE package and the General Head
Boundary package. The recharge value used represents the long-term recharge to the unit
being modeled, and is not indicative of the recharge to the shallow ground water system
during the irrigation season. This shallow recharge appears to drain rapidly into Tuttle
and Calamity Draws. The model thus represents the deeper overburden, and may
underestimate the amount of discharge into the pit during the irrigation season.
The General Head Boundary package was used to simulate the increase in recharge caused by
drawdown during the drawdown calculations. This package calculates leakage on the basis
of the difference in head in the aquifer and in the beds providing recharge, and the
17-10 Revised 04/11/88