GENERAL49836

• Permit No: C1981008
• IBM Index Class Name: General Documents
• Doc Date: 2/28/1989
• Doc Name: PROPOSED DECISION & FINDINGS OF COMPLIANCE FOR PR2
• Permit Index Doc Type: FINDINGS
• Media Type: D
• Archive: No

The mines are located in the Nucla-Naturita coal field. The coal is found in
late Cretaceous sediments. The youngest Cretaceous rocks in the area consist
of Mancos shale, a dark grey marine shale, present only on isolated hilltops
south of the town of Nucla. Two hundred feet of Dakota sandstone outcrops
extensively along the axis of the Nucla syncline. This formation consists of
a yellowish-gray, fissile sandstone and conglomeratic sandstone interbedded
with dark grey shale and coal. There are two coal seams within 100 feet of
the base of the Dakota (Figure 2). The lower Cretaceous unit in the area, the
Burro Canyon Formation, is separated from the Dakota by an unconformity. The
Burro Canyon is comprised of white, grey and light brown sandstones and
conglomerates interbedded with green and purplish shales, siltstones,
mudstones and thin beds of limestone. This formation has a maximum thickness
of 200' and outcrops on rims of canyons west and northwest of the mine. This
formation is difficult to differentiate from the Dakota in this area, and is
commonly considered hydroaeologically similar. The Jurassic Morrison
Formation is composed of 250'-600' of varicolored siltstones and mudstones
with beds of limestone and sandstone.
The Nucla Mine produces coal from both the upper and lower Dakota coal seams.
The upper Dakota coal seam ranges in thickness from 1.6'-3.3' and is separated
from the lower Dakota seam by 7 to 11 feet of interbedded sandstone and
shale. The lower Dakota seam varies in thickness from 5.9' - 7.9'. The base
of the lower Dakota coal is 83' above the top of the Burro Canyon Formation.
The coal dips to - 20 southwest and strikes N 25oW to N 45o W.
The Nucla East Mine will produce coal from the lower Dakota coal seam
exclusively during the initial permit term, and may utilize the upper Dakota
coal seam during subsequent permit terms. The lower Dakota coal seam at Nuc1a
East ranges in thickness from 4.2' to 6.9' and is 99' above the top of the
Burro Canyon Formation. The coal dips 4.5o SW along a strike of N 45oW.
Surface Water
The San Miguel River has its source in the San Juan Mountains. These
mountains are primarily composed of Tertiary volcanic rocks. Iorns
et. al. (1965a) has shown these waters to be of a calcium bicarbonate -
sulfate type during high flow periods. These waters contain less bicarbonate
during low flows while the calcium and sulfate concentrations increase due to
less dilution from snowmelt runoff. The San Miguel River traverses the
interior portions of a basin that is chiefly underlain by Dakota Sandstones,
the Morrison Formation and Mancos Shales. The Morrison Formation and Mancos
Shale have the greatest potential for influencing the river water chemistry.
The San Miguel River drains an area of 1,080 square miles. In addition,
approximately 15,500 acres of irrigated land lies between Placerville and
Naturita, Colorado. It is estimated that 2.8 tons of Total Dissolved Solids
(TDS) per acre are contributed to the waters annually from this area. This
results in increased levels of magnesium, potassium, sulfate and chloride.
Water quality samples taken from the San Miguel River at Naturita, Colorado
have a weighted average of 316 milligrams per liter (mg/1) TDS. Specific
conductance varies between 318 and 730 millimhos (mmhos). The mean sodium
adsorption ratio (SAR) is about 0.7 at Naturita. Water curves indicate that,
90% of the time, flow in this river exceeds 60 cubic feet per second (cfs),
while flows exceed 1000 cfs about 10% of the time. The mean discharge of the
river is 351 cfs. San Miguel River waters are suitable for domestic useage
except at low flow periods when sulfate levels are high.
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