PERMFILE100217

• Permit No: M2002004
• IBM Index Class Name: Permit File
• Doc Date: 7/8/2002
• Doc Name: Response to Preliminary Adequacy Review APPENDIX G H I J
• From: GCC Rio Grande Inc
• To: DMG
• Media Type: D
• Archive: No

The mine and plant site is located on the northeastern dip slope of a low hogback ridge dissected
by north-trending arroyos. The ridge consists of gently dipping Upper Cretaceous limestone and
shale, covered by a thin mantle of unconsolidated colluvial materials and soils.
Robson and Banta (1995) describe the complex factors affecting the fate of precipitation
(meteoric waters) in nonmountainous azeas. The low precipitation rate, combined with relatively
low relief, porous soils and general aridity, causes most precipitation to be retained as soil
moisture, a high percentage of which evaporates or is transpired by plants. Recharge from
precipitation is extremely variable and is affected by aquifer depth, thickness and permeability of
soils and geologic units, slope and aspect, precipitation intensity and duration, temperature, wind,
and relative humidity. Recharge from precipitation can range from zero to several inches, and
might occur only during the slow melting of a winter snowpack or during an extended period of
winter rainfall, when excess soil moisture is maintained. The conditions at the site more often
result in low soil moisture with little or no recharge.
Regional Characterization
The principal regional aquifer in eastern Colorado is the High Plains aquifer (Quaternary and
Tertiary age). The High Plains aquifer is a shallow aquifer system underlying portions of
Colorado, New Mexico, Texas, Oklahoma, Kansas, Nebraska, South Dakota, and Wyoming, and
providing the principal source of water in one of the major agricultural areas of the U.S. The High
Plains aquifer consists of near-surface deposits of unconsolidated or partly consolidated gravel,
sand, silt or clay. Alluvial deposits and Tertiary formations are included (Robson and Banta
• 1995). The Southern High Plains aquifer is isolated from the Northern High Plains aquifer by the
erosion associated with the formation of the Arkansas River valley in southeast Colorado (Aiken
et al. eds. 2000).
Unconsolidated alluvial deposits only form part of the High Plains aquifer where they are
hydraulically connected laterally or vertically to Tertiary deposits. Portions of the Arkansas River
alluvial aquifer in eastern Prowers County, Colorado, downstream of the mine and plant site, are
hydraulically connected to Tertiary deposits (Aiken et al. eds. 2000).
The unconfined alluvia] aquifer associated with the Arkansas River and its tributaries, described
by Aiken et al. eds. (2000), is a principal aquifer in southeastern Colorado. Alluvial groundwater
within the Arkansas River basin moves toward the surface drainages in a downstream direction,
and eastward down the river valley. Alluvial deposits are generally less than three miles wide at
the river and attain a maximum thickness of 250 feet along the valley floor. Alluvial deposits
along tributary streams are even narrower, and can be 0 to 50 feet thick. The water table typically
is 30 feet or less below the land surface along the Arkansas River and its tributaries in Colorado.
The depth to groundwater is related to topography, and is generally shallower in low-lying areas.
The alluvial aquifer is in hydraulic connection with the Arkansas River and its tributaries.
Withdrawals of alluvial groundwater (tributary groundwater, as classified by the State of
Colorado) have impacted base flows of the Arkansas River and its tributaries. During part of the
year, aquifer discharges to the river cease due to extensive withdrawals by irrigation wells, and
streams losing water to the aquifer (Aiken et al. eds. 2000). Irrigation return flows affect the
quality of surface water and alluvial groundwater, contributing nitrates and elevated
concentrations of total dissolved solids (TDS) (Cain 1985).
The Cretaceous Dakota-Cheyenne Group aquifer, also known as the Great Plains Aquifer System,