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HYDROLOGICAL DESCAI PT ION Regional Hydrogeology and Surtace Water Characteristics The Hayden Gulch Terminal, Inc.'s (HGTI) Loadout facility is situated principally (coal handling facility area and majority of the railroad spur) in the Dry Creek drainage basin (tributary to the Yampa River). A small portion of the railroad spur exists in the Yampa River drainage basin. A regional hydrological description of this area is presented in detail in Attachment 7-1. Site Specific Ground Water Characteristics The coal rail ear unloading/truck Loadout facility area of the Hayden Gulch Loadout is situated in alluvial and colluvial materials overlying the Lewis shale of upper Cretaceous age. Two alluvial/shallow Lewis shale wells were constructed in the Loadout area prior to 1993 by the pervious operator, H-G Coal Company. One well, identified as HGDALI, is located upg radient of the Loadout area and the second well, identified as HGDAL2, is located downgradient of the Loadout. During Oc[ober 1993, a new downgradient alluvial well, identified as HGDAL3, was constructed along Dry Creek within the Rail Loop to assure all potential groundwater impacts from the Loadout could be assessed. This downgxadient well was installed to alleviate CDMG concerns regarding the possibility that wells HGDALI and HGDAL2 were partially completed in the Lewis shale. The locations of the three alluvial wells are shown on Exhibit 12-2 in Tab 12. Completion information for monitor well HGDAL3 is presented in Attachment 7-2. Seasonal Variability and Trends. Tables 7-1, 7-2, and 7-3 present the water level and chemistry data (through 1995; see the Annual Hydrology Reports fox more recent data) for wells HGDALI, HGDAL2, and HGDAL3, respectively. Water level ranged from 1.6 to 8.9 feet below ground surface (bgs), 0.7 to 13.5 feet bgs, and 0.9 to 7.5 feet bgs at wells HGDALI, HGDAL2 and HGDAL3, respectively. On 5.4.95, the water level in well HGDALI was reported as -0.42 feet, which indicates that the water level in the casing was above ground surface. Generally, no water level trends or seasonality are evident at the we 11 s, however, water levels tend to be higher during spring snowmelt months which indicates that snowmelt may be recharging the alluvium. Total dissolved solids (TDS), magnesium (Mg), sodium (Na) and sulfate (SO4) concentrations in well HGDALI are generally twice those measured in Well xGDAL2. Following the completion of well HGDAL3 in November 1993, The groundwater quality data demonstrates that a similar groundwater chemistry pattern exists between HGDALI and HGDAL3 as what existed between HGDALI and HGDAL2. The TDS, Mg, Na and SOy concentrations measured in well HGDALI were generally three times the corresponding concentrations measured in well HGDAL3, however, they were generally only twice those measured in well HGDAL2. The TDS concentrations at well HGDALI ranged from 15,000 mg/1 to 60,000 mg/1 while the TDS concentrations at well HGDAL2 ranged from 2,000 mg/1 and 26,000 mg/1 and at well HGDAL3 TR-06 1 Revised OS/OS