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Groundwater Quality in Piceance Creek Basin by G.J. Saulnier Jr. Duke Engineering & Services, Austin, Texas September 7, 1999 Introduction The groundwater quality in the Piceance Creek Basin vanes vertically and areally and is dependent on the geologic unit in which water is found, the local structural geology, and manmade disturbances. The groundwater in Piceance Creek Basin occurs in three principal water-bearing units: in alluvial deposits along Piceance and Yellow Creeks (Welder and Saulnier, 1974); in the Upper Aquifer which consists of the Uinta Formation and that part of the Parachute Creek Member of the Green River Formation between the kerogen-rich Mahogany Zone and the Uinta Formation; and the Lower Aquifer which includes the Parachute Creek Member from the Mahogany Zone to either the base of the formation or to the top of intact saline minerals (Weeks et al., 1974; Welder and Saulnier, 1974; and Saulnier 1978). The lower portion of the Upper Aquifer is known as the "A Groove"; and the upper portion of the Lower Aquifer is referred to as the "B Groove". These zones are lean fractured marlstones which erode easily and, on outcrop, forms indentations above and below the Mahogany Zone. The lower boundary of the Lower Aquifer is called the "Dissolution Surface" and represents the lowest point to where groundwater has dissolved and removed saline minerals from the formation. The part of the Lower Aquifer representing the stratigraphic horizon from which soluble minerals have been dissolved is referred to as the "leached zone" (Welder and Saulnier, 1978). The presence of soluble minerals in the Parachute Creek Member can locally alter groundwater quality if and where flowing groundwater encounters these minerals. The quality of groundwater in Piceance Creek Basin, as reflected in the concentration of total dissolved solids, varies over the basin from fresher water, with dissolved-solids concentrations of 400 to 800 mg/L in the higher elevations and recharge areas of Piceance Creek and its major tributaries, to saline groundwater in the extreme north, groundwater-discharge part of Piceance Creek Basin where dissolved-solids concentrations greater than 30,000 mg/L can be found (Saulnier, 1978). Saline groundwater occurs where there is natural or artificial dissolution of soluble minerals in the Green River Formation. This process contributes dissolved sodium, bicarbonate, and chloride to the groundwater. The following sections describe the water quality in each of the aquifer units in Piceance Creek Basin. Alluvial Aquifer A significant alluvial aquifer is found along Piceance Creek and to a lesser degree alluvial groundwater occurs along Yellow Creek. Weeks et al. (1974) modeled Piceance Creek as the principal drainage in Piceance Creek Basin and Sentember 7. 1999