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1\' -.J A 1'.J ~,\I4.,.. S.ff?.A+Pt ..'.,~ ~~(j: Water Quality The chemical quality of ground water is greatly influenced by the type of rock contacted. Salinity content (dissolved solids or DS) is one of the largest water quality constraints to ground water develop- ment in the Region. Generally, sedimentary rocks in the Region such as shales, limestones, siltstones, and sandstones yield DS ranging from under 1,000 p"rts per million (ppm) to more than 35..,_QOO ppm. Sedimen- tary formations are primarily"ufolind in physiographic basins such as the Green River, Great Divide, Washakie, Piceance Creek, and San Juan basins. It is in these regions that highly saline ground water may be found. Deeper ground water generally has a higher DS content than water in shallower aquifers because it has been in contact with saline formations longer. Formations such as the Paradox (found in the Paradox Valley in Colorado and the Spanish Valley and Canyon Lands in Utah) or parts of the Green River Formation (in the vicinity of Rock Springs, Wyoming) are extensive sources of very saline water. 1 Fresh water usually occurs in shallow aquifers where the land sur- face is above 7,000 feet in altitude, or in permeable sandstones such as the Navajo and Dakota formations (Price and Arnow, 1974). Alluvial aquifers generally yield the best quality water, but some alluvial ground water may be highly saline as a result of hydrologic intercon- nections with saline rocks. In many cases, fresh water found in the Region is separated from saline formations by relatively impermeable rock structures such as the Mahogany'Zone in the Piceance Creek Basin. " ;<::i::;~' In the Piceance Basin, the quality of the ground water is fairly well known. The hydrologic system consists generally of an upper aquifer, above the confining Mahogany Zone, and a lower aquifer below the Mahogany Zone. Most water in the upper aquifer contains less than 2,00 mg/l of dissolved solids; the water in the lower aquifer con- tains as much as 30,000 mg/l of dissolved solids in the northern part of the basin, as well as undesirable quantities of flouride and boron throughout the basin. Various wells and test holes show that methane and hydrogen sulphide gases exist in some places (Welder, 1978) . 1. Water salinity is classified as follows: fresh--less than 1,000 mg/l DS; slightly saline--l,OOO to 3,000 mg/l DS; moderately saline--3,000 to 10,000 mg/l DS; very saline--10,000 to 35,000 mg/l DS; brine--more than 35,000 mg/l DS. <;. :~: )-28