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~ ~ ~ . . . . . . . ~ . ~ ~ ~ ~ ~ . . ~ . ~ . Artificial Recharge of Ground Water in colorado A Statewide Assessment The sedimentary rocks of Colorado are either composed of fragments of pre-existing rocks (clastic) or formed by thc prccipitation of carbonate compounds associated with marine life. The names of the clastic sedimentary rocks are largely derived from the size of the fragments from which they are composed; these include siltstone, sandstone, and conglomerate. The common carbonate rocks are limestone and dolomite. Chemically precipitated carbonate rocks can have very low primary porosity and permeability, but secondary permeability is developed along bedding planes, fractures, and faults by dissolution and enlargement ofthese zones of weakness. Florida's carbonate aquifers produce tremendous amounts of ground water, and contain a number of "underground rivers" where a surface stream disappears and flows through caves. Colorado's carbonate aquifers are best developed in the Eagle Basin, where wells typically yield from I to 3,000 gpm. The topography over carbonate rocks, which is characterized by sinkholes, caverns, and lack of surface streams, is termed karst. AR possibilities in caves in carbonate rocks of Colorado are discussed later in this section. The major sedimentary rock aquifers in Colorado consist predominantly of sandstones and limestones of varying ages. Many of these aquifers are located in structural basins that contain multiple geologic units and aquifers such as the Denver, Raton, San Juan, Paradox, Piceance, and Sand Wash Basins. Due to their convex structure and perimeter outcrop areas, these basins represent multiple aquifer systems and wells may penetrate several geologic units. The majority of sedimentary rock aquifers within the state's structural basins are under confined conditions. The hydraulic characteristics of these aquifers vary significantly with location and depth, with well yields from 50 to 1,500 gpm. Other sedimentary rock aquifers, such as the Cretaceous Dakota and Cheyenne sandstones and the High Plains aquifer, are relatively flat lying and are present throughout large portions of the state. The Dakota-Cheyenne aquifer is most prolific in the southeast portion of Colorado, yielding from 50 to 1,000 gpm, where the aquifer is at, or near, the surface. The High Plains aquifer has been used extensively for irrigation in eastern Colorado, with well yields averaging 300 gpm. The state's major structural basins and sedimentary rock aquifers are presented in Figure VII-2. The aquifers that occupy the mountainous regions of Colorado include fractured crystalline-rock and volcanic rocks, valley-fill deposits, and intermontane park sequences. Fractured igneous and metamorphic rocks, as well as volcanic rocks, form the backbone of the states major mountain ranges and provide much of the domestic water supply in the mountainous regions. Well yields in these aquifers are typically only a few gallons per minute, and fracture porosities are less than one percent. The intermontane parks such as North and South Park, on the other hand, contain thousands of feet of sedimentary rocks that were not eroded during the uplift of the Rocky Mountains. The state's mountainous region aquifers are presented in Figure VII-3. 55