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Common Aquifer Materials Well-sorted sedimentary material (alluvium of the South Platte River) Poorly sorted sedimentary material (Dawson, Denver, and Arapahoe Aquifers) Fractured crystalline rocks {Pikes Peak Granitel Water movement in the subsur- face cannot be seen or measured with the certainty that surface flow can, and thus its occurrence is not as precisely understood. This aspect of ground water also pro- duces many misconceptions; many people think of it in the form of underground lakes and streams. Most ground water occurs as water filling pore spaces between rock grains in sedimentary rocks or in crevices such as fractures and faults in crystalline rocks. Open- ings that exist in rock and soil, such as pore spaces between grains of sand and silt, between particles of clay, or along fractures in crys- talline rock, represent a tremen- dous volume when taken in aggre- gate. These water-filled pores or fractures represent the zone of sat- uration, which man has tapped for water supply since early civiliza- tion. The top of the zone of satura- tion is termed the water table. Colorado Geological Survey ROCKTAlKVol. 5, No.4 ,.. Soluble rock forming material (leadville limestonel Some materials have a greater ability to store and transmit water than others. The amount of water a material can hold depends upon its porosity-the ratio of void space to total volume. Geologic units con- sist of either unconsolidated sedi- ments or consolidated rock. Porosi- ty in granular deposits such as sands or gravel may exceed 40 per- cent of the total rock volume, while fractured, crystalline rock porosity may be one percent or less. The size and degree of interconnection of those openings, or permeability, determine the materials' ability to transmit fluid. The most produc- tive aquifers in the world are com- posed of unconsolida ted sand and gravel and cavernous carbonate rocks. An aquifer is a ground-water reservoir composed of geologic units that are saturated with water and sufficiently permeable to yield water in usable amounts to wells WATER AND ROCKS Arecent article in the Rocky Mountain News (September 14,2002, p. 25A) described a survey the paper conducted of water users in the Denver metro- politan area. The newspaper reported that the residents within the 44 water districts of the seven counties around Denver used an average of 74,410 gallons per per- son during 2001. Along with the 2.5 million residents of the Den- ver area, Colorado also needs water for the other 1.8 million people in the rest of the state, and its industrial and agricultural uses. We now know that much of our state is in its fourth year of drought, and that reservoirs that provide water to the Denver area are at about 50 percent of capaci- ty as we go into the winter sea- son. When we look at all of these facts together, it is no surprise that everyone is talking about water in Colorado this year. But what does geology have to do with water? Why are the scientists at CGS writing about water? In fact, geology is extremely relevant to water, and good understanding of geology con- tributes to a better understanding of our water quality and quantity. Such knowledge can, in turn, lead to better ways to protect, con- serve, store and manage water. Consider the geology of any water system, surface or under- ground, as the vessel that contains the water. The shape, size, and physical and chemical characteristics of that vessel, or the rocks holding the water, play from the director continued on p. 15 J