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' <br />' <br />u <br />� <br />, <br />' <br />' <br />' <br />' <br />� <br />� <br />' <br />' <br />u <br />' <br />' <br />i� <br />' <br />finer grained clays and sands into the central and eastern portions of the valley. This top few <br />thousand feet of valley fill comprises the ground water aquifers that provide water for the <br />thousands of wells that have been drilled and are utilized for agricultural, municipal, and <br />domestic water supply purposes. This aquifer system includes what is commonly called the <br />unconfined and confined aquifer. These aquifer systems are described in more detail in sections <br />1.4.1 through 1.4.3. <br />1.4.1 General Description of Aquifer System <br />The upper few thousand feet of this aquifer consists of an unconfined (ground water table) and <br />multiple confined (artesian) aquifers. The active and usable portion of this aquifer system is not <br />known with certainty and may consist of as much as the top 3,000 to 4,000 feet. Below these <br />depths the weight of overlaying alluvial fill compacts the sands, gravels and clays so yield of <br />water to wells decreases significantly. Water quality also decreases significantly at the deeper <br />depths due to its long resident time in contact with rock material providing an opportunity for the <br />water to dissolve minerals. Poor quality water is also found at shallower depths near the center <br />of the valley where ground water has resided for long periods of time in buried marsh and lake <br />bed environments which contain organic materials. <br />1.4.2 Unconfined Aquifer System <br />An unconfined aquifer is defined as a saturated zone where the water level in a well penetrating <br />it, will not rise above the shallowest adjacent ground water surface. Through out most of the <br />area of the San Luis Valley the depth of the unconfined aquifer extends from 50 to 100 feet <br />below land surface. However, in the southeast portion of the valley, along the outer edges of the <br />valley and along streams and rivers flowing into the valley, the unconfined aquifer can extend to <br />depths of hundreds of feet. <br />1-5 <br />' <br />