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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />For the most part, the upper 50 to 200 feet of the San Luis Valley fill <br />is an unconfined aquifer. The water table in about one-half of the basin is <br />less than 12 feet below the land surface, but it ranges up to 300 feet deep. <br />Below about 200 feet, interbedded volcanic flows (generally in the south- <br />western part of the Valley) and clay layers serve as aquitards which, to <br />varying degrees, confine the deeper ground water. The clay beds are discon- <br />tinuous, particularly in the northern part of the Valley, and allow some <br />hydraulic connection. Piezometric levels in almost all of the confined <br />aquifer are higher than the land surface and free flowing artesian wells <br /> <br />are common. <br /> <br />Recharge to the confined aquifer is primarily from leakage from surface <br />streams which flow out of the surrounding mountains and across the alluvial <br />fans around the Valley perimeter. Absence of the clay beds and the presence <br />of coarse sand and gravel in the fans cause streams leaving the mountains to <br />lose significant portions of their flows. Ground water movement is down dip <br />toward the center of the structural basin and slightly southward into New <br />Mexico. Natural discharge from the confined aquifer is through springs and <br />through upward leakage to the unconfined aquifer. <br /> <br />Natural recharge to the unconfined aquifer occurs through infiltration <br />of snowmelt and direct precipitation on the Valley floor, leakage from sur- <br />face streams, and upward leakage from the confined aquifer. In addition, the <br />unconfined aquifer also receives considerable recharge through percolation <br />of irrigation water, from leakage of irrigation distribution canals, and <br />from uncontrolled flows of artesian wells in the confined aquifer. Since <br />irrigation began in the San Luis Valley, these manmade sources of recharge <br />have caused the water table to rise in much of the unconfined aquifer from <br />depths of 50 to 100 feet to within 5 to 20 feet of the land surface [20]. <br />Ground water movement generally is toward the Rio Grande and other surface <br />streams in the southern part of the Valley and toward the "sump area" in the <br />Closed Basin part of the Valley. A ground water divide lies to the north <br />and roughly parallel to the Rio Grande mainstem. The rise in the water <br />table to near the land surface has changed many streams, including the upper <br />reaches of the Rio Grande, from leaking streams which contribute to ground <br />water storage to receiving streams which collect ground water. <br /> <br />~ <br />~ <br />.0 <br /> <br />27 <br />