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Introduction <br />the San Juan and Sangre de Cristo mountains. Previous authors and investigations have offered water <br />budgets for the valley. These estimates have historically been contested and were therefore not <br />incorporated into this study. <br />The San Luis Valley is part of the Rio Grande depression, anorth-trending series of intermontane <br />basins extending from Texas to central Colorado. Despite its apparent simplicity and lack of <br />topographic relief, the subsurface geology of the valley is complex and not yet fully understood. In <br />general terms, the valley is called a graben, a geologic term meaning adown-dropped block of the <br />earth's crust, bounded on the west side by the San Juan Mountains and on the east side by a major <br />fault at the edge of the Sangre de Cristo range. Over many millions of years the valley has continued <br />to grow in depth, while at the same time filling with sediments and layers of volcanic rock shed from <br />the surrounding mountains and carried to the valley by the Rio Grande, the Conejos River, and other <br />streams. <br />Over time, the pore spaces between the grains of sediments, fractures, and other openings in the <br />volcanic rocks beneath the valley floor have filled with water. These saturated sedimentary and <br />volcanic rock layers comprise the aquifers of the valley, from which numerous wells draw water. <br />Ground water continues to recharge the pore spaces in the aquifers by percolation from surface <br />streams, leakage through canals, and recharge from the mountains surrounding the valley along some <br />of the more permeable rock layers. <br />This complex, interconnected aquifer system is, in many areas, in hydrologic connection with the <br />surface water system. The aquifer system is generally comprised of a shallow unconfined aquifer and <br />a deeper, confined aquifer. The geometry and characteristics of both major aquifers are controlled by <br />the geologic structure and stratigraphy (layering) that have developed over geologic time in the <br />valley. <br />The uppermost water-saturated layer of sand and gravel, down to a depth of about 100 feet across <br />most of the valley, is the unconfined aquifer. Below the unconfined in the central part of the valley <br />are a number of clay layers that serve to separate, although not totally disconnect, the unconfined <br />aquifer from deeper water-bearing layers of sand, gravel, and fractured volcanic rocks. The deeper <br />layers, of which there are many, together make up the confined aquifer because of the overlying and <br />confining clays. Water flows from many wells completed in the confined aquifer due to natural <br />artesian pressure. <br />The volume of storage in the aquifer system of the valley is significant. However, not all of this water <br />is recoverable, and in some locations and at selected depths the quality may limit its uses. The <br />unconfined aquifer functions similar to a surface reservoir with a pattern of rising levels in the spring <br />and early summer caused by recharge from streams, canals, and early irrigation season return flows. <br />These increases are followed by a decline as the streamflow decreases and ground water is pumped <br />largely for agricultural purposes. <br />Agricultural activities account for more than 85 percent of basin water consumption with an estimated <br />638,000 acres under irrigation (HRS 1987). The primary crops are potatoes, carrots, small grains, and <br />alfalfa. <br />Along the edges of the valley there is little, if any, separation between the confined and the <br />unconfined aquifers. This allows both aquifers to be recharged with water from runoff entering the <br />San Luis Valley. However, the relationships between the two aquifers and between the aquifers and <br />a454/report/fmaUintro.doc 07/2&~OS 1-Y <br />