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NEW MEXICO GEOLOGICAL SOCIETY- TWENTy-SECOND FIELD CONFERENCE 131 TABLE 1. Summary of the hydrologic character of the geologic units, San Luis Valley, Colorado. Ii HYDROLOGIC CHARACTER Transmissivity ranges from 1,000 to 250,000 gallons per day per foot (134-33,500 square feet per day). Specific yield is estimated to be 0.20. Transmissivity ranges from 1,500 to 1,500,000 gallons per day per foot (201-201.000 square feet per day) in zones tapped by existing wells. Stor- age coefficient is estimated to be 0.008. Water is under pressure. Not tapped by wells. Prob- ably Dot water bearing. HYDROLOGIC UNIT Unconfined aquifer THICKNESS (FEET) 0-200 PHYSICAL CHARACTER Unconsolidated clay, silt, sand, and gravel. WATER SUPPLY Yields as much as 3,000 gallons per minute to wells. ~ I SYSTEM OR SERIES GEOLOGIC UNlT Holocene to Oligocene Valley fill Confined aquifer Unconsolidated clay, silt, sand, and gravel interbedded with volcanic flows and tuffs. Yields as much as 4,000 gallons pe' minute to wells. 50-30,000 None. Precambrian Crystalline rocks Granite, gneiss, and schist. tering the valley and precipitation on the vaHey floor pro- vide recharge to the unconfined aquifer. Upward leakage from the confined aquifer is also a source of recharge. Dis- charge from this aquifer is by wells, evapotranspiration, and seepage to streams. The principal source of recharge to the confined aquifer is seepage from mountaiu streams that flow across the allu- vial fans flanking the valley floor. The "clay series" is ab- sent at the edge of the valley, permitting recharge to beds that constitute the confined aquifer in the main part of the valley. The mountain streams show significant losses as they cross the porous surface of the fans. For example, seepage measurements made July 6, 1967, on Deadman Creek south of Crestone (northeast part of vaHey) showed that the 7 cfs (cubic feet per second) measured at the can- yon mouth was completely dissipated within about 8 miles; all but I cfs was lost in the first 3.7 miles. The confined aquifer underlies most of the valley and the water has sufficient head to flow at the land surface in an area of approximately 1,400 square miles. The major discharge from the confined aquifer is by wells, springs, and upward leakage through the confining beds into the unconfined aquifer. A small amount may discharge as underflow into New Mexico. The quality of water in the confined aquifer generally is better than that in the unconfined aquifer according to Powell (1958). The concentration of dissolved solids in 41 samples from the confined aquifer ranged from 70 to 437 mg/I (milligrams per liter) and, in 271 samples from the unconfined aquifer, ranged from 52 to 13,800 mg/1. The least mineralized water in the unconfined aquifer occurs on the west side of the valley_ The mineral concentration increases toward the sump area of the closed basin prob- ably because of solution of aquifer materials and by evapora- tive concentration in areas of a shaHow water table. DEVELOPMENT OF WATER SUPPLIES The principal source of water for irrigation in the San Luis Valley between 1880 and 1950 was surface water. A large network of canals was built in 1880-90 to irrigate lands in the eastern and central parts of the closed basin. By 1915 most of the area around Mosca and Hooper (cen- tral part of valley) became waterlogged because of this irrigation. Drainage systems constructed between 1911 and 1921 to reclaim watedogged lands aHeviated some of the problems but created waterlogging in areas downgradient. Other areas are intentionally waterlogged in the process of subirrigation. The subirrigation practice continues because locally it is considered to be essential to successful growth of crops. The number of large-capacity wells (yield more than 300 gallons per minute) in the San Luis Valley has more than quadrupled during the last 20 years. By the end of 1969 there were about 2,920 large-capacity irrigation weHs in the valley_ Of this total, about 2,270 tap the uncon- fined aquifer and about 650 tap the confined aquifer. Of the 650 large-capacity irrigation wells tapping the confined aquifer, 94 range in depth from 1,000 to 2,000 feet, and 19 are over 2,000 feet deep. Most of these deep wells flow, some exceeding 3,000 gallons per minute. In addition to the large-capacity wells, there are more than 7,000 small- capacity wells, most of which provide water for domestic and stock use. Ground-water withdrawal for recent years averaged about 750,000 acre-feet per year. Withdrawal by large- capacity irrigation wells was about 450,000 acre-feet per year and withdrawal by small-capacity wells was an es- timated 300,000 acre-feet per year. In 1970 the unconfined aquifer accounted for 78 percent of the ground water with- drawn from large-capacity wells. Many of the small capacity wells are uncontrolled and flow continuously. Perhaps 150,- 000 acre-feet per year from these wells might be considered waste. It does not contribute to crop production but causes additional waterlogging. The annual water use of the valley is substantial. Despite the abundant supply, water-use practices over the past 100 years have created water problems. Surface-water use has resulted in the waterlogging of large areas of the valley. The valley-fill deposits in the northern part of the valley r -:":-: ,- n \j.... NV,.Jf:..