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('I - , ') I 0 UU.L"''1 '- GROUND \,ATER In March 1978, the Bureau of Reclamation requested the Geological Survey to conduct a study of the water supply potential of the Laramie-Fox Hills aquifer in lower Coal Creek Basin in Adams, Boulder, Jefferson, and Weld Counties. The water would be used to supplement the needs for the communities of Louisville, Lafayette, and Erie, as the town of Superior presently relies entirely on individual wells. The study was 1 imited to the Laramie-Fox Hill s aquifer because it is the major aquifer in the area wi th the most potential for ground-water develop- ment. Calculations indicate that there are about 5 million acre-feet of water in storage in this aquifer. . Yields from 91 wells ranged from 2 to 120 gallons per minute, and averaged 18 gallons per minute. Generally, well yields increased comparably with an increase in aggregate sand thickness. The thickness of the aggregate sand ranged from 15 to 467 feet, and averaged 212 feet. These yields did not necessarily reflect the maximum that could be obtai ned in the area. Most of the exi sti ng well s were constructed for domestic use, and with proper construction and larger capacity pumps, increased yields could be obtained. Water samples were analyzed from 14 wells and total dissolved solids ranged from 171 to 2,910 mg/l and hardness from 8 to 2,000 mg/l. Environmental Protection Agency maximum or recommended limits were exceeded in water from 12 of the 14 wells by at least one of the following: nitrate, selenium, fluoride, dissolved solids concentra- tions, iron, manganese, and sulfate. Based on the 5 million acre-feet of water available in the Laramie-Fox Hills aquifer, there is adequate water to supply the annual demand for 8,000 acre-feet of additional water. However, areas of concern include (1) the possibility that the aquifer may be tributary to Boulder Creek, (2) the required pumpage will exceed the natural recharge to the aquifer and ground-water mining will occur, (3) the potential well yields are low, and (4) the chemical Quality of the water may require treatment if it is to be used as a domestic and municipal water supply. The above consi derati ons tend to rul e out ground water as a vi ab 1 e alternati ve water supply, especially on a large-scale basis. WATER QUALITY The Boulder County (Marshall) landfill is located approximately 1/2-mil e northwest and approximately 120 feet in el evati on above the Coal Creek streambed at the proposed Coal Creek Reservoir site. When the moisture content of the buried refuse in landfills reaches satura- tion, the soluble components are leached into the underlying soil and ground water. Polychlorinated biphenyls (PCBs) have been found in surface water samples taken on the landfill (EPA letter, March 20, 1980). Other priori ty poll utants detected in surface water sampl es coll ected from 27