Geology and ground-water resources of the Anchorage area, Alaska

Water Supply Paper 1773

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The Anchorage area, at the head of Cook Inlet in south-central Alaska, occupies 150 square miles of a glaciated lowland and lies between two estuaries and the Chugach Mountains. Two military bases are in the area; Anchorage is the largest city in Alaska and the chief transportation center for this part of the State. The bedrock in the Anchorage area is chiefly Tertiary shale in the lowland and metamorphic rocks of Mesozoic age beneath the adjacent mountain slopes. Glacial drift which underlies nearly the entire area has an average thickness of several hundred feet and appears to include at least five sheets of deposits, two of which are exposed. The drift consists of till, outwash stream and lake deposits (sand and gravel), and estuarine (and lake) deposits (clay and silt). The stratigraphy and lateral distribution of the deposits are complex, but data at hand s, how that the thickest deposits, including all the estuarine and lake sediment and most of the stream-deposited sediment, are beneath the lowland away from the mountain wall, and that the deposits near the mountains are till and subordinate outwash sediments. Deposits of sand and gravel laid down by outwash streams in channels and on outwash plains are the most important aquifers, and the only ones which yield large quantities of ground water from single beds. Thin layers of sandy or gravelly material in till are also important aquifers although they yield relatively small quantities of water. Bedded sand and silt associated with the estuarine and lake(?) clay commonly becomes unstable during drilling and pumping, and has been successfully developed in only a few wells. Unconfined aquifers are extensive, but permeable saturated material is thin in many places and water supplies available from them are small or undependable in those places. The most important aquifers are confined or artesian. Clay and till form the confining beds: the till is somewhat 'leaky' in many places. Near Anchorage the buried water-bearing beds appear to be interconnected and to form a single artesian system. The water table and piezometric surface slope from the mountain wall of the lowland toward the estuaries, and the flow of the ground water is in that direction. The aquifers are recharged by the infiltration of precipitation at the land surface and of surface water through stream beds: near the mountains the artesian aquifers are probably recharged in part by percolation from the water-table aquifer, and far from the mountains the water-table aquifer is probably recharged in part by upward flow from the underlying artesian aquifers. In several valleys and in a few other places, in the lowland, artesian wells flow at the land surface. The outwash sand and gravel are moderately to very permeable; most of the other water-bearing material are much less permeable. The co- efficient of transmissibility for some single beds of sandy gravel is as high as 60,000 to I00,000 gpd per ft (gallons per day per foot); for the entire section of glacial drift at and near Anchorage it is believed to be of the order of 200,000 gpd per ft. Calculations based on this value for the total section and on the slope of the piezometric surface indicate that in the immediate vicinity of Anchorage about 5 million gpd flows through each mile-wide section of the drift (measured in a northeast-southwest direction, perpendicular to the direction of flow), under normal (nonpumping) conditions. Under conditions of continuous heavy pumping the slope of the piezometric surface is steepened, flow is increased, and additional recharge is induced. The highest yield reported from a well in this area is 2.600 gpm (gallons per minute) with 35 feet of drawdown: the highest reported specific capacity is 180 gpm per ft of drawdown, for a well pumped at. 270 gpm. Only a few wells in the area have been developed for high yields. Well screens have been used

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USGS Numbered Series
Geology and ground-water resources of the Anchorage area, Alaska
Series title:
Water Supply Paper
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U.S. Govt. Print. Off.,
vi, 108 p. :illus., maps (1 col.) diagrs., tables. ;24 cm.