Water resources appraisal of the Anchorage area, Alaska

Abstract

At the present, water use in the Anchorage area amounts to about 21 mgd (million gallons per day); of this amount ground water accounts for about 10 mgd. By 1980, 60 mgd may be required to meet the demand. The greatest potential problem is overpumping the ground-water reservoir resulting in excessive declines in water levels, which, in turn, might lead to salt-water intrusion. A well-laid plan for conjunctive use of surface and ground water seems to be the most promising means of supplying the expected need of 60 mgd, and of preventing saltwater intrusion of the aquifers. Total inflow into the Anchorage hydrologic system amounts to about 180 mgd. Of this amount Ship and South Fork Campbell Creeks contribute about 130 mgd, North Fork Campbell Creek and other streams contribute an estimated 24 mgd, and precipitation and ground-water inflow contribute an estimated 26 mgd. Of the total outflow, which must be equivalent to the inflow, Ship and Chester Creeks contribute about 94 mgd, Campbell Creek about 25 mgd, and ground-water pumpage contributes an assumed amount of approximately 10 mgd. The difference of 50 mgd between inflow and outflow presumably is accounted for; by submarine discharge beneath Cook Inlet and evapotranspiration. Surface and subsurface storage of excess stream discharge during periods of high flow can overcome the problem of water shortages during periods of greatest demand. Surface storage can be accomplished by construction of an additional dam on Ship Creek. Natural ground-water storage can be supplemented by spreading techniques to increase the amount of ground-water recharge. Additional recharge can be provided by returning air-conditioning water to the aquifer through recharge wells. Recharge along the coastline would be a means of maintaining a fresh-water barrier against salt-water intrusion. The area with the greatest potential for ground-water development is along Ship Creek east and north of Mountain View. The alluvial fan east of Mountain View seems favorable for installation of deep wells; and withdrawal of ground water in this area is not likely to result in salt-water intrusion. Similar favorable conditions exist in the alluvial fan areas of North and South Forks Campbell Creek. Infiltration galleries in alluvial deposits along Ship Creek are a relatively inexpensive and convenient means of withdrawing water. To plan for orderly and economical development of the Anchorage area's water resources, geologic and hydrologic studies are needed. An expanded network of stream-gaging stations and observation wells is needed. Deep wells near the coastline are needed to monitor any changes in chemical quality of ground water that would indicate impending salt-water intrusion. Borehole geophysical studies and pumping tests are needed to define the boundaries and hydraulic characteristics of the aquifers. The primary goal of these and other supplementing studies would be to provide the information needed to construct an electric-analog model of the Anchorage hydrologic system. Such a model would provide a means of assessing quantitatively alternative methods of water development.