Fact Sheet 2006–3148
U.S. GEOLOGICAL SURVEY
Fact Sheet 2006–3148
By Edward H. Moran, and Devin L. Galloway
Throughout the city of Anchorage, ground water is pumped from hydrologic units consisting of unconsolidated surficial deposits and metamorphic bedrock underlying hillside areas. The surficial deposits of gravel, sand, silt, and clay range in thickness from several feet to more than 1,500 feet below land surface (Barnwell and others, 1972). These water-bearing materials constitute two surficial aquifers overlying a bedrock aquifer and extend from the foothills of the Chugach Mountains to Cook Inlet and from the Elmendorf and Fort Richardson military bases to Potters Marsh.
The city of Anchorage, part of the municipality of Anchorage, covers about 200 square miles and extends from Ship Creek south of Eagle River, AK, to Potters Marsh just west of Girdwood, AK. About 280,000 people, more than 42 percent of Alaska’s population, live within the city of Anchorage, which accounted for about 44 percent of the State’s population growth in the 1990s (State of Alaska, 2004). Forecasts predict that Anchorage’s population will grow about 35 percent by 2020 (MOA, 2001; Williams, 2004).
Anchorage Water and Wastewater Utility (AWWU) provides locally obtained surface water and ground water to satisfy most of the city’s public water demand. The remaining water is supplied primarily from private domestic wells. In 2002, AWWU delivered on average 27.6 million gallons per day (Mgal/d) of water to more than 52,600 customers, equating to nearly 127 gallons per person per day (AWWU, 2005a). About 83 percent of the water supplied by AWWU in 2002 was surface water obtained from Eklutna Lake (79 percent) and Ship Creek (4 percent). The remaining 17 percent (4.7 Mgal/d) was obtained from ground water pumped from local aquifers (AWWU, 2005b). An additional estimated1 non-AWWU production (4 Mgal/d) pumped from private domestic wells contributed to a total of about 8.7 Mgal/d of ground water pumped from local aquifers in 2002.
1Based on 1972 estimate (USCOE, 1979) adjusted for growth.
As the demand for water increases in proportion to the expected 35-percent growth in population, by 2020, AWWU will need to supply an estimated 37 Mgal/d of water. If ground-water use increased at the same rate as population, ground water would constitute about 6.3 Mgal/d of AWWU water deliveries, and domestic pumpage would contribute to a total of about 11.7 Mgal/d of ground water pumped from local aquifers.
Anchorage has several good-quality sources of water. Eklutna Lake alone is capable of providing more than 100 Mgal/d, but is currently limited by the 35 Mgal/d capacity of the Eklutna Water Treatment Facility (AWWU, 2005a). Average flows in Ship Creek (USGS gaging station 15276000) as measured at the foothills of the Chugach Mountains below the Fort Richardson military base diversion dam were about 93 Mgal/d from 1947–2003 (USGS, 2005). As potential sources these quantities far exceed the projected 2020 demand for water.
Increasing the supply from Ekultna’s Water Treatment Facility would require substantial capital investment and would consume water currently used for electricity production. The water treatment facility could meet the expected 2020 demand; however, by 2030– 40, the expected demand likely would exceed its capacity. AWWU’s Ship Creek supply primarily is limited by the 10.5 Mgal/d annual water right and limits on withdrawals of 3 Mgal/d or less during low-flow winter periods. Because of variable flows and water quality of Ship Creek, its use has been minimized to maintain flows for aquatic and riparian habitat and to mitigate fecal coliform contamination (ADEC, 2004).
An estimated 75 Mgal/d of water on average recharges Anchorage-area aquifers (Barnwell and others, 1972). Ground water flows toward Cook Inlet from recharge areas in the Chugach Mountains and upland areas of the coastal plain deposits. Much of the flow discharges directly into Cook Inlet. Along the way, ground water is recharged by infiltration of streamflow in the upper reaches of the major watersheds, such as Ship Creek, and to a lesser degree throughout Anchorage by infiltration of precipitation through the surficial deposits. In the lowland areas of the coastal plain, ground water discharges into streams, springs, seeps, and wetlands, where it runs off, evaporates, or is transpired by plants (Barnwell and others, 1972; Patrick and others, 1989).
Under natural conditions, over the long term, a flow system is in a dynamic equilibrium, or steady state—the average discharge approximates the average recharge. The equilibrium is dynamic because recharge fluctuates seasonally and annually. Major climate shifts, changes in landscape owing to natural events, such as earthquakes or human alteration, and varying ground-water pumping rates can disrupt the equilibrium for years until a new equilibrium is established. By 1969, pumping from high-capacity wells lowered water levels more than 50 feet in the lower part of Ship Creek basin, resulting in reduced Ship Creek streamflow, and by 10 feet or more over an area of 40 square miles (Miller and Whitehead, 1999). However, ground-water levels in some areas of Anchorage have recovered to predevelopment levels owing to reduced extraction.
Poor water quality can render ground water unusable for drinking. Human activities can introduce undesirable chemicals into aquifers and increase concentrations of the nutrients nitrogen and phosphorous. Regionally, the quality of Anchorage-area ground water generally is good (Glass, 2001). In isolated areas in Anchorage, however, oil and fuel spills and waste-disposal sites have released benzene, xylenes, arsenic, chromium, fluorescein, and sulfate into the ground water (ADEC, 2005). Leachate from septic systems, a landfill, and other disposal sites have introduced coliform bacteria and higher concentrations of iron, manganese, dissolved organic carbon, and chloride in local ground water (Munter, 1987; Munter, and Maynard, 1987).
A sustainable ground-water supply largely is a subjective quantity that depends on the “acceptable” consequences and knowledge of the realized or expected outcomes of ground-water management practices. Whether the consequences and outcomes are “acceptable” is best determined by stakeholders. But choosing the best or most desirable outcomes requires knowledge of the hydrogeologic system and its responses to imposed stresses, such as ground-water pumping.
The development of water resources imposes stresses on flow systems. Poorly planned, ground-water pumping can reduce streamflow that sustains riparian ecosystems, which occurred along Ship Creek in the 1970s and 1980s. Additionally, high rates of ground-water extraction can reduce outflow to coastal areas causing landward migration of saline ground water—saltwater intrusion.
Overuse can deplete a ground-water resource and potentially cause adverse consequences. Maximizing the sustainable use of the ground-water resource in the context of conjunctive-use management can be beneficial. In Anchorage, for example, it may be more economical to substitute local ground water for some Eklutna Lake water. But, depending on the location, timing, and amount of ground-water pumping, increased extractions may be environmentally and economically unacceptable owing to:
Optimal use of a ground-water resource refers to sets of choices or management alternatives that achieve sustainability of the resource while maximizing the overall acceptability of any environmental, economic, or social consequences. For Anchorage, the choices involve the conjunctive use of ground water and surface water—how much of each to use, when, and subject to what constraints. What constitutes “maximizing acceptability of consequences”? These are subjective decisions made by stakeholders, but, ideally, these decisions are based on objective scientific evaluation of alternative outcomes.
The ability to address issues concerning future water-resources management in Anchorage depends on many factors including an understanding of the hydrologic systems. The ability to simulate or predict the short- and long-term responses of the system to imposed stresses, hypothetical or real, is improved by monitoring natural and anthropogenic induced variations in the flow system. As Anchorage grows and the demand for water reaches the capacity of the surface-water resources, good-quality ground water again will become an important source of water. Effective management of these water resources will require:
Hydrologic data collection and analysis can lead to improved understanding of the ground-water flow system. The development of a comprehensive hydrogeologic database that reflects long-term natural and anthropogenic stresses can be used to assess future sustainable management strategies (Alley and others, 1999). These data would include lithologic, geophysical, and hydraulic properties of hydrogeologic units, ground-water levels, and water-level maps for the principal aquifers, ground-water quality, and streamflow characteristics indicative of ground- and surface-water interaction.
Numerical ground-water flow models, such as MODLFOW (Harbaugh, 2005), are important tools that increase an understanding of ground- and surface-water systems. Models constrained by available hydrologic data can simulate periods of historical ground-water development and a means to forecast the consequences associated with future water-use requirements (Reilly and Harbaugh, 2004).
Implementing a strategy or policy for sustainable use of ground-water resources is a complex task. Stakeholders in developed ground-water basins have used simulation-optimization modeling techniques to determine optimal alternatives related to conjunctive-use management problems (Ahlfeld and Mulligan, 2000; Ahlfeld and others, 2005) by coupling stakeholder-defined water-resources objectives and constraints with a physically-based model of the ground-water flow system. Simulation-optimization modeling could facilitate the sustainable use of the combined ground- and surface-water resources in the Anchorage area.
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Send questions or comments about this report to the author, Edward H. Moran, (907) 786-7000, and Devin L. Galloway, (916) 278-9549.
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