Ground Water in the Anchorage Area, Meeting the Challenges of Ground-Water Sustainability

Present and projected water use Ground water is an important component of Anchorage’s water supply. During the 1970s and early 80s when ground water extracted from aquifers near Ship Creek was Anchorage Water and Wastewater the principal source of supply, area-wide declines in ground-water levels resulted Utility (AWWU) provides locally obtained in near record low streamflows in Ship Creek. Since the importation of Eklutna Lake surface water and ground water to satisfy water in the late 1980s, ground-water use has been reduced and ground water has most of the city’s public water demand. contributed 14– 30 percent of the annual supply. As Anchorage grows, given the The remaining water is supplied primar- current constraints on the Eklutna Lake water availability, the increasing demand ily from private domestic wells. In 2002, for water could place an increasing reliance on local ground-water resources. The AWWU delivered on average 27.6 million sustainability of Anchorage’s ground-water resources challenges stakeholders to gallons per day (Mgal/d) of water to more develop a comprehensive water-resources management strategy. 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 hroughout the city of Anchor- A growing municipality water obtained from Eklutna Lake (79 Tage, ground water is pumped from percent) and Ship Creek (4 percent). The The city of Anchorage, part of the hydrologic units consisting of unconsoli- remaining 17 percent (4.7 Mgal/d) was municipality of Anchorage, covers about dated surficial deposits and metamorphic obtained from ground water pumped from 200 square miles and extends from Ship bedrock underlying hillside areas. The local aquifers (AWWU, 2005b). An addi- Creek south of Eagle River, AK, to Potters surficial deposits of gravel, sand, silt, tional estimated1 non-AWWU production and clay range in thickness from several Marsh just west of Girdwood, AK. About (4 Mgal/d) pumped from private domestic feet to more than 1,500 feet below land 280,000 people, more than 42 percent of wells contributed to a total of about 8.7 surface (Barnwell and others, 1972). Alaska’s population, live within the city of Mgal/d of ground water pumped from These water-bearing materials constitute Anchorage, which accounted for about 44 local aquifers in 2002. two surficial aquifers overlying a bedrock percent of the State’s population growth As the demand for water increases aquifer and extend from the foothills of in the 1990s (State of Alaska, 2004). in proportion to the expected 35-percent the to and Forecasts predict that Anchorage’s popula- from the Elmendorf and Fort Richardson tion will grow about 35 percent by 2020 1Based on 1972 estimate (USCOE, 1979) military bases to Potters Marsh. (MOA, 2001; Williams, 2004). adjusted for growth.

1970 2000 Elmendorf and Fort Richardson Military Bases Sh ip Creek ALASKA USGS Gaging Station 15276000

0 20 MI Eagle River Eklutna Lake Cook Inlet City of Anchorage 1.24% 0.56% Girdwood 1.01% 5.39% 0.87% 9.67% Municipality of Anchorage 1.26% 2.12% Potters 22.8% Marsh 42.83%

Chugach Mountains 62.45% 0.84% 46.79% Urban Coastal Markon, 2003 LAND USE Vegetated Roads 2.17% Wetlands Barren Urban growth has changed land-use patterns significantly in the city of Anchorage since Lakes/Ponds 1970. Wetlands and vegetated areas in the city have been replaced by urban development.

U.S. Department of the Interior Fact Sheet 2006–3148 U.S. Geological Survey December 2006 Surface water POPULATION Ground-water availability Ship Creek (Mgal/d) Municipality of Anchorage 300 WATER USE Eklutna Lake (Mgal/d) City of Anchorage An estimated 75 Mgal/d of water Ground water on average recharges Anchorage-area 30 Wells (Mgal/d) 200 aquifers (Barnwell and others, 1972). Ground water flows toward Cook Inlet 20 from recharge areas in the Chugach Moun- tains and upland areas of the coastal plain 100 (Mgal/d) no data deposits. Much of the flow discharges

WATER USE*, 10 directly into Cook Inlet. Along the way, POPULATION, IN THOUSANDS ground water is recharged by infiltration IN MILLION GALLONS PER DAY 0 0 6‘02‘9‘59 ‘65‘60‘29‘39 ‘70 ‘75 ‘80 ‘85 ‘90 ‘95 ‘00 of streamflow in the upper reaches of the Year major watersheds, such as Ship Creek, and *AWWU, 2005a and 2005b and USGS, 1987 to a lesser degree throughout Anchorage Water use in Anchorage keeps pace with a growing population. by infiltration of precipitation through the surficial deposits. In the lowland areas of growth in population, by 2020, AWWU diversion dam were about 93 Mgal/d from the coastal plain, ground water discharges will need to supply an estimated 37 Mgal/d 1947–2003 (USGS, 2005). As potential into streams, springs, seeps, and wet- of water. If ground-water use increased sources these quantities far exceed the lands, where it runs off, evaporates, or is at the same rate as population, ground projected 2020 demand for water. transpired by plants (Barnwell and others, water would constitute about 6.3 Mgal/d Increasing the supply from Ekultna’s 1972; Patrick and others, 1989). of AWWU water deliveries, and domes- Water Treatment Facility would require Under natural conditions, over the tic pumpage would contribute to a total substantial capital investment and long term, a flow system is in a dynamic of about 11.7 Mgal/d of ground water would consume water currently used for equilibrium, or steady state—the aver- age discharge approximates the average pumped from local aquifers. electricity production. The water treat- ment facility could meet the expected recharge. The equilibrium is dynamic 2020 demand; however, by 2030– 40, the because recharge fluctuates seasonally and Surface-water availability expected demand likely would exceed its annually. Major climate shifts, changes in landscape owing to natural events, such Anchorage has several good-qual- capacity. AWWU’s Ship Creek supply as earthquakes or human alteration, and ity sources of water. Eklutna Lake alone primarily is limited by the 10.5 Mgal/d varying ground-water pumping rates can is capable of providing more than 100 annual water right and limits on with- disrupt the equilibrium for years until a Mgal/ d, but is currently limited by the drawals of 3 Mgal/d or less during low- new equilibrium is established. By 1969, 35 Mgal/d capacity of the Eklutna Water flow winter periods. Because of variable pumping from high-capacity wells low- Treatment Facility (AWWU, 2005a). flows and water quality of Ship Creek, ered water levels more than 50 feet in the Average flows in Ship Creek (USGS its use has been minimized to maintain lower part of Ship Creek basin, resulting gaging station 15276000) as measured at flows for aquatic and riparian habitat and in reduced Ship Creek streamflow, and by the foothills of the Chugach Mountains to mitigate fecal coliform contamination 10 feet or more over an area of 40 square below the Fort Richardson military base (ADEC, 2004). miles (Miller and Whitehead, 1999). How- ever, ground-water levels in some areas of Most ground-water recharge occurs through Anchorage have recovered to predevelop- alluvial deposits near the Chugach foothills. ment levels owing to reduced extraction. Chugach Mountains SOUTH Poor water quality can render ground water unusable for drinking. Human activities can introduce undesir- able chemicals into aquifers and increase a re a C concentrations of the nutrients nitrogen rge ampbell Cr. ha rec and phosphorous. Regionally, the quality pal nci City of Anchorage Cook Inlet Pri S of Anchorage-area ground water gener- Metamorphic h ip Unconfined ally is good (Glass, 2001). In isolated C r. Confined areas in Anchorage, however, oil and Sedimentary fuel spills and waste-disposal sites have bedrock Bootlegger Cove Formation Clay/silt released benzene, xylenes, arsenic, chro- mium, fluorescein, and sulfate into the Ground water is pumped Sand/ ground water (ADEC, 2005). Leachate from the surficial aquifers gravel and the bedrock aquifer. from septic systems, a landfill, and other Modified from disposal sites have introduced coliform Miller and Whitehead, 1999 bacteria and higher concentrations of iron, Both unconfined and confined ground- water discharges into Cook Inlet. manganese, dissolved organic carbon, and chloride in local ground water (Munter, NORTH 1987; Munter, and Maynard, 1987). <10 1. Consequences associated with water resources, good-quality ground Net water-level decline 10 to 20 reduced flows to streams, wetlands, water again will become an important in the confined aquifer 20–30 between 1955 and and Cook Inlet; source of water. Effective management of 30–40 March 21, 1969, in feet these water resources will require: 40–50 2. Costs associated with constructing >50 new wells, rehabilitating old ones, and 1. Further knowledge of the hydrogeol- High-capacity well extracting deeper ground water; and ogy, sustainability, and interconnec- tivity of Anchorage’s ground-water 3. Costs incurred for purifying poor- and surface-water systems; quality ground water. Cr. Ship Optimal use of a ground-water 2. Understanding the consequences of ground-water depletion; and Cook Inlet resource refers to sets of choices or City of Anchorage Chester Cr. management alternatives that achieve 3. A set of effective water-resources sustainability of the resource while management objectives and con- Cr. maximizing the overall acceptability straints and optimal conjunctive-use l l e of any environmental, economic, or alternatives. b p m social consequences. For Anchorage, the Hydrologic data collection and Ca choices involve the conjunctive use of analysis can lead to improved understand- ground water and surface water—how ing of the ground-water flow system. The Chugach Mountains much of each to use, when, and subject to development of a comprehensive hydro- 0 2 4 MI what constraints. What constitutes “maxi- geologic database that reflects long-term Miller and Whitehead, 1999 0 4 KM2 mizing acceptability of consequences”? natural and anthropogenic stresses can be These are subjective decisions made by used to assess future sustainable manage- stakeholders, but, ideally, these decisions ment strategies (Alley and others, 1999). Ground-water sustainability are based on objective scientific evalua- These data would include lithologic, A sustainable ground-water sup- tion of alternative outcomes. geophysical, and hydraulic properties of ply largely is a subjective quantity that hydrogeologic units, ground-water levels, depends on the “acceptable” conse- Meeting the challenges of and water-level maps for the principal aquifers, ground-water quality, and quences and knowledge of the realized ground-water sustainability or expected outcomes of ground-water streamflow characteristics indicative of management practices. Whether the The ability to address issues con- ground- and surface-water interaction. consequences and outcomes are “accept- cerning future water-resources manage- Numerical ground-water flow able” is best determined by stakeholders. ment in Anchorage depends on many models, such as MODLFOW (Harbaugh, But choosing the best or most desirable factors including an understanding of 2005), are important tools that increase outcomes requires knowledge of the the hydrologic systems. The ability to an understanding of ground- and sur- hydrogeologic system and its responses simulate or predict the short- and long- face-water systems. Models constrained to imposed stresses, such as ground-water term responses of the system to imposed by available hydrologic data can simu- pumping. stresses, hypothetical or real, is improved late periods of historical ground-water The development of water resources by monitoring natural and anthropogenic development and a means to forecast imposes stresses on flow systems. Poorly induced variations in the flow system. the consequences associated with future planned, ground-water pumping can As Anchorage grows and the demand for water-use requirements (Reilly and Har- reduce streamflow that sustains riparian water reaches the capacity of the surface- baugh, 2004). ecosystems, which occurred along Ship Creek in the 1970s and 1980s. Addition- ally, high rates of ground-water extraction Water budget can reduce outflow to coastal areas caus- A water budget defines the amount of Mgal/d discharged to local streams ing landward migration of saline ground inflow and outflow in a hydrologic unit. flowing to Cook Inlet (Patrick and others, water—saltwater intrusion. Prior to urban development, about an 1989). Overuse can deplete a ground-water average 275 Mgal/d of water moved PRE-DEVELOPMENTWATER BUDGET resource and potentially cause adverse through the surface- and ground-water consequences. Maximizing the sustain- systems in the city of Anchorage. Inflow Ground Water able use of the ground-water resource in Streamflow and precipitation accounted 5% 36% the context of conjunctive-use manage- for 95 percent and ground-water inflow Precipitation ment can be beneficial. In Anchorage, from adjacent bedrock and other outly- Streams for example, it may be more economi- ing areas contributed 5 percent. Stream 59% cal to substitute local ground water for discharge and ground-water flow to Outflow some Eklutna Lake water. But, depend- Cook Inlet accounted for 85 percent Evapotranspiration ing on the location, timing, and amount of the outflow and evapotranspiration Ground Water 15% 22% of ground-water pumping, increased accounted for the remaining 15 percent. extractions may be environmentally and Of the estimated 75 Mgal/d of pre-devel- 63% Streams economically unacceptable owing to: opment recharge to local aquifers, 15 Simulated effects of ground-water withdrawals flow system. Simulation-optimization modeling could facilitate the sustainable on the Anchorage ground-water flow system use of the combined ground- and surface- Some of the potential effects of sustained levels. Simulated ground-water levels water resources in the Anchorage area. pumping on the predevelopment ground- recovered to near pre-pumping condi- Edward H. Moran, water system were simulated using a tions in 7 years following the cessation of USGS, Anchorage, Alaska numerical ground-water flow model (MOD- simulated pumping. The ability to calibrate Devin L. Galloway FLOW) calibrated to pre-1955 conditions the historical response of the flow system USGS, Sacramento, California (Patrick and others, 1989). The model simu- to ground-water development during the lated about 21 years of pumping at a rate period 1955-84 was limited by the available of about 19 Mgal/d from wells distributed hydrogeologic data. throughout the basin, which was based on pumping distributions estimated during the Simulated drawdown and recovery in the aquifer 1970s-80s. Although the simulated effects system at a location between Ship Creek and of pumping are hypothetical and uncali- , southeast of the city of Anchorage brated, the model demonstrated decreased Recovery ground-water discharge to streams and 0 Cook Inlet, increased recharge from 8 streams to the shallow aquifer, increased Shallow Aquifer 16 leakage of water from the shallow aquifer Deep Aquifer Drawdown, in feet in Drawdown, to the deep aquifer, and 20-30 ft draw- surface land below 24 Pumping downs of pre-pumping ground-water 0 8 16 24 32 Years

Implementing a strategy or policy tives related to conjunctive-use manage- for sustainable use of ground-water ment problems (Ahlfeld and Mulligan, resources is a complex task. Stakeholders 2000; Ahlfeld and others, 2005) by cou- in developed ground-water basins have pling stakeholder-defined water-resources Eklutna Lake lies in the Chugach Mountain used simulation-optimization modeling objectives and constraints with a physi- Range and receives much of its recharge from techniques to determine optimal alterna- cally-based model of the ground-water Eklutna Glacier.

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