The Tanana Basin Alluvial Aquifer

Nathan Visser

Spring 2007

GO571 Geohydrology

Dr. Marcia Schulmeister

Introduction

Alaska has more freshwater resources in the form of lakes, streams, rivers, wetlands, glaciers, and groundwater than any other state in the union (ADEC 2007). Even so, these water resources are often either undeveloped or unavailable in population centers. For example, about 16,000 of the homes across over 70 rural Native villages in the state of Alaska do not have access to piped water (Senate Report 109-170, 2005). Although about 23 percent of the freshwater used in Alaska comes from groundwater sources, 90 percent of the state's rural population depends on groundwater for domestic use. In addition, the cities of Juneau, Fairbanks, and about fifty smaller communities depend primarily on groundwater for public supplies. Most of Alaska's groundwater is pumped from only a few unconsolidated- deposit aquifers formed by glacial outwash and river sands and gravel, which are easily developed and generally yield good quality water (USGS 1998). This includes the Tanana Basin Aquifer, an unconsolidated sand and gravel deposit aquifer of the Tanana River Basin which supplies Fairbanks, Alaska and several other local communities in the Interior of Alaska with drinking water (see Figure 1). Technically the region is bounded by the watersheds of the Brooks Range to the north and the Alaska Range to the south.

Figure 1: Unconsolidated Aquifers of Alaska, Including the Tanana Basin Aquifer, Interior Alaska
(From USGS 1998)

Hydrologic Setting

Regional Climate

The Interior region of Alaska has a highly continental climate with the lowest recorded winter temperatures in the state, as well as the warmest summers. Mean annual temperatures average slightly below freezing, with individual years sometimes averaging above freezing. Minimum temperatures are strongly controlled by ground-based inversions, and thus may vary radically over short distances and in response to human modification of the local environment. Trendlines depicting the annual temperature fluctuations in Fairbanks, Alaska are shown in Figure 2. The large range of temperatures reflects the great difference between frigid weather associated with dry northerly airflow from the Arctic to mild temperatures associated with southerly airflow from the Gulf of Alaska, accompanied by winds off the Alaska Range south of Fairbanks (ACRC 2007). The extreme cold found in the region contributes to the presence of permafrost, which greatly influences the flow and presence of groundwater and will be discussed in greater detail below.

Figure 2: Temperature Summary for Fairbanks, Alaska
(From ACRC 2007)

Snow cover is persistent in Fairbanks from October through April, although snowfalls of 4 inches or more in a day occur on average only three times during winter. Blizzard conditions are almost never seen, as winds in Fairbanks are above 20 miles an hour less than 1 percent of the time. Precipitation normally reaches a minimum in spring, and a maximum in August, when rainfall is frequent (see Figure 3). Thunderstorms occur, on average, about eight days during the summer.

Figure 3: Cumulative Precipitation Summary for Fairbanks, Alaska
(From ACRC 2007)

Recharge Mechanisms

Primary recharge to the Tanana Aquifer is by water from dozens of tributaries flowing from the Alaska Range, which contribute to the aquifer by seepage through streambeds (USGS 1998). Secondary recharge occurs by rain and snowmelt, which directly infiltrate the unconsolidated alluvium to the unconfined portions of the aquifer (Wolff and Haring, 1967). Water levels in streams that flow from the Alaska Range bedrock formations onto permeable parts of the alluvium at the base of the mountains are much higher than the water table. Larger streams lose much of their flow to seepage into the aquifer, and some smaller streams lose all of their flow, seemingly disappearing as they contribute to the aquifer (USGS 1998). Regional groundwater flow in the alluvial Tanana Aquifer moves toward the Tanana River and then downstream in the same direction of flow as the river.

Surface Water

Groundwater in the Tanana Aquifer is influenced by seasonal water level changes two major rivers that dominate the surface water landscape in the region, the Tanana and Chena rivers. During the greater part of the year, for example, the water level in a reach of the Tanana River near Fairbanks is higher than that of a nearby reach of the Chena River, a tributary of the Tanana. Under these conditions, the shallower parts of the aquifer between the two rivers receive recharge from the Tanana River and discharge to the Chena River (USGS 1998). Groundwater / surface water interactions control vertical water-table changes, changing flow directions, and reversing vertical flow gradients. Several lakes and rivers in the Tanana River Basin have areas of upwelling, which consist of localized or widespread areas where water flows from groundwater back into the water column. These upwelling areas can appear as gravel-bedded springs, pockmarked areas in sandy or silty areas, or generalized flow up through gravel substrate (Durst, 2000). The local direction of shallow ground-water movement varies from the regional direction, which closely corresponds to the direction of stream flow (UAF 2007). Deep ground-water flow, however, is thought to move under the Tanana River and toward the Chena River at all times of the year.

The Tanana Basin in general is not characterized by many lakes or depression areas, although in the lowland areas of the valleys lakes and wet areas, such as the large Chena Slough, are more common. Lakes and wet areas interact with the groundwater table, which is ordinarily about 10 or 12 feet below land surface (Wolff and Haring, 1967). Deeper lakes and rivers and the circulation of groundwater cause the degradation of permafrost and limit its distribution both vertically and aerially (Anderson, 1970). Water discharges from the aquifer to springs throughout the Tanana Basin, and to the lower reaches of the Tanana River on a region-wide scale (USGS 1998).

Hydrologic Properties of the Aquifer

Geologic Description of the Aquifer

The deposits of the Tanana Aquifer consist of flood-plain alluvium near the Tanana River and its tributaries, and alluvial-fan deposits on the north flanks of the Alaska Range that borders the river basin to the south. The deposits are characterized by approximately two-thirds fine sandy material and one-third gravelly material (Wolff and Haring, 1967). Cederstrom (1963) reported that the aquifer is a complex system of alternating sands, gravel and silt, with its heterogeneity being its "oustanding characteristic." This said, thin lenses of clay and sand can be found in the moraine, although their importance in creating perched aquifers dwindles when the influence of permafrost is taken into account. Overall hydraulic conductivity of the aquifer is typical for sandy gravelly alluvial deposits, reportedly ranging regionally on the scale of 1E-3 to 1 cm/s. Moraines deposited by glaciers in valleys of the Alaska Range intermingle with the alluvium and are slightly less permeable parts of the aquifer. Bedrock underlies the alluvial aquifer, consisting primarily of folded, faulted metamorphic rocks, locally intruded by igneous rocks. The bedrock is generally dense and compact, yielding little water. Where the bedrock is fractured, however, it will yield significant quantities of water to wells. For example, numerous wells completed in bedrock in the uplands north and northeast of Fairbanks yield sufficient water for domestic use (USGS, 1998).

Although the alluvial deposits locally comprise several aquifers separated by leaky confining units of silt and clay or by layers of permafrost, they are usually treated as a single aquifer. The parts of the aquifer formed below the Alaska Range are generally unconfined and quite thick in some places, with more than 600 feet of alluvium near Fairbanks and about 550 feet near the junction of the Delta and Tanana Rivers (USGS 1998). The alluvial deposits north and east of the Tanana River occurs under both unconfined and confined conditions. This part of the aquifer is more susceptible to the influence of permafrost (ground frozen for greater than two years), which is more common north of Fairbanks. In addition, these deposits contain greater quantities of silt and clay, which are more likely to be permanently frozen than beds of sand and gravel. Unfrozen alluvium is present beneath the permafrost, however, in most parts of the aquifer. The water is generally unconfined in the higher parts of the alluvial fans and in the alluvial plains near major streams (USGS, 1998). Permafrost is generally present on the floodplain except near streams or recently abandoned stream channels, where the thermal effects of water exert a dominant control on the permafrost regimen. Typically, the depth to the permafrost table ranges from two to more than four feet feet in undisturbed areas (disturbed areas include those affected by recent forest fire). The thickness of permafrost varies greatly, although depths of 265 feet have been penetrated within the Tanana Basin. Permafrost acts as a confining layer to regional groundwater flow, and wells tapped below a continuous permafrost layer will rise to the piezometric surface (Wolff and Haring, 1967). The aquifer exists above, below, and adjacent to permafrost, and in some locations is within unfrozen zones surrounded by it. This complexity makes it difficult to predict the direction and velocity of ground water flow, as well as its seasonal and annual variability (CRREL 1998). Figure 4 shows the basic features of the Tanana Aquifer near Fairbanks.

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Figure 4: Cross-Section of Tanana Aquifer near Fairbanks
(From USGS 1998)

Chemical Characteristics of Tanana Aquifer Groundwater

The heterogeneity of the Tanana Aquifer system contributes to a wide range of water quality measurements within the Aquifer - the quality of the water can vary greatly over a distance of less than 100 feet. This is in part due to the way in which the aquifer was formed, as part of the filling in of meandering channels, which included debris of all sorts, including organic material from vegetation and even animal carcasses (Wolff and Haring, 1967). However, the chemical quality of water in the Tanana Basin aquifer is generally suitable for most uses.

The groundwater is a calcium bicarbonate or calcium magnesium bicarbonate type, and locally contains concentrations of iron and manganese that are higher than those recommended for drinking water by the U.S. Environmental Protection Agency (USGS, 1998). Wells have also been reported to become clogged due to iron oxidation. The groundwater in the aquifer is generally alkaline and moderately hard in nature. Chloride and fluoride are present, but in small quantities. Sulfate concentrations are generally low relative to the bicarbonate concentrations, due to a reducing environment created by the presence of organic material to great depths in the aquifer. The reducing environment also results in low dissolved oxygen levels. Lower iron and manganese concentrations and higher dissolved oxygen concentrations are often found in the fractured bedrock areas of the aquifer. Groundwater temperature within the Tanana Aquifer averages near freezing, at 34 degrees Fahrenheit. This may contribute to higher CO2 levels and the slightly corrosive nature of the water (Wolff and Haring, 1967).

The Tenana Alluvial Aquifer as a Water Resource

Although surface water resources exist in the Interior of Alaska, these are largely untapped for domestic use. Groundwater is, in fact, the only source of potable water used at Fort Wainwright and the Fairbanks area (Denix, 1997). Groundwater in the Tanana Aquifer, as a resource for domestic use to the people of the Interior of Alaska, is of a vast capacity. Recharge to the deep and long aquifer is dependable, and the source of much of the water is pristine. In fact, Cederstrom (1963) stated that "the limitations that might be encountered in any well developed plan that might be conceived are those imposed by factors such as economics, mechanics of well construction, and quality of water obtained, rather than those pertaining to permeability, recharge, or storage capicity of the sediments." Typical water yields today range from 30 gallons per minute (gpm) to 50 gpm in two-inch wells, and as much as 150 gpm in six-inch wells. The city of Fairbanks alone produces more than 1 billion gallons of water annually from four wells near the Chena River. Althought the water is treated to remove iron and manganese, and sodium hypochlorite is added to provide disinfection within the distribution lines, the water is generally of excellent quality (see Table 1).

Table 1: Drinking Water Quality Parameters for Fairbanks Alaska, 2007
(From Fairbanks, 2007)

Aquifer Contamination

The primary sources of contamination in the Tanana Aquifer stem from Department of Defense (DOD) sites in the Tanana Basin and contaminated sites in the Fairbanks area. The largest DOD site is Fort Wainwright, which is located just east of the city limits of Fairbanks. Approximately 15,000 people live and work at Fort Wainwright. The base obtains drinking water from wells which are regularly and extensively monitored, and results show the water meets drinking water standards. Fort Wainwright was placed on the National Priorities List of "Superfund" sites in 1990. A number of areas of contamination have been addressed through five main operable units, which have for the most part been remediated and cleaned. However, sixteen munitions response sites have recently been identified under the new Military Munitions Response Program, and a housing development begun in 2006 has stumbled across contamination ranging from polychlorinated biphenyls (PCBs) and dioxins/furans to heavy petroleum products (ADEC 2007). The complex hydrogeological conditions occurring beneath the installation have lead to considerable difficulties in locating suspected contaminants and source areas, defining transport paths, and evaluating the fate of contaminant plumes and migration off-site (Denix 1997).

Within the Fairbanks city limits, many localized plumes of contamination typical of urban areas have developed. These plumes include perchloroethylene (PCE) from drycleaners, which has reached the Chena River and is difficult to remediate due to the wide variations in groundwater flow, petroleum from junkyards and mechanics shops, and heavy metals from a recycling plant (ADEC 2007). Although much of this contamination is localized near Fairbanks, a large majority of the population in Interior Alaska receive water from the Fairbanks and University of Fairbanks Water Treatment Plants, which both use wells within the city limits of Fairbanks.

Agencies Responsible for the Protection and Cleanup of Alaska Groundwater

The primary agency involved in the protection of Alaska's water is the Alaska Department of Environmental Conservation. This agency regulates the operations of well installation and groundwater production and treatment, and also regulates the contaminated sites program. The U.S. Army also controls certain aspects of remediation within the limits of Fort Wainwright.

On the federal level, the US Environmental Protection Agency (USEPA) and the Agency for Toxic Substances and Disease Registry (ATSDR) have participated in the protection and cleanup of groundwater in Alaska.

References

Alaska Climate Research Center (ACRC). Precipitation and Temperature Data for Fairbanks, Alaska. Retrieved online at: .

Alaska Department of Environmental Conservation (ADEC). 2007. Retrieved online at:

Anderson, G.S. 1970. Hydrologic Reconnaissance of the Tanana Basin, Central Alaska. .

Cederstrom, D.J. 1963. Ground Water Resources of the Fairbanks Area, Alaska. U.S. Geological Survey Water-Supply Paper, 1590.

Cold Regions Research and Engineering Laboratory (CRREL). 1998. Geological and Geophysical Investigations of the Hydrogeology of Fort Wainwright, Alaska. Report number 98-6.

Denix. 1997. FORT WAINWRIGHT ENVIRONMENTAL CLEANUP AWARD. Retrieved online at: .

Durst, J.D. 2000. FISH USE OF UPWELLINGS. Retrieved online at:

Senate Report 109-170. 1995. .

United States Geological Survey (USGS). 1998. Ground-Water Atlas of the United States, Segment 13, Alaska, Hawaii, Puerto Rico and the U.S. Virgin Islands. Published as USGS Hydrologic Investigations Atlas 730-N.

University of Alaska Fairbanks (UAF). 2007. Ground-Water and Surface-Water Interactions, Fort Wainwright, Alaska. Retrieved online at:

Wolff, E; Haring, R.C. 1967. Natural Resource Base of the Fairbanks - North Star Borough, Alaska. University of Alaska Press, Anchorage.