Prepared in cooperation with the Municipality of Anchorage 

Nitrate Source Indicators in Ground Water of the 

Scimitar Subdivision, Peters Creek Area, 

Anchorage, Alaska 

Water-Resources Investigations Report 00-4137 

Peters Creek 

Study Area 

U.S. Department of the Interior 

U.S. Geological Survey 
Nitrate Source Indicators in Ground Water of the 

Scimitar Subdivision, Peters Creek Area, 

Anchorage, Alaska 

By Bronwen Wang, Pat M. Strelakos, and Brett Jokela


__________________________________________________________________


U.S. GEOLOGICAL SURVEY


Water-Resources Investigations Report 00-4137


Prepared in cooperation with the


MUNICIPALITY OF ANCHORAGE


Anchorage, Alaska 

2000 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director 

Use of trade names in this report is for identification 

purposes only and does not constitute endorsement 

by the U.S. Geological Survey. 

For additional information:


District Chief


U.S. Geological Survey


4230 University Drive, Suite 201


Anchorage, AK 99508-4664


http://ak.water.usgs.gov


Copies of this report may be purchased from:


U.S. Geological Survey


Branch of Information Services


Box 25286


Denver, CO 80225-0286

CONTENTS 

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Background and Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Hydrogeology of the Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Nitrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Waste-Water Indicators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Elevation Survey. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Water Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Water Chemistry Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

Major and Minor Ions, and Nitrate Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

Redox and Nitrate Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

Boron Isotopes and Chlorofluorocarbons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

Nitrate and the Ground-Water System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

Appendix 1. Well-elevation data and characteristics for selected wells in the Scimitar Subdivision . . . . . . . . . 23 

FIGURES 

1. Map showing location of wells studied in Scimitar Subdivision, Peters Creek area, 

Anchorage, Alaska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

2. Diagram of simplified nitrogen cycle, showing sources of nitrogen contamination of ground water . 5 

3. Trilinear diagram for well water samples taken from the Scimitar Subdivision . . . . . . . . . . . . . . . . . 10 

4. Map showing spatial distribution of wells in group A and group B and nitrate distribution in the 

Scimitar Subdivision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

5-8. Graphs showing: 

5. Mean and standard deviation for nitrate, magnesium, chloride, sodium, and boron 

concentrations for well water samples taken from the Scimitar Subdivision . . . . . . . . . . . . . . . . 13 

6. Calcium/(bicarbonate + sulfate) ratio plotted against the sodium/chloride ratio for all water 

samples taken from the Scimitar Subdivision and for group B water samples . . . . . . . . . . . . . . . 14 

7. Nitrate concentration plotted against chloride concentration for group B well water samples 

from the Scimitar Subdivision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

8. Chlorofluorocarbon plotted against nitrate concentrations for well water samples 

taken from the Scimitar Subdivision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

9. Boxplots of the difference between the static water levels and the bedrock elevation for group A 

and B wells sampled by the USGS and for wells with less than 1 mg/L nitrate and greater than 

1 mg/L nitrate sampled by Montgomery Watson for the Scimitar Subdivision . . . . . . . . . . . . . . . . . . 19 

Contents III 
TABLES 

1. Wells sampled for chemical constituents, Scimitar Subdivision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

2. Boron isotope ratios for several aqueous systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

3. Field measurements, major and minor ions, nutrients, and isotope data for ground-water 

samples from the Scimitar Subdivision. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

4. Summary statistics for selected major and minor ions for well water samples from the 

Scimitar Subdivision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

5.	 Maximum expected aqueous concentrations of CFC-11, CFC-12, and CFC-113 for recharge 

elevations of sea level and 1525 meters, a recharge temperature of 0 degrees Celsius, and 

100 percent relative humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

CONVERSION FACTORS, WATER-QUALITY ABBREVIATIONS, AND VERTICAL DATUM 

Multiply by To obtain 

meter (m) 3.281 foot 

kilometer (km) 0.6214 mile 

liter per second (L/s) 15.85 gallon per minute 

In this report, temperature is reported in degrees Celsius (°C), which can be converted to degrees Fahrenheit (°F) by 

the following equation: 

°F = 1.8 (°C) + 32 

WATER-QUALITY ABBREVIATIONS 

mg/L, milligram per liter


μg/L, microgram per liter


μS/cm at 25°C, microsiemens per centimeter at 25 degrees Celsius


μL, microliter


μm, micrometer


ng, nanogram


pg/kg, picogram per kilogram


pptv, part per trillion by volume


VERTICAL DATUM 

Vertical datum is an ellipsoid height determined by GPS observations and referenced to NAD83. For National 

Geodetic Survey data see http://www.ngs.noaa.gov/

IV Contents 
Nitrate Source Indicators in Ground Water of the 

Scimitar Subdivision, Peters Creek Area, 

Anchorage, Alaska 

By Bronwen Wang1, Pat M. Strelakos1, and Brett Jokela2 

ABSTRACT 

A combination of aqueous chemistry, isotopic measurement, and in situ tracers were 

used to study the possible nitrate sources, the 

factors contributing to the spatial distribution 

of nitrate, and possible septic system influence 

in the ground water in the Scimitar Subdivision, Municipality of Anchorage, Alaska. Two 

water types were distinguished on the basis of 

the major ion chemistry: (1) a calcium sodium 

carbonate water, which was associated with 

isotopically heavier boron and with chlorofluorocarbons (CFC’s) that were in the range 

expected from equilibration with the atmosphere (group A water) and (2) a calcium magnesium carbonate water, which was associated 

with elevated nitrate, chloride, and magnesium 

concentrations, generally isotopically lighter 

boron, and CFC’s concentrations that were 

generally in excess of that expected from equilibration with the atmosphere (group B water). 

Water from wells in group B had nitrate concentrations that were greater than 3 milligrams 

per liter, whereas those in group A had nitrate 

concentrations of 0.2 milligram per liter or less. 

Nitrate does not appear to be undergoing extensive transformation in the ground-water system 

and behaves as a conservative ion. The major 

ion chemistry trends and the presence of CFC’s 

in excess of an atmospheric source for group B 

wells are consistent with waste-water influ1

U.S. Geological Survey, Anchorage, Alaska 

2

Montgomery Watson, Anchorage, Alaska 

ences. The spatial distribution of the nitrate 

among wells is likely due to the magnitude of 

this influence on any given well. Using an 

expanded data set composed of 16 wells sampled only for nitrate concentration, a significant 

difference in the static water level relative to 

bedrock was found. Well water samples with 

less than 1 milligram per liter nitrate had static 

water levels within the bedrock, whereas those 

samples with greater than 1 milligram per liter 

nitrate had static water levels near or above the 

top of the bedrock. This observation would be 

consistent with a conceptual model of a lownitrate fractured bedrock aquifer that receives 

slow recharge from an overlying nitrateenriched surficial aquifer. 

INTRODUCTION 

Background and Problem 

Nitrate is essential for plant growth and 

nourishment, but an overabundance can cause a 

number of adverse health and ecological 

effects. Public health issues linked to high 

nitrate intake are cancers, hypertension, and 

rare cases of methemoglobinemia in newborn 

infants. Because of these health concerns, the 

maximum concentration level for nitrate in 

drinking water is 10 mg/L nitrate (as nitrogen). 

Nitrate concentrations in uncontaminated 

ground water generally are less than 2 mg/L (as 

nitrogen) and are derived primarily from 

decomposition of soil organic matter (Mueller 

Introduction 1 
and Helsel, 1996). Human activities can influence the nitrate concentration in ground-water 

systems, and the principal sources of enhanced 

nitrate are (1) animal waste, (2) waste water 

effluent, (3) nitrogen fertilizers, and (4) municipal wastes. 

Elevated nitrate concentrations in well 

water from the hillside areas of Anchorage 

have been a concern of the Municipality of 

Anchorage, Alaska, for years. Contractors for 

the municipality have analyzed the distribution 

of nitrate-containing wells (Bristol Environmental, 1997; Montgomery Watson, 2000) and 

have attempted to correlate the occurrence of 

nitrate with environmental factors such as 

depth of well, depth to bedrock, land-surface 

slope, and depth to water table. They have been 

unable, however, to reliably explain the distribution of the nitrate concentration in the wells 

on the basis of any of the factors evaluated. 

The Municipality of Anchorage, Montgomery Watson, and the U.S. Geological Survey (USGS) agreed to focus intensively on the 

aqueous chemistry and hydrology of one subdivision in which wells commonly contain 

nitrate. Scimitar Subdivision near Peters Creek 

(fig. 1) was selected for this study. 

Wells in the Scimitar Subdivision have a 

spotty distribution of nitrate concentration. 

Water from several wells has exceeded the 10 

mg/L standard. Both on-site wells and septic 

systems are used throughout the subdivision, 

and the likely source of the nitrate is waste 

water effluent. However, nitrate concentrations 

throughout the subdivision did not correlate 

with the depth of well, nor with the duration of 

septic-system operation (Montgomery Watson, 

2000). 

Purpose and Scope 

The objectives of this study were to 

obtain a better understanding of factors contributing to the spatial distribution of nitrate, and to 

evaluate possible nitrate sources and septicsystem influence in the ground water in the 

Scimitar Subdivision. A combination of stable 

isotopic analysis, chemical analysis, and in situ 

tracers was used in this evaluation. 

Eight wells in the Scimitar Subdivision 

were sampled for chemical analysis (table 1; 

fig. 1). In addition to these eight wells, 18 additional wells were surveyed for well-head elevation and location (appendix 1). 

Table 1. Wells sampled for chemical constituents, Scimitar Subdivision 

Location Well number 

(fig. 1) USGS site ID 

Subdivision Block Lot 

1 3 1 1 

2 2 3 18 

3 1 2 9 

4 3 1 6 

5 2 3 15 

6 1 3 7 

7 1 2 2 

8 1 2 3 

612408149252001 

612411149252601 

612417149253303 

612359149253201 

612404149253402 

612404149254301 

612409149254901 

612411149254201 

2 Nitrate Source Indicators in Ground Water of the Scimitar Subdivision, Peters Creek Area, Anchorage, Alaska 
Peters Creek 

149o 149 25' o26'16" 

FAIRBANKS 

ANCHORAGE 

JUNEAU 

Study Area 

A L A S K A 

61o 

25' 

0 

0 0.5 KILOMETER 

0.5 MILE 

P eters Creek 

Peters Creek 

Community Park 

Peters Creek 

Homestead 

Park 

Ski Road 

Whaley ve 

Tulwar Dr 

Seika Dr 

Survey 

Control 

Station 

Glenn Highw a y 

7 

8 3 

2 

1 

4 

6 

5 

Subdivision Boundary 

Well with water chemistry and 

survey data 

EXPLANATION 

Well with survey data only 

1 

A

Figure 1. Location of wells studied in the Scimitar Subdivision, Peters Creek 

area, Anchorage, Alaska (see table 1 for information about wells 1-8). 

Introduction 3 
Hydrogeology of the Study Area 

The hydrogeology of the Peters Creek 

area was previously described by Brunett and 

Lee (1983). Alluvium, glacial alluvium, and 

morainal deposits overlie the bedrock in the 

vicinity of the Scimitar Subdivision. Brunett 

and Lee (1983) reported that bedrock in the 

Peters Creek region is composed of both metamorphic and igneous rocks in a complex lithology that includes the following: 

• the Cretaceous McHugh Complex composed 

of metamorphosed siltstone, graywacke, 

arkose, conglomeratic sandstone, and greenstone associated with chert and argillite; 

• a Mesozoic ultramafic complex composed of 

pyroxenite, peridotite, and serpentinized 

dunite and associated gabbro; 

• the Jurassic Knik River schist composed of 

marble, argillite, metachert, metasandstone, 

and metavolcanic rocks; 

• a Jurassic age granodiorite, diorite, quartz 

diorite formation; and 

• the Tertiary Kenai Group composed of siltstone, sandstone, and coal. 

Wells in the Peters Creek area draw water 

either from the bedrock aquifer or from the 

unconsolidated sediments. Weathered zones 

and fractures are the primary conduits for water 

flow in the bedrock. Wells in this region that 

derive waters from the bedrock have an average 

water yield of 0.38 L/s as opposed to 1.2 L/s for 

the unconsolidated sediments (Brunett and Lee, 

1983). 

NITRATE 

Nitrate is one of the primary forms of dissolved nitrogen in natural waters. It is generally 

the dominant form in waters containing dissolved oxygen, is highly soluble, and is readily 

transported in ground water. Dissolved nitrogen may also be present as ammonium (NH4

+), 

nitrite (NO2

-

), nitrogen (N2), nitrous oxide 

(N2O), and organic nitrogen. Nitrogen is converted to these nitrogen-containing compounds 

through the various processes of the nitrogen 

cycle (fig. 2). Organic nitrogen is converted to 

nitrate through the combined processes of 

ammonification (the decomposition of organic 

matter into ammonium ion) and nitrification 

(the oxidation of ammonium to nitrate). These 

are aerobic processes (meaning they occur 

when oxygen is present) and can lead to an 

accumulation of nitrate in the ground water 

(Burt and Trudgill, 1993; Chapelle,1993). 

Nitrate may be lost from ground water by denitrification, a process whereby nitrate is 

reduced to nitrogen gas (fig. 2). Denitrification 

occurs when oxygen is depleted in the water (a 

condition known as “anaerobic”) and when sufficient dissolved organic carbon exists to stimulate the microorganisms that carry out the 

process. Denitrification, along with dilution, 

advection, and dispersion, is a mechanism by 

which nitrate concentration can be lowered in 

ground water and may be important in the spatial distribution of nitrate in the Scimitar Subdivisio n. To a ddress th e pot en ti a l for 

denitrification to occur in wells in the Scimitar 

Subdivision, the oxidation-reduction (redox) 

status of the well water was inferred from the 

dissolved-oxygen concentration, and from the 

presence of dissolved iron and sulfide. 

WASTE-WATER INDICATORS 

Septic-system influence in a groundwater system can be inferred from the presence 

of analytes typical of septic systems. Several 

researchers have used major and minor ion concentrations to delineate septic-system plumes 

in ground water (Robertson and others, 1991; 

Thurman and others, 1984). The utility of a 

specific ion in distinguishing waste water influence will depend on the chemical characteristic 

of the ambient ground water. Concentrations of 

the major ions chloride, calcium, magnesium, 

4 Nitrate Source Indicators in Ground Water of the Scimitar Subdivision, Peters Creek Area, Anchorage, Alaska 
NH3 (gas) N2 (gas) 

NH3 NH4 

+ NO2 NO3 

- -

Nitrification 

Assimilation 

Nitrogen fixation 

Fertilizer Fertilizer

Death and decay 

Sewage and animal wastes 

Ammonification 

Volatilization 

Dissimilatory nitrate reduction 

Denitrification 

Loss to atmosphere 

Organic-N 

N in living 

organisms 

Natural Sources EXPLANATION 

Anaerobic processes 

Form of nitrogen 

Figure 2. Simplified nitrogen cycle, showing sources of nitrogen contamination of ground water 

(from Seiler, 1996). 

carbonate, sodium, sulfate, and boron all have 

been useful in delimiting septic-system contamination (Robertson and others 1991; Thurman and others, 1984; Thomas and others, 

1999; Seiler, 1996, 1999). 

Vengosh and Keren (1996) found that 

chloride from waste water behaves conservatively. A conservative element, such as chloride, remains in solution and is not removed by 

processes such as sorption, volatilization, or 

chemical reaction. Ground water receiving 

waste water can reflect the chloride concentration of the effluent. Similarly, the concentration 

of boron, a common component in household 

detergents that usually moves through septic 

systems, has been used to identify waste-water 

plumes (Thurman and others, 1984). Boron 

may behave conservatively in aqueous systems 

but it sorbs both on clay and oxyhydroxide surfaces and sorption is a mechanism of removing 

boron from surface water and ground water 

(Bassett and others, 1995; Goldberg and 

Glaubig, 1988, 1986a, 1986b; Vengosh and 

Keren, 1996; Leenhouts and others, 1998). 

Waste-Water Indicators 5 
--------------------------------------

Other changes in the major ion composition of water receiving waste water effluent 

arise from salt addition during household use, 

ion-exchange processes involving these salts, 

and carbonate mineral dissolution caused by 

the increased acidity associated with septic 

effluent (Seiler, 1999). The result of these reactions is an evolution of the ground water receiving the effl ue nt towa rd a decreased 

sodium/chloride ratio and increased calcium/(bicarbonate + sulfate) ratio (Seiler, 

1999; Vengosh and Keren, 1996). 

In addition to the ion concentration, stable isotopic ratios have been used to detect 

waste-water effluent in ground-water systems. 

Leenhouts and others (1998) and Bassett and 

others (1995) have found that boron isotopes 

are useful as an intrinsic tracer in identifying 

potential nitrate sources. The isotopic composition of boron in natural waters depends on the 

isotopic composition of the boron in the geologic framework with which the water is in 

contact. Natural waters can have an isotopic 

composition of boron ranging from about -10 to 

greater than +40 per mil depending on the geologic source. The boron isotope ratios for various aqueous systems are given in table 2. 

Stable isotopic measurements determine 

the ratio of the heavier isotope to the lighter isotope. All isotopic measurements are made relative to an accepted standard and are reported in 

the delta notation. For example, the delta notation for boron is given as: 

11 10 

δ – 11B = 1, 000	 ( B/ B)sample-1 . 11 10 ( B/ B)standard 

These values may be positive or negative and 

represent the isotopic distribution in the sample 

relative to the standard. The boron used in 

detergents is obtained from perborates, which 

are isotopically light relative to other geologic 

materials. Consequently, the boron in waste 

water is typically isotopically lighter (has a 

greater abundance of the light 10B isotope) than 

that found in most ground water. Therefore, the 

isotopic composition of the boron in the well 

water may provide a good tracer for wastewater effluent (Vengosh and others, 1999; Bassett and others, 1995; Leenhouts and others, 

1998). 

Table 2. Boron isotope ratios for several aqueous systems 

δ11B Possible boron source Reference (per mil) 

Seawater


Brackish ground water


Ambient ground water


Geothermal fluids


Treated waste water


39.5 

24.8 to 49.9 

13.3 to 16.7 

27.4 

21.2 to 32.4 

-9.3 to 4.4 

6.0 to 10.1 

2.2 

-2.2 

Spivack and Edmond, 1987 

Vengosh and others, 1999 

Bassett and others, 1995 

Leenhouts and others, 1998 

Vengosh and others, 1999 

Palmer and Sturchio, 1990 

Bassett and others, 1995 

Leenhouts and others, 1998 

Seiler, 1999 

6 Nitrate Source Indicators in Ground Water of the Scimitar Subdivision, Peters Creek Area, Anchorage, Alaska 
Anthropogenic compounds such as methylene blue active substances (MBAS) and chlorofluorocarbons (CFC’s) may also serve as 

waste-water indicators (Thomas and others, 

1999; Seiler, 1996). MBAS are anionic surfactants that are common in sewage effluent 

because they are used in common household 

products, such as detergents. CFC’s are stable, 

synthetic organic compounds used worldwide 

in industrial and refrigerant applications. 

Dichlorodifluoromethane (CFC-12) was first 

introduced in 1931, followed by trichlorofluoromethane (CFC-11), and trichlorotrifluoroethane (CFC-113). Their extensive use has 

resulted in a global atmospheric distribution 

and subsequent introduction into the hydrologic cycle through gas-liquid exchange equilibria. Atmospheric concentrations increased 

from the time of introduction until a production 

moratorium in 1996. Atmospheric mixing 

ratios peaked in the mid- to late 1990’s, but 

have been declining since then (Plummer and 

Busenberg, 1999). 

In addition to the atmospheric source, 

other potential sources of CFC’s in ground 

water are runoff or leaching from urban and 

industrial areas, and sewage effluent (Plummer 

and Busenberg, 1999; Thomas and others, 

1999). The concentrations of CFC’s in ground 

water receiving water from these sources are 

greater than those attributable to the atmospheric source. CFC’s were used extensively in 

household products, such as propellants in 

aerosol cans and solvents, and they appear to be 

retained and slowly released from the septic 

system (Thomas and others, 1999; Seiler, 

1996). 

METHODS 

A combination of chemical analysis and 

stable isotopic analysis was used to determine 

the best procedures for evaluating the nitrate 

distribution and fingerprinting the nitrate 

source in the Scimitar Subdivision. In addition, 

26 wells in the Scimitar Subdivision were surveyed for the well-head elevation and locations; eight of these wells were sampled for 

chemical analysis (fig. 1). 

Elevation Survey 

Latitude, longitude, and well-head elevations for the 26 wells were established in 

August 1999 using a Global Positioning System (GPS) survey. The survey control station is 

located on the overpass structure for North 

Birchwood exit on the Glenn Highway (Alaska 

State Highway 1), approximately 1.6 km southeast of Peters Creek (fig. 1). The monument 

was established in 1985 by the Municipality of 

Anchorage and R&M Consultants. The horizontal coordinates were established by GPS 

observations and adjusted by the National Geodetic Survey in September 1992. The vertical 

component is an ellipsoidal height determined 

by GPS observations and is referenced to NAD 

83. The ellipsoidal height of the survey monument is 110.95 m. Prior to 1992, the vertical 

datum of the monument was referenced to 

NGVD 29 at 100.30 m. This is the mean sea 

level datum and the difference between it and 

the ellipsoidal height is 10.65 m. Data were collected over a point on the top of the well cap or 

sanitary seal, with elevations expressed as 

heights above the ellipsoid. In order to obtain 

elevations above sea level that would correspond to elevations on USGS topographic 

maps of the area, 10.65 m was subtracted from 

the ellipsoidal heights. Horizontal data were 

collected and processed in degrees, minutes, 

and decimal equivalents of seconds. Vertical 

data referenced to the ellipsoid were collected 

and processed in meters. Survey data for all 26 

wells are given in appendix 1. 

Water Chemistry 

Ground-water samples from the Scimitar 

Subdivision were collected using the homeowner’s submersible pump. The pump was 

lifted from the pitless-tee and a galvanizedMethods 7 
steel pipe fitted with a brass hose-bib was 

attached. Teflon sampling lines were attached 

to the hose-bib. This process allowed the samples to be taken from the pump discharge and 

before any pressure or in-house treatment 

tanks. Prior to collection, the well water was 

passed through a flow-through chamber 

attached to a multiparameter probe. Samples 

were collected when pH, temperature, and specific conductance stabilized. The multiparameter probe was then removed prior to sample 

collection. Samples for dissolved oxygen (DO) 

were collected in glass DO bottles. The bottle 

volume was exchanged three times with well 

water before the bottles were stoppered. The 

DO concentration was determined using the 

Winkler titration method (Hach Company, 

1998). 

Field measurements and samples for 

major-ion, trace-element, nutrient, and MBAS 

concentrations were collected according to 

published procedures (Wilde and Radtke, 1998; 

Koterba and others, 1995). Filtered samples 

were passed through a 0.45 μm capsule filter 

and these samples are considered to represent 

the dissolved fraction. Unfiltered samples were 

collected for the MBAS analysis. Sulfide analysis was done in the field using the methods of 

the Hach Company (1998). Major ions, trace 

elements, nutrients, and MBAS were analyzed 

using methods described by Struzeski and others (1996), Burkhardt and others (1995), and 

Fishman and Friedman (1989). 

Samples for boron isotopic analysis were 

filtered and collected in a polycarbonate bottle. 

Prior to analysis, the samples were acidified 

with boron-free acid. Boron isotopes were analyzed by the negative ion thermal mass spectrometry technique at the USGS laboratories in 

Menlo Park, Calif. For each analysis, sufficient 

sample containing approximately 2 ng of boron 

was evaporated in a Teflon beaker and treated 

with 100 μL H2O2 to oxidize organics. The 

dried sample was loaded onto a rhenium filament ribbon and placed into the high vacuum 

source of the mass spectrometer. Filaments 

were heated to approximately 950 °C, and data 

were collected when ion beams stabilized in 

their growth characteristics. Ion beams were 

measured simultaneously at mass 42 (10BO2) 

and 43 (

11BO2) for approximately 30 minutes. 

The reported δ11B is the per mil difference of 

the ratio of the measured 11BO2 to 10BO2 

beams, relative to the reference standard, the 

National Institute of Standards and Technology 

(NIST) 951 boric acid. The correction for oxygen isotope composition is negligible. On this 

mass spectrometer, NIST 951 gives a 11B/10B 

value of 4.004; seawater gives a value of 4.161 

which corresponds to a δ11B of +39.2 relative 

to NIST 951. Precision is ±0.5 per mil. 

Waters for CFC analysis were collected 

using specialized sampling equipment 

designed to isolate the water sample from the 

atmosphere. The samples were collected in 

borosilicate glass vials and sealed under ultrapure nitrogen. Detailed sampling procedures 

and the analytical methods are described by 

Busenberg and Plummer (1992). Concentrations expected from atmospheric sources were 

calculated using the model of Plummer and 

Busenberg (1999). The calculation was made 

using a recharge temperature of 0 °C and 

recharge elevations of sea level and 1525 m. 

WATER CHEMISTRY DATA 

To understand the distribution and potential sources of nitrate in ground-water systems, 

it is generally necessary to evaluate the chemical characteristics of the water, in addition to 

determining the nitrate concentration, for two 

reasons. (1) The potential source of nitrate in 

ground water can lead to differences in water 

chemistry and (2) naturally occurring processes, such as denitrification, can transform 

nitrate into other nitrogen-containing compounds. Physical-chemical data, major and 

minor ion concentrations, nutrients concentrations, anthropogenic compounds, and isotopic 

data were collected in eight wells in the Scimitar Subdivision (table 3sd). 

8 Nitrate Source Indicators in Ground Water of the Scimitar Subdivision, Peters Creek Area, Anchorage, Alaska 
-- -- -- -- -- -- --

--

Table 3. Field measurements, major and minor ions, nutrients, and isotope data for ground-water samples 

from the Scimitar Subdivision 

[°C, degree Celsius; mg/L, milligram per liter; μg/L, microgram per liter; μS/cm, microsiemen per centimeter; pg/kg, picogram per kilogram; 

<, value is less than value shown; --, no data] 

Well 

Property or constituent Unit (Group A, unshaded; Group B, shaded) 

1 3 5 6 8 

Water temperature oC .18 6.68 4.4 5.22 4.58 4.49 4.78 7.19 

pH 8.3 9.1 7.4 7.1 8.1 7.4 7.9 7.3 

Specific conductance μS/cm 303 273 400 538 383 309 342 382 

Dissolved oxygen mg/L <0.1 2.4 4.6 0.6a 4.7 5.7 4.8 6.0 

Alkalinity mg/L as CaCO3 116 110 142 155 97 117 152 150 

Bicarbonate mg/L as HCO3 

- 141 102 173 189 118 143 185 183 

Carbonate mg/L as CO3 

2- 14.6 

Sulfide mg/L as S < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 

Orthophosphate, dissolved mg/L as P < 0.01 0.02 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 

Ammonia + organic nitrogen, 

dissolved 

mg/L as N 0.1 0.1 <0.1 0.1 <0.1 <0.1 0.4 0.1 

Nitrite, dissolved mg/L as N < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 

Ammonia, dissolved mg/L as N 0.03 0.09 < 0.02 0.09 < 0.02 < 0.02 < 0.02 < 0.02 

Nitrate, dissolved mg/L as N < 0.05 0.22 5.9 < 0.05 11 3.3 0.1 5.4 

Calcium, dissolved mg/L as Ca 17 15 53 47 43 34 36 46 

Sodium, dissolved mg/L as Na 43 26 5.7 46 4 4.5 34 5.1 

Magnesium, dissolved mg/L as Mg 6.8 10 17 12 18 14 5.5 16 

Potassium, dissolved mg/L as K 0.6 0.5 0.8 1.2 0.8 0.6 < 0.1 0.7 

Chloride, dissolved mg/L as Cl 4.7 4.2 16 2.8 21 7.5 4.9 14 

Fluoride, dissolved mg/L as F 0.3 0.8 < 0.1 0.2 < 0.1 < 0.1 < 0.1 < 0.1 

Sulfate, dissolved mg/L as SO4 40 31 19 120 18 18 32 18 

MBASb mg/L < 0.02 < 0.02 0.03 < 0.02 0.03 < 0.08c < 0.02 < 0.02 

Boron, dissolved μg/L as B 820 2780 80 730 55 49 1090 64 

Iron, dissolved μg/L as Fe <10 < 10 45 < 10 < 10 50 < 10 < 10 

Manganese, dissolved μg/L as Mn 40 5 2d 90 < 3 9 9 10 

Dissolved organic carbon mg/L as C 1 1.3 1.2 1.6 0.9 1.7 0.4 1.4 

Silica, dissolved mg/L as SiO2 12 16 17 9.5 15 20 13 17 

CFC-11 pg/kg 10 657 2817 70 450 1644 642 2206 

CFC-12 pg/kg 32 391 748 60 333 515 425 2700 

CFC-113 pg/kg 7 122 287 16 97 344 182 541 

δ11B per mil 6.5 5.7 4.7 1.5 -1.2 -1.2 22.2 4 

2 4 7 

4

4

a

Value from multiparameter probe 

bMethylene blue activated substance, unfiltered sample 

c

The laboratory reported a higher detection limit for this sample than the method detection limit of 0.02 mg/L. This can arise from interference in 

a given sample. 

dEstimated 

Water Chemistry Data 9 
Major and Minor Ions, and Nitrate 

Concentrations 

Major ion data indicate that two water 

types are present in the Scimitar Subdivision. 

This is illustrated by the separation of the wells 

into two groups when the major ion data are 

plotted on a trilinear diagram (fig. 3). Wells 1, 

2, 4, and 7 compose one group (group A) and 

wells 3, 5, 6, and 8 compose the other (group B) 

(fig. 4). Sodium accounts for greater than 40 

percent of the cationic charge in water from 

wells in group A as opposed to less than 10 percent in group B (fig. 3). Waters from group A 

classify as calcium sodium carbonate type 

waters, whereas those from group B are calcium magnesium carbonate type waters. 

In addition to sodium, significant differences in chloride, magnesium, boron, and 

nitrate concentrations were found between the 

two groups (table 4, fig. 5). Nitrate, chloride, 

and magnesium concentrations are higher in 

group B, whereas boron and sodium concentrations are higher in group A (fig. 5). The 

increased chloride and magnesium shifts the 

water of group B slightly more toward a calcium magnesium chloride water relative to the 

water of group A. A plot of the calcium/(bicarbonate + sulfate) versus the sodium/chloride 

ratio clearly separates the two groups: group B 

5 

3 

8

6 

4 

2 

7 

1 

5 

3 

8

6 

4 

2 

7 1 

1 2 

7 

6 

8 

3 5 

4 

SULFATE PLUS CHLORIDE 

CALCIUM PLUS MAGNESIUM 

SODIUM 

CARBONATE PLUS BICARBONATE

PERCENT 

PERCENT 

SULFATE

MAGNESIUM 

Group A 

Group B 

100 

80 

60 

40 

20 

0 

100 

80 

60 

40 

20 

0 

100 

80 

60 

40 

20 

0 

100 

80 

60 

40 

20 

0 

100 

80 

60 

40 

20 

0 

100 

80 

60 

40 

20 

0

100 100 

80 80 

60 60 

40 40 

20 20 

0 100 

80 

0

0

CALCIUM CHLORIDE, NO2 + NO3 

PERCENT 

Figure 3. Trilinear diagram for well water samples taken from the Scimitar Subdivision. 

Numbers refer to well number given in table 1. 

60 

40 

20 

0 20 

40 

60 

80 

100 

10 Nitrate Source Indicators in Ground Water of the Scimitar Subdivision, Peters Creek Area, Anchorage, Alaska 
0 

0 0.5 KILOMETER 

0.5 MILE 

1 

2 

3 

8 

7 

4 

5 

6 

Subdivision Boundary 

Wells in Group A 

EXPLANATION 

1 

Wells in Group B 

Wells with survey data only 

Figure 4. Spatial distribution of wells in group A and group B and nitrate distribution in the Scimitar 

Whaley Ave 

Tulwar Dr 

Seika Dr 

Ski Road 

6 

(5.9) Nitrate concentration, 

milligrams per liter 

(<0.05) 

(<0.05) 

(11) 

(0.22) 

(5.9) 

(3.3) 

(0.1) 

(5.4) 

Subdivision. 

has a lower sodium/chloride ratio and a higher 

calcium/(bicarbonate + sulfate) ratio relative to 

group A (fig. 6A). For this trend to represent 

the evolution of group A type water to group B, 

boron must be removed from the native ground 

water during the septic processes and/or subsequent waste-water release. 

Boron may behave as a conservative ion 

but it also sorbs both on clay and oxyhydroxide 

surfaces and sorption is a mechanism of removing boron from surface water and ground water 

(Bassett and others, 1995; Goldberg and 

Glaubig, 1988, 1986a, 1986b; Vengosh and 

Keren, 1996). Iron oxide floc was noted when 

the pumps were pulled from wells 6 and 3. Peak 

sorption of boron onto iron oxide occurs over 

the pH range from 7 to 8 (Goldberg and 

Glaubig, 1985). This is the pH range found in 

the ground-water system in the Scimitar area 

and sorption of boron by iron oxide may 

remove boron from this system. If boron is not 

Water Chemistry Data 11 
Table 4. Summary statistics for selected major and minor ions for well water samples from the Scimitar


Subdivision


(mg/L, milligram per liter; μg/L, microgram per liter; SD, standard deviation; p-value, attained significance level

a

)


Group A Group B 

Constituent Unit p-value 

Mean Median SD Mean Median SD 

Nitrate mg/L as N 0.1 0.08 0.08 6.4 5.7 3.26 0.008 

pH 8.1 8.1 0.8 7.5 7.4 0.37 0.27 

Calcium mg/L as Ca 29 27 15 44 45 7 0.13 

Sodium mg/L as Na 37 39 9 4.8 4.8 0.7 0.0004 

Magnesium mg/L as Mg 8.6 8.4 2.9 16 16 1.7 0.004 

Alkalinity mg/L as CaCO3 140 146 21 161 162 37 0.36 

Chloride mg/L as Cl 4.2 4.5 0.95 15 15 5.6 0.01 

Sulfate mg/L as SO4 56 36 43 18 18 0.5 0.13 

Boron μg/L as B 1350 950 960 61 59 13 0.036 

a

significance determined to an α-value of 0.05, where α is the level of significance. Values less than the reporting limit were set equal to the 

reporting limit for purposes of the analysis. This may bias the mean high relative to the true mean concentration. 

removed, these waters simplyrepresent two distinct systems. However, within group B, there 

is a trend toward a decreased sodium/chloride 

ratio and an increased calcium/(bicarbonate + 

sulfate) ratio as the nitrate concentration 

increases (fig. 6B). This trend is consistent with 

waste-water influence in the wells of group B. 

Nitrate and chloride concentrations in the 

wells in group B vary linearly (fig. 7). The linearity between the nitrate and the chloride suggests that they are derived from the same 

source, and that the nitrate is behaving conservatively and is not sorbed or transformed in the 

system. Elevated chloride could be indicative 

of waste-water influence in wells of group B. 

Redox and Nitrate Concentrations 

Water from wells in group B have nitrate 

concentrations that are greater than 3 mg/L, 

whereas those in group A have nitrate concentrations of 0.2 mg/L or less (table 3). A possible 

mechanism that influenced the nitrate spatial 

distribution is the removal of nitrate through 

denitrification. Denitrification occurs under 

anaerobic conditions. Redox indicators include 

dissolved oxygen, iron, and sulfide concentrations. Dissolved-oxygen concentrations in 

most of the wells are between 2.4 and 6.0 mg/L 

indicating that water in most wells is oxygenated. The exceptions are wells 1 and 4, which 

12 Nitrate Source Indicators in Ground Water of the Scimitar Subdivision, Peters Creek Area, Anchorage, Alaska 
SODIUM, IN MILLIGRAMS PER LITER 

NITRATE, IN MILLIGRAMS PER LITER 

25 

14 

20 

12 

A B 

20 

10 15 

15 

8 

10 

6 10 

4 

5 

5 

2 

0 0 0 

A B A B 

50 2500 

45 2250 

40 2000 

35 1750 

30 1500 

25 1250 

20 1000 

15 750 

10 500 

5 250 

0 0 

A B A B 

Figure 5. Mean and standard deviation for nitrate, magnesium, chloride, sodium, and 

boron concentrations for well water samples taken from the Scimitar Subdivision. MAGNESIUM, IN MILLIGRAMS PER LITER BORON, IN MICROGRAMS PER LITER CHLORIDE, IN MILLIGRAMS PER LITER


Water Chemistry Data 13 
0 

1.0 

CALCIUM/(BICARBONATE + SULFATE) RATIO


0.8 

0.6 

0.4 

0.2 

0 

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 

Group A well 

Group B well 

A 

(3.3) Nitrate concentration, 

in milligrams per liter 

1 

6 

6 

5 

3 

8 

7 

4 

1 

2 

(11) 

(6) 

(5.4) 

(3.3) 

(0.1) 

(<0.05) 

(0.22) 

(<0.05) 

1.0 

0.8 

0.6 

0.4 

0.2 

0 

0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 

B 

(11) 

(6) 

(5.4) 

(3.3) 6 

5 

3 

8 

SODIUM/CHLORIDE RATIO 

Figure 6. Calcium/(bicarbonate + sulfate) ratio plotted against the sodium/chloride ratio for 

(A) all water samples taken from the Scimitar Subdivision and (B) group B water samples. 

Ratios are calculated using milliequivalent per liter concentration units. 

14 Nitrate Source Indicators in Ground Water of the Scimitar Subdivision, Peters Creek Area, Anchorage, Alaska 
12


11 

10 

9 

8 

7 

6 

5 

4 

3 

2 

1 

0 

Group B well 6 

6 

5 

3 

8 

0 5 10 15 20 25 

CHLORIDE, IN MILLIGRAMS PER LITER 

Figure 7. Nitrate concentration plotted against chloride concentration for group B well water 

samples from the Scimitar Subdivision. Least squares regression gives the relation between 

nitrate and chloride in these wells as [nitrate] = 0.5496 [chloride] - 1.6124 with an r2 = 0.89. 

have dissolved-oxygen concentrations of < 0.1 is the nitrogen-containing species that is 

and 0.6 mg/L, respectively. Iron and sulfide expected to dominate under aerobic conditions. 

concentrations in these two wells were below Nitrate is the dominant nitrogen-containing 

the reporting limit (table 3), indicating that the species in the ground water from wells in group 

wells do not reflect strongly reducing condi- B and well 2. Ammonia was the dominant 

tions. nitrogen-containing species in well 4 and 

The presence of oxygen in most wells organic nitrogen was dominant in wells 1 and 7. 

indicates that denitrification may not be an Nitrite was below the detection limit in all 

important mechanism controlling the nitrate wells. 

concentrations in Scimitar Subdivision. Nitrate NITRATE, IN MILLIGRAMS PER LITER


Water Chemistry Data 15 
Boron Isotopes and 

Chlorofluorocarbons 

The isotopic signature of treated waste 

water ranges from -2.2 to 10 per mil (table 2). 

The δ11B values ranged from -1.2 to 4.7 per mil 

for wells in group B and from 1.5 to 22.2 for 

wells in group A (table 3). The generally lighter 

isotopic values for group B are consistent with 

the expected range for waters receiving waste 

water. If boron is sorbed by the iron oxide, isotopic fractionation could alter the isotopic signature. The wide δ11B range for group A 

indicates a complex geologic signature in the 

area. 

The concentrations in water samples of 

the three CFC’s analyzed, CFC-11, CFC-12, 

and CFC-113, are given in table 3. These concentrations are compared with the maximum 

expected aqueous concentration from air-water 

equilibration with atmospheric sources alone 

(table 5, fig. 8). The expected concentration 

depends on the recharge temperature, recharge 

elevation, salinity, and relative humidity. The 

temperature of the water recharging to the 

ground water was assumed to be 0 °C. Because 

gases are more soluble at a lower temperature if 

the actual recharge temperature is greater than 

zero, the expected concentration would be 

lower than the calculated one. Expected concentrations were calculated using both sea level 

and 1525 m (the elevation of the highest ridge 

line above Scimitar). The relative humidity was 

assumed to be 100 percent and, because the 

waters are dilute, the salinity was assumed to be 

0 part per thousand. The maximum expected 

aqueous concentrations of the three CFC’s are 

given in table 5. 

Generally the wells in group B had higher 

concentrations of all three CFC’s than those in 

group A, and were generally at or above the 

concentrations expected from atmospheric 

input during recharge (fig. 8). The exception 

was well 5 in group B. This well had concentrations of all three CFC’s consistent with an 

atmospheric source. Group A wells had CFC 

concentrations at or within the concentration 

range expected from an atmospheric source. 

MBAS was detected in wells 3 and 5 at 

low concentrations, but was not detected in the 

other wells (table 3). Consequently, MBAS 

may have been removed from the effluent by 

sorption on organic matter in the soil or diluted 

with ground water (Thomas and others, 1999). 

Table 5. Maximum expected aqueous concentrations of CFC-11, CFC-12, and 

CFC-113 for recharge elevations of sea level and 1525 meters, a recharge 

temperature of 0 degrees Celsius, and 100 percent relative humidity. 

(m, meter; pptv, part per trillion by volume; pg/kg, picogram per kilogram) 

Maximum Maximum expected Maximum expected 

atmospheric aqueous aqueous 

CFC concentrationa , concentration for concentration for 

partial pressure recharge at sea level recharge at 1525 m 

(pptv) (pg/kg) (pg/kg) 

CFC -11 280 1479 1230 

CFC-12 560 634 527 

CFC-113 90 212 176 

a

Concentrations from Plummer and Busenberg (1999) 

16 Nitrate Source Indicators in Ground Water of the Scimitar Subdivision, Peters Creek Area, Anchorage, Alaska 
3000 

2500 

TRICHLOR

OTRIFLUOROETHANE (CFC-113), DICHLORODIFLUOROMETHANE (CFC-12), TRICHLOROFLUOROMETHANE (CFC-11), 

IN PICOGRAMS PER KILOGRAM IN PICOGRAMS PER KILOGRAM IN PICOGRAMS PER KILOGRAM 2000 

1500 

1000 

500 

0 

3000 

2500 

2000 

1500 

1000 

500 

0 

600 

500 

400 

300 

Contamination source 

Atmospheric source 

Contamination source 

Contamination source 

Atmospheric source 

Atmospheric source 

1 

6 

6 

5 

3 

8 

7 

4 

1 

2 

6 

5 

3 

8 

7 

4 

1 

2 

6 

5 

3 

8 

7 

4 

2 

Group A well 

Group B well 

Sea level used as recharge elevation 

1525 meters used as recharge elevation 

Sea level used as recharge elevation 

1525 meters used as recharge elevation 

1525 meters used as recharge elevation 

Sea level used as recharge elevation 

200 

100 

0 1 

0 2 4 6 8 10 12 

NITRATE, IN MILLIGRAMS PER LITER 

Figure 8. Chlorofluorocarbon plotted against nitrate concentrations for well water samples 

taken from the Scimitar Subdivision. 

Water Chemistry Data 17 
NITRATE AND THE GROUND-WATER 

SYSTEM 

In order to evaluate the nitrate concentrations in relation to the ground-water system in 

the Scimitar Subdivision, the difference 

between the static water level and the top-ofbedrock elevation was calculated for the wells 

in groups A and B. The result of this computation will be negative if the static water level is 

within the bedrock and positive if the static 

water level is above the bedrock. Groups A and 

B were not statistically different (p = 0.4) due 

to a small sample size relative to the variation 

in the difference. However, the computation 

indicated that group B wells (those with higher 

nitrate) had a water-level elevation near or 

above the bedrock and that group A wells generally had a water-level elevation within the 

bedrock (fig. 9). To explore this possibility fur40 

(4) (3) 

WELLS SAMPLED BY THE USGS 

EXPLANATION 

(4) Number of observations 

Outlier data value less than or equal 

to 3 and more that 1.5 times the 

interquartile range outside the quartile 

Data value less than or equal to 1.5 times 

the interquartile range outside the quartile 

75th percentile 

Median 

25th percentile 

20 

0 

STATIC 

WATER LEVEL MINUS BEDROCK ELEVATION, IN METERS
 -20 

-40 

-60 

-80 

-100 

A B GROUP 

40 

(7) (9) 

ALL WELLS SAMPLED BY MONTGOMERY WATSON 

20 

0 

-20 

-40 

-60 

Figure 9. Boxplots of the difference 

between the static water levels and the 

bedrock elevation for group A and B wells 

sampled by the USGS and for wells with 

less than 1 mg/L nitrate (group L) and 

greater than 1 mg/L nitrate (group G) 

sampled by Montgomery Watson for the 

Scimitar Subdivision. 

-80 

-100 

-120 

18 Nitrate Source Indicators in Ground Water of the Scimitar Subdivision, Peters Creek Area, Anchorage, Alaska 

L G GROUP 
ther, additional nitrate and water table data 

from wells sampled by Montgomery Watson 

(2000, appendix 1) were compiled. Sampling 

by Montgomery Watson was concurrent with 

the USGS field work but was limited to nitrate 

analysis. The samples were grouped into wells 

with nitrate concentrations less than 1 mg/L 

(group L) and those with nitrate concentrations 

greater than 1 mg/L (group G). A significant 

difference (p = 0.01) was found between groups 

L and G in the static water level relative to the 

bedrock elevation (fig. 9). Group G wells have 

static water levels near or above the top of the 

bedrock and group L wells have static water 

levels within the bedrock. This observation 

would be consistent with a conceptual model of 

a low-nitrate fractured bedrock aquifer that 

receives slow recharge from an overlying 

nitrate-enriched surficial aquifer. 

A detailed analysis of the ground-water 

system was not within the scope of the current 

study. However, the observations indicate a 

ground-water system that almost certainly has 

the following characteristics: 

• The water table aquifer is a thin layer of saturated unconsolidated materials through 

which ground water flows in a direction parallel to the bedrock surface. In much of the 

area, this aquifer is too thin to produce water 

for wells. 

• Septic effluent and infiltration of precipitation 

recharge the unconfined aquifer. 

• Water levels that are generally lower in the 

bedrock aquifer than those in the water-table 

aquifer indicate downward movement of 

water between the aquifers 

SUMMARY AND CONCLUSIONS 

Two distinct water types were found in 

wells sampled in the Scimitar Subdivision. Of 

the eight wells sampled, four were classified as 

calcium sodium carbonate (group A) and four 

were calcium magnesium carbonate (group B). 

Wells in group B have significantly higher 

nitrate, chloride, and magnesium concentrations than those in group A, whereas wells in 

group A have higher sodium and boron concentrations. The trends in the major ion chemistry 

within group B are consistent with those 

expected from waste-water effluent. Aerobic 

conditions are typical in the aquifer and there is 

no evidence of significant denitrification. Consequently, nitrate behaves as a conservative 

ion. The boron in group A wells is generally 

heavier than that in group B wells. CFC concentrations in group B wells were generally in 

excess of those expected from an atmospheric 

source alone, indicating an additional source 

for these compounds. Urban and industrial 

areas, and waste water effluent are common 

sources of these compounds. In this instance, 

waste-water effluent is the most likely source. 

Using an expanded nitrate concentration 

data set provided by Montgomery Watson, a 

significant difference in the static water level 

relative to bedrock was found. Samples with 

nitrate concentrations less than 1 mg/L had 

static water levels within the bedrock, whereas 

those with nitrate concentrations greater than 1 

mg/L had static water levels near or above the 

top of the bedrock. This observation would be 

consistent with a conceptual model of a lownitrate fractured bedrock aquifer that receives 

slow recharge from an overlying nitrateenriched surficial aquifer. 

On the basis of the major ion chemistry of 

the water and the presence of CFC’s in excess 

of an atmospheric source, group B wells appear 

to have some waste-water influence. The spatial distribution of the nitrate among wells is 

likely due to the magnitude of this influence on 

any given well. 

Summary and Conclusions 19 
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20 Nitrate Source Indicators in Ground Water of the Scimitar Subdivision, Peters Creek Area, Anchorage, Alaska 
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References Cited 21 
APPENDIX 1 

Well-Elevation Data and Characteristics for Selected Wells in the Scimitar Subdivision 

Appendix 1 23

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Appe

n

dix 1. W

ell-ele

v

ation d

ata and characteristics for s

elected wells in the Scimitar Subdivision 

[Bold wells on shaded rows are those with chemical analyses; mg/L, milligram per liter; --, no data] 

Scimitar location aStatic Elevation: elevation aNitrate Well Top of well aStatic water aBedrock minus concen- No. U

SGS site ID Latitude Longitude above sea elevation elevation bedrock tration (fig. 1) Sub 

Blk Lot level (meters) (meters) elevation (mg/L) (meters) (meters) 

1 1 1 612406149252601 61°24'06.11" 149°26'04.37" 148.83 138.2 134.8 3.4 5.13 

7 1 2 2 612409149254901 61°24'07.35" 149°26559.96" 148.84 134.2 110.1 24.1 0.68 8 1 2 3 612411149254201 61°24'09.38" 149°25'59.48" 149.86 130.5 125.8 4.7 5.65-5.42 

1 2 4 612411149255601 61°24'09.10" 149°25'54.42" 150.54 127.5 142.7 -15.1 1.07 1 2 5 612415149255501 61°24'12.31" 149°25'53.23" 149.98 135.4 2.93 1 2 8 612416149254101 61°24'13.87" 149°25'39.98" 166.71 148.2 

3 1 2 9 612417149253303 61°24'12.53" 149°25'39.86" 169.70 147.4 153.0 -5.6 5.71-5.63 

1 2 10 612414149254001 61°24'11.58" 149°25'38.71" 174.42 147.1 142.5 4.6 4.27 

6 1 3 7 612404149254301 61°24'01.93" 149°25'52.04" 189.34 189.6 171.1 18.5 3.37 

1 3 8 612404149255601 61°24'01.13" 149°25'54.91" 188.63 190.0 154.2 35.8 12.7 1 3 10 612401149260201 61°23'59.11" 149°26'00.08" 189.55 149.7 162.4 -12.7 1 3 24 612353149260501 61°23'50.78" 149°26'03.87" 189.58 184.3 151.5 32.8 2 2 25 612354149260201 61°23'52.14" 149°26'00.81" 188.75 179.7 174.3 5.4 2 3 14 612403149254901 61°24'01.10" 149°25'47.95" 190.09 181.1 179.5 1.6 5.95 

5 2 3 15 612404149253402 61°24'03.62" 149°25'46.94" 189.65 169.7 12.2 

2 3 16 612407149254501 61°24'05.02" 149°25'43.43" 191.33 179.4 175.0 4.4 13.2 

24 Nitrate Source Indicators in Ground Water of the Scimitar Subdivision, Peters Creek Area, Anchorage, Alaska 
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Appendix 1. Well-elevation data and characteristics for selected wells in the Scimitar Subdivision—Continued 

[Bold wells on shaded rows are those with chemical analyses; mg/L, milligram per liter; --, no data] 

Scimitar location aStatic Elevation: 

elevation aNitrate Well Top of well aStatic water aBedrock minus concen- No. USGS site ID Latitude Longitude above sea elevation elevation bedrock tration (fig. 1) Sub Blk Lot level (meters) (meters) elevation (mg/L) (meters) (meters) 

2 2 3 18 612411149252601 61°24'06.96" 149°25'37.01" 191.63 134.8 173.0 -38.2 0.78 1 3 1 1 612408149252001 61°24'04.62" 149°25'30.34" 232.81 193.4 214 -20.6 0.62 

3 1 2 612406149253201 61°24'04.11" 149°25'30.86" 232.77 113.2 214.7 -101.5 0.72 3 1 3 612404149253601 61°24'01.22" 149°25'34.95" 232.34 195.1 217.0 -21.9 

4 3 1 6 612359149253201 61°23'57.69" 149°25'39.16" 233.01 131.4 220.3 -88.9 0.634 

3 1 7 612358149254201 61°23'55.60" 149°25'40.81" 232.26 154.6 219.6 -65.0 3 1 8 612356149254501 61°23'53.50" 149°25'43.62" 235.87 197.9 3 1 16 612359149254001 61°23'56.29" 149°25'38.18" 233.07 129.6 233.2 -103.4 0.71 3 2 1 612404149253101 61°24'01.85" 149°25'29.18" 235.52 243.0 237.1 5.88 3 2 2 612402149253401 61°23'59.99" 149°25'33.07" 235.49 201.9 240.7 -38.8 0.70 

aSelected data from Montgomery Watson, 2000 

Appendix 1 25