Distribution of Isotopic and Environmental Tracers in 

Groundwater, Northern Ada County, Southwestern Idaho 

Scientific Investigations Report 2010–5144

Prepared in cooperation with the Idaho Department of Water Resources

U.S. Department of the Interior

U.S. Geological Survey

Distribution of Isotopic and Environmental 

Tracers in Groundwater, Northern Ada 

County, Southwestern Idaho

By Candice B. Adkins and James R. Bartolino

Prepared in cooperation with the Idaho Department of Water Resources

Scientific Investigations Report 2010-5144

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Adkins, C.B., and Bartolino, J.R., 2010, Distribution of isotopic and environmental tracers in groundwater, northern 

Ada County, southwestern Idaho: U.S. Geological Survey Scientific Investigations Report 2010–5144, 30 p.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................1

Previous Investigations........................................................................................................................2

Description of Study Area ............................................................................................................................3

Hydrogeologic Setting and Framework ............................................................................................3

Land Use.................................................................................................................................................5

Climate....................................................................................................................................................5

Methods...........................................................................................................................................................6

Field Methods........................................................................................................................................6

Site Selection ...............................................................................................................................6

Sample Collection........................................................................................................................6

Quality Control..............................................................................................................................7

Laboratory Methods.............................................................................................................................7

Major Ions and Field Water-Quality Parameters....................................................................7

Nutrients........................................................................................................................................7

Oxygen/Deuterium.......................................................................................................................8

Tritium ...........................................................................................................................................8

Radiocarbon..................................................................................................................................8

Chlorofluorocarbons and Dissolved Gases.............................................................................8

Quality Control..............................................................................................................................8

Distribution of Isotopic and Environmental Tracers in Groundwater....................................................9

Major Ions and Field Water-Quality Parameters.............................................................................9

Nutrients...............................................................................................................................................11

Oxygen/Deuterium..............................................................................................................................12

Tritium ..................................................................................................................................................14

Radiocarbon.........................................................................................................................................14

Chlorofluorocarbons and Dissolved Gases....................................................................................16

Application of Tracers in Groundwater....................................................................................................20

Suggestions for Further Study ..................................................................................................................26

Summary and Conclusions.........................................................................................................................26

Acknowledgments.......................................................................................................................................27

References Cited..........................................................................................................................................28
iv

Figures

Figure 1. Map showing location of study area and sample well locations, northern Ada

County, southwestern Idaho ............................................................... 4

Figure 2. Trilinear diagram showing major ion relations of samples collected from

selected wells in northern Ada County, southwestern Idaho,

September–October 2009 .................................................................. 11

Figure 3. Graph showing oxygen and hydrogen isotopes for samples collected from

selected wells in northern Ada County with Local Meteoric Water Line and

Global Meteoric Water Line, southwestern Idaho, 

September–October 2009 .................................................................. 13

Figure 4. Graph showing carbon-13 values becoming less negative with increasing

residence time .............................................................................. 15

Figure 5. Cross section showing age tracers in samples collected from selected wells in 

northern Ada County, southwestern Idaho ............................................. 20

Figure 6. Graphs showing variation of oxygen isotopes and hydrogen isotopes

with altitude in samples collected from selected wells in northern Ada

County, southwestern Idaho ............................................................... 22

Figure 7. Graph showing carbon-14 corrected age and oxygen isotopes in samples

collected from selected wells in northern Ada County, southwestern Idaho ...... 23

Figure 8. Graphs showing age tracers carbon-14, tritium, and chlorofluorocarbons, 

with depth in samples from northern Ada County, southwestern Idaho ............ 24

Figure 9. Graph showing tritium concentration and carbon-14 activity in samples

collected from selected wells in northern Ada County, southwestern Idaho ...... 25

Tables

Table 1. Climatic data for selected weather stations in and near northern Ada County,

southwestern Idaho, 1940–2005............................................................ 5

Table 2. Site information and well construction for wells sampled in northern Ada

County, southwestern Idaho, September and October 2009 ........................... 7

Table 3. Physical parameters for samples collected from selected wells in northern

Ada County, southwestern Idaho, September–October 2009 ........................ 9

Table 4. Major ion and trace metals concentrations for samples collected from

selected wells in northern Ada County, southwestern Idaho,

September–October 2009 .................................................................. 10

Table 5. Nutrient concentrations for samples collected from selected wells in

northern Ada County, southwestern Idaho, September–October 2009 ............... 12

Table 6. Isotope data for samples collected from selected wells in northern Ada

County, southwestern Idaho, September–October 2009 .............................. 13

Table 7. Dissolved gas data and estimated recharge temperature for samples

collected from selected wells in northern Ada County, southwestern Idaho,

September–October 2009 .................................................................. 17

Table 8. Chlorofluorocarbon results and modeled groundwater recharge dates for

samples collected from selected wells in northern Ada County,

southwestern Idaho, September–October 2009 ....................................... 18
v

Conversion Factors and Datums

Inch/Pound to SI

Multiply By To obtain

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8.

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).
vi

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Abstract

Residents of northern Ada County, Idaho, depend 

on groundwater for domestic and agricultural uses. The 

population of this area is growing rapidly and groundwater 

resources must be understood for future water-resource 

management. The U.S. Geological Survey, in cooperation 

with the Idaho Department of Water Resources, used a 

suite of isotopic and environmental tracers to gain a better 

understanding of groundwater ages, recharge sources, and 

flowpaths in northern Ada County. Thirteen wells were 

sampled between September and October 2009 for field 

parameters, major anions and cations, nutrients, oxygen and 

hydrogen isotopes, tritium, radiocarbon, chlorofluorocarbons, 

and dissolved gasses. Well depths ranged from 30 to 580 feet 

below land surface. Wells were grouped together based on 

their depth and geographic location into the following four 

categories: shallow aquifer, intermediate/deep aquifer, Willow 

Creek aquifer, and Dry Creek aquifer. 

Major cations and anions indicated calcium-bicarbonate 

and sodium-bicarbonate water types in the study area. Oxygen 

and hydrogen isotopes carried an oxygen-18 excess signature, 

possibly indicating recharge from evaporated sources or 

water-rock interactions in the subsurface. Chlorofluorocarbons 

detected modern (post-1940s) recharge in every well sampled; 

tritium data indicated modern water (post-1951) in seven, 

predominantly shallow wells. Nutrient concentrations 

tended to be greater in wells signaling recent recharge based 

on groundwater age dating, thus confirming the presence 

of recent recharge in these wells. Corrected radiocarbon 

results generated estimated residence times from modern 

to 5,100 years before present. Residence time tended to 

increase with depth, as confirmed by all three age-tracers. The 

disagreement among residence times indicates that samples 

were well-mixed and that the sampled aquifers contain a 

mixture of young and old recharge. Due to a lack of data, no 

conclusions about sources of recharge could be drawn from 

this study.

Introduction

The sustainability of the groundwater supply in northern 

Ada County is crucial to support the rapid development of 

the area. Between 1970 and 2007, the population of Ada 

County increased 330 percent from 112,230 to 373,406 (U.S. 

Census Bureau, 2009). This trend is projected to continue. 

The Community Planning Association of Southwest Idaho 

(COMPASS) estimates that the population of Ada County 

will increase to 875,960 by 2040 (Church, 2007). Significant 

development is planned for northern Ada County, with a 

proposed 20,000–30,000 residential homes to be constructed 

over the next 10 years (Church, 2007). 

Until recently, the hydrogeology of northern Ada County 

has not been well understood. Studies commissioned by 

developers have improved this situation but, as with many 

technical studies, additional questions have been raised. 

Among those questions relating to the long-term sustainability 

of the aquifer system in northern Ada County is the degree 

to which recent groundwater recharge is present and whether 

there is zonation related to hydraulic differences within 

the aquifer. Because previous studies of the groundwater 

in the area have made limited use of hydrochemistry, a 

limited general characterization using a variety of analyses 

may be useful in adding to the current understanding of the 

groundwater-flow system. Therefore, the U.S. Geological 

Survey (USGS), in cooperation with the Idaho Department 

of Water Resources (IDWR), initiated a study to characterize 

the distribution of isotopic and environmental tracers in 

groundwater of northern Ada County. 

Purpose and Scope

This report describes the initial characterization of 

the distribution of isotopic and environmental tracers in 

groundwater of northern Ada County. It includes brief 

discussions of environmental tracers, how tracers are used 

to better understand groundwater-flow systems, and the 

results from this study. Conclusions in this report about the 

groundwater-flow system are from data collected for this 

study. Finally, suggestions are proposed for the collection of 

environmental-tracer data for future studies.

Distribution of Isotopic and Environmental Tracers in 

Groundwater, Northern Ada County, Southwestern Idaho

By Candice B. Adkins and James R. Bartolino
2 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

Previous Investigations

The chemistry of groundwater at various depths in 

northern Ada County has been discussed at length in various 

reports. Parliman (1982) presented well-inventory and 

groundwater-quality data for 665 sites in Elmore, Owyhee, 

Ada, and Canyon Counties, including a large number in 

northern Ada County. Analyses were done for major ions, 

nutrients, and bacteria. These data are available through the 

USGS National Water Information System (U.S. Geological 

Survey, 2010a). 

Mayo and others (1984) discussed water samples 

collected from shallow and deep aquifers of the Boise 

Valley and Boise foothills that represented nonthermal and 

geothermal waters (geothermal is defined by the IDWR 

as having a temperature greater than 85°F). Samples were 

analyzed for major ions, oxygen and hydrogen stable isotopes, 

carbon-13 and carbon-14, and tritium. Geothermal waters 

tended to have higher Na/Ca ratios than nonthermal water 

samples; nonthermal and geothermal waters tended to have 

the same stable isotope signature. Carbon-14 values ranged 

from 6.6 to 104.6 pmc (percent modern carbon), and tritium 

was detected in some samples. Estimated mean residence 

times in waters along the Boise frontal fault ranged from 0 to 

17,000 years.

Wood and Low (1988) presented work done as part 

of the USGS Snake River Plain (SRP) Regional Aquifer 

System Analysis program. Because the Regional Aquifer 

System Analysis was a regional study, few samples were 

collected and analyzed in northern Ada County. The primary 

focus of this work was to identify the chemical reactions 

between the aquifer matrix and groundwater that control 

solute concentrations in the Snake River Plain aquifer system. 

Radiocarbon analyses were applied to the geothermal system 

to determine that recharge probably occurred during the 

Pleistocene. 

Mariner and others (1989) collected water samples from 

10 geothermal wells and 3 nonthermal-water springs along 

the Boise frontal fault immediately north of Boise. They 

analyzed these samples for major ions, oxygen/deuterium, 

radiocarbon, and dissolved gasses, concluding that all samples 

were chemically identical to those from the Idaho Batholith. 

Two systems were identified, and geothermal groundwater 

recharge was estimated to occur during the Pleistocene, 

15,000–30,000 years before present. 

Baker (1991) described historical and short-term water 

levels, and analyzed 10 samples for major ions from 8 wells in 

the Dry Creek study area. Groundwater was determined to be 

a calcium-sodium-bicarbonate type, and concentrations of iron 

(exceeding 0.3 mg/L) and fluoride (as high as 13 mg/L) were 

high in select samples.

Parliman and Young (1992) described selected wellinventory and water-chemistry data for 718 thermal water 

wells and springs in Idaho, with about 12 of these in northern 

Ada County. Analyses included major ions, nutrients, bacteria, 

trace elements, tritium, oxygen/deuterium, radiocarbon, and 

sulfur isotopes (δ34S). 

Parliman and others (1996) reported analytical results 

from 903 wells sampled in the Boise River Valley between 

1990 and 1995. Sampling was directed primarily toward 

detecting natural and anthropogenic contamination. Analyses 

included major ions, nutrients, bacteria, trace elements, radon, 

volatile organic compounds (VOCs), and pesticides. 

Parliman and Spinazola (1998) presented data for 

884 groundwater samples (27 in the Boise foothills) 

collected by the USGS between 1985 and 1996, as well as 

previous work. Groundwater was classified into three main 

hydrochemical types: nonthermal water was primarily of 

calcium-bicarbonate or calcium-sulfate types, and thermal 

water was primarily a sodium-bicarbonate type. Isotopic and 

environmental-tracer data were mentioned, but discussion was 

limited to noting that carbon-14 data was collected only in 

wells deeper than 500 ft. This water in the nonthermal aquifers 

was a mixture of young and old waters, and water from five 

wells in the geothermal system was recharged approximately 

15,000–28,000 years before present. The remainder of the 

report discusses natural and anthropogenic contamination.

Neely and Crockett (1998) presented water-quality data 

collected from 281 wells (144 shallow, 137 deep, with 250 ft 

being the dividing depth), including a number of wells in the 

northern Ada County area. Data include major ions, nutrients, 

bacteria, trace elements, radon, VOCs, and pesticides. The 

study included an evaluation of network design, trend analysis 

of selected constituents, as well as scatterplots and trilinear 

(Piper) diagrams. Sampling was done as part of the IDWRUSGS Statewide Water Quality Program.

Hutchings and Petrich (2002) sampled 38 wells 

throughout the Treasure Valley, as part of the Treasure Valley 

Hydrologic Project. Only one of the wells (TVHP1) is near 

the northern Ada County area and was analyzed for major 

ions and tritium. Radiocarbon was analyzed in a subset of 

28 wells in the central Treasure Valley, including TVHP1; a 

carbon-14 correction model based on carbon-13 values was 

recommended. The youngest waters were found to be adjacent 

to the Boise foothills, increasing in age towards the western 

edge of the basin near the Snake River with a corresponding 

increase in groundwater mineralization. 

Hutchings and Petrich (2002) made four major 

recommendations for further hydrogeochemical work 

to help define the groundwater flow system. Of these, 

one is applicable primarily to northern Ada County. This 

recommendation is to further study recharge along the Boise 

frontal fault by quantifying the contribution of natural recharge 

to Boise-area aquifers in this area. Among the techniques 

suggested were installation of multilevel monitoring wells, 

additional groundwater-level data, additional geophysical 

and stratigraphic work, and a recharge-focused geochemistry 

study.
Description of Study Area 3

SPF Water Engineering, LLC (2004) defined two general 

aquifers in the foothills north of Eagle in northern Ada County, 

the Willow Creek and the Northern Margin aquifers, which 

have differing hydrochemical and temperature properties. 

In the analysis, 10 wells were used: 4 sampled for this study 

and 6 others sampled by others, including the USGS and the 

IDWR. Analyses were for major ions and arsenic. A trilinear 

(Piper) diagram of the 10 wells indicated waters are primarily 

of calcium-bicarbonate to sodium-bicarbonate types. 

Glanzman and Squires (2009) analyzed data previously 

collected by United Water and consultants between 1991 and 

2008 for 84 samples from 43 wells. Completion information 

from wells was used to assign each sample to one of five 

primary aquifers or groups. Analyses were for major ions 

and arsenic, and a number of trilinear (Piper) diagrams were 

plotted (Piper, 1944). The five aquifers or groups and their 

hydrochemical type were determined to be: the Pierce Gulch 

Sand aquifer, calcium-bicarbonate type; Terteling Springs 

Formation, calcium-bicarbonate to sodium-bicarbonate 

types; Willow Creek, calcium-bicarbonate and sodiumbicarbonate types; Spring Valley Ranch wells (most likely in 

the Tertling Springs Formation), multiple types; and Emmett 

wells (no connection to above mentioned aquifers), calciumbicarbonate-sulfate and sodium-bicarbonate types.

Schlegel and others (2009) analyzed 111 surface-water 

and groundwater samples from the Idaho Batholith, Boise 

frontal fault, and Nampa-Caldwell area (west of Boise) for 

oxygen and hydrogen isotopes. Selected samples also were 

analyzed for major ions, tritium, and carbon-13 and carbon-14. 

Schlegel and others (2009) generated a local meteoric water 

line for the Boise area from these data, and discussed how 

potential climate change has affected the sources of recharge 

to the Boise area.

Description of Study Area

Hydrogeologic Setting and Framework

Northern Ada County is in Treasure Valley in the western 

Snake River Plain, located between the northern Rocky 

Mountains physiographic province and the northern Basin 

and Range Extensional Province (Wood, 1994). The western 

Snake River Plain is a northwestern-trending graben valley, 

28 mi wide and 155 mi long that was formed during the late 

Cenozoic Period. The general geology of the western Snake 

River Plain consists of granitic Idaho Batholith basement rock, 

overlain by 1.6–2.7 mi of Miocene basalt associated with the 

Yellowstone hotspot, and covered by lacustrine and fluvial 

sediments (Mitchell, 1981; Wood and Anderson, 1981; Mayo 

and others, 1984; Wood and Clemens, 2002). The fluvial 

and lacustrine sediments consist of interbedded gravel, sand, 

claystone, and mudstone that were deposited on lakeshores 

in the Snake River Plain during the Miocene and Pleistocene 

(Squires and Wood, 2001). 

The northeastern part of the study area is dominated 

by mountainous portions of exposed Idaho Batholith (Wood 

and Clemens, 2002). The Boise frontal fault delineates the 

contact between the southwestern edge of the Atlanta Lobe 

of the Idaho Batholith and the northeastern edge of the 

western Snake River Plain. The frontal-fault area is a zone 

of northwest-trending high angle normal faults that have 

resulted in an offset of about 800 ft from the plain (Mayo 

and others, 1984; Holdaway, 1994). This “buried” fault 

(Wood and Clemens, 2002), which is apparent on well-logs 

and seismic‐reflection surveys, shows an important contact 

between the Idaho Batholith and water-bearing sediments in 

the western Snake River Plain. 

High permeability, water-bearing sediments in northern 

Ada County occur along paleo-river channels, lakebeds, 

deltas, and alluvial fans (Hutchings and Petrich, 2002). In the 

western Snake River Plain lies the Pierce Gulch Sand aquifer 

that is comprised of stratified sand layers interbedded with 

silt and clay and is believed to be a very productive aquifer 

(Squires and others, 2007). This large aquifer underlies the 

cities of Star, Eagle, and Meridian (fig. 1), and it extends to 

the northwest to at least the city of Payette (Squires and Wood, 

2001; Squires and others, 2007). Underlying the Pierce Gulch 

Sand aquifer is the Terteling Springs Formation aquifer. The 

Terteling Springs Formation aquifer, described by Wood and 

Clemens (2002), is comprised of oolitic sand. It is bounded 

by clays beneath the Pierce Gulch Sand aquifer system to the 

southwest and by bedrock to the northwest. 

The Willow Creek and Dry Creek drainage basins 

(fig. 1), in the foothills of the Idaho Batholith, were identified 

as areas of interest for this study. The Willow Creek aquifer 

is an isolated aquifer in the foothills of the Willow Creek 

drainage basin, consisting of coarse sands and gravels that 

overlie granitic and volcanic bedrock of the Idaho Batholith. 

The Willow Creek aquifer is stratigraphically older and deeper 

than the Pierce Gulch Sand aquifer (Squires and others, 2007), 

and it is related to the sand facies of the Terteling Springs 

Formation. The Dry Creek aquifer is an isolated aquifer 

comprised of gravels, sands, and silts that occupies the Dry 

Creek drainage basin in the foothills of the Idaho Batholith 

(Baker, 1991). Wells sampled in the western Snake River 

Plain for this study were in the Pierce Gulch Sand aquifer 

and shallow unconfined, undifferentiated alluvial material 

overlying the Pierce Gulch Sand aquifer. Because of the 

complex geology of the area, wells in the western Snake 

River Plain are defined by the primary water-bearing zone in 

which the well is completed. Samples in the foothills aquifers 

were collected from wells completed in the Willow Creek and 

Dry Creek aquifers; further details about well selection are 

discussed in sections, “Site Selection,” “Sample Collection,” 

and “Quality Control.” 

For the current study, recharge to the system is assumed 

to be from infiltration of surface water and precipitation. 

Since the 1880s, a canal system has diverted water for 

irrigation from the Boise River through the Treasure Valley. 
4 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

idtac10-0485_fig01

Willow Creek

Dry Creek

Boise River

A

A'

"

"

116°28'30"W 116°23'0"W 116°17'30"W 116°12'0"W

43°44'30"N

43°39'0"N

43°33'30"N

A A'

Boise River

Willow Creek, S. Fork

Willow Creek,

N. Fork

0 1.5 3 6 Miles

0 3 6 1.5 Kilometers

IDAHO

Study area

Ada County

Boise

Eagle

DC1

ID4 & ID5

S3

S1

ID6

S2 ID7

ID1

ID2 & ID3

WC2

Meridian

Meridian

Star

WC1

ADA COUNTY

BOISE COUNTY

GEM COUNTY

CANYON COUNTY

Snake River Plain

Idaho Batholith

S1

ID1

WC1

DC1

Base from U.S. Geological Survey, Datum = NAD83, shaded relief USGS 

90-m gid, 2002, roads from Alpine Enterprise, 2004, hydrography from 

USGS NHD, 1991, 1:1000,000.

EXPLANATION

Shallow aquifer well

Intermediate/Deep aquifer well

Willow Creek aquifer well

Dry Creek aquifer well

Cities

Line of cross section—

See figure 5

Figure 1. Location of study area and sample well locations, northern Ada County, southwestern Idaho.
Description of Study Area 5

About 1,170 mi of major irrigation canals existed throughout 

the Treasure Valley as of 2004 (Urban, 2004). Canals in 

the Treasure Valley have been shown to lose significant 

amounts of water to the groundwater system through seepage 

(Berenbrock, 1997). Infiltration of precipitation probably only 

constitutes a small component of recharge; Newton (1991) 

suggested that only 2 percent of the total annual precipitation 

in the western Snake River Plain actually recharges 

groundwater. 

Groundwater levels in northern Ada County have been 

measured for multiple studies. Shallow groundwater in 

the Pierce Gulch Sand and Willow Creek aquifers moves 

northwest towards the Payette River Valley (Squires, 2008). 

Groundwater levels range from a few feet to 397 ft below land 

surface, depending on the location of measurement and aquifer 

measured. Groundwater levels in some areas are affected by 

seepage from local canals and by infiltration from irrigation; 

mounds near losing portions of the canals are evident in the 

surface of the water table (Dion, 1972). 

Land Use

Land use in northern Ada County is a mixture of sparsely 

populated rangeland, farmland and rapidly developing 

suburban and urban areas (the cities of Eagle and Star, fig. 1) 

(U.S. Department of Agriculture, 2010). Urban development 

is most dense near the city of Eagle; multi-acre lots are more 

common farther from the city and in the foothills. In 2000, 

most land use was classified as rangeland, with the second 

most common land use being agriculture. The remainder 

of the land use is classified as residential, commercial, or 

industrial. Several golf courses exist in the area, as well as 

a small number of concentrated animal-feeding operations 

(dairies and feedlots). Residential development has increased 

significantly in the past 9 years and is projected to continue 

(U.S. Department of Agriculture, 2010).

Climate

Climate in the study area falls into three classes under 

a modified Köppen system. The basin floor of the Treasure 

Valley is classified as BSk, the foothills as Dsa, and the 

mountains as Dsb. For the valley floor, BS indicates an 

arid steppe climate and k indicates that the mean annual air 

temperature is less than 64 °F. For the foothills, Dsa indicates 

a cold, dry, hot-summer climate; for the mountains, and Dsb

is a cold, dry, warm-summer climate (Godfrey, 2000; Peel and 

others, 2007).

The National Weather Service has 78 weather stations 

within a 50-mi buffer of the study area, although only 

37 are active (U.S. Department of Commerce, 2010). Of 

these 37 stations, 6 are within or immediately adjacent to the 

study area and have sufficient long-term data for climatic 

calculations (table 1). As of 2010, two active AgriMet stations 

are within 25 mi of the study area: Boise, Idaho (BOII) and 

Nampa, Idaho (NMI) (Bureau of Reclamation, 2006). In 

addition, two active Natural Resources Conservation Service 

snow-survey sites are in or adjacent to the study area: one is 

an instrumented SNOTEL site and the other is a snow course 

(Natural Resources Conservation Service, 2010).

Table 1. Climatic data for selected weather stations in and near northern Ada County, southwestern Idaho, 1940–2005.

[Climate data from Western Regional Climate Center (2008). Abbreviations: °F, degrees Fahrenheit; in., inches; WSFO, Weather Service Field Office; 

NAVD 88, North American Vertical Datum of 1988]

Station name 

and No. Latitude Longitude

Air temperature (°F)

Mean total 

precipitation 

(in.)

Mean total 

snowfall 

(in.)

Altitude 

(feet above 

NAVD 88)

Dates in operation

Mean

July 

maximum

Mean 

January

minimum

Mean 

annual

Starting 

date

Ending 

date

Boise WSFO Airport 

(101022)

43°34 ́N 116°13 ́W 90.5 22.3 51.2 11.8 19.5 2,860 01-01-1940 12-31-2005

Boise Lucky Peak Dam 

(101018)

43°33 ́N 116°04 ́W 91.6 20.8 51.7 13.7 4.9 2,840 01-12-1951 12-31-2005

Boise 7 N 

 (101017)

43°43 ́N 116°12 ́W 87.9 22.7 49.6 19.0 55 3,890 05-01-1973 12-31-2005

Emmett 2 E 

 (102942)

43°52 ́N 116°28 ́W 91.5 21.9 51.1 13.4 8.8 2,390 08-01-1948 12-31-2005

Kuna 2 NNE 

 (105038)

43°31 ́N 116°24 ́W 88.3 20.2 49.6 9.8 11.9 2,680 08-01-1948 12-31-2005

Nampa Sugar Factory 

(106305)

43°37 ́N 116°34 ́W 91.1 21.5 51.0 11.2 9.6 2,470 10-01-1976 12-31-2005
6 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

Long-term means for select National Weather Service 

stations are listed in table 1. Mean annual air temperatures 

at the six stations range from 51.7 °F at Boise Lucky Peak 

Dam to 49.6 °F at Boise 7 N and Kuna 2 NNE. The coldest 

month in the area is January, with mean low air temperatures 

ranging from 20.2 °F at Kuna 2 NNE to 22.7 °F at Boise 7 N. 

Typically, the warmest month is July, with mean high air 

temperatures ranging from 91.6 °F at Boise Lucky Peak 

Dam to 87.9 °F at Boise 7 N. Mean total precipitation ranges 

from 19.0 in. at Boise 7 N to 9.8 in. at Kuna 2 NNE. July and 

August typically are the driest months; November, December, 

and January are the wettest (Western Regional Climate Center, 

2008).

Methods

Data used for this study were collected from sites 

specifically selected by the USGS and the IDWR based on 

location, well depth, well construction, and other criteria. 

This section provides details about the field methods for 

site selection, sample collection, and quality-control and 

laboratory methods.

Field Methods

Site Selection

A total of 15 samples (table 2) were collected for this 

study during September and October 2009. Of the 15 samples 

collected, 13 were from domestic, irrigation, monitoring, and 

municipal wells at 11 sites; 2 were replicate samples collected 

for quality control. Wells were selected, based on well depth 

and location, to represent the shallow aquifer, the intermediate/

deep aquifer, the Willow Creek aquifer, and the Dry Creek 

aquifer. Samples from the shallow aquifer were collected 

from wells at depths less than 60 ft below land surface in the 

shallow, undifferentiated alluvium of the western Snake River 

Plain. Intermediate/deep samples were collected from wells 

at depths ranging from 144 to 425 ft below land surface in the 

Pierce Gulch Sand aquifer or undifferentiated alluvial aquifers. 

Willow Creek aquifer samples were collected from wells at 

depths ranging from 425 to 580 ft below land surface. One 

sample was collected from the Dry Creek aquifer from a well 

with a depth of 292 ft below land surface. Well construction 

and site information are shown in table 2 and well locations 

are shown in figure 1. 

Three samples were collected from the shallow aquifer, 

seven from the deep/intermediate aquifer, two from the 

Willow Creek aquifer, and one from the Dry Creek aquifer 

(table 2). Two of the seven sites in the deep/intermediate 

aquifer had wells placed at different intervals that allowed 

for sampling at two different depths at the same geographical 

location. Two wells were sampled at each of those two sites 

(ID2 and ID3, ID4 and ID5). Sample names were assigned 

based on the aquifer sampled. Replicate (quality-control) 

samples are listed on all tables for reference underneath the 

primary sample and are labeled with normal site nomenclature 

and the letter “R” to indicate a replicate sample. 

Sample Collection

At the time of sample collection, a closed path was 

established between the groundwater source and sampling 

equipment to prevent contact between sample and air. Where 

a well had an existing submersible pump, samples were 

collected as close to the wellhead as possible through existing 

plumbing. For the four monitoring wells that did not have 

pumps, a portable electric Grundfos Redi-Flo2® submersible 

pump and tygon tubing was used.

 A Hydrolab® MS 5 multiparameter meter was used to 

measure specific conductance, pH, water temperature, and 

dissolved oxygen. The multiparameter meter was connected 

to the discharge line from the well, and field parameters 

were recorded before sample collection. Wells were pumped 

continuously during sample collection. Wells were purged of 

at least three casing volumes, and field parameters (specific 

conductance, pH, and water temperature) stabilized before 

samples were collected. 

Samples were collected in appropriate containers and 

preserved in accordance with the procedures specified in 

the USGS National Field Manual (U.S. Geological Survey, 

variously dated). Tritium and physical parameter bottles were 

rinsed three times with unfiltered water and samples were 

collected without filtration. Samples for nutrients, metal, major 

ions, oxygen-18 and deuterium (hydrogen-2), and carbon-13 

and carbon-14 were filtered using a 0.45-micron capsule filter. 

All filtered sample bottles were rinsed three times with filtered 

water, with the exception of the oxygen-18 and deuterium 

samples. Chlorofluorocarbon (CFC) and dissolved gas samples 

were collected using copper tubing. Field procedures followed 

those outlined by the USGS CFC and dissolved gas guidelines 

(U.S. Geological Survey, 2006, 2009).

Alkalinity titrations were performed in the field using 

an incremental titration method. About 60 mL of sample 

was titrated using 0.1600 N ± 0.0008 N H2SO4 acid through 

a HACHTM manually operated digital titrator. In addition, 

specific conductance, pH, and alkalinity were analyzed by the 

USGS National Water-Quality Laboratory (NWQL).
Methods 7

Quality Control

The quality assurance plan practiced at the USGS Idaho 

Water Science Center (Hardy, 2008) was followed during 

the collection of water samples for this study. To assure 

quality control, the multiparameter meter was calibrated in 

the field before each use and was checked for accuracy at 

the end of each day. In addition, the multiparameter meter 

is checked yearly to ensure its accuracy against known 

reference standards. All field equipment, instruments, and 

sampling ports were cleaned prior to use. Laboratory protocols 

for quality control can be found at the various laboratory 

websites and in the publications listed in section, “Laboratory 

Methods.” No blank samples were collected.

Laboratory Methods

Major Ions and Field Water-Quality Parameters

Major ion, metals, and physical parameters were 

determined at the USGS NWQL in Denver, Colo. Samples for 

metals and some major ions were acidified in the field using 

concentrated nitric acid. Detailed laboratory procedures are 

described in Fishman and Friedman (1989).

Nutrients

Nutrients were analyzed at the NWQL in Denver, Colo. 

All samples were collected in 125 mL brown polyethylene 

bottles and chilled. Laboratory methodologies are described in 

detail in Fishman (1993). 

Table 2. Site information and well construction for wells sampled in northern Ada County, southwestern Idaho, September and 

October 2009.

[Sample name: Location of wells sampled are shown in figure 1. Sample names were assigned based on depth and aquifer sampled. Sample names with 

“R” indicate replicate sample. Depth of well and screening information obtained from well logs associated with USGS well No. Abbreviations: USGS, U.S. 

Geological Survey; NAVD 88, North American Vertical Datum of 1988; ft, foot; –, no data]

Sample 

name

USGS 

station No. USGS well No. Latitude Longitude

Altitude 

(ft above 

NAVD 88)

Sample 

date

Sample 

time

Depth 

of well

 (ft below 

land 

surface)

Top of 

screen 

(ft below 

land 

surface)

Depth to 

water 

(ft below 

land 

surface) 

Shallow aquifer

S1 434256116215901 04N 01E 05BDAC1 434256 1162159 2,600 09-25-09 1205 43 – 18.0

S2 434224116223501 04N 01E 06DDD1 434224 1162235 2,575 09-24-09 1020 30 – 6.6

S3 434342116190401 05N 01E 34ADCD1 434342 1161904 2,670 09-25-09 1005 54 42 13.3

Intermediate/Deep aquifer

ID1 434613116273801 05N 01W 16DBC1 434613 1162738 2,730 09-30-09 1140 400 290 216.3

ID2 434412116272704 05N 01W 28DDC4 434412 1162727 2,610 09-23-09 1205 425 – 104.6

ID3 434412116272705 05N 01W 28DDC5 434412 1162727 2,610 09-23-09 1040 144 – 92.3

ID4 434048116184403 04N 01E 14CCB3 434048 1161844 2,582 09-22-09 1150 250 210 1.1

ID5 434048116184404 04N 01E 14CCB4 434048 1161844 2,582 09-22-09 1350 177 130 3.5

ID5R 434048116184404 04N 01E 14CCB4 434048 1161844 2,582 09-22-09 1355 177 130 3.3

ID6 434225116214601 04N 01E 05CDDD1 434225 1162146 2,600 10-30-09 1040 340 183 55.0

ID7 434208116253401 04N 01W 11BDAA1 434208 1162534 2,512 09-24-09 1240 357 270 0.0

Willow Creek aquifer

WC1 434836116255001 06N 01W 35CCA1 434836 1162550 2,860 09-24-09 1450 580 – 397.1

WC2 434650116280802 05N 01W 09CCD2 434650 1162808 2,680 09-21-09 1320 425 – 334.7

WC2R 434650116280802 05N 01W 09CCD2 434650 1162808 2,680 09-21-09 1325 425 – 334.6

Dry Creek aquifer

DC1 434343116151601 05N 02E 31ADDD1 434343 1161516 2,765 09-25-09 1440 292 – 85.0
8 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

Oxygen/Deuterium

Oxygen and hydrogen stable isotope samples were 

analyzed at the USGS Reston Stable Isotope Laboratory in 

Reston, Va. Samples were collected in 60 mL glass bottles 

with polyseal caps. Hydrogen-isotope-ratio analyses were 

performed using the hydrogen equilibration technique (Coplen 

and others, 1991). Results for stable isotopes are in units 

of per mil (‰, parts per thousand), and expressed in delta 

notation (δ) as derived from comparing the sample to the 

Vienna Standard Mean Ocean Water (VSMOW) standard 

(Coplen, 1996). The two standard deviation (2-σ) accuracy 

of oxygen and hydrogen isotope results were 0.2 and 2‰, 

respectively (Révész and Coplen, 2008a, 2008b). 

Tritium

Tritium samples were analyzed by the USGS Isotope 

Research Laboratory in Menlo Park, Calif. (U.S. Geological 

Survey, 2010b). Samples were collected in 1,000 mL 

polyethylene bottles with polyseal cone caps. Samples were 

treated by electrolytic enrichment and liquid scintillation 

decay counting methods. The two standard deviation (2-σ) 

accuracy of tritium results was ±0.3 TU. The minimum 

detection limit is 0.7 TU. Results were reported by the 

laboratory in picocuries per liter (pCi/L) and were converted 

to tritium units for this report using the equation:

1TU 3.23pCi/L. = (1)

Any results indicating a negative tritium activity or 

activity less than 2 pCi/L were reported as less than 0.7 TU to 

indicate tritium-dead water. 

Radiocarbon

Carbon-isotope samples were analyzed at the National 

Ocean Sciences Accelerator Mass Spectrometer Facility 

in Woods Hole, Mass. Samples for carbon isotopes were 

collected in a safety-coated 1-L glass bottle out of contact 

with the atmosphere. Carbon dioxide gas was extracted from 

the sample by acid hydrolysis and reduced to graphite that 

was made into targets for accelerator mass spectrometry. 

Process blanks were used for calibration. This method can 

measure samples as old as 60,000 years with an error of less 

than 0.4 pmc (Woods Hole Oceanographic Institution, 2003). 

Results for carbon-14 are reported in pmc, and mean residence 

times are calculated from the decay equation. Samples 

reported as “modern” imply that recharge has occurred later 

than the year 1950 (Woods Hole Oceanographic Institution, 

2003). Results for carbon-13 are reported in per mil (‰, 

parts per thousand) notation, and they are expressed in delta 

notation (δ) as derived from comparing the sample to the 

Vienna Pee Dee Belemnite (VPDB) standard (Woods Hole 

Oceanographic Institution, 2003).

Chlorofluorocarbons and Dissolved Gases

CFCs and dissolved gas samples were analyzed at the 

Reston Chlorofluorocarbon Laboratory in Reston, Va. Each 

CFC sample was collected in five 125-mL glass bottles, 

capped with aluminum foil-lined caps, sealed with electrical 

tape, and stored upside down. Dissolved gas samples were 

collected in two 160-mL septum glass bottles and plugged; 

gasses were allowed to escape during sampling through a 

hypodermic needle in the septum.

CFC results were determined in the laboratory using the 

purge-and-tap gas chromatography procedure with an electron 

capture detector. Dissolved gas samples were allowed to 

equilibrate with headspace, and the resulting gas was analyzed 

on a gas chromatograph. 

Quality Control

Because method detection limits are low, each analytical 

method has a corresponding minimum reporting limit. 

Sample concentrations less than the reporting limit are noted 

in the tables with the less than (<) sign, and are assumed 

to be smaller than the laboratory’s ability to analyze with 

certainty. Lower reporting limits are listed beneath each 

constituent; isotopes, dissolved gases, and CFCs do not have 

a reporting limit. In addition, samples that have an estimated 

concentration (denoted as “E” in tables) indicate that results 

are less than the lowest calibration standard and are only an 

estimate of the concentration. 

Replicate samples were collected at two sites and 

results are listed beneath the primary sample in all tables. At 

both sites, the primary and replicate samples were within a 

10 percent relative percent difference (RPD) from each other 

in all constituents using the formula:

s R S R

s

R

 RPD [V V ) / ((V V ) / 2)]*100,

where

V is the value of the primary sample, and

V is the value of the replicate sample.

= � + (2)

For this study, replicate samples were required to fall 

within a RPD of 10 percent, thus the replicate samples for all 

constituents indicate that the data are acceptable.
Distribution of Isotopic and Environmental Tracers in Groundwater 9

Distribution of Isotopic and 

Environmental Tracers in Groundwater

Major Ions and Field Water-Quality Parameters

Relations and spatial patterns in water quality resulting 

from differing flow paths, recharge sources, and the presence 

of geothermal water are often evident from major ion 

chemistry and parameters, such as specific conductance, 

pH, dissolved oxygen, and alkalinity. Such data also may 

be used to generate trilinear (Piper) diagrams and to define 

hydrochemical facies (Hill, 1940; Piper, 1944; Back, 1966). 

Water type as determined by major ion chemistry frequently 

is used to determine major geochemical reactions occurring 

in different parts of an aquifer, as well as reactions along 

a flowpath. This approach has been used by many of the 

previous geochemical studies in northern Ada County (Neely 

and Crockett, 1998; SPF Water Engineering, LLC, 2004; 

Glanzman and Squires, 2009).

Field parameter results varied considerably, depending 

on the aquifer sampled (table 3). Specific conductance in 

the samples ranged from 198 to 863 μS/cm; conductivity 

values were highest in the Dry Creek aquifer (863 μS/cm) 

and shallow aquifer (470 μS/cm) samples and lowest in the 

intermediate/deep aquifer and Willow Creek aquifer samples. 

The pH values of samples ranged from 6.2 to 8.0; pH values 

were lowest in the shallow aquifer samples and highest in the 

Willow Creek aquifer samples. Water temperatures ranged 

from 56 to 85 °F; water temperatures were highest in the Dry 

Creek aquifer. Dissolved-oxygen concentrations ranged from 

0.1 to 9.6 mg/L; dissolved oxygen concentrations typically 

were higher in the shallow aquifer and intermediate/deep 

aquifer samples than the Willow Creek and Dry Creek aquifer 

samples. 

Table 3. Physical parameters for samples collected from selected wells in northern Ada County, southwestern Idaho, 

September–October 2009.

[Sample name: Location of wells sampled are shown in figure 1. Sample names with “R” indicate replicate sample. Abbreviations: Lab, laboratory; °F, degrees 

Fahrenheit, μS/cm, microsiemens per centimeter; <, less than]

Sample

name

USGS station No.

Specific 

conductance pH Water 

temperature,

field 

(°F) 

Air 

temperature 

(°F)

Dissolved 

oxygen 

(mg/L)

Alkalinity Bicarbonate, 

field

 (mg/L)

Carbonate, 

field 

(mg/L) Field 

(μS/cm)

 Lab

 (μS/cm) Field Lab

Field 

(mg/L as 

CaCO3

)

Lab 

(mg/L as 

CaCO3)

Shallow aquifer

S1 04N 01E 05BDAC1 198 206 6.8 7.4 58.5 86.0 6.5 90 90 109 <0.1

S2 04N 01E 06DDD1 312 326 6.5 6.8 56.1 73.4 6.4 96 98 118 <0.1

S3 05N 01E 34ADCD1 470 478 6.9 7.2 56.5 67.1 2.1 146 151 178 <0.1

Intermediate/Deep aquifer

ID1 05N 01W 16DBC1 297 297 6.9 6.9 70.0 48.2 0.1 119 123 145 <0.1

ID2 05N 01W 28DDC4 204 280 7.1 7.1 64.2 78.8 2.6 118 120 144 <0.1

ID3 05N 01W 28DDC5 264 274 6.2 7.1 58.8 77.0 9.6 115 113 140 <0.1

ID4 04N 01E 14CCB3 321 330 7.0 7.3 58.3 68.0 0.1 149 149 182 <0.1

ID5 04N 01E 14CCB4 260 272 6.9 7.3 56.5 73.4 0.1 125 130 153 <0.1

ID5R 04N 01E 14CCB4 264 273 6.2 6.7 58.8 77.0 9.6 115 131 140 <0.1

ID6 04N 01E 05CDDD1 248 259 7.0 7.0 55.9 41.0 5.2 106 112.6 129 0.1

ID7 04N 01W 11BDAA1 232 241 7.1 7.7 63.0 83.3 0.3 104 108 126 <0.1

Willow Creek aquifer

WC1 06N 01W 35CCA1 244 248 8.0 8.0 85.3 85.1 0.1 77 83 92 <0.1

WC2 05N 01W 09CCD2 219 221 7.0 7.2 74.5 69.8 0.1 104 103 127 <0.1

WC2R 05N 01W 09CCD2 260 221 6.9 7.3 58.3 68.0 0.1 125 103 153 <0.1

Dry Creek aquifer

DC1 05N 02E 31ADDD1 863 846 7.5 7.9 57.2 89.6 0.9 203 230 247 < 0.1
10 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

Major ion chemistry (table 4) indicates that the water 

types for all samples collected in this study are either a 

calcium-bicarbonate or a sodium-bicarbonate water type. 

A trilinear diagram (fig. 2) is used to indicate the water 

types present in the study area. Water type is determined 

by calculating the most dominant species in each sample in 

terms of milliequivalents per liter (meq/L) to give each ion an 

equivalent weight. The lower right triangle shows calculations 

for anions, the lower left triangle shows calculations for 

cations, and the center diamond of the trilinear diagram 

displays the relation of the seven major species on one 

diagram. Each point represents one single water analysis 

and the water type. Water type for shallow aquifer and 

intermediate/deep aquifer samples were calcium-bicarbonate. 

The water type of samples from the Willow Creek aquifer and 

the Dry Creek aquifer was sodium-potassium-bicarbonate, 

commonly simplified to sodium-bicarbonate due to chemical 

similarities; therefore, this water type will be labeled as 

sodium-bicarbonate hereinafter.

Samples also were analyzed for silica, iron, and 

manganese. Silica concentrations ranged from 36.9 to 

52.6 mg/L in shallow aquifer samples, 30.3 to 42.6 mg/L 

in intermediate/deep aquifer samples, 43.1 to 44.7 mg/L in 

Willow Creek aquifer samples, and was 55.9 mg/L in the 

single sample from the Dry Creek aquifer. Iron concentrations 

in the northern Ada County were less than the detection limit 

in shallow aquifer samples (less than 4 μg/L), and ranged 

from less than 4 to 1,020 μg/L in deep aquifer samples. Iron 

concentrations ranged from 94.2 to 316 μg/L in Willow 

Creek aquifer samples, and measured 4.8 μg/L in the Dry 

Creek aquifer sample. Manganese concentrations were less 

than 0.8 μg/L in shallow aquifer samples. Concentrations 

of manganese ranged from less than 0.2 to 40.1 μg/L in 

intermediate/deep aquifer samples and 30.8 to 183 μg/L in 

Willow Creek aquifer samples. Manganese concentration was 

0.8 μg/L in Dry Creek aquifer sample. 

Table 4. Major ion and trace metals concentrations for samples collected from selected wells in northern Ada County, southwestern 

Idaho, September–October 2009.

[Sample name: Location of wells sampled are shown in figure 1. All major ions and trace metals are filtered. Abbreviations: mg/L, milligrams per liter; 

E, estimated; <, less than]

Sample

name

USGS station No. Calcium

(mg/L)

Magnesium 

(mg/L)

Sodium 

(mg/L)

Potassium 

(mg/L)

Chloride 

(mg/L)

Sulfate 

(mg/L)

Fluoride 

(mg/L)

Silica 

(mg/L as 

SiO2

)

Iron 

(μg/L)

Manganese 

(μg/L)

Reporting 

limit of 

analyte

0.01 0.006 0.06 0.03 0.06 0.09 0.04 0.1 2 0.1

Shallow aquifer

S1 04N 01E 05BDAC1 21.1 3.92 14.7 1.84 1.81 6.4 0.25 41.3 E4 0.3

S2 04N 01E 06DDD1 35.7 8.49 12.3 4.14 15.10 13.9 0.22 36.9 E3 E0.2

S3 05N 01E 34ADCD1 53.5 13.70 25.9 3.47 8.68 72.2 0.84 52.6 <4 0.7

Intermediate/Deep aquifer

ID1 05N 01W 16DBC1 30.7 7.43 19.1 2.28 5.25 25.3 0.55 39.3 1020 13.8

ID2 05N 01W 28DDC4 30.8 5.72 18.0 1.99 3.13 21.3 0.54 32.2 <4 E0.2

ID3 05N 01W 28DDC5 32.4 5.90 13.9 2.71 4.85 10.5 0.23 42.6 <4 E0.2

ID4 04N 01E 14CCB3 39.7 7.45 18.8 1.82 2.78 21.2 0.38 31.8 597 40.1

ID5 04N 01E 14CCB4 33.0 4.37 16.8 1.24 3.68 6.6 0.36 30.3 18 14.5

ID5R 04N 01E 14CCB4 32.8 4.34 16.3 1.20 3.69 6.6 0.36 30.3 19 15.3

ID6 04N 01E 05CDDD1 31.1 5.53 14.3 1.58 2.64 12.6 0.41 34.0 6 0.2

ID7 04N 01W 11BDAA1 28.9 5.03 13.8 1.76 2.25 14.1 0.39 31.7 155 3.9

Willow Creek aquifer

WC1 06N 01W 35CCA1 23.2 0.27 28.3 1.36 4.16 32.1 1.57 43.1 94.2 30.8

WC2 05N 01W 09CCD2 18.2 3.38 21.5 2.29 2.88 6.9 0.29 44.7 316 183

WC2R 05N 01W 09CCD2 18.8 3.50 22.2 2.30 2.87 7.0 0.27 43.4 319 183

Dry Creek aquifer

DC1 05N 02E 31ADDD1 0.09 E 0.008 1.81 257 6.19 101 0.33 55.9 4.8 0.8
Distribution of Isotopic and Environmental Tracers in Groundwater 11

Nutrients

Elevated concentrations of nutrients, such as nitrogen and 

phosphorus, indicate the presence of recharged water affected 

by human activities (for example, agriculture or wastewater 

disposal) (Hallberg and Keeney, 1993). An understanding of 

the sources and sinks of nutrients in groundwater is important 

because nutrients in high concentrations may have deleterious 

effects on the environment and on human health. The nutrients 

sampled for this study were ammonia (as nitrogen), nitrite (as 

nitrogen), nitrite plus nitrate (as nitrogen), and orthophosphate 

(as phosphorus).

Ammonia concentrations were detected in samples from 

the intermediate/deep aquifer, the Willow Creek aquifer, 

and the Dry Creek aquifer (table 5). Nitrite concentrations 

were not detected in any of the samples. Nitrite plus nitrate 

as nitrogen (hereinafter referred to as nitrate) concentrations 

were highest in shallow aquifer samples (7.01 mg/L); other 

nitrate concentrations were high (greater than 1.0 mg/L) 

in intermediate/deep aquifer samples. Orthophosphate 

concentrations were highest in the intermediate/deep aquifer 

samples (0.402 mg/L). Orthophosphate was the only nutrient 

at detectable concentrations in every sample. 

idtac10-0485_fig02

Magnesium (Mg) 

Calcium (Ca)

Sulfate (SO4) 

Chloride (Cl) + Fluoride (F) 

Sodium (Na) + Potassium (K) 

Chloride (Cl) + Fluoride (F) +

Sulfate (SO4) 

Calcium (Ca) + Magnesium (Mg) 

Bicarbonate (HCO3) + Carbonate (CO3) 

100 80 60 40 20 0 

0 0 

0 

20 

40 

60 

80 

100 

100 

80 

60 

40 

20 

0 

0 20 40 60 80 100 

100 

100 

100 

80 

60 

40 

20 

0 

0 

20 

40 

60 

80 

100 

20 

80 

40 

60 

60 

40 

80 

20 

80 

20 

60 

40 

40 

60 

20 

80 

PERCENTAGE OF TOTAL, IN MILLIEQUIVALENT PER LITER

Shallow aquifer 

EXPLANATION

Intermediate/Deep aquifer 

Willow Creek aquifer 

Dry Creek aquifer 

Figure 2. Major ion relations of samples collected from selected wells in northern Ada County, 

southwestern Idaho, September–October 2009.
12 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

Oxygen/Deuterium

The presence and concentrations of oxygen-18 and 

deuterium (hydrogen-2) in precipitation vary in response 

to sources of precipitation, altitude, season, temperature, 

amount of evaporation, and other factors. Thus, oxygen-18 

and deuterium can provide information regarding the seasonal 

timing and altitude of precipitation recharging groundwater. 

The ratio of oxygen-18 and deuterium to their lighter isotopes 

(hereinafter referred to as the signature) can indicate changes 

to water that have occurred since precipitation, such as 

evaporation or increased water-rock interaction (Coplen, 

1993). The Local Meteoric Water Line (LMWL) can be used 

to describe a continuum in the isotopic signature of local 

precipitation and its variation from the Global Meteoric Water 

Line (GMWL) as defined by Craig (1961). For this study, the 

LMWL representing the Boise area is taken from the modern 

LMWL as determined by Schlegel and others (2009), with an 

equation of δ2H=6.94*δ18O–10.7. 

The results of stable isotope analyses for all samples 

(table 6) were plotted on a delta diagram along with the 

modern LMWL as determined by Schlegel and others (2009) 

and the GMWL as determined by Craig (1961) (fig. 3). 

Oxygen-18 (δ18O) values ranged from -16.39 to -15.00‰, 

and deuterium (δ2H) values ranged from -129 to -119‰. 

Many of the samples plot below and to the right of the 

LMWL, displaying a preferential enrichment or excess of 

oxygen-18 as compared to deuterium relative to the LMWL. 

Oxygen-18 excess can be a product of evaporation (generating 

a slope between 3 and 6) or water-rock interaction from 

low temperature or geothermal waters (increases only the 

oxygen-18 signature) (Clark and Fritz, 1997). Samples from 

the foothills aquifers and intermediate/deep aquifer all plot 

below the LMWL, and two of the shallow aquifer samples 

(S2, S3) plot below the LMWL. 

Table 5. Nutrient concentrations for samples collected from selected wells in northern Ada County, 

southwestern Idaho, September–October 2009.

[Sample name: Location of wells sampled are shown in figure 1. Abbreviations: E, estimated; mg/L, milligrams per liter; 

<, less than]

Analyte USGS station No. Ammonia as N 

(mg/L)

Nitrite as N 

(mg/L)

Nitrate+Nitrite

as N 

(mg/L)

Orthophosphate 

as P 

(mg/L)

Reporting limit of analyte 0.02 0.002 0.04 0.008

Shallow aquifer

S1 04N 01E 05BDAC1 <0.02 <0.002 1.76 0.193

S2 04N 01E 06DDD1 <0.02 <0.002 7.01 0.27

S3 05N 01E 34ADCD1 <0.02 <0.002 3.23 0.278

Intermediate/Deep aquifer

ID1 05N 01W 16DBC1 <0.02 <0.002 <0.04 0.041

ID2 05N 01W 28DDC4 <0.02 <0.002 0.17 0.071

ID3 05N 01W 28DDC5 <0.02 <0.002 2.07 0.402

ID4 04N 01E 14CCB3 0.054 <0.002 <0.04 0.011

ID5 04N 01E 14CCB4 <0.02 <0.002 E0.02 0.033

ID5R 04N 01E 14CCB4 <0.02 <0.002 E0.02 0.033

ID6 04N 01E 05CDDD1 <0.02 <0.002 1.36 0.096

ID7 04N 01W 11BDAA1 <0.02 <0.002 <0.04 0.048

Willow Creek aquifer

WC1 06N 01W 35CCA1 0.062 <0.002 <0.04 0.024

WC2 05N 01W 09CCD2 0.18 <0.002 <0.04 0.153

WC2R 05N 01W 09CCD2 0.18 <0.002 <0.04 0.153

Dry Creek aquifer

DC1 05N 02E 31ADDD1 <0.02 <0.002 0.77 0.157
Distribution of Isotopic and Environmental Tracers in Groundwater 13

Sample 

name

USGS station No. δ2

H H2O 

(‰, VSMOW)

δ18O H2O 

(‰, VSMOW)

Tritium 

(TU)

14C 

uncorrected 

(pmc)

δ13C 

(‰, VPDB)

Corrected 

age (years)

Uncorrected 

age (years)

Shallow aquifer

S1 04N 01E 05BDAC1 -124 -16.39 5.8 110.72 -17.84 Modern Modern

S2 04N 01E 06DDD1 -122 -15.89 5.0 106.08 -17.89 Modern Modern

S3 05N 01E 34ADCD1 -119 -15.00 4.0 100.31 -15.76 Modern Modern

Intermediate/Deep aquifer

ID1 05N 01W 16DBC1 -124 -15.71 < 0.7 68.33 -13.63 1,900 3,059

ID2 05N 01W 28DDC4 -124 -15.74 < 0.7 81.44 -13.03 1,000 1,649

ID3 05N 01W 28DDC5 -123 -16.00 4.8 102.84 -17.31 Modern Modern

ID4 04N 01E 14CCB3 -129 -16.59 < 0.7 89.87 -14.07 560 857

ID5 04N 01E 14CCB4 -129 -16.31 6.2 106.78 -16.93 Modern Modern

ID5R 04N 01E 14CCB4 -129 -16.28 6.0 106.52 -17.20 Modern Modern

ID6 04N 01E 05CDDD1 -123 -15.95 6.1 102.56 -14.45 Modern Modern

ID7 04N 01W 11BDAA1 -129 -16.11 < 0.7 74.64 -13.16 1,400 2,350

Willow Creek aquifer

WC1 06N 01W 35CCA1 -124 -15.55 < 0.7 35.00 -12.30 8,400 8,433

WC2 05N 01W 09CCD2 -128 -16.12 < 0.7 31.47 -11.73 5,100 9,287

WC2R 05N 01W 09CCD2 -128 -16.12 < 0.7 31.13 -11.80 5,200 9,374

Dry Creek aquifer

DC1 05N 02E 31ADDD1 -123 -15.90 1.4 67.02 -13.34 2,000 3,215

Table 6. Isotope data for samples collected from selected wells in northern Ada County, southwestern Idaho, September–October 

2009.

[Sample name: Location of wells are shown in figure 1. “Modern,” indicates water recharged since the 1950s. Abbreviations: ‰, per mil; TU, tritium units; 

<, less than]

Figure 3. Oxygen and hydrogen isotopes for samples collected from selected wells in northern Ada County with Local 

Meteoric Water Line and Global Meteoric Water Line, southwestern Idaho, September–October 2009.

y = 8x + 10 

y = 6.94x – 10.7 

-150 

-145 

-140 

-135 

-130 

-125 

-120 

-115 

-110 

-105 

-100 

-19.0 -18.0 -17.0 -16.0 -15.0 -14.0 -13.0 

Shallow aquifer

Intermediate/Deep aquifer

Willow Creek aquifer

Dry Creek aquifer

Global Meteoric Water Line (Craig, 1961)

Local Meteoric Water Line 

(Schlegel and others, 2009)

Linear (Wood and Low, 1988) 

δ2H H2O (‰ VSMOW)

δ18O H2O (‰)
14 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

Tritium

The short-lived radiogenic isotope tritium, 3H (half-life 

of 12.43 years), is incorporated into the water molecule during 

atmospheric contact. It is the only isotope that is capable of 

dating groundwater itself (Clark and Fritz, 1997). Tritium is 

produced in small concentrations naturally in the atmosphere 

by cosmic radiation. However, atmospheric weapons testing 

between 1951 and 1963 introduced high concentrations that 

have been used as a means of dating groundwater recharge. 

The maximum activity of tritium in continental precipitation 

was approximately 1,000 TU (tritium units) during these 

years. Tritium activity in pre-1951 recharge water has decayed 

to less than 0.8 TU, and it was about 8 TU in continental 

precipitation in Salt Lake City in 2001 (International Atomic 

Energy Agency, 2001). Tritium incorporated in precipitation 

can give an estimate of the time that the sample was in contact 

with the atmosphere; tritium concentrations in samples greater 

than 0.8 TU indicate modern recharge or mixing with modern 

waters (Clark and Fritz, 1997). Tritium concentrations in 

samples less than 0.8 TU are considered to be tritium-dead; 

they indicate recharge prior to 1951 (Michel, 1989; Plummer 

and others, 1993; Clark and Fritz, 1997). Analysis of tritium/

helium ratios (3H/3He) in groundwater is an ideal tool in 

systems with piston flow. However, because of the complex 

nature of the groundwater system in north Ada County and 

the high likelihood of mixing of water, this analysis was not 

performed.

Tritium activities throughout northern Ada County ranged 

from less than 0.7 to 6.2 TU. Tritium activities in samples 

from the shallow aquifer were 4.0 to 5.8 TU, indicating 

modern recharge in all samples (table 6). Tritium activities in 

samples from the intermediate/deep aquifer samples were less 

than 0.7 to 6.2 TU, with the highest activities in the shallow 

zones of the monitoring wells (ID3, ID5) and in ID6. The 

Willow Creek aquifer contained tritium-dead samples with 

tritium activities of less than 0.7 and 0.4 TU. The sample 

from the Dry Creek aquifer contained 1.4 TU. Modern 

water, or some proportion of modern water as determined 

by tritium activity, was found in samples in the shallow 

aquifer, intermediate/deep aquifer, and Dry Creek aquifer. 

Tritium‐dead water was found in some of the samples from the 

intermediate/deep aquifer and in all samples from the Willow 

Creek aquifer. 

Radiocarbon

The primary method of dating groundwater recharge 

in the 1,000–40,000 year range is carbon-14 (half-life of 

5,730 years) dating. Carbon in groundwater is derived from 

various components of the carbon cycle, and carbon isotopes 

can give useful information about interactions between 

groundwater and its environment. Most of the carbon-14 

dissolved in groundwater is derived from carbon dioxide 

present in the soil zone. As recharge water passes through 

carbon-14 rich dissolved inorganic carbon in the soil zone, it 

incorporates carbon-14, although carbonate dissolution along 

a groundwater flowpath may dilute the carbon-14 activity, 

thereby requiring correction techniques. These techniques may 

range from the relatively simple (statistical, alkalinity, and 

carbon-13 mixing model) to advanced geochemical modeling. 

For this study carbon-13 (measured as δ13C) was measured to 

evaluate whether significant carbonate dissolution is occurring 

along the flowpath (Coplen, 1993; Clark and Fritz, 1997).

Carbon-13 (δ13C) results ranged from -17.89 and 

-11.73‰ throughout the entire system (table 6). The δ13C 

values ranged from -17.89 to -15.76‰ for the shallow aquifer 

samples, -17.31 to -13.03‰ for the intermediate/deep aquifer 

samples, -12.30 to -11.73‰ for the Willow Creek aquifer 

samples, and the value was -13.34‰ for the Dry Creek aquifer 

sample. Carbon-13 values were the most positive in samples 

from the foothills aquifers and were the most negative in the 

samples from the shallow parts of the aquifer, which suggests 

that a large portion of dissolved carbon is contributed from 

soil carbon dioxide. Less negative carbon-13 values may be 

attributed to increased interaction with the geologic matrix; 

longer residence times correlated with these more positive 

carbon-13 values verify this trend (fig. 4).

Carbon-14 values ranged from 31.47 to 110.72 pmc 

throughout the study area (table 6). Carbon-14 values ranged 

from 100.31 to 110.72 pmc in samples from the shallow 

aquifer, and all samples indicated modern recharge. Carbon-14 

values ranged from 68.33 to 106.78 pmc in samples from the 

intermediate/deep aquifer indicating varied residence times. 

The two samples that indicated modern recharge were from 

monitoring wells in the intermediate/deep aquifer that were 

less than 55 ft below land surface that do not experience 

frequent pumping; the other well indicating modern recharge 

was deeper than 100 ft and is frequently pumped. Carbon-14 

values were lowest in the Willow Creek aquifer samples 

(31.47–35.00 pmc). The carbon-14 value was 67.02 pmc 

for the Dry Creek aquifer sample. Uncorrected ages were 

calculated by the laboratory and were obtained using the decay 

equation:

14 14

14

14

 t 8, 267 ln ( C / C),

where

is the age of the water sample, in years,

C is the measured carbon-14 activity in 

groundwater, and

C is the carbon-14 activity of water in modern

so

a a0

t

a

a0

= �

il (rounded to 100 pmc).

(3)

To account for carbonate dissolution and to obtain a 

more accurate minimum residence time than allowed by 

simply the decay equation, carbon-14 measurements were 

corrected using a δ13C mixing model (Clark and Fritz, 

1997). The closed system model (developed by Pearson 
Distribution of Isotopic and Environmental Tracers in Groundwater 15

and White, 1967) is a useful tool for correcting carbon-14 

age calculations because carbon-13 measurements provide 

a tracer of carbonate evolution in groundwater. The δ13C 

mixing model was used in a similar geochemical study in 

Treasure Valley, Idaho (Hutchings and Petrich, 2002), and the 

carbon-13 values they suggested were used for the model in 

this report. This δ13C mixing model allows for incorporation 

of dissolved inorganic carbon during carbonate dissolution, 

and it accounts for fluctuations in the dissolved-inorganiccarbon pool, which can alter the carbon-14 concentration. This 

model assumes that carbonate dissolution takes place under 

closed-system conditions; open-system conditions were not 

accounted for in this study, but Hutchings and Petrich (2002) 

suggested that correcting for open-system conditions would 

result in values similar to the closed system. The δ13C mixing 

model‐correction factor is calculated using the equation:

 q C C /( C C

where

q is t

= − (δ δ δ δ DIC carb soil carb ) − ), 13 13 13 13

he dilution factor,

C is the measured C in groundw DIC δ δ 13 13 ater,

C is the C of calcite being dissolved, and carb δ δ

δ

13 13

13 13 C is the C of soil carbon dioxide. soil δ

(4)

Hutchings and Petrich (2002) suggested using 0.0‰ as 

the δ13C value of dissolved calcite and -23‰ as the δ13C value 

of soil carbon dioxide in the Treasure Valley study. These 

same values were used in this study.

The correction factor was calculated for each sample and 

multiplied by the uncorrected age to obtain a corrected age 

using the following equation from Clark and Fritz (1997):

14 14 t 8, 267 ln ( C / q 0 C). = � a a (5)

Corrected carbon-14 apparent ages (table 6) show the 

same trends as the uncorrected values. Corrected carbon-14 

apparent ages indicate modern water in the shallow aquifer, 

modern to 1,948 year old water in the intermediate/deep 

aquifer, 4,847–5,090 year old water in the Willow Creek 

aquifer, and 2,004 year old water in the Dry Creek aquifer. 

idtac10-0485_fig04

0 

1,000 

2,000 

3,000 

4,000 

5,000 

6,000 

7,000 

8,000 

9,000 

-20.00 -18.00 -16.00 -14.00 -12.00 -10.00 

CARBON-14 AGE, IN YEARS (CORRECTED) 

δ

13C (‰ VPDB) 

Shallow aquifer

Intermediate/Deep aquifer

Willow Creek aquifer

Dry Creek aquifer 

Figure 4. Carbon-13 values becoming less negative with increasing residence time. 
16 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

Chlorofluorocarbons and Dissolved Gases

Chlorofluorocarbons (CFCs) are stable volatile 

organic compounds (VOCs) that were first produced in the 

1930s for use in refrigeration and that have been used in 

the production of cleaning solvents, foams, plastics, and 

other synthetic materials (Clark and Fritz, 1997). CFCs 

are released into the atmosphere and are stable in the 

atmosphere. Because historical atmospheric concentrations 

are known, by measuring concentrations of CFC-12 

(dichlorodifluoromethane), CFC-11 (trichlorofluoromethane), 

and CFC-113 (trichlorotrifluoroethane) in groundwater, it is 

possible to identify groundwater recharged since about 1941, 

1947, and 1955, respectively. Analyses of selected dissolved 

gases in groundwater (N2, Ar, CH4, O2, and CO2) provide gas 

and excess air concentrations, thus allowing estimation of 

the recharge temperature. Both excess air concentration and 

recharge temperature are used in the analysis of CFC data to 

reduce uncertainty in the estimated age. Modeled recharge 

dates given are based on the time that groundwater entered 

the aquifer and became isolated from the unsaturated zone 

(Plummer and others, 1993).

Major dissolved gases were used to determine recharge 

temperature and to calibrate calculations for CFC data. The 

calculations involved in determining recharge temperature 

depended on recharge altitude, thus, a sensitivity analysis 

for recharge altitude was performed as described below. 

Two separate analyses were performed for each sample for 

quality‐control purposes. The results indicate the sample 

name and a lowercase letter for reference, for example S2(a) 

(table 7). One sample (WC1) was lost during processing, so 

only one analysis is available at this well. None of the samples 

were found to have excess nitrogen gas that could skew 

results; therefore, they did not need to be corrected for this 

parameter. Estimated recharge temperatures ranged from 41 to 

83.3 °F. Reported recharge temperatures are based on recharge 

occurring 1,000 ft above the land-surface altitude at the well 

(table 7) because this is the estimated recharge altitude. These 

recharge temperatures were used for CFC calculations.

Results from CFC analysis (table 8) were calculated 

based on recharge temperatures and altitudes (table 7). If two 

different recharge temperatures were generated, the average of 

the two temperatures was used in the CFC model. Out of the 

five bottles collected in the field for each CFC analysis, only 

two or three were analyzed in the laboratory. Results listed in 

table 8 indicate the sample name and an uppercase letter for 

reference, for example S2(A). Each CFC-model lists estimated 

recharge dates and indicates the possibility of contamination. 

All samples were determined to contain the presence of 

CFCs; modeled recharge dates range from the late-1940s to 

the mid-1990s. Dual recharge dates are possible for CFC-11, 

CFC-12, and CFC-113 due to the turnover in the CFC mixing 

ratios in the atmosphere (one concentration is equivalent to 

two different points in time). Although alternative dates are 

possible for some of the samples collected, the data from all 

three CFCs were used to calculate the recharge date. Recharge 

dates for samples in the shallow aquifer fall within the mid1980s to late-1980s. Samples in the intermediate/deep aquifer 

indicate recharge from the late-1940s to 1990. Samples in the 

Willow Creek aquifer indicate recharge from the late-1940s 

to early 1950s, and the Dry Creek aquifer sample indicates 

recharge in the early 1960s. In the two monitoring well sites 

with nested wells (ID2 and ID3, ID4 and ID5), different CFC 

recharge dates were generated for different depths; older dates 

correlated with the deeper water.

Because of the complex nature of the groundwater 

system, recharge altitudes were difficult to determine. 

Therefore, a sensitivity analysis was used to ascertain how 

recharge temperature changed over a range of altitudes and, 

in turn, how this affected CFC results. The minimum recharge 

altitude used was the altitude of the well, and the maximum 

altitude used was 7,000 ft (the highest altitude of the Dry 

Creek drainage basin). The range of recharge altitudes resulted 

in a maximum temperature difference of 11 °F. The varying 

recharge temperatures and altitudes were then used in the 

CFC model. Varying these parameters only slightly affected 

recharge dates; the greatest difference was only 7 years. 

Changing recharge altitude and recharge temperature did not, 

however, affect the overall presence of CFCs, which indicates 

the presence of modern water in all samples.
Distribution of Isotopic and Environmental Tracers in Groundwater 17

Table 7. Dissolved gas data and estimated recharge temperature for samples collected from selected wells in northern Ada County, 

southwestern Idaho, September–October 2009.

[Well name: Location of wells are shown in figure 1. ft, feet; °F, degrees Fahrenheit; cc STP/L, cubic centimeters per liter of pressure at standard temperature 

and pressure; mm Hg, millimeters of mercury]

Sample

name Legal name

Gases, in milligrams per liter Assigned 

recharge 

altitude 

(ft)

Recommended 

recharge 

temperature 

(°F)

Excess air 

(cc STP/L)

Barometric 

pressure

(mm Hg) Nitrogen 

(N2

)

Argon

(Ar)

Oxygen 

(O2)

Carbon 

dioxide

(CO2

)

Methane

(CH4

)

Shallow aquifer

S1(a) 04N 01E 05BDAC1 22.43 0.7432 4.57 25.01 0.0000 1,097 43.9 5.4 665.9

S1(b) 04N 01E 05BDAC1 22.26 0.7396 4.79 24.98 0.0000 1,097 44.0 5.3 665.9

S2(a) 04N 01E 06DDD1 18.28 0.6586 4.43 66.60 0.0000 1,090 46.8 1.8 666.5

S2(b) 04N 01E 06DDD1 18.21 0.6571 4.41 66.45 0.0000 1,090 46.8 1.7 666.5

S3(a) 05N 01E 34ADCD1 18.84 0.6409 1.13 42.23 0.0000 1,119 52.4 3.6 664.2

S3(b) 05N 01E 34ADCD1 18.87 0.6401 1.39 41.83 0.0000 1,119 52.7 3.6 664.2

Intermediate/Deep aquifer

ID1(a) 05N 01W 16DBC1 20.05 0.6642 0.23 19.91 0.0000 1,137 51.6 4.7 662.7

ID1(b) 05N 01W 16DBC1 19.94 0.6600 0.24 20.13 0.0000 1,137 52.1 4.7 662.7

ID2(a) 05N 01W 28DDC4 18.76 0.6640 0.99 14.77 0.0000 1,100 47.2 2.4 665.6

ID2(b) 05N 01W 28DDC4 18.88 0.6675 0.99 14.68 0.0000 1,100 46.9 2.5 665.6

ID3(a) 05N 01W 28DDC5 21.67 0.7029 7.56 39.52 0.0000 1,100 49.4 5.8 665.6

ID3(b) 05N 01W 28DDC5 21.72 0.7036 7.08 39.65 0.0000 1,100 49.5 5.9 665.6

ID4(a) 04N 01E 14CCB3 19.23 0.6582 0.24 26.40 0.0000 1,092 50.2 3.5 666.3

ID4(b) 04N 01E 14CCB3 20.23 0.6718 0.50 26.33 0.0000 1,092 51.0 4.7 666.3

ID5(a) 04N 01E 14CCB4 20.28 0.6978 0.26 24.29 0.0078 1,092 45.7 3.6 666.3

ID5(b) 04N 01E 14CCB4 19.95 0.6902 0.26 24.11 0.0068 1,092 46.1 3.4 666.3

ID5R(a) 04N 01E 14CCB4 20.09 0.6947 0.26 24.14 0.0076 1,092 45.7 3.4 666.3

ID5R(b) 04N 01E 14CCB4 20.06 0.6916 0.27 24.05 0.0069 1,092 46.2 3.5 666.3

ID6(a) 04N 01E 05CDDD1 20.23 0.7218 5.55 18.57 0.0000 1,097 41.0 2.5 665.9

ID6(b) 04N 01E 05CDDD1 20.06 0.7171 5.50 18.57 0.0000 1,097 41.3 2.4 665.9

ID7(a) 04N 01W 11BDAA1 19.24 0.6772 0.25 16.05 0.0000 1,070 46.5 2.7 668.0

ID7(b) 04N 01W 11BDAA1 19.12 0.6783 0.25 16.02 0.0000 1,070 45.8 2.4 668.0

Willow Creek aquifer

WC1(a) 06N 01W 35CCA1 14.35 0.4622 0.17 1.83 0.0070 1177 83.9 3.4 659.6

WC2(a) 05N 01W 09CCD2 15.71 0.5216 0.19 13.63 0.0397 1122 70.7 3.2 663.9

WC2(b) 05N 01W 09CCD2 15.67 0.5210 0.20 13.67 0.0393 1122 70.7 3.1 663.9

WC2R(a) 05N 01W 09CCD2 17.20 0.5465 0.56 14.07 0.0369 1122 70.9 4.7 663.9

WC2R(b) 05N 01W 09CCD2 15.81 0.5240 0.20 13.80 0.0383 1122 70.5 3.3 663.9

Dry Creek aquifer

DC1(a) 05N 02E 31ADDD1 19.01 0.6500 0.24 12.31 0.0010 1148 50.7 3.5 661.9

DC1(b) 05N 02E 31ADDD1 19.58 0.6623 0.30 12.58 0.0005 1148 50.1 3.9 661.9
18 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho Table 8. Chlorofluorocarbon results and modeled groundwater recharge dates for samples collected from selected wells in northern Ada County, southwestern Idaho, September–October 2009. [Sample name: Location of wells sampled are shown in figure 1. Abbreviations: ft, feet; °F, degrees Fahrenheit; pmol/kg, picomole per kilogram; CS, contaminated sample; NP, not possible] Sample name Assumed recharge altitude (ft) Calculated recharge temperature (°F) Chlorofluorocarbons (pmol/kg) Model piston dates (excess air corrected) Recommended age based on Date recharged CFC-11 CFC-12 CFC-113 CFC-11 CFC-12 CFC-113 Shallow aquifer S1(A) 3,600 44.0 2.233 3.301 0.360 1978 NP 1984 NP 1986 NP CFC-12, CFC-113 Mid 1980s S1(B) 3,600 44.0 2.212 3.210 0.355 1977 NP 1984 NP 1986 NP S2(A) 3,575 46.8 2.402 15.376 0.385 CS CS 1988 NP 1988 NP CFC-12, CFC-113 Late 1980s S2(B) 3,575 46.8 2.340 16.017 0.371 CS CS 1987 NP 1988 NP S2(C) 3,575 46.8 2.380 15.537 0.364 CS CS 1988 NP 1987 NP S3(A) 3,670 52.6 1.815 2.496 0.195 1978 NP 1985 NP 1984 NP CFC-12, CFC-113 Mid 1980s S3(B) 3,670 52.6 1.845 2.888 0.206 1980 NP 1986 NP 1984 NP Intermediate/Deep aquifer ID1(A) 3,730 51.9 0.008 0.161 0.041 1959 NP 1945 NP 1972 NP CFC-12 Late 1940s to early 1950s ID1(B) 3,730 51.9 0.008 0.165 0.039 1960 NP 1945 NP 1972 NP ID2(A) 3,610 47.1 0.031 0.124 0.014 1957 NP 1949 NP 1963 NP CFC-12 Late 1940s to early 1950s ID2(B) 3,610 47.1 0.027 0.126 0.001 1957 NP 1949 NP 1953 NP ID2(C) 3,610 47.1 0.035 0.111 0.035 1957 NP 1950 NP 1970 NP ID3(A) 3,610 49.5 3.869 4.667 0.226 1988 2007 CS CS 1984 NP CFC-11, CFC-113 Mid to late 1980s ID3(B) 3,610 49.5 3.878 4.713 0.227 1988 2006 CS CS 1984 NP ID4(A) 3,582 50.6 0.031 0.154 0.103 1959 NP 1949 NP 1978 NP CFC-12 Late 1940s to early 1950s ID4(B) 3,582 50.6 0.022 0.176 0.064 1960 NP 1948 NP 1975 NP ID4(C) 3,582 50.6 1.146 5.719 0.222 CS CS 1976 NP 1984 NP ID5(A) 3,582 45.9 3.492 0.178 1.769 1959 NP CS CS CS CS CFC-12 About 1990 ID5(B) 3,582 45.9 2.654 0.179 1.729 1959 NP 1989 NP CS CS ID5R(A) 3,582 45.9 2.515 0.173 1.562 1959 NP 1989 NP CS CS CFC-12 Mid to late 1980s ID5R(B) 3,582 45.9 2.150 0.190 1.714 1960 NP 1986 NP CS CS ID5R(C) 3,582 45.9 1.000 0.138 1.837 1958 NP 1973 NP CS CS ID6(A) 3,600 41.2 1.607 2.952 0.106 1975 NP 1977 NP 1976 NP all CFCs Mid 1970s ID6(B) 3,600 41.2 1.569 2.584 0.102 1974 NP 1976 NP 1976 NP ID6(C) 3,600 41.2 56.373 2.825 0.101 1975 NP CS CS 1976 NP ID7(A) 3,512 46.7 0.012 0.132 0.032 1957 NP 1946 NP 1969 NP CFC-12 Late 1940s to early 1950s ID7(B) 3,512 46.7 0.008 0.131 0.033 1957 NP 1945 NP 1969 NP
Distribution of Isotopic and Environmental Tracers in Groundwater

19

Sample 

name

Assumed 

recharge 

altitude 

(ft)

Calculated 

recharge 

temperature 

(°F)

Chlorofluorocarbons (pmol/kg) Model piston dates (excess air corrected)

Recommended age 

based on

Date 

recharged CFC-11 CFC-12 CFC-113 CFC-11 CFC-12 CFC-113

Willow Creek aquifer

WC1(A) 3,860 83.9 0.012 0.207 0.039 1965 NP 1949 NP 1978 NP CFC-12 Late 1940s to early 1950s 

WC1(B) 3,860 83.9 0.015 0.269 0.035 1967 NP 1949 NP 1977 NP

WC2(A) 3,680 70.7 0.018 0.200 0.101 1963 NP 1949 NP 1983 NP CFC-12 Late 1940s to early 1950s 

WC2(B) 3,680 70.7 0.014 0.156 0.078 1962 NP 1948 NP 1981 NP

WC2R(A) 3,680 70.7 0.021 0.179 0.085 1963 NP 1950 NP 1981 NP CFC-12 Late 1940s to early 1950s 

WC2R(B) 3,680 70.7 0.017 0.171 0.071 1962 NP 1949 NP 1980 NP

Dry Creek aquifer

DC1(A) 3,765 50.4 0.247 0.150 0.044 1959 NP 1963 NP 1972 NP CFC-12, CFC-11 Early 1960s

DC1(B) 3,765 50.4 0.230 0.155 0.043 1959 NP 1962 NP 1972 NP

DC1(C) 3,765 50.4 0.213 0.177 0.072 1960 NP 1962 NP 1976 NP

Table 8. Chlorofluorocarbon results and modeled groundwater recharge dates for samples collected from selected wells in northern Ada County, southwestern Idaho, September–October 2009.—Continued[Sample name: Location of wells sampled are shown in figure 1. Abbreviations: ft, feet; °F, degrees Fahrenheit; pmol/kg, picomole per kilogram; CS, contaminated sample; NP, not possible]
20 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

Application of Tracers in Groundwater

Groundwater flowpaths and flow directions for this 

study were assumed to follow the three primary flowpaths 

as determined by Squires (2008). These flowpaths are from 

the foothills aquifer systems downgradient into the alluvial 

aquifer system of the Snake River Plain (generally south to 

southwest), from shallower portions of the alluvial aquifer 

into deeper portions of the alluvial aquifer, and to a general 

northwesterly flow in the alluvial aquifer system of the 

Snake River Plain. A cross section (fig. 5) was constructed by 

selecting wells along the general flow direction overlain with 

data from this study highlighting differences in age tracers 

with depth. Although this cross section does not address 

geologic influences on groundwater geochemistry, it does 

demonstrate groundwater age variation at different depths.

Groundwater chemistry near recharge areas reflects the 

chemistry of recharge sources. As water moves downgradient, 

it interacts with the aquifer matrix and evolves from its initial 

chemistry. Groundwater in northern Ada County has a number 

of recharge sources including precipitation, infiltration of 

irrigation water, infiltration of canal water, and possibly 

other surface-water sources. Each of these recharge sources 

potentially has different water chemistry, which may account 

for variations in water chemistry and recharge age across the 

study area. Due to the limited nature of this study, potential 

recharge sources, such as surface water and precipitation, 

were not sampled. Thus, changes in chemistry are assumed 

to be associated only with movement along the groundwater 

flowpaths. 

Major ion chemistry of groundwater (table 4, fig. 2) 

varies throughout the study area and indicates interaction 

with local geologic units. Sodium and potassium were 

ELEVATION ABOVE NORTH AMERICAN 

VERTICAL DATUM OF 1988

Base map from U.S. Geological Survey,

1/3-Arc Second National Elevation Dataset,

2009. 

A A'

EXPLANATION

Shallow aquifer well and identifier

Intermediate to deep well and identifier

Dry Creek aquifer well and identifier

Residence time (years) as determined by corrected C-14 (as of 2009)

Recharge date as determined by modeled chlorofluorocarbon (CFC) data

Relative recharge period as determined by tritium activity

Nitrate concentration, in milligrams per liter

Wells with age tracers Age tracers

1,400

Late

1980s

Pre1951

7.01 Land surface

Late 1940s to early 1950s

Pre-1951

< 0.04

1,000

0

2,000

3,000

4,000

5,000

6,000

FEET

1,400

Modern

Modern

DC1

Modern

Mid 1980s

Post 1951

2,000

Early 1960s

Post 1951

0.77

ID7 S2

ID6

S3

0 20,000 40,000 Feet

 Mid-1970s

Late 1980s

Post 1951

Post 1951

1.36

7.01

3.23

Figure 5. Age tracers (carbon-14, CFC, tritium, and nitrogen) in samples collected from selected wells in northern Ada 

County, southwestern Idaho.
Application of Tracers in Groundwater 21

combined on the trilinear diagram and in water-type 

discussions because of their similar chemistry. As water moves 

downgradient from the foothills aquifers, water type changes 

from sodium‐bicarbonate to calcium-bicarbonate. Samples 

from differing depths in the Snake River Plain all indicate 

calcium‐bicarbonate type water; samples from the shallow 

aquifer and the intermediate/deep aquifer are dominated by 

calcium and bicarbonate. As water moves downgradient from 

the foothills aquifer systems into the alluvial aquifers, calcium 

concentrations tend to increase. The geochemical change 

from the foothills aquifer system could indicate deeper water 

mixing with water from the shallow aquifer, or dissolution 

of calcite and other calcium-rich aquifer materials. This 

geochemical change also could indicate that the foothills 

aquifers and alluvial aquifers are not hydrologically connected 

and that they do not allow for geochemical evolution. 

However, this investigation does not have enough information 

to address this issue. Due to the possibility of mixing and 

the complex geology of the alluvial aquifers, neither the 

exact source of calcium nor the connection between wells 

can be determined from the samples collected in this study. 

Additional samples collected from along a discrete flowpath 

and detailed geologic information could help to determine the 

source of major ions in groundwater. No distinct trend was 

determined by silica, iron, and manganese.

High nutrient concentrations in groundwater can be 

linked to anthropogenic activities, and they are used to 

help identify modern recharge and its presence throughout 

a groundwater system. The maximum contamination limit 

as determined by the EPA is 10 mg/L (U.S. Environmental 

Protection Agency, 2010). None of the samples in this 

study had high enough concentrations of nitrate to be 

near this hazardous level. Concentrations of nitrogen and 

orthophosphate were highest in samples from the shallow 

aquifer (table 5, fig. 5), indicating modern recharge. Some 

samples containing high concentrations of nitrogen and 

orthophosphate also contain high concentrations of chloride 

and potassium; evaporation, the application of artificial 

fertilizers (such as potassium chloride), or influence of septic 

tanks could explain these high concentrations. Additional 

samples of recharge water could help to determine if 

evaporation or application of fertilizers are influencing 

concentrations in groundwater. Deeper and older age-date 

waters tended to have low concentrations of both nitrogen and 

orthophosphate, confirming that high concentrations of these 

two nutrients reflect more recent recharge.

Oxygen and hydrogen isotopes were analyzed to help 

elucidate the source of recharge and aquifer interactions 

in northern Ada County groundwater. Due to the lack of 

isotopic data for precipitation, variations between seasonal 

contributions of precipitation could not be determined. 

Altitude and oxygen (fig. 6A) and hydrogen isotope (fig. 6B) 

values were plotted against each other to try to determine 

if the altitude effect (Clark and Fritz, 1997) contributed 

to isotopic variation. However, no obvious trend exists 

indicating that increasing isotope values are correlated with 

increasing altitude. Carbon-14 results were plotted against 

oxygen isotopes (fig. 7) to determine if climactic variations 

due to a changing climate regime affected the stable isotopes 

or indicated paleorecharge. No trend was apparent; oxygen 

isotopes have a broad range and more negative values are 

not correlated with increased age as would be expected from 

climactic variations.

All but one sample (S1) plotted to the right of the 

LMWL, indicating oxygen-18 excess or preferential 

enrichment (fig. 3). Oxygen-18 excess can result from 

evaporation or water-rock interaction. It is highly likely that 

this trend is due to evaporation because sources of recharge 

(inland precipitation and irrigation water) are likely to 

carry a signature indicating evaporation prior to recharge. 

However, sources of recharge were not sampled, so the effect 

of evaporation cannot be determined. Samples showing 

oxygen-18 excess also could indicate water-rock interaction 

with geothermal or low-temperature waters (fig. 7). Warmer 

waters greater than 68°F that could be related to geothermal 

interaction (Mitchell, 1981) could indicate this trend; many of 

the geothermal waters along the Boise frontal fault have been 

proposed to have experienced intense hydrothermal interaction 

(Schlegel and others, 2009). The three warmer waters greater 

than 68°F present in this sampling (ID1, WC2, WC1) all show 

the oxygen-18 excess trend, favoring geothermal interaction as 

the cause of oxygen-18 excess. Longer residence time leading 

to increased water-rock interaction also could contribute to 

oxygen-18 excess. This factor may be considered only if 

older waters showed this trend. However, samples indicating 

modern recharge (S2, S3, ID3, ID5) also showed a trend 

of oxygen-18 excess, inferring that extensive water-rock 

interaction with low-temperature waters is not the cause of 

oxygen-18 excess.

Distinct water groups were not defined by oxygen and 

hydrogen isotopes. Similar isotopic values existed for samples 

from the shallow aquifer, intermediate/deep aquifer, and 

foothills aquifers. This similarity could indicate that recharge 

from the foothills aquifers is contributing to the groundwater 

system of the intermediate/deep aquifer. No distinct 

difference was found among aquifer groups, indicating that 

the groundwater in northern Ada County is complex and well 

mixed; age dating information, discussed below, also indicates 

that groundwater is well mixed. Further investigation would 

help to determine contribution from various recharge sources 

and potential interactions between aquifer groups. Due to the 

varied nature of oxygen-18 values and the limited information 

on isotopic values of recharge sources, oxygen and hydrogen 

isotopes were not helpful in this study for establishing the 

source or elevation of recharge, or for mixing of groundwater 

from different aquifers. 
22 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

idtac10-0485_fig06ab

2,450 

2,500 

2,550 

2,600 

2,650 

2,700 

2,750 

2,800 

2,850 

2,900 

-16.80 -16.60 -16.40 -16.20 -16.00 -15.80 -15.60 -15.40 -15.20 -15.00 -14.80 

-130 -128 -126 -124 -122 -120 -118 

A. δ18O variations with altitude

2,450 

2,500 

2,550 

2,600 

2,650 

2,700 

2,750 

2,800 

2,850 

2,900 

ALTITUDE OF WELL SITE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988 (NAVD 88)

δ2H H2O (‰ VSMOW)

Shallow aquifer

Intermediate/Deep aquifer

Willow Creek aquifer

Dry Creek aquifer 

B. δ2H variations with altitude

Figure 6. Variation of (A) oxygen isotopes and (B) hydrogen isotopes with altitude in 

samples collected from selected wells in northern Ada County, southwestern Idaho.

Tritium, carbon-14, nutrients, and CFCs are useful age 

tracers of groundwater residence times, and they can aid 

in identifying locations of focused recharge. Variations in 

residence time give insight into flowpaths and recharge areas. 

Comparing these various age tracers at specific locations 

(fig. 5) can help in understanding the use of each tracer. 

Carbon-14 dating generates mean residence time on the scale 

of thousands of years, whereas CFCs, nutrients, and tritium 

can detect modern recharge. Groundwater with detectable 

tritium (greater than 0.7 TU) is indicative of waters recharged 

within the past 60 years; however, tritium information 

cannot be used to determine the source of recharge because 

precipitation and surface water may be tritium enriched. 

Tritium can be diluted in a well-mixed system when waters 

of various age (tritium-enriched modern water, tritium-dead 

water) blend; therefore, using tritium may not be helpful 

for identifying areas of modern recharge or for identifying 

recently recharged waters in the same way as CFC dating. 

Tritium activities in northern Ada County (table 6, 

fig. 5) reveal the presence of modern water in the shallow, 

intermediate/deep, and Dry Creek aquifers. Modern water 

(tritium activity greater than 0.8 TU) was found in every 

sample in the shallow aquifer and the Dry Creek aquifer, 

indicating that a portion of water in these aquifers has 
Application of Tracers in Groundwater 23

been recharged in the past 60 years. Tritium activities in 

the intermediate/deep aquifer indicate that some waters 

have experienced modern recharge, but that some are 

tritium‐dead. Both samples from the Willow Creek aquifer 

were tritium‐dead. Tritium-dead samples either could indicate 

recharge prior to atomic testing or could imply that tritium 

has been diluted by mixing. For this study, tritium is best 

interpreted in conjunction with CFC and carbon-14 data to 

determine if any modern recharge is occurring.

Analyses of CFCs and dissolved gases indicate that 

every sample collected in northern Ada County (table 8, 

fig. 5) contained at least a portion of modern water. Estimated 

recharge dates generated from CFC modeling ranged from 

the 1940s to the 1990s. Shallower samples tended to have 

evidence of more recent recharge; all samples collected at 

depths less than 180 ft showed evidence of recharge from the 

mid-1980s or later. Samples in the foothills aquifers show 

evidence of recharge from the late-1940s to the early 1960s, 

although recharge dates within the intermediate/deep aquifer 

vary from the late-1940s to the mid-1970s. CFC results imply 

that the shallow aquifer has experienced recharge within the 

past 30 years, although the intermediate/deep aquifer and 

foothills aquifers have experienced recharge within the past 

70 years. Recharge dates indicate older water downgradient 

from the shallow aquifer, confirming the flowpaths suggested 

by Squires (2008). 

Carbon-14 results indicate varied residence times 

throughout the system. Groundwater “age” should be thought 

of as the mean residence times of water molecules sampled 

because groundwater usually is a mixture of water molecules 

that recharged at different locations and different times. 

Carbon-14 dating indicates that samples from the shallow 

aquifer contain modern recharge, as do the shallow portions 

from the monitoring wells. Corrected ages in the deeper 

portion of the deep/intermediate aquifer indicate residence 

times on the order of 1,000–2,000 years, while the Willow 

Creek aquifer has residence times around 5,000 years. 

Carbon-14 residence times imply that older water is present in 

the study area but because the groundwater seems to be well 

mixed and only a few samples were taken, it is possible that 

even older water is present than the corrected dates generated 

for this study. Residence times tend to increase with depth; 

variations in residence time along a flowpath could not be 

determined from this study.

The use of a suite of age tracers confirms age trends in 

the study area, allowing for insight into the various ages of 

waters present. Mean residence times tend to increase with 

depth (fig. 8A), confirming that water flows from shallow 

to deep portions of the aquifer. This trend is seen somewhat 

with tritium (fig. 8B) and CFC data (fig. 8C). Young waters, 

as identified by these tracers, are consistent with elevated 

concentrations of nutrients (anthropogenic contaminants). 

idtac10-0485_fig07

0 

1,000 

2,000 

3,000 

4,000 

5,000 

6,000 

-16.80 -16.60 -16.40 -16.20 -16.00 -15.80 -15.60 -15.40 -15.20 -15.00 -14.80 

δ18O H2

O (‰)

CARBON-14 CORRECTED AGE, IN YEARS

Shallow aquifer

Intermediate/Deep aquifer

Willow Creek aquifer

Dry Creek aquifer 

Figure 7. Carbon-14 corrected age and oxygen isotopes in samples collected from 

selected wells in northern Ada County, southwestern Idaho.
24 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

idtac10-0485_fig08

0 

100 

200 

300 

400 

500 

600 

700 

9,000 8,000 7,000 6,000 5,000 4,000 3,000 2,000 1,000 0 

WELL DEPTH, IN FEET BELOW LAND SURFACE 

CARBON-14 CORRECTED AGE, IN YEARS 

A. Carbon-14 age at depth 

0 

100 

200 

300 

400 

500 

600 

700 

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 

TRITIUM ACTIVITY, IN TRITIUM UNITS 

B. Tritium activity at depth 

0 

100 

200 

300 

400 

500 

600 

700 

1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 

CFC MODELED RECHARGE DATE 

C. CFC recharge date at depth 

Shallow aquifer

Intermediate/Deep aquifer

Willow Creek aquifer

Dry Creek aquifer 

Figure 8. Age tracers (A) carbon-14, (B) tritium, and (C) 

chlorofluorocarbons, with depth in samples from northern Ada County, 

southwestern Idaho.
Application of Tracers in Groundwater 25

The presence of CFCs in every sample and tritium in many 

samples indicates that even samples that have residence times 

of thousands of years through carbon-14 dating contain at 

least some modern water. Samples with older carbon-14 ages 

also have older recharge dates as determined by CFC dating, 

indicating that this modern water took years to reach this 

portion of the aquifer. Similarly, comparing concentrations of 

tritium and radiocarbon (fig. 9) indicates that age tracers are in 

agreement and are able to identify relatively younger or older 

waters. No distinct pattern is seen between solute chemistry 

data and ages as determined by carbon-14 dating. 

Mixing is likely to occur due to the highly permeable 

and interbedded composition of the aquifers in this study. The 

presence of young and old waters in the same sample indicates 

a well-mixed groundwater system that has the potential for 

infiltration of young water into older groundwater. Modern 

recharge typically is dominant in shallower samples, and it 

was found in every sample in the study. However, carbon-14 

dating indicates that samples that have experienced modern 

recharge may also have a mean residence time of up to 

thousands of years. These older residence times are typical of 

deeper samples in the study area and, they could be a product 

of depth or compartmentalization. The varying age dates 

indicate that some areas of the intermediate/deep aquifer have 

complex recharge systems and that these systems experience 

focused but relatively small volumetric modern recharge. 

Some areas of the intermediate/deep aquifer (ID6) and the 

entire shallow aquifer seem to be dominated by modern 

recharge. Due to the small number of samples collected for 

this study, geochemical changes along a flowpath or with 

increased residence time are difficult to determine. Similarly, 

determination of sources of recharge was difficult due to lack 

of data about recharge sources. 

0.0 

1.0 

2.0 

3.0 

4.0 

5.0 

6.0 

7.0 

0 20 40 60 80 100 120 

Younger waters

Older waters

TRITIUM, IN TRITIUM UNITS

CARBON-14, IN PERCENT MODERN CARBON

Shallow aquifer

Intermediate/Deep aquifer

Willow Creek aquifer

Dry Creek aquifer 

Figure 9. Tritium concentration and carbon-14 activity in samples collected from selected 

wells in northern Ada County, southwestern Idaho.
26 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

Suggestions for Further Study 

Future geochemical studies involving the isotopic tracers 

used in this study could help to improve understanding of the 

hydrologic system of northern Ada County. More samples 

spanning a larger area and taken from various depths could 

help to determine regional groundwater trends. Future 

sampling strategies should aim to collect a large number of 

samples and target samples from wells close to one another 

and in the same stratigraphic unit to determine geochemical 

changes along a determined flowpath. Future sampling 

efforts also should aim to stratify well selection by various 

anthropogenic contaminants (such as septic system density or 

fertilizer application intensity) to help determine sources of 

anthropogenic compounds. In addition, more samples need 

to be obtained from the foothills aquifers to understand the 

hydraulic connectivity between these aquifers and those in 

the Snake River Plain. Sampling monitoring wells at various 

depths proved to be a useful technique in this study and should 

be utilized in future studies.

Although major ion chemistry did not add to the 

understanding of the system in this study, collecting major ion 

chemistry of source waters in addition to that of groundwater 

could help to elucidate sources of recharge to the system. 

Geochemical modeling can use saturation indices to help 

to identify minerals that groundwater interacts with and the 

trend of chemical evolution that groundwater experiences. 

Saturation indices for samples collected for northern Ada 

County study were examined along an assumed flowpath 

to determine major geochemical reactions; however, no 

consistent patterns were recognized, possibly because of the 

varied nature of the aquifer matrix and interactions between 

aquifers. In future studies, saturation indices may be used 

to examine major geochemical reactions expected along 

flowpaths and may yield further insight into the groundwater 

system.

Future studies also could include sampling oxygen and 

hydrogen isotopes of recharge sources to assess recharge 

sources and chemical evolution of recharge water. Oxygen 

and hydrogen isotopes of precipitation and source water 

(such as the Boise River, canals, and urban irrigation sources) 

should be collected at various altitudes and throughout various 

seasons to assess altitude and seasonality of recharge so as 

to account for evaporation, altitude, and seasonal effects. 

Sampling for oxygen and hydrogen isotopes also could help 

to determine whether the oxygen-18 excess trend seen in this 

study is attributed to recharge from evaporated sources or 

water-rock interaction. 

Understanding of the system was improved by including 

age tracers in this study; future studies should include 

age‐dating techniques, but can minimize the number of 

methods used while still gaining useful information. Nutrient, 

CFC, and tritium data all were used to identify modern waters, 

and they detected similar trends. Because tritium can be 

diluted in a well-mixed system, and because tritium analyses 

are expensive, it may not be the best tool for detecting modern 

recharge. Similarly, nutrient data can be diluted easily and is 

not always found in modern recharge. Nutrient data also are 

highly sensitive to redox conditions and nitrate concentrations 

can be lowered by reducing conditions. For this study area 

involving mixed groundwaters, CFC analyses were the 

most useful tracer of modern water because this tracer does 

not decay, can be detected in small amounts, and is evenly 

distributed across the study area. In similar study areas, 

especially in a similar region with well-mixed groundwater, 

CFC age-dating techniques may be the most cost-effective and 

efficient indicator of modern recharge.

In addition, age-dating techniques that were used in 

this study could be improved in a number of ways for more 

accurate dating in future studies. If tritium activities are 

sampled in the future, analyzing tritium activities in local 

precipitation could help to refine the relative age of recharge. 

Carbon-14 age estimates could be improved by constraining 

the age correction model used in this study. Soil and carbonate 

δ13C values were approximated for this study and were 

taken from previous modeling efforts. The values of δ13C in 

local soil and carbonate samples could help to constrain the 

δ13C mixing model correction model, and decrease error in 

carbon-14 age estimates. 

Summary and Conclusions

 In this study, general chemistry and geochemical tracers 

were measured from a group of wells in northern Ada County 

to understand the groundwater-flow system. Isotopic and 

environmental tracer data were collected from 13 wells at 

11 sites, and the data were used to determine geochemical 

relations and relative recharge dates of the groundwater. 

Major-ion chemistry was able to determine a shift in 

water type from the foothills aquifers (sodium-bicarbonate) 

into the plains aquifers (calcium-bicarbonate). However, 

due to the limited number of samples, lack of data about 

source waters, and minimal geologic information about the 

study area, the source of this water type change could not be 

identified. Major-ion chemistry did not seem to provide any 

information that could be used to determine relations between 

aquifer groups, relative recharge date of water, source of 

water, or geochemical evolution of water. In future studies, 

major-ion chemistry may be useful if samples of source water 

and additional groundwater samples can be obtained and 

saturation indices are examined. In addition, simple mixing 

models may help to explain the dominance of certain ions. In 

this study, however, major ion chemistry did not contribute 

significantly to the overall understanding of the groundwater 

system. 
Acknowledgments 27

Nutrient data in this study helped to identify samples that 

have elevated concentrations of nitrate and orthophosphate 

and that have most likely experienced modern recharge. 

Shallow samples seemed to have high concentrations of 

nitrate, indicating that they had been evaporated or recharged 

from modern waters. Although this method is not able to give 

an approximate date of recharge, sampling for nutrient data 

indicates recharge that has occurred during the use of modern 

agricultural practices or during urban development. Modern 

water, as determined by nutrient data, generally agreed with 

water dated with tritium and CFC analyses. However, nutrient 

data did not identify all modern recharge as determined by 

these methods. Nutrient analysis may not always identify 

modern recharge (especially if there is little agriculture), but it 

is a low-cost alternative to age-dating. 

Oxygen and hydrogen isotopes indicated a pattern of 

oxygen-18 excess, signaling that groundwater within the study 

area has experienced either evaporation before recharge or 

water-rock interaction. However, due to the lack of oxygen 

and hydrogen isotope information for recharge sources, the 

cause of this trend could not be determined. Conclusions 

about recharge sources or recharge environment could not be 

determined by oxygen and hydrogen data. Without further 

sampling of source waters, oxygen and hydrogen isotopes are 

not helpful in determining relations between water groups or 

in identifying sources of recharge. However, if further studies 

are conducted and sources of water are sampled, oxygen 

and hydrogen isotopes may prove to be a useful method of 

determining the source of recharge to the system or of waters 

that may have had geothermal interaction.

Tritium data indicated that modern to older, tritiumdead water existed in northern Ada County. Tritium activities 

generally agreed with age dates as determined by CFCs 

and carbon-14 dating, indicating that modern recharge is 

occurring in the shallow aquifer, portions of the intermediate/

deep aquifer, and in the Dry Creek aquifer, although older 

(tritium‐dead) samples were found in the intermediate/deep 

aquifer and in the Willow Creek aquifer. Due to the similar 

range of age dates generated by tritium and CFC data analyses 

and the relatively high expense of these analyses, it would be 

logical to perform only one of these analyses. No sampling 

or field constraints would prefer one analysis to another. 

However, this study has shown that groundwater in northern 

Ada County is well-mixed and could dilute tritium activity 

in water; some samples showed modern water as indicated 

by CFCs but not by tritium, possibly due to dilution or 

radioactive decay. To gain the most accurate information about 

modern recharge, CFC dating would be preferred over tritium 

dating. 

Dissolved gas and CFC data determined that modern 

water was present in every single well sampled in this study. 

These data, coupled with carbon-14 and tritium data, indicate 

that water molecules of different ages exist in the same portion 

of an aquifer. The discrepancies among the recharge ages 

indicate that the system is well-mixed, but that, in general, 

older water tends to be deeper in the system and younger water 

tends to be shallower in the system. In addition, CFC data 

confirmed assumed groundwater flowpaths of water moving 

downward into the system from shallower portions of the 

aquifer. Measuring for CFCs and dissolved gases was helpful 

in identifying the presence of modern recharge throughout the 

system, and this technique should be used in future studies in 

northern Ada County. However, only when CFC data were 

coupled with carbon-14 data was it apparent that the system 

was well-mixed; future studies should include both of these 

tracers for identification of mixed groundwater. 

Groundwater in northern Ada County had recharge 

dates of thousands of years ago to modern recharge through 

carbon-14 dating. In this study, carbon-14 dates were corrected 

using the carbon-13 mixing model. In future studies, collecting 

for carbon-13 and carbon-14 isotopes would be helpful in 

constraining recharge dates. Sampling local carbonate and 

soil CO2 could help to produce more accurate age estimates. 

Carbon-14 is helpful in indicating that old water does exist in 

the system and that it may make up a significant amount of the 

available water in the aquifer. 

This study is limited in its conclusions due to the limited 

amount of data collected; although it was able to determine 

that the northern Ada County study area contains well-mixed 

groundwater that has been recharged by modern and older 

water. Sources of recharge and complex relations among 

aquifers were unable to be assessed with certainty. To establish 

sources of recharge more accurately, additional samples would 

need to be collected.

Acknowledgments

The authors thank Dennis Owsley and Sean Vincent of 

IDWR; Ed Squires and Loren Pearson of Hydro Logic, Inc.; 

Roger Dittus of United Water; Deb Parliman, Jim Schaeffer, 

Alvin Sablan, and Rhonda Weakland of the USGS; and the 

landowners who gave us permission to sample their wells.
28 Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

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Adkins and Bartolino— Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho—SIR 2010–5144